{TIES}:-

ago;

This is to certify that the
dissertation entitled

EXPLORATORY SYNTHESIS AND CHARACTERIZATION
OF NEW MULTINARY BISMUTH CHALCOGENIDES
RELATED BY PHASE HOMOLOGIES

presented by
Jun Ho Kim

has been accepted towards fulfillment
of the requirements for the

degree In Chemistry

 

 

Jami/V

Major Professor’ 5 Signature

S/lc /2oo€

Date

 

MSU is an Affirmative Action/Equal Opportunity Institution

LIBRARY
Michigan State

University J

\U

 

_._.-.— -c-—-—.—o-n--a--u-o-o-n-:— —

-._ -._--.-u-n--o-n-.-.-.-._ —.

..-.--.-.- -.-.- — ..

PLACE IN RETURN BOX to remove this checkout from your record.
TO AVOID FINES return on or before date due.
MAY BE RECALLED with earlier due date if requested.

 

DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2/05 p:/ClRC/DaleDue.indd-p.1

 

EXPLORATORY SYNTHESIS AND CHARACTERIZATION OF NEW
MU LTINARY BISMUTH CHALCOGENIDES RELATED BY PHASE
HOMOLOGIES

By

Jun Ho Kim

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Department of Chemistry

2006

ABSTRACT
EXPLORATORY SYNTHESIS AND CHARACTERIZATION OF NEW

MULTINARY BISMUTH CHALCOGENIDES RELATED BY PHASE
HOMOLOGIES

By
Jun Ho Kim

Bismuth chalcogenide chemistry has been extensively studied for the past decades
since the BizTe3 was discovered as the best thermoelectric (TE) material at room
temperature. Ternary and quaternary bismuth chalcogenide system is apparently fertile
area to provide a variety of interesting structures. The studies in this dissertation were
mostly focused on exploration of the new quatemary compounds with complex
compositions containing additional transition metals. Structural and physical
characterization and crystal growth of new compounds were also performed.

Synthetic investigations were carried out in the A/M/Bi/Q (A = K, Rb, Cs; M =
Ag, Cd, Cu; Q = S, Se) and M/Bi/S (M = Ag, Cd, Pb, Sb) systems. A wide variety of new
phases were discovered that vary in composition and structure.

Investigation of various transition metals (Ag, Cd) with Bi in the alkali metal
chalcogenides resulted in a series of novel structure and compositions that define the
homologous series A2[M5+,,Seg+,,] (A = Rb, Cs; M = Bi, Ag, Cd; n = l, 2, 3, 4) as well as
the family of AM6Se9 (A = Rb, Cs; M = Bi, Ag or Cd). As a group the phases promote
better understanding of structural relationships and even enhance the predictive ability to
ultimately design targeted compounds. Other compounds in the ternary and quaternary
systems such as CdBi4S7, Cdo_63Pbo,ngi5$9, ,B-CsAngi35Se6 (x S 0.5), A2-2xAgl-xBi3+xQ6

(A = K, Rb, CS; Q = 8, Se), Al+xCdl+xBi3-XS6 (A = K, Rb), A2CUBi3Q6 (A = K, Rb: C5; Q

= S, Se), and Rb2.76Ago.6gBI4,3sseg were found by reactions of Bi with transition metals
such as Ag, Cd, Pb and Cu in the presence of alkali metal chalcogenides. Interestingly,
CdBi4$7 and Cd0,6ngo.3zBi589 are derived by tropochemical cell-twinning of galena type
slabs with a mirror symmetry as a twinning operation. The two layered compounds with
A2M4Q5 stoichiometry, Rb1,7Ago.85Bi3_1SS6 and Rb1,6Ago,gBi3,2Se6, display the possibility
of ion-exchange properties with Ag+/Pb2+ ions in the solution state.

The compounds AngxBi3_xSS (x = O, 0.3), CdBi4S7, and Cdo_6ngo,32Bi589 were
evaluated as potential thermoelectric materials. Since the Ag and Cd possess more
covalent character for bonding with the chalco gen atoms compared to the alkali metal,
this class of compounds was expected to exhibit more narrow energy gaps and showed

interesting TE properties including very low thermal conductivities.

ACKNOWLEDGMENTS

First of all, I am deeply grateful to my advisor, Professor Mercouri G. Kanatzidis,
for his help, support, guidance, and encouragement during the past five years. I especially
appreciate the opportunity to join in this advanced lab and chance to work with
intellectual professor.

I would like to acknowledge my committee members, Professors James
McCusker, Professor Subhendra. D. Mahanti, and Professor David P. Weliky, for their
precious guidance. I would also like to thank Professor Tim Hogan and his students Sim
Loo and Jarrod Short for the charge transport property measurements, and Daniel Bilc for
the electronic structure calculations.

I also want to thank all the past and current group members with their lovely
support, companionship, and everything from them. I have no words to express my
gratitude to Dr. Duck-Young Chung who helped me with his sincere support from the
beginning.

In concluding, I would like to thank to all my family. Especially, my mother has
encouraged me with deeply trust and invaluable support. Furthermore I have been

cheered by great love from my wife Wan Soon, and two sons Kyu Joon and Kyu Young.

iv

TABLE OF CONTENTS

page
LIST OF TABLES ............................................................................ xii
LIST OF FIGURES ........................................................................... xvi
LIST OF ABBREVIATIONS ............................................................... xxiii
Chapter 1. Thermoelectric Materials and Multinary Bismuth Chalcogenides ....... l
1. Thermoelectric concepts ......................................................... 1
2. Multinary Bismuth chalcogenides .............................................. 6
3. Synthesis method .................................................................. 14

Chapter 2. Crystal Growth, Thermoelectric Properties and Electronic Structure of

AgBi 3S 5 and AngxBi 3.xS 5 (x=0. 3) .......................................................... 24
1. Introduction ........................................................................ 24
2. Experimental Section ............................................................. 25

Reagents ........................................................................... 25
Synthesis .......................................................................... 26

Ag Powder ................................................................... 26
AgBi385 ....................................................................... 26
Ango.3Bi2_7SS ............................................................... 27

3. Physical measurements ........................................................... 27
Electron Microscopy ............................................................ 27
Differential Thermal Analysis ................................................. 27
Solid-State UV/vis Spectroscopy .............................................. 28
Charge Transport and Thermal Conductivity Measurements .............. 28

Powder X-ray Diffraction ...................................................... 29

Single-crystal X-ray Crystallography ......................................... 29
Band structure calculation ......................................................... 3O

4. Results and Discussion ........................................................... 31
Synthesis and Crystal Growth .................................................. 31
AgBi385 ....................................................................... 31
AngxBi3-x85 ................................................................. 31
Structure Description ............................................................ 34
Energy gaps and electronic band structure calculations .................... 38
Thermoelectric properties ...................................................... 45

5. Concluding Remarks ................................................................ 48

Chapter 3. A New Chalcogenide Homologous Series A 2[M 5+nSe9+nl (A = Rb, Cs; M = Bi,

Ag, Cd) ......................................................................................... 53
1. Introduction ........................................................................ 53

2. Experimental Section ............................................................. 54
Reagents ............................................................................. 54

Synthesis ............................................................................ 54

Ag Powder .................................................................... 55

fl-CsBi3Se5 and CstBi3Se6 ............................................... 55

szAgljBIlsSCn, and CSzAg|_5BI7_5Se|3 ................................. 56

CSAgo_5BI3_5886 ............................................................... 56

vi

3. Physical measurements ........................................................... 57
Electron Microscopy ............................................................ 57

DifferentialThermalAnalysis.................................................... 57

Solid-State UV/vis Spectroscopy .............................................. 57
Infrared Spectroscopy ............................................................ 58
Charge transport measurements ................................................ 58
Powder X-ray Diffraction ...................................................... 58
Single-crystal X-ray Crystallography ......................................... 58
4. Results and Discussion ........................................................... 69
Homologous series and Structure description ................................ 69
Thermoelectric properties ......................................................... 77
5. Concluding Remarks .............................................................. 77

Chapter 4. Crystal Growth and Thermoelectric Properties of CdBi4S 7 and

CdofingMszSg ............................................................................... 81
l . Introduction ......................................................................... 81

2. Experimental Section ............................................................. 83
Reagents ............................................................................. 83

Synthesis .......................................................................... 83

CdBi4$7 ....................................................................... 83
Cdo,6ngo_ngi589 .............................................................. 84

3. Physical measurements ........................................................... 84
Electron Microscopy ............................................................ 84

vii

Differential Thermal Analysis ................................................. 85

Infrared Spectroscopy ........................................................... 85
Charge transport measurements ................................................ 85
Powder X-ray Diffraction ...................................................... 85
Single-crystal X-ray Crystallography ......................................... 86
4. Results and Discussion ........................................................... 92

Synthesis, thermal analysis and crystal grth 92

Structure Description ............................................................ 93
CdBi4S7 ....................................................................... 96
CdofingongisSg ............................................................ 99

Charge Transport Properties and Energy Gaps ............................. 102

5. Concluding Remarks .............................................................. 106

Chapter 5. Structural diversity in the Quaternary Bismuth Selenides AM6Se9 (A = Rb, Cs ,'

M= Bi, Ag or Cd) ............................................................................. 110
1. Introduction ........................................................................ 110

2. Experimental Section ............................................................. 112
Reagents. ......................................................................... 1 12

Ag Powder ................................................................... l 12

Synthesis .......................................................................... 1 12
CsAgo_5Bi5.5Se9 ............................................................... l 12
Rbo,95Cdo,35Bi5,45Se9 ........................................................... l 13

RdeBi5Seg ................................................................... 1 l3

viii

Bridgman grth for Rb0,95Cdo,3sBi5.45Se9 .............................. 114

3. Physical measurements ........................................................... 114
Electron Microscopy ............................................................. 114
Differential Thermal Analysis ................................................. 115
Infrared Spectroscopy ........................................................... 115
Charge transport measurements ................................................ 115
Powder X-ray Diffraction ...................................................... 1 15
Single-crystal X-ray Crystallography ......................................... 116

4. Results and Discussion ........................................................... 128
Synthesis and Crystal Growth ................................................. 128
Structure Description ........................................................... 128

CsAgo,5Bi5.SSe9 ............................................................... 130
Rbo.95Cdo,35Bi5_458e9 ........................................................... 132
RdeBIsSCg ................................................................... 136
Charge Transport Properties and Energy Gaps .............................. 137

5. Concluding Remarks ................................................................ 143

Chapter 6. Structural Diversity and Characterization of Novel Quaternary Bismuth

chalcogenide AM4Q6, A 2M4Q6 and A 2M6Q9 (A = K, Rb, Cs ; M = Bi, Ag, Cu, Cd; Q = S,

Se) .............................................................................................. 147
l . Introduction ........................................................................ 147

2. Experimental Section ............................................................. 149
Reagents ........................................................................... 149

ix

Ag Powder ................................................................... 149

Synthesis ............................................................................ 150
B-CsAgo,5Bi3,5Se6 ............................................................... 150
K1_36Ago,93Bi3,07S6 .............................................................. 150
K1_34Ag0.92Bi3,ogSe6 ............................................................................. 150

RblegogsBIluSg 151

Rb16AgogBl32366 151

Cs.,7Ago,gsBi3,15S6 ............................................................ 151

Cs] _5Ago_7sBi3_ZSSe(, ............................................................. 152
Rb1,34Cd1,34Bi2_66S6 .......................................................... 152
KL22CszzBlzjgs6 ............................................................ 152
RbZCuBi3Seé. ................................................................ 153
CszCuBi3S6. .................................................................. 153
Rb2_76Ago,6gBi4,358e9 .......................................................... 153

3. Physical measurements ........................................................... 154
Electron Microscopy ............................................................ 154
Differential Thermal Analysis ................................................. 154
Solid-State UV/vis Spectroscopy .............................................. 154
Infrared Spectroscopy ........................................................... 155
Charge transport measurements ................................................. 155
Powder X-ray Diffraction ...................................................... 155
Single-crystal X-ray Crystallography ......................................... 155

4. Results and Discussion ........................................................... 176

Synthesis and Crystal Growth ................................................. 176
Structure Description ........................................................... 178
fl-CSAgo_5BI3_5Se6. ............................................................. I 78

Hexagonal phases A2-2xAg.-xBi3+xQ6 (A = K, Rb, Cs; Q = S, Se) and

A1+xCd1+xBi3-xS6 (A = K, Rb) ............................................................ 180
AzCuBi3Q6 (A = K, Rb,Cs; Q = S, Se) .............................................. 183
Rb2,76Ago_6gBi4.35Se9 ............................................................ 186
Charge Transport Properties and Energy Gaps .............................. 190

Solution Ion-Exchange Properties of Rb1,7Ago_35Bi3,ISS6 and

Rb1,6Ago,3Bi3_2Se¢5. . .. . .. . .. . .. . .. . .. . .. . .. . .. . I96

comparison 0f the AM4Q6, A2M4Q6 and A2M6Q9 .......................... 200
5. Concluding Remarks .............................................................. 203
Chapter 7. Conclusions and Future Work .................................................. 207

xi

Table 2-1.

Table 2-2.

Table 2-3.

Table 2-4.

Table 3-1.

Table 3-2.

Table 3-3.

Table 3-4.

Table 3-5.

Table 3-6.

Table 3-7.

Table 3-8.

LIST OF TABLES

Page

Crystallographic Data for synthesized AgBi3S5 and Ango,34Bi2.66S5

and previous AgBi3S5 ......................................................... 32
Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for AgBi385. U(eq) is defined as one third of

the trace of the orthogonalized Uij tensor .................................. 33
Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for Ang0_34Bi2_6685. U(eq) is defined as one

third of the trace of the orthogonalized Uij tensor ......................... 33

Bond lengths [A] and angles [°] for AgBi385 and Ango_34Bi2,66S5. . 41

Summary of crystallographic data for members of A2[M5+,.Se9+,,]
Ifl-CSBI3SCs, szCdBi6Sen, CSCdBI3SCe, and szAg15Bi75Se13. . 60

Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for fl-CsBi3Se5. U(eq) is defined as one third
of the trace of the orthogonalized Uij tensor ............................... 62

Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for szCdBi6Seu. U(eq) is defined as one third
of the trace of the orthogonalized Uij tensor ............................... 62

Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for CstBi3Se6. U(eq) is defined as one third
of the trace of the orthogonalized Uij tensor ............................... 63

Atomic coordinates (x 104) and equivalent isotropic displacement
parameters (Azx 103)for szAg1,5Bi7_5Sel3. U(eq) is defined as one

third of the trace of the orthogonalized Uij tensor ........................ 63
Bond lengths [A] and angles [°] for ,B-CsBi3Se5 ........................... 64
Bond lengths [A] and angles [°] for szCdBi6Se“ ....................... 65
Bond lengths [A] and angles [°] for CstBi3Se6 ......................... 66

xii

Table 3-9. Bond lengths [A] and angles [°] for szAgljBi7jse|3 ................... 67
Table 3-10. Summary of crystallographic data for members of A2[M5+,.Seg+n]

and their band gaps ............................................................ 68
Table 4-1. Crystallographic Data for synthesized CdBi4S7 and CdofingongisSg. 87
Table 4-2. Atomic coordinates ( x 104) and equivalent isotropic displacement

parameters (A2x103)for CdBi487. U(eq) is defined as one third of

the trace of the orthogonalized Uij tensor .................................. 88
Table 4-3. Atomic coordinates ( x 104) and equivalent isotropic displacement

parameters (A2x103)for CdogngngBisSg. U(eq) is defined as one

third of the trace of the orthogonalized Uij tensor ......................... 88
Table 4-4. Bond lengths [A] and angles [°] for CdBi4S7 .............................. 89
Table 4-5. Bond lengths [A] and angles [°] for CdofingongisSg .................... 90
Table 4-6. Anisotropic displacement parameters (A2 x 103) for CdBi4S7. The

anisotropic displacement factor exponent takes the form: -21r2

[h2a*2Un+...+2hka*b* U12] .......................................... 91
Table 4-7. Anisotropic displacement parameters (A2 x 103) for Cdo,6ngo.3gBi5S9.

The anisotropic displacement factor exponent takes the form: -21t2

[hza’l'zUn+...+2hka*b*U12]..... ................................................ 91
Table 5-1. Crystal data and structure refinement for CsAgo.5Bi5_5Se9,

Rbo_95Cdo_35B15.45869, and RdeBIsSCg ............................................. I 17

Table 5-2. Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for CsAgo.5Bis_5Se9. U(eq) is defined as one
third of the trace of the orthogonalized Uij tensor ......................... 118

Table 5-3. Atomic coordinates ( x 104) and equivalent isotropic displacement

Table 5-4.

Table 5-5.

Table 5-6.

parameters (Azx 103) for Rbo,95Cdo_35Bi5,4SSeg. U(eq) is defined as one
third of the trace of the orthogonalized Uij tensor ......................... 119

Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for RdeBisseg. U(eq) is defined as one third

of the trace of the orthogonalized U5,- tensor ............................... 120
Bond lengths [A] and angles [°] for CsAgo.5Bi5,SSe9 .................. 121
Bond lengths [A] and angles [°] for RbogstgogsBIsAsSeg .............. 122

xiii

Table 5-7.

Table 5-8.

Table 5-9.

Table 5-10.

Table 6-1.

Table 6-2.

Table 6-3.

Table 6-4.

Table 6-5.

Table 6-6.

Table 6-7.

Table 6-8.

Table 6-9.

Table 6-10.

Bond lengths [A] and angles [°] for RdeBisseg ......................... 123

Anisotropic displacement parameters (Azx 103 for CSAgo.5Bi5_5Se9.
The anisotropic displacement factor exponent takes the form:
-21t2[h2 a*2U11+...+2hka*b* U12] .................................... 125

Anisotropic displacement parameters (Azx 103) for
Rbo_95Cdgo,35Bi5.45Se9. The anisotropic displacement factor exponent

takes the form: -21r2 [ h2 a*2Un + + 2 h k a* b* U12] ................. 126

Anisotropic displacement parameters (Azx 103) for RdeBIsSCg.
The anisotropic displacement factor exponent takes the form:
-21t2[h2a*2Uu+...+2hka* b* U12] .................................... 127

Crsytal data and structure refinement for AM4Q6, A2M4Q6
and A2M6Q9 .................................................................... 157

Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for ,B-CsAgo.5Bi3,5Seg. U(eq) is defined as one
third of the trace of the orthogonalized Uij tensor ......................... 161

Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (A2x103)for A2-2xAg1-xBi3+xQ6. U(eq) is defined as one
third of the trace of the orthogonalized Uij tensor ......................... 162

Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (A2x103)for A1+xCd1+xBi3-xS6. U(eq) is defined as one
third of the trace of the orthogonalized Uij tensor ......................... 161

Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (A2x 103) for szCuBi3Se6.U(eq) is defined as one third
of the trace of the orthogonalized Uij tensor .............................. 162

Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (A2x 103) for CszCuBi3se.U(eq) is defined as one third
of the trace of the orthogonalized Uij tensor .............................. 163

Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (A2x103) for Rb2_76Ago,69Bi4,3sseg. U(eq) is defined as one

third of the trace of the orthogonalized Uij tensor ......................... 163
Bond lengths [A] and angles [°] for ,B-CsAgo,5Bi3,SSe6 .................. 165
Bond lengths [A] and angles [°] for A2-2xAg.-xBi3+xQ6 .................. 166
Bond lengths [A] and angles [°] for A1+xCd1+xBi3-xS6 .................... 167

xiv

Table 6-11.

Table 6-12.

Table 6-13.

Table 6-14.

Table 6-15.

Table 6-16.

Table 6-17.

Table 6-18.

Table 6-19.

Table 6-20.

Table 6-21.

Table 6-22.

Bond lengths [A] and angles [°] for szCuBi3Se6 ........................ 168
Bond lengths [A] and angles [°] for CszCuBi3S6 .......................... 169
Bond lengths [A] and angles [°] for Rb2.76Ago,69Bi4,gsse9 ............... 170

Anisotropic displacement parameters (Azx 103) for fl-CsAg0,5Bi3.5Se6.
The anisotropic displacement factor exponent takes the form:

-21t2[h2a*2U11+... +2hka*b* U12] ................................ 172

Anisotropic displacement parameters (A2 x 103) for A2-2xAg1-xBi3+xQ6.
The anisotropic displacement factor exponent takes the form:
-213“? a*2U“+...+2hka* b* U12] .................................... 173

Anisotropic displacement parameters (A2 x 103) for A1+xCd1+xBi3-xS6.
The anisotropic displacement factor exponent takes the form:
-27t2[h2a*2U11+...+2hka*b* U12] .................................... 172

Anisotropic displacement parameters (A2x 103) for Rb2CuBi3Se6
The anisotropic displacement factor exponent takes the form:

-21t2[h2a*2U11+... +2hka* b* U12] ................................ 174

Anisotropic displacement parameters (A2x 103) for CszCuBi386
The anisotropic displacement factor exponent takes the form:

-27t2[h2a*2U11+... +2hka* b* U12] ................................ 174

Anisotropic displacement parameters (A2 x 103) for
Rb2,76Ago,6gBi4_gSSe9. The anisotropic displacement factor
exponent takes the form: -21t2[h2 a*2Un + + 2 h k a* b* U12 ]. . 175

Unit cell parameters and volumes for several AM4Q6 structures ....... 194

Unit cell parameters and volumes for several A2M4Q6 structures and

A2M6Q9 ......................................................................... 195

Comparison of specific interlayer distances (001) and EDS data of the
products obtained from the ion-exchange reaction of a)
RbI.7AgO.8SBi3.ISS6 and b) Rb1.6Ago.8Bi3.2S€6 With A8N03

and Pb(NO3)2 .................................................................. I99

XV

Figure 1-1.

Figure 1-2.

Figure 1-3.

Figure 1-4.

Figure 1-5.

Figure 1-6.

Figure 2-1.

Figure 2-2.

Figure 2-3.

LIST OF FIGURES

Page
Schematic representation of a) the Peltier effect for cooling devices
and b) Seebeck effect for power generation devices ...................... 2

Various building units from the NaCl structure observed in multinary
bismuth chalcogenides : a) NaCl(NaCll°°)- es, b) SbZSe3(NaCl'°0)-
type, c) BizTe3(NaCl'“)-type, d)Cd12-(NaCl 11) and e) galena

(N aCl3 I 1)-type, (black circles bismuth atoms, white circles chalcogen
atoms) ........................................................................... 9

Various building blocks(shaded) based on different “cuts” of the
NaCl-type structure. The diagram is view down with [011] plane.

Black and white circles are bismuth and chalcogen atoms,

respectively ..................................................................... 10

Variations of the NaCl1 I 1-type blocks encountered in multinary

bismuth chalcogenides: a) Cdlz-type blocks in CdBi4S7 and
CdoggpbongIsSg, b) BizTe3-type DIOCKS in AM6869 (A = Rb, CS;

M = Bi, Ar or Cd), A2-2xM1-xBi3+xQ6 (A = K, Rb, Cs; M = Ag;

Q = S, Se) c) modified BizTe3-type in A2[Bi5+nSe9+,,] (A = K, Rb)

(gray circles bismuth atoms and white circles chalcogen atoms) ....... 11

A member-generating scheme illustrating successive additions of

MSe units to a M6Selo layer in the homologous subseries
Am[M1+1Sez+1]2m[l\/I4+,,Seg+,,] for l=2. Small white spheres denote Se,

large light-gray spheres A, and middle-gray spheres M. Question

marks indicate predicted but as of yet undiscovered compositions. . 16

a) Composition of reaction tube b) General Bridgman furnace setting

and temperature profile with height c) well grown polycrystalline

ingots of a bismuth chalcogenide compound from Bridgman furnace

(see chapter 2, 3, 4, 5 and 6) and (1) Speed controller with various
diameter wire guide which makes one revolution a day .................. 17

Ingots of a) AgBi3S5 and b) Ang0,3Bi2,7S5 grown in Bridgman
furnace ........................................................................... 34

XRD patterns of AgBi3S5 (a) of polycrystalline powdered sample,

(b) calculated from the crystal structure, (o) well grown ingot sample
with an X-ray beam along a direction on ab plane, ((1) b direction on

ab plane, (e) b direction on bc plane and (f) c direction on bc plane... 36

Projection of the structure of AgBi3S5 down the b-axis. Two slabs
can be described by slightly distorted layers out along the (311) face

xvi

Figure 2-4.

Figure 2-5.

Figure 2-6.

Figure 2-7.

Figure 2-8.

Figure 2-9.

Figure 3-1.

Figure 3-2.

Figure 3-3.

Figure 3—4.

of NaCl-type structure. The slab (11) includes five octahedra per one
diagonal octahedral chain. In the structure of Ango3Blz785 the Bi(l)
and Bi(2) sites are disordered with Sb atoms .............................. 37

A scheme of local coordination environment of Ag(l ), Ag(2) and
Bi(2) atoms in Ag31335 ....................................................... 38

Electronic band structure of AgBi3S5 with spin-orbit interaction
included (Eg = 0.17 eV) ...................................................... 43

Density of states (DOS) of AgBi3ss. (A) Total DOS, partial atomic
DOS of (B) bismuth atoms, (C) S] and S4, and (D) silver atoms ........ 44

Solid-state UVN is spectra for (a) AgBi385 and (b) AngogBiqus
respectively ..................................................................... 45

Variable temperature thermopower, electrical conductivity and
thermal conductivity for (a) AgBi3ss and (b) Ango.3Bi2_7ss.
(I = K1019 . = Kc, ‘= Kb“) .................................................... 47

(a) Variable temperature thermopower and electrical conductivity
data for AgBi3SS. (b) Thermal conductivity for AngOJBIZJSS
measured with the thermal diffusivity technique at high temperature. 49

Structural evolution of the homologous series A2[M5+,,Se9+,,]

(A = Rb, Cs; M = Bi, Ag, Cd; n = 1, 2, 3, 4). The various sizes

of the NaCll ' l-type ([M5+nSeg+n]) units as a function of integer n

are shown. With every increase of n by 1 a unit of “MSe” is added

to produce the next member(shown with a different color). The
particular modules in each case are shown within shaded

parallelograms .................................................................. 71

Comparison between two successive homologues to show

relationship. Projection of a) ABi3Ses (A = Rb, Cs; n = 1) down

the b-axis with ana space group and b) Rb2CdBigsen (n = 2) down

the c-axis with Pnnm space group. The NaCll ' l-type building units

with n = 1 and n = 2 are highli ted in both structures. M1 and M3

sites are mixed occupied by Bi + with Cd2+ ................................ 72

Solid-state electronic absorption spectra for all homologs ............... 74

Top: ingot of (a) fl-CsBi3Se5 and (b) CstBi3Se6 grown in a

Bridgman furnace. Bottom: The SEM image of oriented ,B-CsBi38e5
ingot. The direction of crystal growth is the b-axis in the structure and
micro cracks are shown inside the white circles ........................... 75

xvii

Figure 3-5.

Figure 4-1.

Figure 4-2.

Figure 4-3.

Figure 4-4.

Figure 4-5.

Figure 4-6.

Figure 4-7.

Temperature dependece of the thermopower for single crystal sample

of ,B-CsBi3Se5 and CstBi3Se6 ............................................... 76
Differential thermogram of the CdofingongisSg phase showing

melting and recrystallization events. Heating/cooling rate was

10 °C/min ....................................................................... 94

a) SEM images of CdBi4S7 (left) and Cd0.68Pb0_82Biss9 (right). b)

Ingot of CdBi4S7 grown in a Bridgman fumace. c) SEM image

of well grown ingot of Cdo,6ngo.32Bi589 phase prepared from
CdHPbei4S7 (x = 0.7) ....................................................... 95

Derivation of the unit structures from the galena type slabs (A and
A’) based on tropochemical cell twinning. (Dark gray circles bismuth
atoms and gray circles chalcogen atoms) a) A and A’ are joined with
sharing one anion atom. b) A and A’ are displaced around half
octahedron difference. c) It is modified from a) and has a metal ion

in the center of trigonal prism. d) It is also modified from a) and metal

atom reside inside of trigonal prism ......................................... 97

Projection of the structure of CdBi4S7 down the b-axis. The structure

is composed of two types of slab described as slab C (N aCl3 1 1-type)
and slab D (Cdlz-type) ........................................................ 98

Projection of the structure of Cdo,63Pbo,32Bissg down the a-axis.
The structure is composed of two types of slab described as slab C
(Nac13‘ l-type) and slab D (CdIz-type) ...................................... 101

Solid-state infrared absorption spectra showing band gap transitions
for a) Cdo,63Pbo,32Bi589 and b) CdBi4S7. The band gaps in each case
are estimated from the crossing point of the solid lines shown in

each spectrum .................................................................. 104

Variable-temperature thermoelectric power data for a) I CdBi4S7,

xviii

Figure 5-1.

Figure 5-2.

Figure 5-3.

Figure 5-4.

Figure 5-5.

Figure 5-6.

Figure 5-7.

0 CdBI4S7 (5% B1283 doping) and A CdBI4S7 (10% 31283 doping),
b) I CdofingongisSg phase prepared from Cd1-bexBi4S7 (x = 0.5) and
0 CdQfingngBIsSg phase prepared from Cd1-bexBi4S7 (x = 0.7) ...... 105

Derivation of the structures; a) CsAgo,5Bi3_5Se6, b) Rbo.95Cdo,35Bi5,4sseg
and c) RdeBisseg, fi'om the two BIzTC3 type slabs, two BiSe units
and Sb2Se3 type slabs with different arrays. a, b, c and d in the circles
on the SbZSe3 type slab represent the possible sites and the
combinations in the parenthesis show four possible arrays

of assembly ..................................................................... 129

Projection of the three dimensional structure (top) and polyhedral
representation (bottom) of CsAgo_5Bi5,5Se9 down the b-axis. The Cs

ions are in the tricapped trigonal prismatic spaces ........................ 131

A scheme of local coordination environment of Bi(2)/Ag(2), and

Ag(3)/Bi(2) atoms in CsAgo,sBi5.5Se9 ....................................... 132

Projection of the three dimensional structure(top) and polyhedral
representation(bottom) of Rbo,95Cdo,35Bis_4SSe9 down the b-axis.
The Rb ions are in the bicapped trigonal prismatic space ............... 133

Projection of the structure of PbsBi6Sel4 down the b-axis (top) and
polyhedral representation of PbsBi6Se|4 down the b-axis (bottom).

M5 in a circle is in a bicapped trigonal prismatic space .................. 135

A scheme of local coordination environment of Cd(6)/Bi(6) atoms in

RbogstogsBisasSCg. (a Cd-Se bonds) ....................................... 136

Projection of the two dimensional structure(top) and polyhedral
representation(bottom) of RdeBisseg down the b-axis. Shaded
rectangle area show cis formation between two Bi-Se octahedra

and Sb28e3 type slab ........................................................... 138

xix

Figure 5-8.

Figure 5-9.

Figure 5-10.

Figure 6-1.

Figure 6-2.

Figure 6-3.

Figure 6-4.

Figure 6-5.

Figure 6-6.

Solid-state infrared absorption spectra showing band gap transitions
for a) CsAgo.5Bi5_SSe9 at 0.30 eV, b) Rbo_95Cdo.35Bi5,45Se9 at 0.51 eV,
and c) RdeBi5869 at 0.49 eV. The band gaps in each case are
estimated from the crossing point of solid lines shown in each

spectrum ........................................................................ 1 39

Ingot of Rbo_95Cdo_3sBi5.4sseg grown in a Bridgman furnace.

The ingot was out along the direction parallel and perpendicular to

the crystal grth .............................................................. 140
Temperature dependence of the Seebeck coefficient for a single crystal

Of Rbo,95Cdo,3sBis.4sseg ........................................................ I41
Ingot of CsAgo,5Bi3.5Se6 grown in a Bridgman furnace .................. 177

Projection of the structure of a-CstBi3Se6 down the b-axis. The
shaded area is NaCl type (2X2) block ....................................... 179

Projection of a) the structure of and b) A+ arrangements in
A2-2xAg1-xBi3+xQ6 (A = K, Rb, Cs; Q = S, Se) and A1+xCd1+xBi3-xS6
(A = K, Rb) With inter layer distances ObelegQgsBiylsSe. . . . . . . . 182

Selected area electron diffraction patterns revealing a hexagonal
P lattice viewed along crystallographic [001] direction for
a) Rb1.7Ago.35Bi3,1ss6 and b) RblegogBiyzseqs ........................... 184

a) Projection of AzCuBi3Q6 (A = K, Rb, Cs; Q = S, Se) down the
b-axis and the BizTe3-type building block is shown within shaded
parallelograms. b) The coordination geometry of two distinct Rb(3)
atoms in szCuBigseG. c) Polyhedral representation of the structure
of AzCuBi3Q6 (A = K, Rb, Cs; Q = S, Se) ................................. 185

a) Projection 0f Rb2_76Ago.6gBi4,35Se9 down the b-axis. The shaded

XX

Figure 6-7.

Figure 6-8.

Figure 6-9.

Figure 6-10.

Figure 6-11.

Figure 6-12.

area indicate the BizTe3-type building blocks. b) Polyhedral
representation of the structure of of Rb2.76Ago.6gBi4,3sseo.
c) The arrangements of Ag(l) and Ag(2) atoms in the tetrahedral

coordination .................................................................... 1 87

Solid-state UV/vis and infrared absorption spectra showing band gap
transitions for a) B-CSAgo.sBi3,5866 at 0.51 eV, b) szCuBi3Se6 at
0.56 eV, c) C82CuBi3Se6 at 0.62 eV, and d) Rb2,76Ago_6gBi4_3sseg at
0.51 eV. The band gaps in each case are estimated from the crossing

point of the solid lines shown in each spectrum ........................... 188

Solid-state UV/vis absorption spectra showing band gap transitions
for a) K1,36Ago,93Bi3,07S6 and K1_34Ag0.92Bi3,ogse6 at 1.17 and 0.59 eV,
b) Rb1,7Ago.3sBi3,1586 and Rb1,6Ag0.3Bi3,2Se6 at 1.03 and 0.72 eV,

c) Csl,7Ago,3sBi3.1586, Cs],5Ago,7sBi3,25Se6, at 1.01 and 0.57 eV, and
d) Rb1_34Cd1_34Bi2.66S6, K122Cd122312jgs6 at 1.33 and 1.22 eV.

The band gaps in each case are estimated from the crossing point of

the solid lines shown in each spectrum ..................................... 191

Temperature dependence of the Seebeck coefficient for an ingot of

,B-CsAngi35Se6 with various x values (O for x = 0.1, I for x = 0.3,
A for x = 0.5) .................................................................. 193

Comparison of diffraction patterns of a) original Rb1,7Ago.35Bi3_1586
and the products obtained from the ion-exchange reaction with b)

Pb(NO3)2 and C) AgNO3 ...................................................... I98

Comparison of diffraction patterns of a) original Rb1_6Ag0,3Bi3_p_Se6
and the products obtained from the ion-exchange reaction with b)
Pb(N03)2 and c) AgNO3 ...................................................... 198

Structural diversities based on the following formulas AM4Q6,
A2M4Q6, and A2M6Q9 with various mono or divalent metal ions

xxi

and chalcogen ions. M represent several possible metal ions for
their structures including Bi atoms as primary metal ions ............... 202

xxii

LIST OF ABBREVIATIONS

TE Thermoelectric

CCD Charge Coupled Device

DTA Differential Thermal Analysis

EDS Energy Dispersive Spectroscopy

IR Infrared Spectroscopy

SEM Scanning Electron Microscopy
TEM Transmission Electron Microscopy

UV/V is Ultraviolet/Visible Spectroscopy

xxiii

CHAPTER 1

Thermoelectric Materials and

Multinary Bismuth Chalcogenidcs.

1. Thermoelectric concepts

The current energy consuming system has generated a lot of problems, especially
environmental disasters, which demand a proper solution from the scientific community.
Solid state chemists have proposed various and long-term solutions to meet our energy
needs while maintaining the quality of our environment such as photovoltaics, fuel cells,
thermoelectrics and batteries, which are concerned in energy storage or conversion based
on a coupling of chemical, thermal and/or electrical phenomena within the solid state.
One of the promising areas is thermoelectrics, which can convert thermal energy into
electrical energy or use electrical energy to move heat.I

The thermoelectric phenomenon was found in 1821 by Seebeck and in 1834 by
Peltier. A typical schematic of a thermoelectric couple is shown in Figure 1-1. It is
composed of two electrically conducting materials, n-type and p-type, which are joined to
make a junction. When the current flows as shown with the direction of arrows, the
electrons in the n-type material flow from the junction to the base, while the holes in the
p-type material flow from the junction to the base, which is known as the Peltier effect,

see Figure 1-1 a). When heat is applied to the junction, both negative and positive carriers

a)

 

 

b)

 

 

Figure 1-1. Schematic representation of a) the Peltier effect for cooling devices and b)

Seebeck effect for power generation devices.

transport heat to the base and a voltage difference is generated at the two base electrodes,
which is called the Seebeck effect, see Figure 1-1 b).

The advantages of thermoelectric devices are that they are highly reliable, light in
weight, small, quiet, environmentally fiiendly, and give precise temperature control. The
thermoelectric devices have been used especially in medical applications, laboratory
equipments and space missions as cooling and power sources, for which the cost and the
efficiency were not so important as the availability and the reliability. In addition, our
group has recently been trying to develop power generators using newly developed
thermoelectric materials from the waste heat of automobiles. Therefore improving the
efficiency, reducing the cost and increasing the applications of thermoelectric devices are
of strong interest.

The performance of a thermoelectric device can be described by the

dimensionless thermoelectric figure of merit:

2
ZT=S0T
K

 

where S is the thermopower or Seebeck coefficient, a the electrical conductivity, K the
thermal conductivity and T is the temperature. The thermal conductivity K‘ has
contribution fi'om lattice vibrations, to, and electrons, Ke, which are called the lattice
thermal conductivity and carrier thermal conductivity, respectively. Thus K = K, + IQ.
Therefore, a high thermoelectric figure of merit requires high electrical conductivity, high
thermopower, and low thermal conductivity. However, increasing the thermoelectric
power S for materials also leads to a simultaneous decrease in the electrical conductivity
and an increase in the electrical conductivity leads to a comparable increase in the

electronic contribution to the thermal conductivity because of the Wiedeman-Franz (WF)

lawz. So all these properties (S, a, x) determined by the particular electronic structures
and scattering of charge carriers (holes and electrons) are not independently controllable
parameters.

For the several decades since the late 1950s, the best values of ZT were ~1 in
binary metal chalcogenides, BizTe3, PbTe, szTeg, and their solid solutions, which are
doped narrow band-gap semiconductors having large thermopowers and electrical
conductivities but low thermal conductivities. 3 To be competitive compared to
conventional refrigerators and generators, TE materials require ZT > 3. 4 Therefore,
several attempts to improve ZT values have been made by including various concepts
such as quantum confinement (QC)5 and phonon glass electron crystal (PGEC)6.

The concept of a PGEC minimizing lattice thermal conductivity, suggested by
Slack, is that the material conduct electricity like a crystalline solid but heat like a glass.
Materials with PGEC characteristic such as skutterudites7 and clatharates8 have atoms in
the cages or the tunnels in the crystal structure, which have weak chemical bonds and
work as a rattler in solid lattice that results in dramatic reduction of the solid’s lattice
thermal conductivity without a deterioration of the electronic mobilities. For example, the
skutterudite CeFe3.5Coo,5Sb12 was reported to have ZT ~l .35 at ~900K9.

The quantum confinement (QC) proposed by Hicks et. all” is that new physical
phenomena are introduced into the thermoelectric figure of merit as the dimensionality is
decreased from 3D to 2D (quantum well), 1D(quantum wire), and OD(quantum dot)
crystalline solids. They bring out an importance of anisotropic effective mass through a

parameter E given by:

 

1 2k T 3” k 2
B: 372( h? l "'xmymz‘fgt‘x

Where m; is the effective mass of the carriers in the i direction, ,ux is the carrier mobility
along the transport direction, and rq is the lattice contribution to the thermal conductivity.
For an anisotropic three dimensional single band case and band degeneracy of y, when
the thermal and electrical currents travel in the same direction the figure of merit ZT
increases with a parameter B. Thus, in order to increase the value of Z, large effective
masses, high carrier mobility, and low lattice thermal conductivity are necessary.ll It has
recently been reported that nanostructured thin-film superlattices of BlzTe3 and szTe3
have ZT ~2.4 at room temperature, 12 whereas PbseoggTerz/PDTC quantum dot
superlattices have ZT ~1.6.l3

In addition, the presence of heavy elements and solid solutions leads to mass
fluctuation scattering of the lattice phononsl4 which generates randomness of the mass,
size, and charge of the atoms on a particular lattice position and can strongly scatter
lattice phonons carrying heat. So, in principle, a huge increase in ZT can be achieved by
going to lower dimensions, which is due not only to the enhanced thermopower and
electrical conductivity resulting from the change in the density of states but also the
reduced lattice thermal conductivity caused by the increased phonon scattering.

In pursuit of increasing the thermopower of material without depressing the
electronic conductivity, Boltzmann transport theory provides a general understanding of

the thermopower that is expressed in the Mott equation” :

sz dln0'(E)
. e . (IE 5:5’

 

s = fi
3
where 0(E) is the electrical conductivity determined as a function of band filling. The

electronic conductivity 0' = 0(E) 5:5, where Ef is the Fermi energy. If the carrier

scattering is independent of energy, then 0(E) is just proportional to the density of states
at E. In the general case, S is a measure of the difference in 0(E) above and below the
Fermi surface, specifically through the logarithmic derivative of 0' with E. Therefore
promising thermoelectric materials with high thermopower may require higher
compositional and structural complexity contributing to complex electronic structures
since the thermopower of a material is a measure of the asymmetry in electronic structure
and scattering rates near the Fermi level.

The previous compounds BaBiTe3, '6 CsBi4Te(,,l7 KBi(,_33Sm,18 K2Bigsl3,'8 fl-
K2B13Sel3, '9 K2,sBig.5Se14,'9 Ag1-be13SbTe20, 20 have been synthesized and showed
interesting and promising thermoelectric properties such as low thermal conductivity
without significantly decreasing the electrical conductivity and high thermopower due to
complex composition and structures. To further explore the effects of structural and

compositional complexity we investigated the Bi-Q system.

2. Multinary Bismuth chalcogenides

Solid state bismuth chalcogenides may be valuable candidates for thermoelectric
research. They have an abundant compositional and structural diversity from mineral to
synthetic chalcogenides such as sulfo- and selenosalts : PbBizs421, PbBi48722, PbBi4sgz3,
PbBi6SmZ4, P13231235”, PbgBi681126, P11331236”, Fineness“, 911,131,313”, Phasing”,
PbBiSe23o, PbB12Se43', PbBi4Se73', szBiZSe532°, Pb3Bi4Se932, PbgBi6Se1733, PbgBi4Se1533,
Sn4Bi2Se734, SnBi4Se735, CdBizs436, CdBi4S7 3°,Cd2,8318_.s,5 36, cazaiésll 36, ternary NaCl
type ABin (A = Li, Na, K; Q = S, Se)”, alkali metal bismuth chalcogenides RbBiQ2(Q

= 3, Se)”, flrCSBiSZ”, 103135540, KB16_33S.o'8, 193188.93, 1111131335“, CsBi38542, or-

KzBiSSeBls, ,B-KzBigSenlg, K2581853e14‘9, AzBigsen (A = Rb, Cs)“, CsBi3_67Se,4-”,
CszBi7Se12‘“, CS3Bi7Se1245, AxBi4Se7 (x = 1, 2; A = Rb, Cs)46, ABi3Q5 (A = Rb, Cs; Q -- S,
Se, Te) 47 , CSBi4Te6”, quaternary bismuth chalcogenides KzBi3.,beSe13 48 , K2-
,beBigSen‘g, KzBi3Se13.xS,5°, APbBi3Se6, (A = K, Rb, Cs)“, APbBi3S6, (A = Rb, Cs)“,
CszBi2ZnSe552, CszBizMss (M = Zn, Cd, Mn)”, AMBiS4 (A = Rb, Cs; M = Si, Ge)“,
K3Bi5Cuzsm (A = K, Rb, Cs), C3812CuS4 (A = K, Cs), RbBi2,66CuSe5, and CsBiAgzs355,
Tropochemical cell-twinning KxSn6-2xBi2+xSe9 and KSnsBisseuSé, the megaseries of
A,,[M.+,Se2.,]2,,,[M2,.,,Se2.n+,,], A,+,M’3-2,Bi7.,Se,4 (A = K, Rb, Cs; Sn, Pb)”, AHXM’4-
ZIM”7+xSels (A = K, Rb; M’ = Sn, Pb, M” = Bi, Sb)”, Cs1.,Sn,.,Big.,Se,5”, C515-
3xBi9.5+xS€1559, Ar-xM’s-xBi11+xS€20 (A = K, Rb, CS; Sn, Pb)57,A1—xM4-xBi11+xSezi (A = K,
Rb, Cs)“, K,-,Sn5-,Bin.,Se226'_ K..,Pb,.,Bn...,Se2262, A,.,Sn9.,,Bi....,Se26 (A = K, Rb,
Cs)“, new homologous series of CstmBi3Te5+m CsMBi3Te6 and CstBi3Te7 (M = Pb,
Sn)“, CSPb3B13Teg and CSPD4BI3T6965, and alkali earth bismuth chalcogenides SrBiSe366,
Sr4Bi6Se1367, 01-,B-BaBiZS468, BaBi28e443, BaBiSe369, Ba3Bi6,67Se;37°, Ba3MBi6Se13 (M =
Sn, Pb)”, BaBiTe37', SrBiTe372. However, not all of them have been well studied with
physical and structural characterization.

There are some reasons for structural diversity in bismuth chalcogenide
compounds. The bismuth atom can adopt several different coordination environments
from 3 to 9 coordination number with the trigonal pyramidal, square pyramidal,
octahedral and trigonal prismatic-type polyhedra. Important in affecting the local
geometry of bismuth atom is the 6s2 pair of electrons which can cause stereochemical
distortion in the bismuth coordination (when sp3 hybridization is present) or adopt a

symmetrical octahedral coordinating geometry (caused by hybridizing with energetically

adjacent p and d orbitals). This property is the result of the most adaptable coordination
geometry in the periodic table. Therefore, it is interesting to observe the behavior of Bi“
and its role in stabilizing various structure types. Among the various Bi-Q coordination
geometries, BiQ6 octahedral coordination is the most abundant. Furthermore octahedral
and square pyramidal geometry when combined can produce several common building
fragments such as NaCl-(NaCl'OO), Sb28e3-(NaC1'OO), BizTe3-(NaClm), Cdlz-(NaClm)
and galena typeS(NaCl3 I I), all of which are based on the NaCl-type structure but derived
by excising along different directions of the NaCl structure type, see Figure 1-2 and 1-3.

1'”) type building fragments known as good thermoelectric

For example, BizTeg-(NaC
material units are found with different size such as Cdlz-type in CdBi4S7 73 and
Cd0.68Pb0_ngISS973, BizTeg-type in AM6Se9 (A = Rb, Cs; M = Bi, Ar or Cd)74 and A2-
2,M,-,Bi3.,o, (A = K, Rb, Cs; M = Ag; Q = S, Se)75 and modified BizTe3-type in
A2[Bi5+,,Seg+,,] (A = K, Rb)“, see Figure 1-4.

Therefore the ultimate purpose of understanding the building units based on the
structures can be extended to design and predict crystalline solids with definitive
stoichiometries, compositions and structures. The concept of “homologous series” helps
to identify close structural and compositional relationships using a general formula. The
term “homologous series” was first used by Magneli to characterize transition metal
oxides that are expressed by general formulae and built on common structural
principles.77 The Aurivillius phases BizAn-1B,,O3,,+3 (A = Na, K, Ca, Sr, Ba, Pb, Ln, Bi, U,
Th etc and B = Fe, Cr, Ga, Ti, Zr, Nb, Ta, Mo, W etc.)78 and the Jacobson-Dion phases
A[A’,,-;B,,o3,,+;] (A = Li, Na, K, Rb, Cs, T1, NH4; A’ = Ca, Nd; B = Nb)79 related to rutile

and perovskite type lamellar oxides are examples of well known homologous series,

a)

b)

C)

d)

6)

 

Figure 1-2. Various building units from the NaCl structure observed in multinary
bismuth chalcogenides : a) NaCI(NaC11°°)-types, b) SbZSe3(NaCll°°)-type, c)
BizTe3(NaCl”')-type, d) Cdlz-(NaClm) and e) galena (NaCl‘m)-type, (black circles

bismuth atoms, white circles chalcogen atoms).

 

 

 

 

 

Figure 1-3. Various building blocks(shaded) based on different “cuts” of the NaCl-
type structure. The diagram is view down with [011] plane. Black and white circles

are bismuth and chalcogen atoms, respectively.

b)

 

Figure 1-4. Variations of the NaClm-type blocks encountered in multinary bismuth
chalcogenides: a) CdIz-type blocks in CdBi4$7 and Cdo_6ngo,32Bi5S9, b) BizTe3-type
blocks in AM6869 (A = Rb, Cs; M = Bi, Ar or Cd), A2-2xM1_xBi3+xQ6 (A = K, Rb, Cs;
M = Ag; Q = S, Se) c) modified BizTe3-type in A2[Bis+,.Seg+,,] (A = K, Rb) (gray

circles bismuth atoms and white circles chalcogen atoms).

where the integer n determines the thickness of the Slabs; moreover, the gustavite-

8' pavonite82 series and (CdS),,(B12S3),,,83 are also

lillianite series,80 the kobellite series,
known as sulfosalt homologous series. When compounds can be recognized and grouped
in a series of homologs defined by their structural modules, we then have a powerful way
of correlating and understanding large classes of materials thereby allowing useful
generalizations and predictions“. From this point of view, new series of compounds
Am[M1+ISez+z]2m[M2/+,,Se2+31+,,] (A = K, Rb, Cs, Sr. Ba; M = Sn, Pb, Eu, Bi, Sb)84a),
CstmBi3Te5+m85 and (szTe3),,,-(Sb2),,86 are of good example of homologous series from
our group. The megaseries Am[M1+zse2+1]2m[M21+nSez+31+,,], for example, is composed of
NaCl'OO-type [M;+/Sez+1]2m and NaClm-type [M21+,,Se2+31+n] slabs, which are
interconnected to create frameworks with tunnels accommodating the alkali metal (Am)
ions. The size of each module can be tuned by changing integer l, m and n while retaining
the Sites for alkali metals, see Figure 1-5. Therefore, many compounds in this series have
been successfully targeted for preparation after their structure and composition was
predicted by the general formula. Not only will this promote better understanding of their
interrelationships, but more critically it will enhance predictive ability and will prove to
be an important design tool for bulk solid-state materials.

Although some structural and compositional information can be found in the

80'83, relatively limited

literature for the mineral or synthetic sulfosalt compounds
information can be found about the thermoelectric properties of these materials.
Especially no sulfide classes, even structurally diverse, have been studied well with the

thermoelectric point view while some of the selenides and telluride classes have shown

promising properties such as Biz-bexTe3-ySey, TlgBiTe(, 87 and Ag1-belgstezozo. In

12

general, the energy band gaps of bismuth sulfide classes are wider than what is
considered to be optimum for TE performance because the pertinent materials tend to be
exceedingly resistive due to their strong ionic interactions between the alkali metal ions
and the [Bixsy]z' framework. Instead, desirable energy gaps for TB applications up to
1000 °C are thought to be <~0.6 eV. In order to include the bismuth sulfide class of
materials in thermoelectric investigations it is preferable to produce systems with smaller
semiconductor gaps by replacing, partially or totally, the alkali metals with other less
electropositive metals such as Ag+ , Pb2+ and Cd2+ capable of stronger interactions with
the [Bixsy]z‘ framework. We then expect them to be quite within the realm of possible
new bulk solid-state thermoelectric compounds with narrow energy band gap and higher
carrier mobility and it will help to understand thermoelectric properties based on the
crystal structure, electronic structure and composition.

The multinary compounds ,B-KzBi38e13, CsBi4Te6, Ag;-belgste20,
Am[M1+18ez+1]2m[M21+,.Se2+31+,.], CstmBi3Te5+m and (szTe3)m’(Sb2),, reported previously
have demonstrated promising thermoelectric properties based on comprehension of the
correlation between various crystal structures, compositions and individual
thermoelectric properties such as thermopower, electrical conductivity, and thermal
conductivity. This inspired us to try to extend this work to more complicated quaternary
systems and sulfosalts. To investigate new promising thermoelectric materials we
explored the ternary A/Bi/Q and quaternary A/M/Bi/Q (A = K, Rb, Cs; M = Cu, Ag, Cd;
Q = S, Se) systems and ternary M/Bi/S and quaternary M/M’/Bi/S (M = Ag, Cd; M’ = Sb,
Pb) system in terms of increasing the thermopower S by having complex structures,

reducing the thermal conductivity K by incorporating electropositive elements such as

13

alkali metals and solid solutions, and adapting the energy band gap by replacing alkali
metals with transition metal ions. The following chapters will Show the compounds
synthesized as well as investigations of their physicochemical, charge transport and
spectroscopic properties.

In particular, in Chapter 2 and 4 describe the mineral sulfosalts, AngxBi3-xss (x =
0, 0.3), CdBi487, and Cdo,6ngo,32Bissg, with crystal growth and thermoelectric properties.
In Chapter 3 we present a new chalcogenide homologous series A2[M5+,,Se9+nl (A = Rb,
Cs; M = Bi, Ag, Cd) that are formed by the NaClm-type slabs tuned by changing n. In
Chapter 5 we present the structural diversity of novel quaternary bismuth selenide
AM6Se9 (A= Rb, Cs ; M= Bi, Ag or Cd) systems where we found a number of different
polymorphs. In Chapters 6 we present complex two- or three-dimensional quaternary
structures that are formed by the incorporation of several transition metals into the

bismuth chalcogenide framework.

3. Synthesis method

The synthetic methods used in the preparation of bismuth chalcogenide
compounds are quite different from those used by organic, organometallic, coordination
and even metal oxide ones. The most widely used method for the synthesis of inorganic
materials follows an almost universal route that involves heating the components together
at high temperature over an extended period. Generally the metal chalcogenide
compounds are not stable while in air at the high temperature. Therefore, we usually used
evacuated fused Silica tubes for preventing unwanted oxidation. In this work we used

variety of synthesis techniques including the moderate temperature, polychalcogenide

14

flux method“. The molten salt (AzQx flux) method has been used for the exploration of
new materials with new structural types involving heavy elements such as Ba, Sr, Bi, Pb,
Sn, Se, Te with alkali metal ions. The traditional direct combination method at high
temperature and pelletized method at relatively lower temperature were also employed
with various temperature profiles. Especially, for incongruent melting compounds, to
avoid undesired byproducts in the targeted compounds we chose reaction temperatures
much lower than the melting points.

In addition, high quality samples for the measurement of TE properties have been
grown in selected cases. We have applied the Bridgman growth method for producing
large crystals”, see Figure 1—6. The Bridgman grth technique is basically a controlled
freezing process taking place under liquid - solid equilibrium conditions. The growth also
takes place under a temperature gradient, and the mechanism is to produce a Single
nucleus from which a Single crystal will propagate and grow. This is achieved by
allowing the solid - liquid interface to move slowly until the whole molten charge is

solidified.

15

AI-XM,5-XBII 1418622

m [Ml-HS e2+1] 2m [M4+nseS+n] for [=2

.- - O
I _
143-14.1,, m — 2

A ’ ‘7“ ‘ In,
A ffigfiXfifimKAf 9))» 2.1 1 :1
I :

MGSem-layer (n= 2) "I r)

+ 2 MSe if}: ‘1.

eg) KSnBi; 18613 Kl+xM 2 2xB17+xSCI3

+1MSe

b
S
fo‘dg,‘ “k. “1

+1MSe

It“ L
. " s \, [7. “If ‘ h‘ p
. 0 ~ - - :1‘1 x
. 1... °.- u .' . ' ' - “VF; ." ’1' - PA“! '~
.‘ .. 1 . s r. ‘I. ‘-. A‘. \h‘ :h“ la \‘ k I‘L'.\.LJ.
‘ I I ‘u‘ ‘ T ,B i . ' 7T“ 7“
, \ a 1 1 . b O . A\’ :\/(:: ' I ‘ TV‘ I; B r 7
‘ ‘h‘ 5 s "3 9 o > .L ‘ T) ’ 7“T3w~”~7u‘T‘rm
.1.‘ ... "I KKK sack ~sa‘fiqf‘seg41
‘ I; i" e . . B 0'“ (I 1‘ U‘. .‘l T
I c I“ ~ ‘~ ' I " x s ' x “I" ' ‘l' -
I It a! I‘ l" .“g‘ 9 ‘ I 3‘ I \‘ k (2": . .‘~ \ I’1,(:\l I" t: ‘1.“ .B.
s I o o . , .. .‘ M S l 4 .1. .1. \ . ‘2. V, .2. . ~
\ \ ‘.~ \ ...‘ ‘ ' . \ ‘_
. . ...,,\ 1 re, ., 1 8 e12- ayer “(11: ) 1*,- ... "7 ,/ , ..~\ ~..-.
’ “\ ‘ ““,. T . y“’.\.‘ 3]) B )tLXTD B r Ti,“ 0‘
.“‘~¢ sssss . 7 I". -»¢,l I Ari. [‘1 "'4, - r. ’
2 ‘7 . . t .1 . . . . g , I ) - , ‘144 t 7wh
‘ ’ \ . .5” ‘ P I . .p‘ /"'*~ 5. ‘-,t "‘-.
. I "I‘ I. "~ C. B I» ‘ '1. . ARK . ‘ '\ VIP, ,- '
5 ‘ 1‘ X M" \, i“ ‘ A u 7‘ '- .-_‘/‘-~~
\_ n . .1' MY; \ ”B I‘f \ ;.. -. B
9)» - ‘ ' \ J." “txr's‘f

Al+xM 3- 2xBi7+xsel4

cg) K23n7B114SC29

. L B I .. a .‘ B 3‘ . ~.-.
‘ ‘ O ‘ \
. O o“ k . B I‘ g b .. p .- I\ .3. x. . 0‘ \T . l.‘ .
'5 .¥ . g _‘ . L. L ' \ ~‘ .‘I ‘ . . tr 1 K. . .. A. _ ‘ ‘- \ .
. . c t v . 9 ~ - ‘o' .
C. O . -_.L. 1 I . . In . \ B . . 1 .‘O‘ . 1 ( .‘ . \ . .\ .1
s 1 . 5 ~ . B s 1 \ . ‘ t a ‘ , L g . ' ‘
\ \ ‘ B L §. . ‘ ‘ ‘. - ’\ . g..‘ ‘
90m,- .‘"“\ I I.‘ .\ y ’\c . ‘ .".\ ‘ l.‘\ot ‘
~ « . . “ Q ~ . \ " 5.. ' s. .‘ S \ ‘ $ \, .‘
.‘r. ",\ " \ s .L .L "‘0‘ ., '“I . .“9\ .‘
i s. Q'.‘ ‘ s. 1 \ " . - a .- ' . \. . g . 0 ... ‘ . - . I
t l r i .1 \ ‘. 5 \I ' . .
O . 1. 1 ‘ . y._ \ ‘. . .‘ .3 \ g \ '1‘ i‘ ‘5 L . ‘ . \."1. ~\ \ .‘ .\ ‘ \.‘ .5 .‘ . _\ ‘s . 1 ‘, ‘1. k 1
R . . b s . ~ ._ ‘ . 8 g . B\ ‘ \~ . 5- g \ ‘ t . I . '\ . ‘ I ' ‘ 1 I
, .. 1 . u -\_ . g \ \ . 1 5‘ . ‘ \‘ L O . \ g I . I . ~ .
\ t . . b c O i 1 fl . , n. g s. . . I , - L e . . - I ‘ '
. O b . .5 \. \ ‘ s .~ I ’ ‘ h . , s \ . O \ .k \ \‘. I - ‘ ~‘ B . I‘ ' ' ~ I ' ‘. B
3 ~. ". ' \ g \. 1‘ ._ ‘. B \ ;. ~ \. § ‘_ “ _.‘ ‘7‘ ‘ I ....‘ .‘ . 1 ‘5. ~..\ . s
. B \ B... . "?‘~\" ‘2 .1 . ‘ “\ ‘ V ‘ ‘ ‘. ‘ . ‘
‘9‘. . YO. g \ .~ I: ...“"‘9_ . .5" I‘. ' T“,
t. . . v \ s.‘ ‘1 “ : ‘ '\-‘ ‘ . ‘ . 1 . ‘ . Q ‘ .
,.,‘ \ The“ .s 10 14.. .- so. ‘1‘ ~o, ‘1 .‘m
x s .‘y a ‘ ..,..~ ~ . ~.‘ .. ’.‘.‘ ‘
'- IT “ A B ' . ‘ s . O ‘ . 0 ~ . ‘I‘ . I‘ . ‘-
. '. b ‘, ' . . . ‘ ‘1 . \ “ 1 ' g ‘1
\ .,‘ ' \ s . ‘ I. ~ . . I 1 ‘1 O \\ g . Q ‘- '
. 5 k . B \ . n . ‘ a .7 I
‘1 . ‘ \ .\ . . " L t . i . . . I: .‘ I. x: .‘ I:
~ ‘. s ‘ . \ \ . U 1“. x. L I .‘

A1+xM 4 2xB17+xS€15

Figure 1-5. A member-generating scheme illustrating successive additions of MSe

units to a M5Selo layer in the homologous subseries A,,,[M1+;Sez+1]2m[M4+,,Seg+,,] for

l=2. Small white spheres denote Se, large light-gray Spheres A, and middle-gray

spheres M. Question marks indicate predicted but as of yet undiscovered

compositions.

l6

d)
652‘ Speed controller

a)
Hook for wire {

l

 

iPt or Cu wir

Glass rod I . CD

b)

 

  
  
 

iifmm‘llfiiunm m
41-4 .4
G

l l 11 1 -.

1

‘ 1

:1 1’“. .1

i 9"”

. -.
,1
9" '
V. _.
'-

 

 

 

 

 

T‘f
Silica tube 2 E;
(point end) m :55?)
O
r °“

    
 
 

 

 

200 400 600 800
Temperature (°c)

Figure 1-6. a) Composition of reaction tube b) General Bridgman furnace setting and
temperature profile with height o) well grown polycrystalline ingots of a bismuth
chalcogenide compound from Bridgman fumace (see chapter 2, 3, 4, 5 and 6) and d)

Speed controller with various diameter wire guide which makes one revolution a day.

References

1 CRC Handbook of Thermoelectrics, Introduction, Edited by D.M. Rowe, Ph.D., D.Sc.,
CRC Press, 1995.

2 Kittel, C. Introduction to Solid State Physics, 7th ed.; John Wiley & Sons, Inc.: New
York, 1996; 166.

3 Kanatzidis, M. G. Semicond. Semimet. 2001, 69, 51-100

4 a) Tritt, T. M. Science, 1996, 272, 1276-1277 b) Disalvo, F. J. Science, 1999, 285, 703-
706

5 Hicks, D.; Dresselhaus, M.S. Phys. Rev. B, 1993, 47, 12727-12731.

6 a) Slack, G. A. “New materials and Performance Limits for Thermoelectric Cooling” in
CRC Handbook of Thermoelectrcis edited by Rowe, D. M. CRC Press, Boca Raton, 1995,
407-440. b) Slack, G. A. in “Solid State Physics”, eds. Ehrenreich, H.; Seitz, F. ;Tumbull,
D. Academic, New York. 1997, Vol. 34, 1.

7 Sales, B. C.; Manddrus, D.; Williams, R. K. Science 1996, 2 72, 1325-1328

8 a) Nolas G. S. ; Cohn, J. L.; Slack, G. A.; Schujman, S. 8., Appl. Phys. Lett. 1998, 73,
178-180. b) Nolas G. S.; Slack, G. A.; Morelli, D. T.; Tritt, T. M.; Ehrlich, A. C., J. Appl.
Phys. 1996, 79, 4002-4008.

9].P. Fleurial et. al. Proc. 15th Int. Conf on T hermoelectrics. IEEE, Piscataway, NJ,
1996.

'0 a) Hicks L. D.; Dresselhaus M. S. Phys. Rev. B 1993, 4 7, 16631-16634. b) Hicks L. D.;
Harman T. C.; Dresselhaus M. S. Appl. Phys. Lett. 1993, 63, 3230-3232.

11 Dresselhaus M. 8.; Lin Y. M.; Cronin S. B.; Rabin 0.; Black M. R.; Dresselhaus G.;
Koga T. Recent Trends In Thermoelectric Materials Research Iii Semiconductors and
Semimetals 2001, 71, 1-121.

'2 Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O’Quinn, B. Nature 2001, 413, 597-
602.

” Harman, T. C.; Taylor, P. J.; Walsh, M. P.; LaForge, B. E. Science 2002, 297, 2229-
2232.

” Euken, A.; Kuhn, (1,2. Anorg. Alleg. Chem. 1928, 134, 193.

‘5 Mott, N. E; Jones, H. The theory of the Properties of Metals and Alloys, Dover
Publications: NewYork, NY.

18

 

'6 Chung, D.-Y.; Jobic, S.; Hogan, T.; Kannewurf, C. R.; Brec, R.; Rouxel, J .; Kanatzidis,
M. G. J. Am. Chem. Soc. 1997, 119, 2505-2515.

'7 a) Chung, D.-Y.; Hogan, T.; Brazis, P.; Rocci-Lane, M.; Kannewurf, C. R.; Bastea, M.;

Uher C.; Kanatzidis, M. G. Science 2000, 287, 1024-1027. b) Chung, D.-Y.; Hogan, T.;

Brazis, P.; Rocci-Lane, M Brazis, P.; Ireland, J. R.; Kannewurf, C. R.; Bastea, M.; Uher
C.; Kanatzidis, M. G. J. Am. Chem. Soc. 2004, 126, 6414-6428.

'8 a) Kanatzidis, M. G.; McCarthy, T. J.; Tanzer, T. A.; Chen, L.-H.; Iordanidis, L.;

Hogan, T.; Kannewurf, C. R.; Uher, C.; Chen, B. Chem. Mater. 1996, 8, 1465-1474. b)

Kanatzidis, M. G. ; McCarthy, T. J .; Tanzer, T. A.; Chen, L.-H.; Hogan, T.; Kannewurf, C.
R.; Iordanidis, L. Mater. Res. Soc. Symp. Proc. 1996, 410, 37-43. c) Chung, D.-Y.;

Hogan, T.; Schindler, J .; Iordanidis, L.; Brazis, P.; Kannewurf, C. R.; Chen, B.; Uher, C.;

Kanatzidis, M. G. Mat. Res. Soc. Symp. Proc. 1997, 478, 333-344.

‘9 a) Chung, D.-Y.; Choi, K-.S.; Iordanidis, L.; Schindler, J. L.;Brazis, P. W.; Kannewurf,

C. R.; Chen, B.; Hu, 8.; Uher C.; Kanatzidis, M. G. Chem. Mater. 1997, 9, 3060-3071. (b)
Kanatzidis, M. G.; DiSalvo, F. J. Nav. Res. Rev. 1996, 4, 14-22.

20 a) Hsu, K. -F.; L00, 8.; Guo, F .; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.;

Polychroniadis. E. K.; Kanatzidis, M. G. Science 2004, 303, 818-821 b) Quarez, B.; Hsu,

K.-F.; Pcionek, R. ; Frangis, N. ; Polychroniadis, E. K.; Kanatzidis, M. G. J. Am. Chem.

Soc. 2005, 12 7, 9177-9190.

21 a) Iitaka, Y.; Nowacki, W. Acta Crystallogr. 1962, 15, 691-698 b) Takeuchi, Y.;
Takagi, J. Proc. Jpn. Acad. 1974, 50, 222-225.

22 Takeuchi, Y.; Takagi, J.; Yamanaka T. Proc. Jpn. Acad. 1974,50, 317-321.
23 Takeuchi, Y.; Ozawa, T.; Takagi, J. Z. Kristallogr. 1979, 150, 75-84.

2“ Otto, H. H.; Strunz, H. N. Jb. Miner. Abh. 1968, 108, 1-9.

25 Srikrishnan, T.; Nowacki, w. z. Z. Kristallogr. 1974, 140, 114-136.

26 Tilley, R. J. D.; Wright, A. C. J. Solid State Chem. 1986, 65, 45-62.

27 Takagi, 1.; Takeuchi, Y. Acta Crystallogr., B 1972, 28, 649- 651.

2“ Matzat, E. Acta Crystallogn, B 1979,35, 133-136.

2" Takeuchi, Y.; Takagi, J. Proc. Jpn. Acad. 1974, 50, 76-79.

19

 

3° Palatnik, L. S.; Konovalov, O. M.; Gladkikh, N. T.; Kolesnikov, V. N. Phys. Metal
Metallogr. 1961, 15, 36-39.

3' Agaev, K. A.; Semiletov, s. A. Soviet Phys. - Crystallogr. 1968, 13, 201-203.
32 Agaev, K. A.; Talybov, A. G.; Semiletov, S. A. Kristallografiya 1966, 11, 736-740.
33 Liu, H.; Chang, L. L. Y. Am. Miner. 1994, 79, 1159-1166.

34 Adouby, K.; Perez Vicente, C.; Jumas, J. C.; Fourcade, R.; Abba Toure', A. Z.
Kristallogr. 1998, 213, 343-349.

35 Perez Vicente, C.; Tirado, J. L.; Adouby, K.; Jumas, J. C.; Abba Toure', A.; Kra, G.
Inorg. Chem. 1999, 38, 2131-2135.

36 Choc, W.; Lee, S.; O’Connell, P.; Covey A. Chem. Mater. 1997, 9, 2025-2030.

37 a) Boon, J. w. Rec Trav. Chim. Pays-Bas, 1944, 63, 32. b) Glemser, o. ; Filcek, M. Z.
Anorg. Allg. Chem, 1955, 279, 321-323. c) Gattow, G.; Zemann, J. Z. Anorg. Allg.
Chem. 1955, 279, 324-327.

3" Voroshilov, Y. v.; Peresh, E. Y.; Golovei, M. I.1norg. Mater. 1972, 8, 777-778

39 McCarthy, T. J.; Ngeyi, S.-P.; Liao, J.-H.; DeGroot, D.; Hogan, T.; Kannewurf, C. R.;
Kanatzidis, M. G. Chem. Mater. 1993, 5, 331-340

40 McCarthy, T. J .; Tanzer, T. A.; Kanatzidis, M. G. J. Am. ChemSoc. 1995, 117, 1294-
1301.

“ Schmitz, 1).; Bronger, w. Z.Natureforch. 1974, 29b, 438-439

42 Kanischeva, A. S.; Mikhailov, J. N.; Lasarev, V. B.; Trippel, A. F. Dokl. Akad. Nauk.,
SSSR (Kryst) 1980, 252, 96-99.

43 Iordanidis, L.; Brazis, P. W.; Kyratsi, T.; Ireland, J .; Lane, M.; Kannewurf, C. R.; Chen,
W.; Dyck, J. S.; Uher, C.; Ghelani, N. A.; Hogan, T.; Kanatzidis, M. G. Chem. Mater.
2001, 13, 622-633.

‘4 Iordanidis, L.; Kanatzidis, M. G. J. Am. Chem. Soc., 2000, 122, 8319-8320

‘5 Cordier, G.; Schafer, H.; Schwidetzky, C. Rev. Chim. Miner. 1985, 22, 676-683.

“6 Iordanidis, L.; Kanatzidis, M. G. Angew. Chem. Int. Ed. 2000, 39, No.11, 1927-1930.

20

 

‘7 Iordanidis, L.; Bilc, D.; Mahanti, s. 1).; Kanatzidis, M. G. JAm Chem Soc 2003, 125,
13741-13752.

48 Kyratsi, Th.; Dyck, J. S.; Chen, W.; Chung, D.-Y.; Uher, C.; Paraskevopoulos, K. M.;
Kanatzidis, M. G. J. Appl. Phys. 2002, 92, 965-975.

49 Kyratsi, T.; Chung, D.-Y.; Ireland, J. R.; Kannewurf, C. R.; Kanatzidis, M. G. Chem.
Mater., 2003, 15(15), 3035-3040.

5" Kyratsi, T.; Kanatzidis, M.G. Z. Anorg. Allg. Chem. 2003, 629, 2222-2228.

5' Chung, D.-Y.; Iordanidis, L.; Rangan, K. K.; Brazis, P. W.; Kannewurf, C. R.;
Kanatzidis, M. G. Chem. Mater. 1999, 11, 1352-1362

52 Yao, J. Y.; Ibers, J. A. Acta Crystallogr. 2004, E60, 1111-1113 Part 9.

53 Huang, F. Q.; Somers, R. C.; McFarland, A. D.; Van Duyne, R. P.; Ibers, J. A. J. Solid
State Chem., 2003, 174, 334-341.

54 Yao, J. Y.; Deng, B.; Ellis, D, E,; Ibers, J. A. Inorg. Chem. 2002, 41, 7094-7099.

55 a) Huang, F. Q.; Mitchell, K.; Ibers, J. A. J. Alloys Compounds 2001, 325, 84-90. b)
Yang, Y. T.; Brazis, P.; Kannewurf, C. R.; Ibers, J. A. J. Solid State Chem, 2000, 155,
243-249.

56 Mrotzek, A. Kanatzidis, M. G. Inorg. Chem. 2003, 42(22), 7200-7206.

57 Mrotzek, A.; Iordanidis, L.; Kanatzidis, M. G. Inorg. Chem. 2001, 40, 6204-6211.

58 Choi, K.-S.; Chung, D-.Y.; Mroztek, A.; Brazis, P. W.; Kannewurf, C. R.; Kanatzidis,
M. G. Chem. Mater. 2001, 13 (3): 756-764.

59 Mrotzek, A.; Iordanidis, L. Kanatzidis, M. G. Chem. Commun. 2001, 1648-1649.

60 Mrotzek, A.; Chung, D.-Y.; Ghelani, N.; Hogan, T.; Kanatzidis, M. G. Chem. Eur. J.
2001, 7, 1915-1926.

61Mrotzek, A.; Chung, D.-Y.; Hogan, T.; Kanatzidis, M. G. J. Mater. Chem, 2000, 10:
(7) 1667-1672.

62 Mrotzek, A.; Chung, D.-Y.; Hogan, T.; Kanatzidis, M. G. J.Mater. Chem. 2000, 10,
1667-1672.

6’ Mrotzek, A.; Kanatzidis, M. G. J. Solid State Chem, 2001, 167, 299-301.

21

 

64 Hsu, K. F.; Chung, D.-Y.; Lal, S.; Mrotzek, A.; Kyratsi, T.; Hogan, T.; Kanatzidis, M.
G. J. Am. Chem. Soc., 2002, 124, 2410-2411.

65 Hsu, K.-F.; Lal, S.; Hogan, T.; Kanatzidis, M. G. Chem. Commun. 2002, 1380-1381.

66 Cook, R.; Schafer, H. Rev. Chim. Miner. 1982, 19, 19-27.

67 Cordier, G.; Schafer, H.; Schwidetzky, C. Rev. Chim. Miner. 1985,22, 631-638.

6" Aurivillus, B. Acta Chem. Scand. 1983, A37, 399-407.

‘59 Volk, K.; Cordier, G.; Cook, R.; Schafer, H. Z. Naturforsch. 1980, 35b, 136-140.

70 Wang, Y. C.; DiSalvo, F. J. Chem. Mater. 2000, 12, 1011-1017.

7' a) Chung, D.-Y.; Jobic, S.; Hogan, T.; Kannewurf, C. R.; Bree, R.; Rouxel, J.;
Kanatzidis, M. G. J. Am. Chem. Soc. 1997, 119, 2505-2515. (b) Chung, D.-Y.; Jobic, S.;
Hogan, T.; Kannewurf, C. R.; Bree, R.; Rouxel, J .; Kanatzidis, M. G. Mat. Res. Soc.
Symp. Proc. 1997, 453, 15-22.

72 R. Cook and H. Schafer, in Studies in Inorganic Chemistry Vol. 3, edited by R.
Metselaar, H. J. M. Heijligers and J. Schoonman (Elsevier Scientific Publishing
Company, Amsterdam, 1983), p. 757-760.

73 See Chapter 4

74 See Chapter 5

75 See chapter 6

76 See Chapter 3

77 a) Magneli A.; Acta Crystallogr. 1953, 6, 495 b) Andersson S.; Sundholm A.; Magneli,
A. Acta Chem. Scand. 1959, 13, 989.

78 a) Aurivillius B. Ark. Kemi, 1949, 1, 463. b) Frit B. ; Mercourio J. P. J. Alloys Comp.
1992,188,27.

79 a) Dion M. ; Ganne M. ; Tournoux M. ; Ravez J. Rev. Chim. Miner. 1984, 21, 92. b)
Jacobson A. J. ; Johnson J. W. ; Lewandowski J. T. Inorg. Chem. 1985, 24, 3727.

8" a) Takagi, J. Takeuchi, Y. Acta Crystallogr. 1972,1328, 369 b) Makovicky, E.,Neues
Jahrb. Mineral. 1989, 160,269.

3‘ Zakrzewski, M. A.; Makovicky, E. Can. Mineral. 1986,24, 7.

22

 

82 Ilinca, G.; Makovicky, E. Eur. J. Mineral. 1999, 114,691.
83 Choe W.; Lee 8.; O’Connell P.; Covey A. Chem. Mater. 1997, 9, 2025-2030.

8“ a) Mrotzek A.; Kanatzidis M. G. Acc. Chem. Res., 2003, 36, 111-119. b) Kanatzidis M.
G. Acc. Chem. Res., 2005, 38, 359-368.

35 Hsu K. F.; Lal s.; Hogan T.; Kanatzidis M. G. Chem. Commun. 2002, 13, 1380-1381.
86 Poudeu P. F. P. ; Kanatzidis M. G. Chem. Commun., 2005, 21 , 2672-2674.

87 Wolfing, B.; Kloc. C.; Teubner, J .; Bucher, E. Phys. Rev. Lett. 2001, 86, 4350-4353.
8* Kanatzidis, M. G.; Sutorik, A. C. Prog. Inorg. Chem. 1995,43, 151-265.

89 Kyratsi, T.; Chung, D.-Y.; Choi, K.-S.; Dick, J. S.; Chen, W.; Uher, C. and Kanatzidis,
M. G. Mat. Res. Soc. Symp. proc. 2000, 626, Z8.8.1- 28.8.6, See Chapter 2, 3, 4, 5, and 6.

23

CHAPTER 2

Crystal Growth, Thermoelectric Properties and Electronic

Structure of AgBi3SS and AngxBi3-x85 (x=0.3)

1. Introduction

Bismuth chalcogenide chemistry has been extensively studied since the BizTe3.,,Sex and
Biz-bexTe3 alloys showed high thermoelectric (TE) figures of merit ZT."2 In recent years,
intense efforts focused on discovering new thermoelectric materials have been devoted to
the class of ternary and quaternary alkali metal bismuth chalcogenides. From the
chemistry standpoint this class of materials has proven to be remarkably large and has
contributed many complex compositions and structures favorable for high TE
performance.3 Some examples include CSBl4TC6,4 B-KzBigSCl3,5 K2_sBi8,5Se14,5 BaBiTe3,6
K1-,Sn5-,Bi.1+xSe22,7 A1.,M1-2,Bi7.,3e.5 (A = K, Rb; M = Sn, Pb),8 A2BigSet3 (A = Rb,
Cs), 9 CsMBi3Te6, and CstBi3Te7 (M = Pb, Sn) '0. Recently a silver containing
compound, Ag1-bethbTezo,” showed a large figure of merit (ZT) of ~2 at 800K.

In comparison to the selenides and tellurides, most of the alkali metal bismuth
sulfides exhibit wide energy band gaps and strong ionic interactions between the alkali

metal ions and the [BixSy]z' framework. For example, the alkali metal containing B-,y-

(383182,12 ’Y-RbBl3Ss,l3 KBi335,l4 KB16,33S|0,15 and K231381315 show band gaps Of ~1.1 -

24

1.4 eV. In general, these gaps are wider than what is considered to be optimum for TE
performance because the pertinent materials tend to be exceedingly resistive. Instead
desirable energy gaps for TE applications up to 1000 °C are thought to be <~0.6 eV. In
order to include the bismuth sulfide class of materials in TE investigations it is preferable
to produce systems with smaller semiconductor gaps. One way to do so is to replace,
partially or totally, the alkali metals with other less electropositive metals capable of
stronger interactions with the [BixSy]z' framework such as Ag+. This is the reason we
examined AgBi3S5. The known Ag/Bi/S compounds exhibit a variety of structural types
and compositions.'6 These include AgBiSz,l7 AgBigSo,18 Ag3Bi7812,'9 and AgBi3,S520
which have not been studied with respect to their physicochemical and electrical charge
transport properties. ’
Herein we report new results on the synthetic pavonite, AgBi3S5, and its
derivative AngxBi3-xS5 (x = 0.3) and evaluate their potential as thermoelectric materials.
The crystal structure refinements, crystal growth, physico-chemical properties, band
structure calculations and exceptionally low thermal conductivity of these materials are

presented.
2. Experimental Section
Reagents. Chemicals were used as obtained: bismuth chunks (99.999% Noranda,

Canada), sulfur powder (sublimed, Spectrum Chemical Mfg. Corp., Gardena, CA),

antimony shot form ( 99.9%, Noranda, Canada)

25

Synthesis. The products are air and water stable and all manipulations were
carried out in air. For all compounds the yield was quantitative. The purity and
homogeneity of the products were verified by comparing the X-ray powder diffraction
patterns to those calculated by the crystallographically determined atomic coordinates.

Ag Powder. A silver coin (99.999%) was dissolved in nitric acid. The solution
was neutralized to a pH of 7 with ammonium hydroxide. Sodium borohydride was added
to reduce the Ag ions to a black precipitate of Ag metal powder. The precipitate of silver
was filtered and washed thoroughly with water and dried in a vacuum oven at 150 °C.
The obtained fine powder of Ag was identified by powder X-ray diffraction.

AgBi385. A mixture of Ag powder (1.294 g, 12 mmol), Bi (7.523 g, 36 mmol),
and S (2.024 g, 63 mmol) was loaded in a fused silica tube (13 mm diameter) and
subsequently flame-sealed at a residual pressure of <10‘4 mbar. The tube was carefully
placed in a flame of natural gas-oxygen torch until the mixture was well melted. The tube
was removed from the flame and let solidify in air. A metallic black polycrystalline ingot
of AgBi385 was obtained. A quantitative microprobe analysis using Energy Dispersive
Spectroscopy (EDS) was performed on a Scanning Electron Microscope (SEM) on
several single crystals of AgBi385 gave the approximate composition of AgoosBiggoSs. In
order to grow highly oriented crystal specimens for the thermoelectric property
measurements, the product was ground and loaded in a silica tube (13 mm diameter) with
a point end and sealed under vacuum. The tube was heated to 800 °C in a Bridgman
furnace and descended at a rate of 3.25 mm/h through a sharp (100 °C/cm) temperature
gradient.21 A pure and well oriented ingot (35 mm long, 11 mm diameter) of AgBi3S5

was obtained.

26

Ango,3Bi2,-;Ss. A mixture of elemental Ag powder (1.294 g, 12 mmol), Sb (0.438
g, 3.6 mmol), Bi (6.771 g, 32.4 mmol), and 8 (2.024 g, 63 mmol) was loaded in a fused
silica tube (13 mm diameter) and subsequently flame-sealed at a residual pressure of <10’
4 mbar. The mixture was carefully molten in a natural gas-oxygen torch as above. After
quenching in air, a black silvery polycrystalline ingot of Ang03B12JS5 was obtained.
SEM/EDS analysis on several single crystals of Ango,3Biz,7S5 showed the approximate
composition of Ag0,93Sbo,2Bi3_45S5. The Bridgman technique was used to obtain highly

oriented crystalline ingots of AngogBiszs using the same condition as AgBi385.

3. Physical measurements

Electron Microscopy. Quantitative microprobe analysis for the compounds was
performed with a JEOL JSM-64OOV Scanning Electron Microscope (SEM) equipped
with a Noran Vantage Energy Dispersive Spectroscopy (EDS) detector. Data were
collected for 30 sec using an accelerating voltage of 20kV. All reported results are an
average of measurements on at least three different crystals.

Differential Thermal Analysis. Differential thermal analysis (DTA) was
performed with a computer-controlled thermal analyzer (Shimadzu DTA-50). A 20 mg of
ground crystals were sealed in silica ampoule under vacuum. A silica ampoule containing
the equal mass of alumina was placed on the reference side of the detector. The sample
was heated to the desired temperature a 10 °C/min, isothermed for 2 min and then cooled
at 10 °C/min. The heating program was recycled to check reproducibility of the thermal

behavior of the sample. The reported melting point is the peak temperature. After DTA,

27

the sample was examined by powder X-ray diffraction to identify if any decomposed
product formed during heating/cooling cycles.

Solid-State UV/vis Spectroscopy. Optical diffuse reflectance measurement was
made at room temperature with a Shimazu UV-3101 PC double-beam, double-
monochromator spectrometer operating in the 200 ~ 2500 nm region. The instrument was
equipped with an integrating sphere and controlled by a personal computer. BaSO4
powder was used as reference (100% reflectance). Absorption data were calculated from
the reflectance data using the Kubelka-Munk function.22

Charge Transport and Thermal Conductivity Measurements. A four sample
measurement system was used to simultaneously measure electrical conductivity,
thermoelectric power, and thermal conductivity.23 To fully characterize the figure of
merit, the properties were measured for each sample over the selective temperature range
of interest (system capability is 4.2 - 475 K). To alleviate offset error voltages and
increase the density of data points, a slow-ac technique was used with a heater pulse
period of 720 see.24 The pulse shape was monitored, in situ, to determine temperature
stabilization, and the sample chamber was maintained at a pressure less than 10'5 Torr for
the entire measurement run. A rectangular sample with dimensions 3 mm x 3 mm x 5
mm was mounted in the standard four-probe configuration for the thermal conductivity,
and the heater current was adjusted for an average temperature gradient of 1 K. The
sample stage and radiation shield were gold-coated copper to minimize radiation effects
and to maintain temperature uniformity. All electrical leads were 25 pm in diameter with
lengths greater than 10 em to minimize thermal conduction losses. Data acquisition and

computer control of the system were maintained under the LabVIEW 25 software

28

environment. For higher temperature measurements of thermoelectric power and
electrical conductivity, a single sample measurement system with system capabilities up
to 800 K was used.26 This system utilizes single ended thermocouples for concurrently
monitoring the temperature gradient and voltage gradient on the sample, and also utilizes
the slow pulsing technique described above.

To obtain the thermal conductivity from 300 to 800 K, we measured the thermal
diffusivity (or) using the laser flash technique. The thermal conductivity (K) values were
calculated as a product of these quantities, i.e. K = and. where Cp is the specific heat and
d is the sample’s density. The bulk density (d) values were calculated from the sample’s
geometry and mass (12 mm in diameter and 2.3 mm thick) and the specific heat (Cp) was
measured on a 12 mm in diameter and 1.0 mm thick sample using differential scanning
calorimetry.27

Powder X-ray Diffraction. A calibrated CPS 120 INEL X-ray powder
diffractometer equipped with a position-sensitive detector, operating at 40kV/25mA with
a flat geometry and employing graphite monochromatized Cu K01 radiation, was used to
obtain powder patterns of starting materials and all products.

Single-crystal X-ray Crystallography. A Bruker SMART Platform CCD
diffractometer was used for data collection at room temperature. The individual frames
were measured with an omega angle rotation of 03° and an acquisition time of 30 sec for
each crystal. The SMART28 software was used for the data acquisition and SAINT28
software for data extraction and reduction. An analytical absorption correction was
performed using face indexing and the program XPREP in the SAINT software package,

followed by a semiempirical absorption correction based on symmetrically equivalent

29

reflections with the program SADABst. Structural solution and refinements were
successfully done using the SHELXTL28 package of crystallographic programs. The
structures were solved with direct methods.

The data collection was performed by selecting the crystals from the interior of
the Bridgman-grown ingots. The complete data collection parameters, details of the
structure solution, and refinement for AgBi385 and Ango3BIz785 are given in Table 2-1
and compared with the previously reported data for AgBi3S520. The fractional coordinates
and temperature factors (Ueq) of all the atoms with estimated standard deviations are
given in Tables 2 and 3.

The previously reported structure for pavonite, AgBi3S5, was determined using
intensity data from integrated Weissenberg photographs and X-ray powder diffraction.
The structure solution was accomplished using the 1101 data to construct a Patterson
function p(u,0,w) which gave R = 11% from 510 reflections. In contrast, the new
refinement for the synthetic AgBi3S5 provides significantly more accurate atomic
coordinates, and bond lengths and angles and much lower R values (3 ~ 4%) from ~3000
reflections.

Band structure calculation. The electronic structure calculations were
performed using the self-consistent full-potential linearized augmented plane wave
method (LAPW) 29 within density functional theory (DFT), 30 using the generalized
gradient approximation (GGA) of Perdew, Burke and Emzerhof31 for the exchange and
correlation potential. The values of the atomic radii were taken to be: 2.3 an. for Ag and
S atoms, and 2.6 an. for Bi atoms, where an. is the atomic unit (0.529 A). Convergence

of the self-consistent iterations was performed for 20 k points inside the irreducible

30

Brillouin zone to within 0.0001 Ry with a cutoff of -6.0 Ry between the valence and the
core states. Scalar relativistic corrections were included and spin-orbit interaction was
incorporated using a second variational procedure.32 The calculations were performed
using WIEN2K program.33 It is necessary to use the more accurate atomic coordinates

obtained by the new refinement to achieve meaningful results in the DFT calculations.

4. Results and Discussion

Synthesis and Crystal Growth. AgBi385 and AngxBi3-xS5 were synthesized by
reacting the elemental mixtures (Ag:Bi:S = 12325.25, Ag:Sb:Bi:S = l:x:3-x:5.25) in a
torch flame. A slight excess of S was added to compensate a loss of sulfur vaporized
from the top surface of the molten mixture during the reaction. The AngxBi3-,,85 series
of compounds with several x values (up to x = 1) were investigated. The AngxBi3-xS5
with x = 0.1, 0.2, 0.3, and 0.5 produced pure solid solutions, while the x = 1 provided a
mixture of the AngxBi3.xS5 solid solution and Bi283. This is not surprising since Ang3S5
(i.e. x = 3) is not a stable compound. AgBi385 and AngojBIszs appear to melt
congruently at 735 and 723 °C, respectively. For both compounds a comparison of the X-
ray powder diffraction patterns before and after the DTA experiments showed no
significant phase change.

For thermal and electrical conductivity measurements we grew large crystals of
AgBi385 and Ango,3Bi2,7S5 using the Bridgman technique. The obtained ingots show

well grown highly oriented characteristics, Figure 2-1.

31

.s :NANeKEaENQ- ”.525 u 23 .__ok__w\__...~_ 1 52w 1 E.

 

026

name 5:8
e

44 SS
.8546

< 524.2
< 8.9.3

4 3:32

END
own—00:02

2 com soc “mamma-

m-< .o mvmé. can thN

>2 mo .1. NM? .vaod .11. mm
M: ~ mo H NMB 6966 .1. 5H

mk- .o ace.” can 8N6
vawmoood

fixed .1. NM? .286 H LM
886 H NM? .586 H La

83 a:
6232: mfoio:
mm no mougcm¢m8_ xtfifiésm
as ”.8 .8 18
3.9.06 1 95E 2: 363° u 955 2.:
when wnnm
euxnva- anvrva-
muvorva 08.1%-
A vavs- :uveuvK a-
.33 9 and Rama 9 Man
use 25 x w; x 86 was mod n mod 4 3o
oee_ oom_
use e33 use 55%
same $3 Mans 83
e 4
me. 2%.an me. 53%
.6283 .Emfleo ”a
< 63?: < 569.2 1 o
< 2 358.4 < @243 u e
< $80: 4. 52%.: u e
E8 88
OMS—00:02 08:02.52
6. Sam M Ema
Snow :83
£338.83 flame.

20: can anon Emu “mowemq
520808 8:05me

3% =3 823 m
SHEER: 829: m .eeE
Nu co 5.3885500
£30883 \ 85868 \ Sea
3508 “5805qu

.NN.wN u 805 9 30:20—an0
fiasco—Moe 309539:
938:8 30:8qu

3923 5?:

5:00:00 8% now owned 923.
came. 3903

88:

Homo—bog downtown?
3083033 bison

N

oEBo>

806586 :8 E5
9.on 8QO

8893 3900
288095...

Emma? «Eaton
«3&8 RoEqfim

 

swam—me. 3353 23 £35.... 2.1%? e5 omen—wee 8585.5 he so: agenda—.330 .3 632.

32

Table 2-2. Atomic coordinates ( x 104) and equivalent isotropic displacement

parameters (Azx 103) for AgBi385. U(eq) is defined as one third of the trace of the

orthogonalized Uij tensor.

 

 

x y z U(eq) occupancy
Bi(l) 7392(1) 0 1110(1) 20(1) 1
Bi(2) 9740(1) -5000 2165(1) 21(1) 1
Bi(3) 12198(1) -10000 3894(1) 25(1) 1
Ag(l) 10000 -10000 0 40(1) 1
Ag(2) 1 0000 -5000 5000 30(1 ) 1
8(1) 8625(2) -5000 551(2) 19(1) 1
S(2) 8426(3) 0 2590(2) 19(1) 1
8(3) 5992(3) -5000 1519(3) 35(1) 1
S(4) 10768(3) -5000 3617(2) 23(1) 1
S(5) 1 1508(2) -10000 5338(2) 16(1) 1

 

Table 2-3. Atomic coordinates ( x 104) and equivalent isotropic displacement

parameters (Azx 103) for AngognBiz,“Ss. U(eq) is defined as one third of the trace of

the orthogonalized Uij tensor.

 

 

x y z U(eq) occupancy
Bi(1)/Sb(l) 7389(1) 0 1116(1) 20(1) O.871(4)/0.129
Bi(2)/Sb(2) 4732(1) -10000 2171(1) 20(1) 0.785(4)/0.215
Bi(3) 7192(1) -15000 3892(1) 26(1) 1
Ag(l) 5000 -5000 0 25( 1) l
Ag(2) 5000 -10000 5000 29(1) 1
8(1) 8643(3) -5000 542(3) 22(1) 1
S(2) 8438(3) O 2586(3) 21(1) 1
8(3) 6017(4) -5000 1529(4) 34(1) 1
S(4) 5764(3) -10000 3615(3) 22(1) 1
S(5) 6514(3) -15000 5333(3) 16(1) 1

 

33

The natural crystal habit of these compounds is to grow as long planks and in the ingots
the long axis (crystallographic b-axis) lies parallel to the Bridgman translation axis.
These ingots were cut along the direction parallel and perpendicular to the crystal growth.
Experimental evidence that a very high degree of crystal orientation was achieved in the
ingots was obtained from X-ray diffraction data taken on cut specimens along different
directions, Figure 2-2. The presence of a certain class of reflections when the X-ray beam
is incident along one direction (e. g. (h21) in Figure 2-21) and their complete absence when
the beam is incident along a perpendicular direction (e.g. in Figure 2-2d) is proof that a

nearly perfect (estimated at >96%) crystallographic orientation has been achieved.

0))

   

Figure 2-1. Ingots of a) AgBi3S5 and b) AngogBiis grown in Bridgman fumace.

Structure Description. AgBi335 has a strongly anisotropic three-dimensional
framework composed of two types of slabs which can be described as an assembly of

blocks excised from the cubic NaCl structure type. These blocks are two-dimensional

34

slabs excised by slicing perpendicular to the [311] direction of the NaCl lattice, Figure 2-
3.

The thinner slab (slab I) is composed of single [Ang octahedron sandwiched by
two square pyramids of [8185]. The thicker slab (slab II) is made of distorted galena-type
structure34 with one [Ang and four [8ng} octahedra per one diagonal octahedral chain.
The two slabs are interconnected through sharing atom S(4). This modular construction
gives the compound a highly anisotropic morphology and electronic structure.

The structure has three crystallographically independent Bi atoms. Bi(l) is in
slightly distorted octahedral site with distances from 2.708(3) to 2.959(3) A to the
coordinated S atoms. Bi(2) is also in an octahedral site of sulfur atoms with bonding
distance from 2.664(4) to 2.947(3) A. Bi(3) is in slab I and has five normal covalent
bonds with neighboring S atoms at a square pyramidal coordination (SbZSeg-type) and
two additional longer interactions with S(2) atoms in slab II at 3.445(3) A; namely Bi(3)
has one bond with S(5) at 2.609(3) A, two bonds with S(4) atoms at 2.795(3) A and two
with S(5) at 2.888(2) A. Ag(l) sits in a slightly distorted octahedral site in slab II with
two Ag(l)-S(3) bonds at 2.741(6) A and four Ag(1)-S(1) bonds at 2.917(2) A. Ag(2) is in
a flattened octahedral site with four Ag(2)-S(5) bonds at 2.877(2) A and two short Ag(2)-
S(4) bonds at 2.560(4) A, Figure 2-4. Even though the S(4) atoms serve as bridges
between the two slabs and the Ag(2)-S(4) and Bi(2)-S(4) bond distances are shorter,
Table 2-4 and Figure 2-4. The equivalent isotropic displacement parameters of Ag atoms
are relatively larger. It can be rationalized if we consider that there may be some rattling
of Ag atoms going in the large octahedral pockets. A low temperature data collection on

AgBi3S5 may show Ag atoms settling into the sides of the octahedron.

35

 

92 _.
l N L 3...
N" N—L
a) .55:
h)
c:
01
h
no
a, 8

928
“"830

 

‘1.
n

 

 

 

 

L
(D
A 937
of 5"
\
612'

 

l

MISU91U|

a
819
{5.1
«—-BOZ

l
1316
:‘f
f

 

’3
?~——— ZLS
Ape—112
l
081

 

 

 

ZOZ

 

 

 

10 20 3'0 ' 40 -50 60.710.810.90
2theta, deg

Figure 2-2. XRD patterns of AgB13S5 (a) of polycrystalline powdered sample, (b)
calculated from the crystal structure, (c) well grown ingot sample with an X-ray beam

along a direction on ab plane, ((1) b direction on ab plane, (e) b direction on be plane

and (i) c direction on be plane.

36

 

Slab I

 

 

 

 

 

Slab II

 

 

Figure 2-3. Projection of the structure of AgBi3SS down the b-axis. Two slabs can be
described by slightly distorted layers cut along the (311) face of NaCl-type structure.
The slab (11) includes five octahedra per one diagonal octahedral chain. In the
structure of Ango_3Bi2_785 the Bi(l) and Bi(2) sites are disordered with Sb atoms.

   
 

51
2.917(2) A
2.741(6) A

 

53
Figure 2-4. A scheme of local coordination environment of Ag(l), Ag(2) and Bi(2)

atoms in AgBi385.

All members of the solid solutions AngxBi3-xS5 we prepared are isostructural to
AgBi3S5. In the selected structure Ango3B12jS§ the Sb atoms occupy two bismuth sites
Bi(l) and Bi(2) on the slab II with 13% and 21%, respectively. The structure of
AngogBisz§ has slightly smaller unit cell parameters than AgBi385 because of Sb
substitution, table 2-1. Furthermore the smaller unit cell may creates smaller room for all
metals comparing with AgBi385 which show mostly smaller bonding distances between
metal atoms and sulfur atoms and at the same time achieve optimum packing. Especially
the equivalent isotropic displacement parameters of Ag(l) and Ag(2) are much smaller
than those of AgBi3S5 which may have little rattling of Ag atoms, Table 2-3 and 2-4.

Energy gaps and electronic band structure calculations. Electronic band
structure calculations can be an important tool to explore the properties of materials. It
not only can rationalize the observed properties but can also provide guidance for further
modifications toward a desired direction. To the best of our knowledge band structure
calculations on AgBi3S5 (pavonite) have not been reported. Thus we first carried out
electronic band structure calculations to understand the influence of the crystal structure

on the electronic structure and properties of AgBi3S5 and AngogBizaSs. We also

38

examine how each element contributes to the conduction band and valence band structure
near the Fermi energy level.

Electronic structure calculations show that AgBi385 is an indirect narrow band-gap
semiconductor with a energy gap of ~0.l7 eV, Figure 2-5 and 6A. Density of
states(DOS) analysis shows that the high valence band states in the range from -0.75 to 0
eV consist mostly of p states of S(4) and S(5) atoms which are hybridized with (1 states of
Ag(2), Figure 2-6C and D. This suggests a 2-dimensional hole transport in Slab I since
S(4), S(5) and Ag(2) atoms are located in it. The bottom of the conduction band consists
of p states of Bi(l) and Bi(2) atoms with very small contribution from Bi(3) atoms,
Figure 2-6B. The Bi(l) and Bi(2) p states are hybridized with the p states of S(l), S(2),
and 8(3) atoms in the range from 0 to 1 eV, suggesting that the electron transport is
mostly confined within Slab 11. Therefore, the electron and hole transports should be
separated in space.

From the projected density of states calculations (Fig. 6D) we find that the filled
d-states of Ag(l) and Ag(2) lie surprisingly high in energy and in the same region as the
S p bands. This results in a strong mixing of the Ag (1 states and the S p states and leads
to two a rather narrow hybridized valence band. Due to the different local environments
of Ag(l) and Ag(2), the Ag(2) associated band is about 0.75 eV higher than the Ag(l)
associated band. As a result the top of the valence band and hence the hole transport takes
place in slab I in which the Ag(2) atom resides, Fig 3. The narrow valence band leads to a
rapidly increasing density of states near the valence band maximum which suggests that
if this system could be hole-doped, it could show a very large thermopower. It will be

interesting to test this prediction by making hole-doped samples.

39

Furthermore, the mixing of the Bi p states with these hybridized Ag—S states leads
to splitting of the p bands near the conduction band bottom associated with different Bi
atoms (Fig. GB). The Bi(3) p-bands get pushed above the Bi(l) and Bi(2) p bands because
of the proximity of the mixed Ag(2)-S bands. As a result, the bottom of the conduction
band has primarily Bi( 1), Bi(2) p character and the resulting electron carriers move
predominantly in Slab II, as discussed in the previous paragraph. Clearly the Ag atoms
play a very important role (although indirect) in determining the nature of the states near
the band gap region. In this regard the Ag systems greatly differ from their alkali
counterparts. In the K systems the K d states are not present whereas in the Rb(Cs)
systems the Rb(Cs) (1 states are much lower in energy (core states) than the S d states. 13

The optical absorption properties of AgBi385 and Ango,3Bi2,7SS were examined
with solid state optical absorption spectroscopy. The spectra in the UVN is range show
intense absorptions for both AgBi385 and AngogBiszs around 0.6 eV, Figure 2-7. The
difference between calculation and measurement is not unusual because the LDA/GGA
band calculation has a tendency to underestimate the gap energy.35 The difference in the
direct band gap (~0.2 eV) and the optical gap (~O.6eV) can be due to variety of reasons.
We know that LDA/GGA band gaps tend to be smaller than the true band gaps and this
may explain the discrepancy.35 There is however another possibility. If we look at the
total DOS (Fig. 6A) we see that there is a sharp rise in the DOS of the conduction band at
about 0.4-0.5 eV above the conduction band bottom. Absorption to these states may be
the origin of the observed optical absorption edge at ~0.6 eV. A careful calculation of the
optical response function including the energy dependence of the optical matrix element

will be able to shed light on this issue.

40

 

 

22 25-258.-va 55.8 25-55-25 8:58 25-25-25
2:85 25-255245 2 :38 2.5-65-2.5 @mN. a 2 :mA :525
econ; 25-25535 8:552 2.5-55-va
35.8 25-258.-va e x SEN 5W5?
€85 $5-25...-va 2:35 25235-25 N x 2.38N 2.5-2:?
6on5 25-255245 2:35 2:3sz-25

2:862 25-233-va 2 :36: 25-2sz24; 2. x SEN 25-25...
6%.; 53:32.5 2 :33 5335.25 N x GVEN 25-25..-
avoflwo @mANVmiam €85 25-65-25

2: $3358.24; 22x3: 25-28525 N x ANzwwN 25.55

N x 632 2.5-55

8:882 2525525 2:33 5335-245 883 25-55
8:25 2525525 8:38 ANvaszANVm
@6ch 2525.425 $5.8 2555-245 ANKEN 25-55

2: 25-25525 N x SEN 5385
65.8 2525525 2:55 25-35-25 N x SEN 52:5
63% 25-25.1725 25.5 25-35-25 $83 $5.25

2 :862 25-25525 8:58: 25-35-25

2 :23” 25-25-25 532 25.25
635 6305-2me 2&3: 252525 N x 583 2.525
€58 25-55-24; €55 25-35-25 N x 5823 25.25

2 :382 25.55.?5 2:58 25-2 :5AN:m €83 ANVmA :5

£453

 

884532.53. one 5453 .8. C 8&5. E: 5... sauce. 2:5 .3 can...

41

pmlmuza

 

 

8:85 8585585
8:55 85-85585
8:388 85-85585 8:825 25-85-85 8 x 855N 85.855
52 85-85585 8:25 25-85-85 N x 8582 85-85<
8:85 85-85585 8:852 25-85-85
8:55 85-85585 8:885 85-85-85 8 x 855N 25.255
52 8585585 855 85-85-85 N x 8558 85-25<
8:5.N: 85-85-85
2 258.8 25-25525 8:55 85-85-85 N x 855N 85-85
2 :55 25-25525 8:55 85-85-85 N x 852 85-85
8:852 25-25525 2 :85 85-85-85 853 85-85
8:55 25-25585
8:28.88 25-25585 8:55 25-25-85 8558 25-85
8:852 85-25585 8:55 25-25-25 N x 855N 85-85
8:83: 25-25-85 N x 85: 85-85
2 :55 85-85-85 8 :55 85-25-85 855N 85-85
855 85-85-85 8:38: 85-25-25
8:35 85-85-85 8585 85-25-25 8583 25-25
8:85 85-85-85 8:55 85-25-85 N x 8595 85-25
8:85 85-85-85 8:55 25-25-25 N x 85N5N 25-25
2 :55 85-85-85 8:5. 8 25-25-85 853 85-25
53.53.25?

 

.VnN 03.5. 65.5.30

42

1.0

 

 

 

 

O
0
~51“

 

 

 

Energy (eV)

 

 

 

 

 

 

 

    

 

 

 

 

 

 

 

A

A F

-1.0_ In i};

IYVZ

 

Figure 2-5. Electronic band structure of AgBi385 with spin-orbit interaction included
(Eg = 0.17 eV).

43

(0
0|

 

 

   

 

 

 

 

 

 

 

 

 

 

ID __
£130 (A) tota OS
to ' .-
a)
52 25 - .
m
5’ 20 - -l
<1:
; 15 -
OJ ‘ .
7. 1o - .
a: .
co 5 .
“I;
o A . n n
1 ,4 B partial DOS: Bi1 p —— 4
( ) partial DOS: Bi2 p .....
E 1 .2 . pa ial DOS: BIS p ............... q
S 1 _ _
CU
>O.8 . _
a:
”0.6 - .
(D
530.4 -. .
m C
0.2 . . 4
o lama 2-:
1.4 . C partial DOS: 81 p —— .
( ) partial DOS: S4 p .....
E12 . i ‘
3 1 _ l .
CU
>O.8 — l ‘
CD
”0.6 - -
a) D
330.4 . g .3 -
a, .a L.
0.2 - -
0 MA ° , ' 5—~-
7 ' D partial DOS: A91 (:1 _ ‘
6 - ( ) partial DOS: A92 d ----- 4
E
o 5 - .
(U
> 4 ' 4
Q)
a, 3 r -
Q)
a: 2 - ‘
‘5;
1 _ .
0 4 -1‘0 A #u ’5'“ A 50

 

Energy [eV]

Figure 2-6. Density of states (DOS) of AgBi385. (A) Total DOS, partial atomic DOS of
(B) bismuth atoms, (C) S] and S4, and (D) silver atoms.

(a) (b)

 

 

 

Absorption coefficient (arbitrary unit)

 

Absorption coefficient (arbitrary unit)

 

V é ' é ' a I l l l l ' I
Energy (eV)

 

 

O
a:
\1

' 5 4
Energy (eV)
Figure 2-7. Solid-state UV/Vis spectra for (a) AgBi385 and (b) AngojBlzJSs

respectively.

Thermoelectric properties. Thermopower measurements on samples cut from
oriented polycrystalline ingots were carried out along the crystal grth direction (i.e.,
crystallographic b-axis). The thermopower of AgBi385 is negative and increases almost
linearly from -25 uV/K at 80 K to -l60 uV/K at 700 K, Figure 2-8(a) and 9(a). The
negative value (n-type) indicates that the predominant carriers are electrons, and charge
transport in this compound is accomplished by carriers moving predominantly through
Bi-p orbitals near the conduction band bottom as suggested by the results of the
electronic band calculations.

Electrical conductivity measurements were also performed along the direction of
crystal growth. The conductivity of the AgBi385 ingot was relatively high and exhibited
negative temperature dependence with the value decreasing almost linearly from 660
S/cm at 80 K to 134 S/cm at 700 K. This is a typical behavior for a degenerate

semiconductor, Figure 2-8(a). It is possible that the degree of doping varies in ingots of

45

these materials since the electrical conductivities between two separate measurements at
a low and a high temperature range showed approximately 100 S/cm gap at room
temperature, Figure 2-8(a) and 9(a). Variations in electrical conductivities were observed
in oriented ingot sample of AgBi385 screened by scanning probe conductivity 36
measurements at room temperature. The scanned electrical conductivity values varied
from 244 S/cm at one crystal domain to the almost twice the value with 415 S/cm at
another domain which was only 0.2 m away. Further studies regarding anisotropy of
AgBi385 with better grown samples are planned.

Among the AngxBi3-,-Ss solid solutions the compound with x = 0.3 was selected
for measuring charge transport properties. This material showed slightly lower electrical
conductivity and higher thermopower than AgBi385 implying a lower number of carriers.
The room temperature values were 260 S/cm for the conductivity and -98 uV/K for the
thermopower, Figure 2-8(b). The thermopower of Ango,3Bi2_7S_:, increases almost linearly
from -35 uV/K at 80 K to -150 pV/K at 400 K and the conductivity decreases from 344
S/cm at 80 K to ~200 S/cm at 400 K.

The thermal conductivity of AgBi385 was observed at ~1.5 W/m-K at room
temperature and it increases as temperature rises from 80 K to 300 K, Figure 2-8(a). The
thermal conductivity can be divided into two contributions, electronic Kele and lattice Klan.
37 Because the room temperature electronic conductivity is <3OO S/cm the electronic
contribution is only a small fraction of the total and the lattice thermal conductivity
dominates heat transport in these materials. The rising thermal conductivity with rising
temperature observed in the data is due to irradiative losses (which begin to appear

around 200 K) inherent in the measurement. The thermal conductivity of AngOJBiZJSS

46

 

 

 

 

 

 

A700
8 I
o
5 I .
V6504 . O
3‘ ' '
-'-| . .
.2 ' I
3600- I .
g —>
o
8550- ‘— ' 0
U . .
7e- . .,
.8500. I
H
3.3. 4 -.
mm ' I ' I ' r V U ' I ' I
50 100 150 200 250 300 350
Temperature (K)
N-o-J )
E
o . o
\ O
m C
V3505 . "o ,
>x ' I .
.*-' - II. I
.E ' “a
*9300- I a. 0 .
E: ‘ ~— --
2504
U
E
'uZOO" I
H
.5}.
“-1150

 

Temperature (K)

Figure 2-8. Variable temperature thermopower, electrical conductivity and thermal

conductivity for (a) AgBi385 and (b) AngogBinSs. (I = Km, 0 = Kc, A: Klan).

50'16035'260'2530'360'50'460'

 

 

 

 

 

 

 

Temperature (K)

47

-20 Q2
‘ 8
, \
30:3 III'
' 3; II...
.40
up. .
'83 II AAAA‘M“
o ..- A
.52 31-4 . A‘
4,022 1“
Ac
. '20
-7ORE‘
‘ Va 0........
_80 D ......
go I l I I i
so 100 150 200 250 300
Temperature(K)
A
5.4
E
-50
532‘ :
I
o o I An
2.3 25‘“
$3 811 I]:
‘90
-150
is.
v-e,
..0~.. . .
-2oo 50 "u“

50'160'150‘2601250'360'350160‘450

 

 

showed a similarly low value (~1.6 W/m-K) at room temperature to that of AgBi385,
Figure 2-8 (b). Again it irradiative losses are evident in the measurement which raise the
thermal conductivity.

The true value of thermal conductivity was obtained (on Ang0_3Bi2.7SS) using
with a different method27 (i.e. thermal diffusivity technique) that is not subject to
irradiative losses. These measurements actually showed a very low thermal conductivity
of less than 1 W/m-K at the temperature, Figure 2-9(b). In comparison with the room
temperature values obtained with the steady state technique (Figure 2-8(b)), the observed
difference of ~O.76 W/m-K is attributed irradiative losses. At 800 K an exceptionally low
value of 0.75 W/m-K is attained, Figure 2-9(b). These values are consistent with the low
crystal symmetry, complexity of the crystal structure and the presence of heavy atoms
(e.g., Bi) in the structure.

The fact that the lattice thermal conductivities of AgBi385 and AngogBinSs are
similar indicates a 10% participation of Sb atoms in the Bi atom sites of solid solution is
not enough to cause a significant reduction in the lattice thermal conductivity in this

system.
5. Concluding Remarks

The synthesis, crystal growth, thermoelectric properties of AgBi385 and its solid
solution AngogBiszs and electronic band structure of AgBi385 were studied for the first

time. Both AgBi385 and AngogBinSs show degenerate n—type semiconducting behavior

with relatively high electrical conductivities and extremely low thermal conductivities.

48

(a) (b)

 

 

 

 

 

 

 

 

E -50 322.0

0 0

E4004 N E

b - . . M5-

:Egoo . 100g e?

‘5 ‘ . s e .2.

'O 4 I . ‘ ———> O 310.

g: l S I I I I

0200 *— I : a C‘- i I I I II

I 0 450A 0

.3 I . 'r: U

U I I < ._.0.5-‘

'C‘. I I I I R a

8100- v E

.2 o

m,...,j,..-200f50.o.v.......v.
300 400 500 600 700 300 400 500 600 700 800

Temperature (K) Temperature (K)

Figure 2-9. (a) Variable temperature thermopower and electrical conductivity data for
AgBi385. (b) Thermal conductivity for Ango.3Bi2_785 measured with the thermal
diffusivity technique at high temperature.

The electronic band structure calculation of AgBi385 suggests that the electron transport
is mostly confined within Slab II. As the result of substitution of Sb in slab II the
electrical conductivity is slightly reduced and thermopower increased but the thermal
conductivity did not change significantly. This study emphasizes the importance of
crystal growth of AgBi385. The calculations suggest that the silver d-states are involved
near the Fermi level and influence the charge transport properties in this material. They
also suggest a high thermopower in this system should p-type doping be achievable. The
controlled substitution with other elements in the slabs consisting of AgBi385 and
modification of the structure by partially replacing Ag with alkali metal, copper, or
thallium could help to further modulate the thermoelectric properties in this class of

compounds.

49

References

' ZT = SZO'T/K, where S is the Seebeck coefficient, 0 is the electrical conductivity, T is
the temperature and K is the thermal conductivity, which includes electron and phonon
contributions.

2 CRC Handbook of Thermoelectric Materials. Rowe, D.M..Ed., CRC Press, Inc.: Boca
Raton, FL, 1995.

3 (a) Kanatzidis, M. G. Semicond. Semimet. 2001, 69, 51-100, (b) Chung, D-. Y.;
Iordanidis, L.; Choi, K.-S. and Kanatzidis, M. G., Bull. Kor. Chem. Soc. 1998, 19, 1283-
1293., (c) Kanatzidis, M.G.; Mahanti, S. D.; Hogan, T. P. Chemistry, Physics, and
Materials Science of Thermoelectric Materials: Beyond Bismuth T elluride.; Kluwer
Academic/Plenum Publishers: New York, 2003; p 35, (d) Mrotzek, A.; Kanatzidis, M.
G.;Acc. Chem. Res. 2003; 36, 111-119.

4 Chung, D-. Y.; Hogan, T.; Brazis, P. W.; Kannewurf, C. R.; Bastea, M.; Uher, C.;
Kanatzidis, M. G. Science 2000, 287, 1024-1027.

5 Chung, D. -Y.; Choi, K. —S; Iordanidis, L; Schindler, J .L. ; Brazis, P. W.; Kannewurf, C.
R.; Chen, B.; Hu, 8.; Uher, C.; Kanatzidis, M. G. Chem. Mater., 1997, 9, 3060-3071.

6 Chung, D.-Y.; Jobic, S.; Hogan, T.; Kannewurf, C. R.; Brec, R.; Rouxel, J .; Kanatzidis,
M. G. J. Am. Chem. Soc. 1997, 119, 2505-2515.

7 Mrotzek, A; Chung, D. —Y.; Hogan, T; Kanatzidis, M. G. J. Mater. Chem, 2000, 10,
1667-1672.

8 Choi, K. —S.; Chung, D. —Y.; Mrotzek, A.; Brazis, P.; Kannewurf, C. R.; Uher, C.; Chen,
W.; Hogan, T.; Kanatzidis, M. G. Chem. Mater. 2001, 13 (3): 756-764.

9 Iordanidis, L.; Brazis, P. W.; Kyratsi, T.; Ireland, J.; Lane, M.; Kannewurf, C. R. Chen,
W.; Dyck, J. S.; Uher, C.; Ghelani, N. A.; Hogan, T.; Kanatzidis, M. G. Chem. Mater.
2001, 13, 622-633.

'0 Hsu, K. —F.; Chung, D. —Y.; La], 8.; Mrotzek, A.; Kyratsi, T.; Hogan, T.; Kanatzidis M.
G. J. Am. Chem. Soc., 2002, 124, 2410-2411.

” Hsu, K. -F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.;
Polychroniadis. E. K.; Kanatzidis, M. G. Science 2004, 303, 818-821.

'2 McCarthy, T. J.; Ngeyi, S.-P.; Liao, J.-H.; DeGroot, D.; Hogan, T.; Kannewurf, C. R.;
Kanatzidis, M. G. Chem. Mater. 1993, 5, 331-340.

'3 Iordanidis, L; Bilc, D; Mahanti, s. D. and Kanatzidis, M. 0.; J. Am. Chem. Soc. 2003;
125, 13741-13752.

50

 

'4 McCarthy, T. J.; Tanzer, T. A.; Kanatzidis, M. G. J. Am. Chem. Soc. 1995, 117, 1294-
1301.

'5 (a) McCarthy, T. J .; Tanzer, T. A.; Chen, L.H.; Iordanidis, L.; Hogan, T; Kannewurf, C.
R.; Uher, C; Chen, B; Kanatzidis, M. G. Chem. Mater. 1996, 8, 1465-1474. (b) Chen, B;
Uher, C; Iordanidis, L; Kanatzidis, M. G. Chem. Mater. 1997, 9, 1655-1658.

'6 Makovicky, E. Neues Jahrb. Mineral. Abh. 1989, 160(3), 269-297.

'7 (a) Wemick, J .H. Am. Mineral.1960, 45, 591-598. (b) Wernick, 1H. J. Mat. 56- 1968, 3a
498-501.

‘8 Mumme, W.G. Neues Jahrbuch fuer Mineralogie 1990,193-204.
‘9 Herbert, H.K.;Mumme, W.G. Neues Jahrbuchfuer Mineralogie. 1981, 69-80.
2° Makovicky, B.;Mumme, W.G.;Watts, J.A. Can. Mineralogist 1977, 15, 339-348.

2‘ Kyratsi, T.; Chung, D.-Y.; Choi, K.-S.; Dick, 1. S.; Chen, W.; Uher, C. and Kanatzidis,
M. G. Mat. Res. Soc. Symp. proc. 2000, 626, Z8.8.1- 28.8.6.

22 Kubelka-Munk fianction: a/S = (l-R)2/2R, where a is the absorption coefficient, S is
the scattering coefficient, and R is the reflectance at a given wavenumber.

23 Hogan, T.; Ghelani, N.; Loo, S.; Sportouch, S.; Kim, S.-J.; Chung, D.-Y. Kanatzidis, M.
G. Proc. Int. Conf Thermoelectr., 1999, p. 671-674.

2‘ Maldonado, o. Cryogenics, 1992, 32, p. 908-912.
25 LabVIEW, Version 5.0, National Instruments, Austin, TX, 1999.

26 Loo, 8.; Short, J.; Hsu, K. —F.; Kanatzidis, M. G.; Hogan, T. Mat. Res. Soc. Symp.
Proc., 2004, 793, S9.4.1-9.

27 The high temperature thermal conductivity measurements for a well grown
polycrystalline ingot sample of AgBi385 were accomplished by Thermophysical
Properties Research Laboratory inc., West Lafayette IN 47906, USA (www.tprl.coml).

28 SMART, SAINT, SHELXTL: Data Collection and Processing Software for the
SMART-CCD system; Siemens Analytical X-ray Instruments Inc.: Madison, WI, 1997.

29 Singh, D.; Planewaves, Pseudopotentials, and the LAPW method (Kluwer Academic,
Boston, 1994).

51

 

3" a) Hohenberg, P. and Kohn, w. Phys. Rev., 1964, 136, B864. b) Kohn, w. and Sham, L.
ibid, 1965, 140, A1133.

3‘ Perdew, J. P.; Burke, K. and Emzerhof, M. Phys. Rev. Lett., 1996, 77, 3865-3868.
’2 Koelling, D. D. and Harmon, B. J. Phys. c, 1980, 13, 6147.

33 Blaha, P.; Schwarz, K.; Madsen, G.; Kvasnicka, D. and Luitz, J. WIENZK, An
Augmented Plane Wave + Local Orbitals Program for Calculating Crystal Properties
(Karlheinz Schwarz, Tech. Univ. Wien, Vienna, 2001).

3“ Takeuchi, Y.; Takagi, J. and Yamanaka, T. Z. Kristallogr. Bd. 1974, 140, 249-272.

35 Aulbur, W. B.; Jonsson, L. and Wilkins, J. W. Solid State Phys. Edited by Ehrenreich,
H. and Spaepen, F. Academic Press, New York, 2000, Vol 54, p11.

36 The typical four point probe method used for measuring the electrical conductivity
utilizes two voltage leads at some fixed spacing. The scanning conductivity system
translates one of the voltage leads along the length of the sample, and the corresponding
slope of voltages collected versus distance scanned between voltages is used to calculate
the electrical conductivity. If a discontinuity in the slope exists, this may indicate a
change in the homogeneity of the material.

37 The thermal conductivity is generally represented by the sum of electronic (Kale) and
lattice (Klan) contributions. The Klan is estimated by the difference between total and
electronic thermal conductivity calculated by the Wiedemann-Franz law. Kittel, C.
Introduction to Solid State Physics, 7th ed.; John Wiley & Sons, Inc.: New York, 1996; p
166.

52

CHAPTER 3

A New Chalcogenide Homologous Series A2[M5+,,Se9+,,] (A = Rb,

Cs; M = Bi, Ag, Cd).

1. Introduction

The excellent thermoelectric properties of BizTe3 near room temperature have
motivated extensive studies in bismuth chalcogenide chemistry over the past decade. "3 In
recent exploration of new compounds incorporating alkali metals, extraordinarily diverse
structures and compositions have emerged. One of the notable features in these
compounds is that they are built with relatively few common structural motifs. When
compounds can be recognized and grouped in series of homologs defined by their
structural modules, we then have a powerful way to correlate and understand large
classes of materials, thereby allowing useful generalizations and predictions. Some
examples of homologies are the megaseries of Am[M|+ISCZ+1]2m[M2]+nSez+3l-m] (A = K, Rb,
Cs, Sr, Ba; M = Sn, Pb, Eu, Bi, Sb),4 CstmBi3Te5+m,5 (szTe3),,,t(Sb2),,6 and the
gustavite—lillianite series7 and the kobellite series8 of mineral sulfosalts. The archetypal
modules in these series are built from structural units excised from the NaCl- and szse3-
type lattices.9 These modules are uniquely expressed in each homology by a predictable
evolution in size. The megaseries Am[M1+ISe2+1]2m[M21+,,Se2+31+,,], for example, is

composed of NaCl-type [M1+ISe2+;]2m and [M21+,,Se2+31+,,] slabs, which are interconnected

53

to create frameworks with tunnels accommodating the alkali metal (Am) ions.4 The size of
each module can be tuned by varying the integers l, m and n while retaining the sites for
alkali metals.

The general insights obtained by understanding the building principles in
homologies have implications in the rational design of solid state compounds. This
directed us to search for new homologies and identify their members. In this paper, we
describe the compounds ,B-CsBi3Se5, szCdBigseu, CsAgo,5Bi3_SSe6, CstBi3Se6,
szAg15Bi758e13 and CszAg1,5Bi7,5Se13, all of which can be organized under the novel
homologous series A2[M5+,,Seg+,,] (A = Rb, Cs; M = Bi, Ag, Cd; n = 1, 2, 3, 4). The

previously reported 7-RbBI3SCle can now be viewed as a member of this series.

2. Experimental Section

Reagents. Chemicals were used as obtained: bismuth chunks (99.999% Noranda,
Canada), selenium shots (99.999%, Noranda Canada), cadmium powder (99.999%, -
200mesh Cerac), rubidium metal (99.8% Johnson Matthey Co., Ward Hill, MA), cesium
metal (99.8% Johnson Matthey Co., Ward Hill, MA).

Synthesis. All manipulations were carried out under a dry nitrogen atmosphere in
a Vacuum Atmospheres Dri-Lab glovebox and in a Schlenk line. For all compounds the
yield was quantitative. AZSe(A = Rb, Cs) were obtained by stoichiometric reactions of
elemental alkali metals and selenium in liquid NH3. The purity and homogeneity of the
products were verified by comparing the X-ray powder diffraction patterns to those

calculated by the crystallographically determined atomic coordinates.

54

Ag Powder. A silver coin (99.999%) was dissolved in nitric acid. The solution
was neutralized to a pH of 7 with ammonium hydroxide. Sodium borohydride was added
to reduce the Ag ions to a black precipitate of Ag metal powder. The precipitate of silver
was filtered and washed thoroughly with water and dried in a vacuum oven at 150 °C.
The obtained fine powder of Ag was identified by powder X-ray diffraction.

fl-CsBi3Se5 and CstBi3Se6. A mixture of Cs metal (1.407 g, 10.6 mmol), Bi
(6.637 g, 31.8 mmol), and Se (4.179 g, 53 mmol) for ,B-CsBi3Se5 and a mixture of Cs
metal (1.156 g, 8.7 mmol), Cd powder (0.978 g, 8.7 mmol), Bi (5.453 g, 26.1 mmol), and
Se (4.121 g, 52.2 mmol) for CstBi3Se6 were loaded in fused silica tubes (13 mm
diameter) and subsequently flame-sealed at a residual pressure of <10”4 mbar. The tubes
were heated at 750 °C for 2 h with rocking, followed by cooling to 550 °C at a rate of
20 °C h'1 then to room temperature in 10 h. Lustrous polycrystalline ingots made from
needle-like crystals randomly oriented were obtained in quantitative yield. A quantitative
microprobe analysis using Energy Dispersive Spectroscopy (EDS) was performed on a
Scanning Electron Microscope (SEM) on several single crystals of ,B-CsBi3Se5 and
CstBi3Se6 gave the approximate composition of Csl_17Bi3,098e5 and Csl,21Cd1_ogBi3,o.iSe6
respectively. In order to grow highly oriented crystal specimens for the thermoelectric
property measurements, the products were loaded in silica tubes (13 mm diameter) with a
point end and sealed under vacuum. The tubes were heated to 750 °C in a Bridgman
fumace and descended at a rate of 4.17 mm/h through a sharp (100 °C/cm) temperature
gradient.ll Pure and well oriented ingots (30 ~ 40 mm long, 11 mm diameter) of ,6-

CsBi3Se5 and CstBi3Se6 were obtained.

55

szCdBuSen. A mixture of szse (0.1499 g, 0.6 mmol), Cd powder (0.0337 g,
0.3 mmol), Bi (0.5016 g, 2.4 mmol), and Se (0.4027 g, 5.1 mmol) was loaded in a fused
silica tube (9 mm diameter) and subsequently flame—sealed at a residual pressure of <10“:
mbar. The thoroughly mixed elements was heated at 750 °C for 72 h, followed by cooling
to 550 °C at a rate of —5 °C h", and then to 50 °C in 10 h. The shiny silvery
polycrystalline ingot of szCdBissen was obtained after washing away any impurities
with dimethylfonnaide (DMF), diethyl ether (over 90 %) and identified by X-ray powder
diffraction. SEM/EDS analysis on several single crystals of szCdBi6Se11 showed the
approximate composition of Rb2,77Cd.,03Bi6_lgSe. 1.

szAg1,5Bi7,5Se13, and CszAg1,5Bi7,SSe13. A mixture of szse (0.0875 g, 0.35
mmol), Ag powder (0.0566 g, 0.53 mmol), Bi (0.5486 g, 2.6 mmol), and Se (0.3316 g,
4.2 mmol) for szAg.,5Bi7,5Se13 and a mixture of Cszse (0.1034 g, 0.3 mmol), Ag
powder (0.0485 g, 0.45 mmol), Bi (0.4702 g, 2.3 mmol), and Se (0.2843 g, 3.6 mmol) for
cesium analogue were loaded in fused silica tubes (9 mm diameter) then flame-sealed at a
residual pressure of <10“1 mbar. The starting materials were heated as above the
temperature profile. Shiny silvery polycrystalline ingots were obtained and each phase
was identified by X-ray powder diffraction. A quantitative microprobe analysis with a
SEM/EDS system, performed on different crystals, gave the average compositions
Rb2.59Ag1.3sBi7.17Se13 and €82.32Ag1.83Bi7.153613, respectively.

CsAgo.sBi3,5Se5. A fast cooling (10 h from isotherm temperature 750 °C to 50 °C)
reaction for CszAg1.5Bi7,5Se13 produced CsAgo.53i3_5Se6 (~90% yield) were analyzed by

XRD.

56

3. Physical measurements

Electron Microscopy. Quantitative microprobe analysis for the compounds was
performed with a JEOL J SM-6400V Scanning Electron Microscope (SEM) equipped
with a Noran Vantage Energy Dispersive Spectroscopy (EDS) detector. Data were
collected for 30 sec using an accelerating voltage of 20kV. All reported results are an
average of measurements on at least three different crystals.

Differential Thermal Analysis. Differential thermal analysis (DTA) was
performed with a computer-controlled thermal analyzer (Shimadzu DTA-50). A 20 mg of
ground crystals were sealed in silica ampoule under vacuum. A silica ampoule containing
the equal mass of alumina was placed on the reference side of the detector. The sample
was heated to the desired temperature a 10 °C/min, isothermed for 2 min and then cooled
at 10 °C/min. The heating program was recycled to check reproducibility of the thermal
behavior of the sample. The reported melting point is the peak temperature. After DTA,
the sample was examined by powder X-ray diffraction to identify if any decomposed
product formed during heating/ cooling cycles.

Solid-State UV/vis Spectroscopy. Optical diffuse reflectance measurement was
made at room temperature with a Shimazu UV-3101 PC double-beam, double-
monochromator spectrometer operating in the 200 ~ 2500 nm region. The instrument was
equipped with an integrating sphere and controlled by a personal computer. BaSOa
powder was used as reference (100% reflectance). Absorption data were calculated from

the reflectance data using the Kubelka-Munk function.11

57

Infrared Spectroscopy. Optical diffuse reflectance measurements were made on
the finely ground sample at room temperature. The spectrum was recorded in the infrared
region (6000-400 cm") with the use of a Nicolet MAGNA-IR 750 Spectrometer equipped
with a Collector Diffuse Reflectance of Spectra-Tech. Inc. The reflectance versus
wavenumber data were used to estimate a material’s and gap by converting reflectance to
absorption data as described previously”.

Charge transport measurements. The Seebeck coeffiecient of polycrystalline
samples was measured between 300 and 700 K by using a SB-100 Seebeck Effect
Measurement System, MMR Technologies. The electrical conductivity measurements
were performed in the usual four-probe geometry at room temperature.

Powder X-ray Diffraction. A calibrated CPS 120 INEL X-ray powder
diffractometer equipped with a position-sensitive detector, operating at 40kV/25mA with
a flat geometry and employing graphite monochromatized Cu Kor radiation, was used to
obtain powder patterns of starting materials and all products.

Single-crystal X-ray Crystallography. Single crystals of ,B-CsBi3Se5,
RbZCdBi68e1 1, CsAg0,5Bi3,5Se6, CstBi3Se6, szAguBimSen and CszAg1,5Bi7_SSel3
were mounted on the tip of a glass fiber. The intensity data were collected on a Bruker
SMART Platform CCD diffractometer with graphite monochromatized Mom radiatin at
room temperature. The individual frames were measured with an omega angle rotation of
03° and an acquisition time of 30 sec for each crystal. The SMART13 software was used
for the data acquisition and SAINT software for data extraction and reduction. An
analytical absorption correction was performed using face indexing and the program

XPREP in the SAINT software package, followed by a semiempirical absorption

58

correction based on symmetrically equivalent reflections with the program SADABS.
Structural solution and refinements were successfully done using the SHELXTL package
of crystallographic programs. The structures were solved with direct methods.

The complete data collection parameters, details of the structure solution, and
refinement for ,B-CsBi3Se5, RbZCdBi6Sen, CstBi3Se6, and szAgmBinSel; are given
in Table 3-1. The fractional coordinates and temperature factors (Ueq) of all the atoms

with estimated standard deviations are given in Tables 3-2 ~ 3-9.

59

Table 3-1. Summary of crystallographic data for members of A2[M5+,.Se9+n] :fl-
CsBi3Se5, szCdBkSeu, CstBi3Se6, and szAglsBi-uSen.

 

Empirical formula CsBi3Se5 szCdBi6Sen

Formula weight 1154.65 2405.78

Temperature 293(2) K 173 K

Wavelength 0.71073 A 0.71073 A

Crystal system Orthorhombic Orthorhombic

Space group ana Pnnm

Unit cell dimensions a = 22.740(13) A a = 12.385(3) A
b=4.171(2)A b=23.839(6)A
c=12.472(7)A c=4.1124(10)A

Volume 1183.0(11) A3 1214.1(5) A3

Z 4 2

Density (calculated) 6.483 Mg/m3 6.581 Mg/m3

Absorption coefficient 62.867 mm'l 64.636 mm'l

F(000) 1896 1988

Theta range for data collection 1.71 to 27.98° 1.71 to 28.29°

Index ranges -28<=h<=29, -5<=k<=51, -16<=h<=15, -31<=k<=31,
-16<=l<=16 -5 <=l<=5

Reflections collected 8168 10010

Independent reflections 1542 [R(int) = 0.0688] 1681 [R(int) = 0.1763]

Completeness to theta = 2829" 94.80% 97.60%

Refinement method F ull-matrix least-squares on F 2

Data/restraints / parameters 1542 / 0 /55 1681 /0 / 65

Goodness-of-fit on F2 1.188 0.929

Final R indices [I>25igma(l)] R18 = 0.0670, wR2 = 0.1688 R1“ = 0.0640, wR2 = 0.1497

R indices (all data) R18 = 0.0922, wR2 = 0.1965 R13 = 0.1565, wR2 = 0.1751

Largest diff. peak and hole 4.173 and -4.842 e. A3 3.868 and -6.326 e. A‘3

 

”R1 = leFol - lFell/EllFoll- wR2 = {Z[W(Foz -Fe2)2]/Z[MF62)2]} "2-

60

Continue. Table 3-1.

 

Empirical formula

Formula weight
Temperature
Wavelength

Crystal system
Space group

Unit cell dimensions

Volume
Z
Density (calculated)

Absorption coefficient
F(000)
Theta range for data collection

Index ranges

Reflections collected
Independent reflections
Completeness to theta = 2829"
Refinement method

Data / restraints / parameters
Goodness-of-fit on F2

Final R indices [I>28igma(l)]
R indices (all data)

Largest diff. peak and hole

Extinction coefficient

Cchanse5
1346.01

293 K

0.71073 A
Orthorhombic
ana

a = 26.512(8) A
b = 4.1192(13) A
e = 12.396(4) A
1353.7(7) A3

4

6.604 Mg/m3

59.161 nnn'l
2224

1.54 to 28.24°
-34<=h<=34, -5<=k<=5,
-15<=l<=16

10826

1819 [R(int) = 0.0621]
95.20%

szAgl5Bi753€13
2926.57

173(2) K
0.71073 A
Orthorhombic
Pnnm

a = 12.386(2) A
b = 27.642(5) A
c = 4.1107(8) A
1407.4(5) A3

2

6.906 Mg/m3

67.966 mm-l

2418

1.47 to 28.29°.
-16<=h<=15, -36<=k<=36,
-5<=l<=4

1 1237

1956 [R(int) = 0.0922]
97.90%

Full-matrix least-squares on F2

1819/0/70

1.116

R18 = 0.0503, wR2 = 0.1269
R1” = 0.0856, wR2 = 0.1568

2.827 and -3.671 e. A3

1956 / 0 / 86

0.799

R1“ = 0.0389, wR2 = 0.0831
R1“ = 0.1021, wR2 = 0.0923
3.907 and -2.548 e.A'3

0.00037(3)

 

“R1 = leFol - lFell/XllFoll- wR2 = {>3[W(Fo2 -Fe2)2]/Z[W(Foz)2]}m-

61

Table 3-2. Atomic coordinates ( x 104) and equivalent isotropic displacement

parameters (Azx 10’) for ,B-CsBi38e5

the orthggonalized Uitensor.

. U(eq) is defined as one third of the trace of

 

 

x y z U(eq) occupancy
Bi(l) 4762(1) 2500 8634(1) 19(1) 1
Bi(2) 4256(1) -2500 5938(1) 18(1) 1
Bi(3) 3763(1) -7500 3063(1) 20(1) 1
Cs(1) 2880(1) -17500 -115(2) 31(1) 1
Se(l) 3439(2) -7500 5286(3) 18(1) 1
Se(2) 301 1(2) -12500 2474(3) 25(1) 1
Se(3) 3880(2) -2500 8058(3) 20(1) 1
Se(4) 4367(2) 2500 10846(3) 18(1) 1
Se(5) 5192(2) 2500 6390(3) 15(1) 1

 

Table 3-3. Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for szCdBkSe". U(eq) is defined as one third of the trace
of the orthogonalized Uij tensor.

 

 

x y z U(eq) occupancy
Bi(1)/Cd(1) 5000 5000 0 9(1) 0.733(9)/0.267
Bi(2) 2248(1) 5477(1) -5000 12(1) 1
Bi(3)/Cd(3) 9503(2) 5910(1) -10000 12(1) 0.645(7)/0.355
Bi(4) 6675(1) 6404(1) -5000 12( 1) 1
Se(l) 8904(4) 6659(2) -5000 14(1) 1
Se(2) 4432(3) 5812(2) -5000 14(1) 1
Se(3) 1674(4) 6272(2) -10000 16(1) 1
Se(4) 6142(4) 7141(2) -10000 18(1) 1
Se(S) 2760(3) 4579(2) 0 1 1(1) 1
Se(6) 0 5000 -5000 16(2) 1
Rb(l) 8586(4) 7841(2) -10000 38(1) 1

 

62

Table 3—4. Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for CstBi3Se6. U(eq) is defined as one third of the trace of

the orthogonalized Llij tensor.

 

x y z U(eq) occupancy
Bi(1)/Cd(1) 210(1) -2500 8615(1) 17(1) 0.604(5)/0.396
Bi(2)/Cd(2) 619(1) -7500 5840(1) 21(1) 0.810(5)/0.190
Bi(3)/Cd(3) -1009( 1) 2500 6891(1) 22( 1) 0.595(5)/0.405
Bi(4) -1438(1) -2500 9693(1) 18(1) 1
Cs(1) -2183(1) #2500 12844(2) 36(1) 1
Se(l) -1677(1) 7500 7476(2) 19(1) 1
Se(2) 894(1) -7500 8049(2) 19( 1) 1
Se(3) 1301(1) -2500 5302(2) 21(1) 1
Se(4) 551(1) -2500 10844(2) 19( 1) 1
Se(5) -2088(1) 2500 10275(2) 24(1) 1
Se(6) -21 1(1) -2500 6400(2) 23(1) 1

 

Table 3-5. Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103)for szAg15Bi75Se13. U(eq) is defined as one third of the
trace of the orthogonalized Uij tensor.

 

 

x y z U(eq) occupancy

Bi(l)/Ag(1) 5000 5000 10000 15(1) 0.665/0.335
Bi(2) 2208(8) 4603(2) 5000 15( 1) 0.718
Ag(2) 2250(40) 4677(13) 5000 15(1) 0.282
Bi(3) -568(2) 4219(1) 0 16(1) 0.926
Ag(3) -570(50) 4068(14) 0 16(1) 0.074
Bi(4) 6690(2) 3863(1) 5000 16(1) 0.772
Ag(4) 6820(20) 3750(7) 5000 16(1) 0.228
Bi(5) 3898(1) 3446(1) 0 14(1) 1

Rb(l) 779(2) 2796(1) -10000 23(1) 1

Se(l) 6099(2) 3199(1) 0 14(1) 1

Se(2) 3313(2) 2825(1) -5000 17(1) 1

Se(3) 1676(2) 3965(1) 0 15(1) 1

Se(4) -1061(2) 3543(1) -5000 15(1) 1

Se(5) 4468(2) 4299(1) 5000 13(1) 1

Se(6) 2792(2) 5377(1) 10000 15(1) 1

Se(7) 0 5000 5000 12(1) 1

 

63

Table 3-6. Bond lengths [A] and angles [°] for ,B-CsBi3Se5.

 

Bi(1)-Se(4)
Bi(1)-Se(4)
Bi(1)-Se(5)
Bi(1)-Se(3)

Bi(2)-Se(3)
Bi(2)-Se(1)
Bi(2)-Se(5)
Bi(2)-Se(5)

Bi(3)-Se(2)
Bi(3)-Se(1)
Bi(3)-Se(4)
Bi(3)-Se(5)

Cs(1)-Se(4)
Cs(1)-Se(2)
Cs(1)-Se(1)
Cs(1)-Se(3)
Cs(1)-Se(2)

Se(4)-Bi(1)-Se(4)
Se(4)-Bi(1)-Se(4)
Se(4)-Bi(1)-Se(5)
Se(4)-Bi(1)-Se(5)

2.902(4)
2.948(3) x 2
2.965(4)

2.982(3) x 2

2.779(4)
2.909(3) x 2
3.034(3) x 2
3.163(4)

2.795(3) x 2
2.870(4)
3.087(4)
3.234(3) x 2

3.588(5)
3.627(5)

3.686(4) x 2
3.836(4) x 2
3.855(5) x 2

89.93(10)
90.0702)
178.7902)
8921(9)

Se(4)-Bi(1)-Se(3)
Se(4)-Bi(1)-Se(3)
Se(4)-Bi(1)-Se(3)
Se(5)-Bi(1)-Se(3)
Se(3)-Bi(1)-Se(3)

Se(3)-Bi(2)-Se(1)
Se(1)-Bi(2)-Se(1)
Se(3)-Bi(2)-Se(5)
Se(1)-Bi(2)-Se(5)
Se(1)-Bi(2)-Se(5)
Se(5)-Bi(2)-Se(5)
Se(3)-Bi(2)-Se(5)
Se(1)-Bi(2)-Se(5)
Se(5)-Bi(2)-Se(5)

Se(2)-Bi(3)-Se(2)
Se(2)-Bi(3)-Se(1)
Se(2)-Bi(3)-Se(4)
Se(1)-Bi(3)-Se(4)
Se(2)-Bi(3)-Se(5)
Se(2)-Bi(3)-Se(5)
Se(1)-Bi(3)-Se(5)
Se(4)-Bi(3)-Se(5)
Se(5)-Bi(3)-Se(5)

91.17(10)
178.73(10)
9058(9)
8969(9)
88.76(12)

94.01(10)
91.6103)
92.2300)
173.28(11)
9043(8)
86.84(11)
174.51(13)
8982(9)
8379(9)

96.5405)
95.5501)
92.1101)
168.48(12)
170.3900)
9134(9)
8913(9)
8208(9)
80.3200)

 

64

Table 3-7. Bond lengths [A] and angles [°] for szCchSeu.

 

Bi(1)-Se(2)
Bi(1)-Se(5)

Bi(2)-Se(2)
Bi(2)-Se(3)
Bi(2)-Se(6)
Bi(2)-Se(5)

Bi(3)-Se(1)
Bi(3)-Se(3)
Bi(3)-Se(5)
Bi(3)-Se(6)

Bi(4)-Se(4)
Bi(4)-Se(1)
Bi(4)-Se(2)
Bi(4)-Se(5)

Rb(1)-Se(2)
Rb(1)-Se(1)
Rb(1)-Se(4)
Rb(1)-Se(3)

Se(2)-Bi(1)-Se(2)
Se(2)-Bi(1)-Se(2)
Se(2)-Bi(1)-Se(2)
Se(2)-Bi(1)-Se(5)
Se(2)-Bi(1)-Se(5)
Se(5)-Bi(1)-Se(5)

2.910(3) x 4
2.951(4) x 2

2.821(4)
2.884(4) x 2
3.007208)

3.036(4) x 2

2.823(4) x 2
2.823(5)
3.035(5)
3.051007) x 2

2.785(3) x 2
2.827(5)
3.115(4)
3.194(4) x 2

3.377(7)
3.511(6)

3.775(6) x 2
3.783(6) x 2

89.91(13)
90.0903)
180
90.0500)
8995(10)
180.0008)

Se(2)-Bi(2)-Se(3)
Se(3)-Bi(2)-Se(3)
Se(2)-Bi(2)-Se(6)
Se(3)-Bi(2)-Se(6)
Se(2)-Bi(2)-Se(5)
Se(3)-Bi(2)-Se(5)
Se(3)-Bi(2)-Se(5)
Se(6)-Bi(2)-Se(5)
Se(3)-Bi(2)-Se(5)
Se(5)-Bi(2)-Se(5)

Se(1)-Bi(3)-Se(1)
Se(1)-Bi(3)-Se(3)
Se(1)-Bi(3)-Se(5)
Se(3)-Bi(3)-Se(5)
Se(1)-Bi(3)-Se(6)
Se(1)-Bi(3)-Se(6)
Se(3)-Bi(3)-Se(6)
Se(5)-Bi(3)-Se(6)
Se(6)-Bi(3)-Se(6)

Se(4)-Bi(4)-Se(4)
Se(4)-Bi(4)-Se( 1)
Se(4)-Bi(4)-Se(2)
Se(1)-Bi(4)-Se(2)
Se(4)-Bi(4)-Se(5)
Se(1)-Bi(4)-Se(5)
Se(2)-Bi(4)-Sc(5)
Se(4)-Bi(4)-Se(5)
Se(5)-Bi(4)-Se(5)

92.9002)
90.9404)
174.2202)
91.1600)
89.9602)
175.9202)
91.82(9)
8579(9)
175.9202)
85.2703)

93.5005)
93.2602)
90.0102)
175.2305)
173.520 1)
9068(7)
9144(8)
8504(8)
8475(6)

95.1906)
95.4803)
94.2703)
165.5304)
9226(9)
86.7801)
82.1601)
l71.96(11)
80.1402)

 

65

Table 3-8. Bond lengths [A] and angles [°] for CstBi3Se6.

 

Bi(1)-Se(2)
Bi(1)-Se(4)
Bi(l)-Se(4)
Bi(1)-Se(6)

Bi(2)-Se(3)
Bi(2)-Se(2)
Bi(2)-Se(6)
Bi(2)-Se(6)

Bi(3)-Se(1)
Bi(3)-Se(3)
Bi(3)-Se(6)
Bi(3)-Se(4)

Bi(4)-Se(5)
Bi(4)-Se(1)
Bi(4)-Se(2)
Bi(4)-Se(4)

Cs(1)-Se(5)
Cs(1)-Se(2)
Cs(1)-Se(1)
Cs(1)-Se(5)
Cs(1)-Se(3)

Se(2)-Bi(1)-Se(2)
Se(2)-Bi(1)-Se(4)
Se(2)-Bi(1)-Se(4)
Se(2)-Bi(1)-Se(4)
Se(4)-Bi(1)-Se(4)
Se(4)-Bi(l)-Se(4)
Se(2)-Bi(1)-Se(6)

2.831(2) x 2
2.906(3)
2.960(2) x 2
2.964(3)

2.821(2) x 2
2.834(3)
2.979(3)
3.092(2) x 2

2.811(2) x 2
2.827(3)
3.015(2) x 2
3.060(3)

2.781(2) x 2
2.820(3)
3.150(3)
3.197(2) x 2

3.579(4)
3.593(4)

3.687(3) x 2
3.801(3) x 2
3.872(3) x 2

9335(9)
9212(7)
176.77(7)
8920(6)
8976(7)
8819(8)
9065(7)

Se(4)-Bi(1)-Se(6)
Se(4)-Bi(1)-Se(6)

Se(3)-Bi(2)-Se(3)
Se(3)-Bi(2)-Se(2)
Se(3)-Bi(2)-Se(6)
Se(2)-Bi(2)-Se(6)
Se(3)-Bi(2)-Se(6)
Se(2)-Bi(2)-Se(6)
Se(3)-Bi(2)-Se(6)
Se(6)-Bi(2)-Se(6)
Se(6)-Bi(2)-Se(6)

Se(1)-Bi(3)-Se(1)
Se(l)-Bi(3)-Se(3)
Se(1)-Bi(3)-Se(6)
Se(1)-Bi(3)-Se(6)
Se(3)-Bi(3)-Se(6)
Se(6)-Bi(3)-Se(6)
Se(1)-Bi(3)-Se(4)
Se(3)—Bi(3)-Se(4)
Se(6)-Bi(3)-Se(4)

Se(5)-Bi(4)-Se(5)
Se(5)—Bi(4)-Se(1)
Se(5)-Bi(4)—Se(2)
Se(1)-Bi(4)-Se(2)
Se(5)-Bi(4)-Se(4)
Se(1)-Bi(4)-Se(4)
Se(2)-Bi(4)-Se(4)
Se(5)-Bi(4)-Se(4)
Se(4)-Bi(4)-Se(4)

l75.96(9)
8734(7)

9380(9)
9364(7)
9072(7)
173.61(9)
l74.52(6)
8804(7)
9130(6)
87.200)
8353(8)

9422(9)
9433(7)
8963(6)
174.06(8)
8989(7)
8619(8)
9077(7)
172.50(9)
8465(7)

95.5700)
9653(8)
9308(7)
165.67(8)
9190(6)
8784(7)
8122(6)
17085(7)
8021(7)

 

66

Table 3-9. Bond lengths [A] and angles [°] for szAg15Bi7,SSe13.

 

Bi( 1 )-Se(5)
Bi( 1 )-Se(6)

Bi(2)-Se(3)
Bi(2)-86(5)
Bi(2)-Se(7)
Bi(2)-Se(6)

A3(2)-Se(6)
Ag(2)-Se(3)
Ag(2)-36(5)
Ag(2)-Se(7)

Bi(3)-Se(4)
Bi(3)-Se(3)
Bi(3)-Se(6)
Bi(3)-Se(7)

Ag(3)-Se(4)
Ag(3)-Se(3)

Bi(4)-Se(1)
Bi(4)-Se(4)
Bi(4)-Se(6)
Bi(4)-Se(5)

Ag(4)-Se(4)
Ag(4)-Se(1)

Bi(5)-Se(2)
Bi(5)-Se(1)
Bi(5)-Se(3)
Bi(5)-Se(5)

Rb(l)-Se(3)
Rb(1)-Sc(1)
Rb(1)-Se(2)
Rb(1)-Se(4)
Rb(1)-Se(2)

2.9013(18) x4
2.927(3) x2

2.787(4) x2
2.923(11)
2.947(10)
3.054(4) x2

290(2) x2
293(2) x2
294(6)
293(6)

2.843(2) x2
2.866(3)
2.973(3)
3.0627(10) x2

259(3) x2
280(6)

2.853(3) x2
2.923(4)
3.0008(3) x2
3.003(4)

268(2)
2.711(16) x2

2.7732(19) x2
2.811(3)
3.104(3)
3.207(2) x2

3.418(4)
3.455(3) x2
3.503(4)

3.699(3) x2
3.753(3) x2

Se(5)-Bi(1)-Se(5)
Se(5)-Bi(1)-Se(5)
Se(5)-Bi(1)-Se(5)
Se(5)-Bi(1)-Se(6)
Se(5)-Bi(1)-Sc(6)
Se(6)-Bi(l)-Se(6)

Se(3)-Bi(2)-Se(3)
Se(3)-Bi(2)-Se(5)
Se(3)-Bi(2)-Se(7)
Se(5)-Bi(2)-Se(7)
Se(3)-Bi(2)-Se(6)
Se(3)-Bi(2)-Se(6)
Se(5)-Bi(2)-Se(6)
Se(7)-Bi(2)-Se(6)
Se(6)-Bi(2)-Se(6)

Se(6)-Ag(2)-Se(6)
Se(6)-Ag(2)-Se(7)
Se(6)-Ag(2)-Se(3)
Se(6)-Ag(2)—Se(3)
Se(7)-Ag(2)-Se(3)
Se(3)-Ag(2)-Se(3)
Se(6)-Ag(2)-Se(5)
Se(7)-Ag(2)-Se(5)
Se(3)-Ag(2)-Se(5)

Se(4)-Bi(3)-Se(4)
Se(4)-Bi(3)—Se(3)
Se(4)-Bi(3)-Se(6)
Se(3)-Bi(3)-Se(6)
Se(4)-Bi(3)-Se(7)
Se(4)-Bi(3)-Se(7)
Se(3)-Bi(3)-Se(7)
Se(6)-Bi(3)-Se(7)
Se(7)-Bi(3)-Se(7)

Se(4)-Ag(3)-Se(4)
Se(4)-Ag(3)-Se(3)

180.00(7)
9021(7)
8979(7)
8854(6)
91 .46(6)
180

95.0609)
926(2)
909(2)
174.8(2)
174.6108)
9016(6)
885(2)
87.6(2)
84.6005)

90.2(10)
90.9(12)
179.2(17)
9040(6)
88.5(12)
890(9)
91.202)
176.9(13)
89.3(11)

9259(8)
9272(7)
9274(8)
172.10(9)
175.86(5)
9155(4)
87.14(6)
8700(5)
8430(3)

105.105)
100.1(14)

 

67

Continue Table 3—9.

 

Se(1)-Bi(4)-Se(1)
Se(1)-Bi(4)-Se(4)
Se(l)-Bi(4)-Se(5)
Se(4)-Bi(4)-Se(5)
Se(1)-Bi(4)-Se(6)
Se(l )-Bi(4)-Se(6)
Se(4)-Bi(4)-Se(6)
Se(5)-Bi(4)-Se(6)
Se(6)-Bi(4)-Se(6)

Se(4)-Ag(4)-Se(1)
Se(l )-Ag(4)-Se( 1)

9220(11)
9284(8)
9132(10)
173.9903)
175.51(9)
9070(5)
90.4400)
8518(7)
86.2000)

101.7(5)
98.6(8)

Se(2)-Bi(5)-Se(2)
Se(2)-Bi(5)-Se(l)
Se(2)-Bi(5)-Se(3)
Se(l)-Bi(5)—Se(3)
Se(2)-Bi(5)-Se(5)
Se(2)-Bi(5)-Se(5)
Se(1)-Bi(5)-Se(5)
Se(3)-Bi(5)-Se(5)
Se(5)-Bi(5)-Se(5)

9566(9)

9592(7)
9312(7)

16650(8)
l7086(6)

9214(5)
8799(6)
81 .67(6)
7972(6)

 

Table 3-10. Summary of crystallographic data for members of A2[M5+,.Se9+,,] and

 

 

their band gaps
Fomula n so 2 a(A) b(A) e(A) Band
gap (eV)

y-RbBi3Se5 1 ana 4 21.956(7) 4.136(1) 123570) 0.8
,B-CsBi3Se5 ana 4 22.740(13) 4.171(2) 124720) 0.63
szCdBi6Sen 2 Pnnm 2 12.385(3) 23.839(6) 4.1124(10) 0.74
CsAgojBisSSee 3 ana 4 26.537(11) 4.1311(18) 12.392(5) 0.54
CstBi3Se6 ana 4 26.512(8) 4.1192(13) 12.396(4) 0.4
Rb2A81.5Bi7_5Sel3 4 Pnnm 2 12.386(2) 27.642(5) 4.1107(8) 0.56
CszAg1,5Bi7,5Se13 Pnnm 2 12.432(8) 28.55308) 4.136(3) 0.6

 

68

4. Results and Discussion
Homologous series and Structure description

The ternary fl-CsBi3Se5, erBi3Ses and the quaternary szCdBi6Se1 1,
CsAgosBi35Se6, CstBi3Se6, szAgl_sBi7_SSel3 and CszAgisBiuSeig present a new “
aufbau” motif according to the homologous series A2[M5+,,Seg+nl (n = 1, 2, 3, 4). This is a

I'll-type module which evolves with n and gives

simple series defined by a single NaC
rise to unique layers. The structural evolution, member organization and hierarchy are
shown in Figure 3-1. The NaClm-type [M5+,,Se9+,,] (or [M5Se9 + n ‘MSe’]) units, which
are infinite in one direction, repeat side by side to build up infinite slabs with thicknesses
defined by the value of n (i. e. the number of ‘MSe’ units). The alkali metal ions reside in
capped trigonal prismatic sites in the spaces between the slabs which present stepped
surfaces.

The structures of all compounds were confirmed by single crystal and powder X-
ray diffraction studies and the refined unit cell parameters and space groups are listed in
Table 3-1. Successive members in this series can be differentiated just by the increased
thickness (~2 A) of NaCll ' l-type modules when it increases. It is interesting to note that
the members with an odd n number crystallize in the space group ana while those with
an even n number in Pnnm. The alternating symmetry change is caused by the sequential
addition of 'MSe' units in the [M5+,,Se9+,.] layer.

The isostructural erBi3Ses and fl-CsBi3Se5 are the first members (n = 1) of this
series, shown in Figure 3-2. In terms of the homology they can be expressed as AzBibselo
(A = Rb, Cs) featuring the [MGSemf’ (M = Bi) modules. This module is three “BiSe6”

octahedra wide and two octahedra thick, and is propagated by linking with identical

69

neighboring modules through sharing an edge of the Bi(1)—Se octahedron to form a
stepwise slab. The structure has three crystallographically different Bi atoms. In )9
RbBi3Se5, for example, Bi(l) is in the least distorted octahedral site with Bi—Se distances
at 2.864(4) — 2.961(4) A and Se—Bi—Se angles at 88.58(14)° — 91.05(8)°. Bi(2) is in a
slightly distorted octahedron (approximately a square pyramid) with one short bond at
2.773(3) A, four bonds between 2.901(3) and 3.012(4) A, and one long bond at 3.131(4)
A, which faces trans to the short bond. The Bi(3) octahedron is distorted along a pseudo
three-fold axis forming three short bonds at 2.840(5) A to Se(l) and 2.784(4) A to two
Se(2) atoms and three long bonds at 3.064(4) A and 3.227(4) A to Se(4) and Se(S),
respectively. As in all members of this series Rb+ atoms are in a bicapped trigonal
prismatic coordination with Rb—Se distances between 3.413(6) and 3.796(6) A.
szCdBiése“ is the second member (n = 2) in the series A2[M5+,,Se9+,,]. It too has
a layered framework but the slabs are assembled from a wider module, namely the
[M78e11]2‘ (M = Bi, Cd), see Figure 3-1 and 3-2. This module is three and four “MSe6”
octahedra wide and two octahedra thick. All metal atoms except Bi(4) are in distorted
octahedral sites with bonding distances to Sc atoms from 2.822(3) to 3.0512(16) A. The
Bi(4) octahedron, as the Bi(3) in y-RbBi3Ses, shows the highest distortion with three short
bonds ranging from 2.784(3) A to 2.828(5) A and three long ones from 3.114(4) A to
3.195(4) A. The szCdBi6Se11 has Cd atoms mixed in two bismuth sites, Bi( 1) and Bi(3),
at the fraction of 27% and 36%, respectively, to preserve charge neutrality. Therefore, the
formally Cd2+ ions are situated in an octahedral environment of Se atoms, which is a

rather unusual coordination for this ion which generally prefers a tetrahedral environment.

70

.mfifiwofizflam 33% £593 56% 2a ammo some E 8:69: @39th 2E. A928 Beach? a 5999 850530358 :8: 05 02605
9 @035 m9 :omE: no EB a 9 .3 .9 mo 03205 9.65 593 .552? 98 = nowoafi mo 45:25.9 a on SE: QeéwEZU cab-_:_OuZ 08 .«o
mafia. msotg 2C. .9. .m .N .9 n = 60 .w< .5 n 2 ”mo 5% n «b T+oom=+92fi< moflum game—080: 05 mo cows—go Ego—Em .TM 2:»:—

G u 5 B92 9.5.9.222

 

m~omm.£mn._m<~mo 91 J . .. . . .. nommzwi
m_omm.£mn._m<~nm JMAWM4¢JMJM+ “/17 -1, .,_., o
N/WW fl... 0 m
con 5 J89“: Eommé somammmwmomwmwl

     

om: _+

1. .34
um? _+
at 559.213.4992

JJN...“ i .4.4../..4.J 514/9. 974.41 .4/

 

JLW. . 1W. .. 9 .9. -9 .91/
ka“ . K.‘ O 0

1.1.1.14. . .1 «A. . J44. m2 9+
19.11.19 ..,..1.1 19.11.14 2 u 11,892.. Eomas:

71

.48 55 1mm 9e 8538 Bee ea 8% A: one :2 42385 see a Banzai as N u a e5 _ u .1 593 ea: wages
25-. _ .5mZ 2E. .94on 8QO 55¢ 593 958.0 05 :38 Q n 5 :omfiEOBM 3 v5 95% 3QO 35¢ 593 38.0 05 :38

2 n 2 $0 .59 .1. 3 mommmmde Am mo 98:00.85 diocese—2 Beam 8 moswoBEo: 038083. 993 50933 99859980 .Nim 95$.”—

9 1... sass: _ u .1

St:— Uflz /

 

72

szAg1.5Bi7_5Sei3 is the fourth member (n = 4) of the A2[M5+nSe9+,,] series. It has
structural characters in common with the above members but of course features the larger
size [M98613]2— (M = Bi, Ag) modules. These are four “MSe6” octahedra wide and three
octahedra thick. Four of the Bi sites are mixed with Ag at the rate of 33%, 28%, 7% and
22%, which preserves charge neutrality.

As the result of the defined structural and compositional relationship from the
three compounds above, the second member conceptually derives from the first by
adding one neutral ‘MSe’ unit on the surface of the [M6Sem]2‘ unit as shown in Figure 3-

1. The third member derives from the second by a similar process. Successive additions

(n 2 3) of neutral MSe equivalents to the [M6Se10]2‘ unit are easily predicted to produce

new member compounds as follows.

A2[M68e110]—LN£°l—> A2[M736211]—mS—81—>A2[M386312]M—bAfiMgSeJfl—pwse]
n = n = n = n =

A2[M10Se14] etc.
n = 5

Based on the above analysis, we searched for new members predicted by the
series A2[M5+,,Se9+,,]. CsAgo,5Bi3,5Se6, CstBi3Se6 and CszAg1_5Bi7,5Sel3 were discovered
by such targeted synthetic reactions. The new isostructural compounds CsAgo_5Bi3,5Se6
and CstBi3Se6 are members with n = 3, and CszAg1.5Bi7,5Se13 is a member with n = 4
according to the scheme of Figure 3-1. This structural evolution leads to n = 5, AM5Se7
depicted in Figure 3-1, which is predicted to exist.

All homolog compounds presented here are valence precise and are narrow band
gap semiconductors. Their energy gaps determined spectroscopically are in the range of

0.4—0.8 eV, see Table 3-10 and Figure 3-3.

73

Absorption coefficient (arbitrary unit)

 

a) szAg1_5Bi7Se13
b) CstBi3Se6
c) [3-CsBi3Se5
d) CS138051355336
e) CS2Agl5f3i7s‘313
f) szCdBi68ell

3)

b)

 

 

 

 

Oi

Energy (eV)

Figure 3-3. Solid-state electronic absorption spectra for all homologs

74

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3-4. Top: ingot of (a) ,B-CsBi3Se5 and (b) CstBi38e6 grown in a Bridgman
fumace. Bottom: The SEM image of oriented fl-CsBi3Se5 ingot. The direction of
crystal growth is the b-axis in the structure and micro cracks are shown inside the

white circles.

75

 

100- ' CSCdBi3866
g .
> 0_ ° B-CsB13Se5
3 I I .
H I I I I I . I I
.03-100- ".._ _ .
o ' ' I
E
05-200-
0 C
U ' O I I .

C

Jag-3004 . . ° 0 o
a)
8 ‘ . . I . . I ' O 0
£400-

-500

 

 

 

300 ' 400 ' 500 ' 600 ' 700
Temperature (K)

Figure 3-5. Temperature dependece of the thermopower for single crystal sample of
fl-CsBi3Se5 and CstBi3Se6.

76

Thermoelectric properties. We succeeded in preparing highly oriented
polycrystalline ingots of ,B-CsBi3Se5 and CstBi3Se6 using a vertical Bridgman grth
technique,l4 Figure 3-4. The crystalline orientation in these ingots was very high with the
short ~4 A axis of the crystal being parallel to their length. Preliminary thermopower and
electrical conductivity measurements on rectangular samples were carried out along the
crystal grth direction (i. e. crystallographic b-axis). At room temperature the
thermopower was —40 and —235 [IV K“1 for ,B-CsBi3Se5 and CstBi3Se6 respectively and
increased steadily with rising temperature, Figure 3-5. Their corresponding electrical
conductivities (four-probe) were 1.3 and 1.0 S cm", respectively, also the micro cracks
shown in Figure 3-4 can be responsible. The negative thermopower and low electrical
conductivity suggest n-type semiconductor character with a small number of electrons as
the charge carriers. Despite the comparable conductivity, the much larger thermopower
of the Cd compound is noteworthy and may point to a potential promise of octahedral

Cd-containing chalcogenides for thermoelectric investigations.

Concluding Remarks

A new homologous series A2[M5+,,Se9+,,] (A = Rb, Cs; M = Bi, Ag, Cd; n = 1—4)
was established which can precisely describe, structurally and compositionally, a large
number of complex phases including ,B-CsBi3Se5, szCdB16Se“ and szAg15Bi75Se13.

ll 1 1-type block and can describe in a

This series is based on a single evolving module NaC
unified fashion a relative large number of seemingly unrelated compounds. The

predictive ability of the homology was then exploited to produce CsAgo_sBi3,5Se6,

CstBi3Se6 and CszAgl5Bi75Sel3. Low dimensional structures such as these may be

77

suitable for doping investigations aimed at optimizing the thermoelectric properties.15

Further investigations on the A2[M5+,,Seg+,,] series could lead to higher order members in

this group.

78

References

l. a) CRC Handbook of Thermoelectric Materials. Rowe, D.M. Bd., CRC Press, Inc.:
Boca Raton, FL, 1995 b) Polvani, D. A.; Meng, J. F.; Shekar, N. V. C.; Sharp, J. and
Badding, J. V. Chem. Mater. 2001, 13, 2068-2071. c) Venkatasubramanian, R.; Siivola,
B.; Colpitts, T.; O’Quinn, B. Nature 2001, 413, 597-602. d) Shelimova, L. B.; Karpinskii,
O. G.; Svechnikova, T. B.; Avilov, E. S.; Kretova M. A. and Zemskov, V. S. Inorg.
Mater. 2004, 40, 1264-1270.

2. a) Chung, D.-Y.; Choi, K.-S.; Iordanidis, L.; Schindler, J. L.; Brazis, P. W.; Kannewurf,
C. R.; Chen, B.; Hu, 8.; Uher, C.; Kanatzidis, M. G. Chem. Mater., 1997, 9, 3060-3071.
b) Iordanidis, L.; Brazis, P. W.; Kyratsi, T.; Ireland, J.; Lane, M.; Kannewurf, C. R.;
Chen, W.; Dyck, J. S.; Uher, C.; Ghelani, N. A.; Hogan, T.; Kanatzidis, M. G. Chem.
Mater. 2001, 13, 622-633.

3. a) Huang, F. Q.; Mitchell, K.; Ibers, J. A. J. Alloys Compd., 2001, 325, 84-90. h) Yao,
J .; Deng, B.; Ellis, D. E.; Ibers, J. A. Inorg. Chem. 2002, 41 , 7094-7099. c) Wang, Y. C.;
Hoffmann, R.; DiSalvo, F. J. J. Solid State Chem. 2001, 156, 230-236. (1) Wang, Y. C.;
DiSalvo, F. J. Chem. Mater. 2000, 12, 1011-1017. e) Poudeu, P. F. P.; Sohnel, T.; Ruck,
M.; Z. Anorg. Allg. Chem. 2004, 630, 1276-1285.

4 a) Mrotzek, A.; Kanatzidis, M. G. Acc. Chem. Res., 2003,36, 111419. b) Kanatzidis, M.
G. Acc. Chem. Res., 2005, 38, 359-368.

5 Hsu, K. F.; Lal, S.; Hogan, T.; Kanatzidis, M. G. Chem. Commun. 2002, 13, 1380-1381.
6 Poudeu, P. F. P.; Kanatzidis, M. G. Chem. Commun., 2005, 21, 2672-2674.

7 a) Takagi, J .; Takeuchi, Y. Acta Crystallogr. 1972, 328, 369. b) Makovicky, E. Neues
Jahrb. Mineral. 1989, 160, 269.

8 Zakrzewski, M. A.; Makovicky, E. Can. Mineral. 1986,24, 7.

9 The most basic modules constructing the structures are NaCl- and BizTe3-type units,
which are often described as NaCl'OO- and NaCll ' l-type, respectively, because they derive
by slicing a NaCl lattice perpendicular to the [100] and [111] directions.

'0. Iordanidis, L.; Bile, D.; Mahanti, s. D.; Kanatzidis, M. G. J. Am. Chem. Soc., 2003,
125, 13741-13752.

11 . Kubelka-Munk function: cl/S = (1-R)2/2R, where a is the absorption coefficient, S is
the scattering coefficient, and R is the reflectance at a given wavenumber.

'2 McCarthy, T. J. ; Ngeyi, S. —P.; Liao, J .-H.; Degroot, D.; Hogan, T.; Kannewurf, C. R.;
Kanatzidis, M. G. Chem. Mater. 1993, 5, 331-340.

79

 

'3 . SMART, SAINT, SHELXTL: Data Collection and Processing Software for the
SMART-CCD system; Siemens Analytical X-ray Instruments Inc.: Madison, WI, 1997.

‘4 Kyratsi, T.; Chung, D.-Y.; Choi, K. —S.; Dick, J. S.; Chen, W.; Uher, C.; Kanatzidis, M.
G. Mat. Res. Soc. Symp. Proc. 2000, 626, 28.8.1.

'5 Hicks, L. D.; Dresselhaus, M. 8. Phys. Rev. B 1993,47, 12727-12731.

80

CHAPTER 4

Crystal Growth and Thermoelectric Properties of

CdBi4S7 and Cd0.63Pb0.32Bi589

1. Introduction

The BizTe3-xSe,, and Bi2-bexTe3 alloys showed high thermoelectric (TE) figures
of merit ZT"2 and much exploratory work in the field of thermoelectric materials has
been probed ever since. For the high ZT thermoelectric materials, high electrical
conductivity, high thermopower and low thermal conductivity are necessary but they are
not independently controllable parameters. One way suggested to increase the ZT is to
minimize thermal conductivity while retaining good electronic and thermopower
properties. The thermal conductivity (K) has a contribution from lattice vibrations (1(1) and
carrier thermal conductivity (Kc), namely K = Kc + K]. One of the most fascinating
approaches to reduce thermal conductivity is introducing the concept of phonon glass
electron crystal.3 A compound with “rattling” atoms such as atoms in a cage or tunnel
structure produces a phonon damping effect that results in dramatic reduction of the
solid’s lattice thermal conductivity. In addition, introducing large non-periodic mass
fluctuations in the crystal lattice (i.e. solid solution) and increasing the lattice period (i.e.
large unit cell parameters) are also superior manoeuvres for decreasing thermal

conductivity. Furthermore, based on this strategy, promising ternary and quaternary

81

compounds of alkali metal bismuth chalcogenides and their analogues materials such as
CsBioTe6, 4 B-KzBi3Se13, 5 K25BiMSen,5 BaBiTe3, 6 K,-,8n5-,,Bin1,,Se22, 7 AMM..-
2,Bi7.,.8e.5 (A = K, Rb; M = Sn, Pb),8 AzBigsel3 (A = Rb, Cs),9 CsMBi3Te6, and
CSMzBI3T67 (M = Pb, Sn)lo were investigated for thermoelectric applications. Besides
alkali metal bismuth chalcogenides, recently a silver containing compound, Ag]-
beingTezo,” showed an excellent figure of merit (ZT) of ~2 at 800K and very small
band gap (~0.26eV).

In general, the desirable energy gap for TB performance up to 1000 °C is
considered to be <~0.6 eV. However, the alkali metal bismuth sulfides such as [3-,y-
CsBiSz,'2 y-RbBigss,” KBi3Ss,” KBi6_33Sm,15 and K2BigS1315 exhibit wide energy band
gaps (~1.1 — 1.4 eV) due to strong ionic interactions between the alkali metal ions and the
[BixSy]z‘ framework. In order to make the bismuth sulfide family more attractive for TB
investigations it is necessary to have less electropositive metal ions instead of alkali metal
ions. An example of this are the well known sulfosalts of the gustavite-lillianite series”,
the kobellite series”, and the pavonite series18 with Cu+, Agli,19 Pb2+, and Cd2+ which are
generated by tropochemical cell-twinning of galena type slabs (NaCl-type) cut
perpendicular to the (311) direction with a mirror as twinning operation. When these
compounds can be explained in terms of a difference in the sequence of certain type slabs
(here galena type) defined by their structural modules and sizes, we then have a powerful
way of correlating and understanding large classes of materials thereby allowing useful
generalizations and predictions. The Pb4,6sBi20_9S3620 phase, for example, was predicted
including their cell parameters and even formulae by understanding the basic twinning

structure PbBl4S721.

82

Due to the extreme thermodynamic stability of phases of CdS and B1283, only a
few ternary compounds in Cd/Bi/S system, such as CdBiZS4, CdBi4S7, CdzBi6S”, and
Cd2,3Big_.S.5, are known 22 . They have not been studied with respect to their
physicochemical and electrical charge transport properties. Herein we report new results
of CchS7 and CdoengngBisSg and we assess their potential as thermoelectric materials.
The crystal structure refinements, crystal growth, and physico-chemical properties are

presented.

2. Experimental Section

Reagents. Chemicals were used as obtained: bismuth chunks (99.999% Noranda,
Canada), sulfur powder (sublimed, Spectrum Chemical Mfg. Corp., Gardena, CA),
cadmium powder (99.999%, -200mesh Cerac), lead powder (99.999%, 200mesh, Cerac).

Synthesis. The products are air and water stable and all manipulations were carried
out in air. To avoid the thermally stable phases such as CdS and B1283 the starting
materials were ground and pelletized under high pressure. The purity and homogeneity of
the products were verified by comparing the X-ray powder diffraction patterns to those
calculated by the crystallographically determined atomic coordinates.

CdBi487. A mixture of Cd powder (0.5620 g, 5 mmol), Bi (4.1796 g, 20 mmol), and
S (1.1221 g, 35 mmol) was ground and pressed with 12mm diameter die at a pressure
about 14,000 psi at room temperature for 10 min. The pellet was loaded in a fused silica
tube (18 mm diameter) and subsequently flame-sealed at a residual pressure of <104
mbar. The tubes were then heated at 650 °C for 2 days and cool down to room

temperature within 10 h. Shiny black needle type crystals on the pellet of CdBi4S7 were

83

obtained. A quantitative microprobe analysis using Energy Dispersive Spectroscopy
(EDS) was performed on a Scanning Electron Microscope (SEM) on several single
crystals of CdBiaS7 gave the approximate composition of CdLooBiamsm In order to grow
highly oriented crystal specimens for the thermoelectric property measurements, the
product was ground and loaded in a silica tube (9 mm diameter) with a point end and
sealed under vacuum. The tube was heated to 800 °C in a Bridgman furnace and
descended at a rate of 6.25 mm/h through a sharp (100 °C/cm) temperature gradient.23 A
pure and well oriented ingot (35 mm long, 7 mm diameter) of CchS7 was obtained.
Cdo,6sto,32Bi5S9. A mixture of Cd powder (0.2360 g, 2.1 mmol), Pb powder (0.1865
g, 0.9 mmol), Bi (2.6332 g, 12.6 mmol), and S (0.7021 g, 21.95 mmol) was ground and
pressed with 12mm diameter die at a pressure about 14,000 psi at room temperature for
10 minutes. The pellet was loaded in a fused silica tube (18 mm diameter) and
subsequently flame-sealed at a residual pressure of <104 mbar. The tubes were then
heated at 650 °C for 2 days and cool down to room temperature within 10 h. Shiny black
needle type crystals on the pellet of Cd(yégpbonglsSg were obtained. A quantitative
microprobe analysis with a SEM/EDS system, performed on different crystals, gave the

average composition of Cdo_95Pbo,95Bi6,3S<;.

3. Physical measurements
Electron Microscopy. Quantitative microprobe analysis for the compounds was
performed with a JEOL J SM-6400V Scanning Electron Microscope (SEM) equipped

with a Noran Vantage Energy Dispersive Spectroscopy (EDS) detector. Data were

84

collected for 30 sec using an accelerating voltage of 20kV. All reported results are an
average of measurements on at least three different crystals.

Differential Thermal Analysis. Differential thermal analysis (DTA) was performed
with a computer-controlled thermal analyzer (Shimadzu DTA-50). A 20 mg of ground
crystals were sealed in silica ampoule under vacuum. A silica ampoule containing the
equal mass of alumina was placed on the reference side of the detector. The sample was
heated to the desired temperature a 10 °C/min, isothermed for 2 min and then cooled at
10 °C/min. The heating program was recycled to check reproducibility of the thermal
behavior of the sample. The reported melting point is the peak temperature. After DTA,
the sample was examined by powder X-ray diffraction to identify if any decomposed
product formed during heating/cooling cycles.

Infrared Spectroscopy. Optical diffuse reflectance measurements were made on the
finely ground sample at room temperature. The spectrum was recorded in the infrared
region (6000-400 cm'l) with the use of a Nicolet MAGNA-IR 750 Spectrometer equipped
with a Collector Diffuse Reflectance of Spectra-Tech. Inc. The reflectance versus
wavenumber data were used to estimate a material’s band gap by converting reflectance
to absorption data using the Kubelka-Munk function.24

Charge Transport Measurements. Room temperature conductivity measurements
were performed in the usual four-probe geometry. The Seebeck coefficient was measured
between 300 and 700 K by using a SB-lOO Seebeck effect measurement system, MMR
Technologies, Inc.

Powder X-ray Diffraction. A calibrated CPS 120 INEL X-ray powder

diffractometer equipped with a position-sensitive detector, operating at 40kV/25mA with

85

a flat geometry and employing graphite monochromatized Cu K01 radiation, was used to
obtain powder patterns of starting materials and all products.

Single-crystal X-ray Crystallography. A Bruker SMART Platform CCD
diffractometer was used for data collection at room temperature. The individual frames
were measured with an omega angle rotation of 03° and an acquisition time of 30 sec.
The SMART25 software was used for the data acquisition and SAINT” software for data
extraction and reduction. An analytical absorption correction was performed using face
indexing and the program XPREP in the SAINT software package, followed by a
semiempirical absorption correction based on symmetrically equivalent reflections with
the program SADABSZS. Structural solution and refinements were successfully done
using the SHELXTL25 package of crystallographic programs. The structures were solved
with direct methods.

The data collection was performed by selecting the crystals fi'om the surface of
the palletized ingots. The complete data collection parameters, details of the structure
solution, and refinement for CdBi4S7 and CdoangonzBisSg are given in Table 4-1. The
fractional coordinates, bond lengths, bond angles and temperature factors (Ueq) of all the
atoms with estimated standard deviations are given in Tables 4-2 ~ 4-7.

The previously reported CdBi487 structure22 was determined using intensity data
from Guinier powder pattern, which corresponded with those calculated CdBi4S7 phase
assumed to be isotypic YSS726. The new single crystal refinement for CdBi487 described
here provides significantly accurate atomic coordinates, and bond lengths and angles in

comparison with the previous work.

86

Table 4-1. Crystallographic Data for synthesized CdBi4S7 and CdQ68PbQ8281589.

 

Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group

Unit cell dimensions

Volume

Z

Density (calculated)
Absorption coefficient

F (000)

Theta range for data collection

Index ranges

Reflections collected
Independent reflections
Completeness to theta=28.26°
Refinement method

Data / restraints / parameters
Goodness-of-fit on F2

Final R indices [I>2sigma(l)]
R indices (all data)

Extinction coefficient

Largest diff. peak and hole

CdBi.,s7

1172.74

293(2) K

071073 A

Monoclinic

C2/m

a = 131430) A a: 90°
b = 4.0129(8) A

0: 105.115(3)°

c =11.782(2)Ay = 90°
5999(2)A3

2

6.492 Mg/rn3
61.374 mm"
984

1.79 to 28.26°
-17<=h<=17,
14<=1<=13
2490

777 [R(int) = 0.0324]

91.6 %

Full-matrix least-squares on F2
777 J 0 J 40

1.130

R13 = 00335,sz = 0.0831
R13 = 0.0366, wR2 = 0.0895
0.0034(3)

2.835 and -2.631 e. A"

-5<=k<=5, -

Cd0.68Pb0.8zBiss9
1579.53

293(2) K

0.71073 A
Orthorhombic

Cmcm

a = 4.0238(10) A a: 90°

b = 13.184(4) A 0: 90°

c = 59339(15)A y = 90°
3147.904)A3

8

6.666 Mg/rn3

66.476 mm"

5270

1.37 to 28.28°

-5<=h<=5, -16<=k<=14, -
78<=l<=78

10299

2099 [R(int) = 0.0483]

93.3 %

2099 J 0 / 108
1.186

R1 = 0.0572, wR2 = 0.1137
R1 = 0.0690, wR2 = 0.1181
0.000088(7)

3.559 and -4.822 e. A"

 

“RI = 2114.1 — lFell/leFoll- wR2 = {EMF02 4.2121021144021211”?

87

Table 4-2. Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (A2x103)for CdBi4S7. U(eq) is defined as one third of the trace of

 

 

the orthogonalized Uij tensor.
x y z U(eq) occupancy

Bi(l) 1217(1) 0 9306(1) 18(1) 1
Bi(2) 3192(1) 0 6640(1) 22(1) 1
Cd(l) 5000 -5000 5000 21(1) 1
S(l) -354(3) 0 7030(3) 18(1) 1
S(2) 3445(2) 0 4512(3) 16(1) 1
8(3) 0 -5000 10000 43(2) 1
S(4) 2623(3) 0 1 1448(3) 17(1) 1

 

Table 4-3. Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (A2x103)for Cdo_63Pbo,32Bi589. U(eq) is defined as one third of the
trace of the orthogonalized Uij tensor.

 

 

x y z U(eq) occupancy

M(l) 0 838(1) 1791(1) 19(1) 1

M(2) 0 1378(1) 132(1) 18(1) 1

M(3) -5000 3358(1) 1276(1) 25(1) 1

M(4) -5000 -1395(1) 2072(1) 21(1) 1

M(S) -5000 -1033(8) 645(2) 22(2) 0.7(2)
M(SS) -5000 -990(30) 600(20) 36(7) 0.3(2)
M(6)/Cd(6) 0 1 163(1) 967(1) 23(1) 0.314(5)/0.686
M(7) -10000 -3658(2) 2472(4) 40(4) 1

S(l) 0 -47(5) 2203(1) 17(1) 1

S(2) -5000 271 1(5) 278(1) 17(1) 1

8(3) -5000 -2236(8) 2500 20(2) 1

S(4) 0 341(5) 565(1) 21(2) 1

S(5) -5000 -448(5) 1652(1) 18( 1) 1

S(6) -5000 2587(5) 869(1) 15(1) 1

8(7) -10000 4711(5) 1053(1) 15(1) 1

S(8) 0 2015(5) 1366(1) 19(1) 1

S(9) -10000 -2850(6) 1984(2) 31(2) 1

S(10) -5000 0 0 44(4) 1

 

88

Table 4-4. Bond lengths [A] and angles [°] for CdBi4S7.

 

Bi(1)-S(4)
Bi(1)-S(4)
Bi(1)-S(3)
Bi(l)-S(1)

Bi(2)-S(2)
Bi(2)-S(4)
Bi(2)-8(3)

Cd(1)-S(1)
Cd(1)-S(2)

S(4)-Bi(l )-S(4)
S(4)-Bi( l )-S(4)
S(4)-Bi( l )-S(3)
S(4)-Bi(1)-S(3)
S(4)-Bi(1)-S(3)
S(3)-Bi( 1 )-S(3)

2.712(3)
2.801(2) x2
2.8167(5) x2
2.926(3)

2.614(2)
2.726(2) x2
2.996(2) x2

2.553(4) x2
2.815(2) x4

8998(9)
91.4900)
9174(5)
178.25(7)
8881(5)
9085(2)

S(4)-Bi(1)-S(1)
S(4)-Bi(1)-S(1)
S(3)-Bi(1)-S(l)

S(2)-Bi(2)-S(1)
S(1)-Bi(2)-S(l)
S(2)-Bi(2)-S(2)
S(1)-Bi(2)-S(2)
S(1)-Bi(2)-S(2)
S(2)-Bi(2)-S(2)

S(1)-Cd(1)-S(l)
S(1)-Cd(1)-S(2)
S(1)-Cd(1)-S(2)
S(2)-Cd(l)-S(2)
S(2)-Cd(1)-S(2)
S(2)-Cd(1)-S(2)

178.2200)
9126(9)
8701(5)

8444(9)
94.81(10)
7969(9)
163.4200)
8842(7)
8409(8)

180
8376(9)
9624(9)
180

9094(9)
8906(9)

 

89

Table 45. Bond lengths [A] and angles [°] for Cdogngongissg.

 

M(1)-S(l) 2.706(7) M(7)-M(7) 033(5) S(3)-M(4)-S(5) 177.0(2)
M(l)-S(5) 2.759(5) x2 M(7)-S(3) 2.754(7) x2 S(3)-M(4)-S(l) 90.0209)
M(l)-S(9) 2.890(6) M(7)-S(9) 309(3) S(5)-M(4)-S(l) 8791(17)
M(l)-S(9) 2.890(5) S(l)-M(4)—S(l) 921(2)
M(1)-S(8) 2.962(7) S(1)-M(l)-S(5) 90.3807) S(3)-M(4)-S(9) 842(2)
S(5)-M(l)-S(5) 93.6(2) S(5)-M(4)-S(9) 979(2)
M(2)-S(2) 2.713(7) S(1)-M(1)-S(9) 843(2) S(l)-M(4)-S(9) 1742(2)
M(2)-S(2) 2.807(5) x2 S(5)-M(l)-S(9) 174.2(2) S(1)-M(4)-S(9) 88.3105)
M(2)-S(10) 2.8221(8) x2 S(5)-M(1)-S(9) 88.82(14) S(1)-M(4)-S(9) 174.2(2)
M(2)-S(4) 2.910(7) S(1)-M(l)-S(9) 843(2) S(9)-M(4)-S(9) 907(2)
S(9)-M(1)-S(9) 88.3(2)
M(3)-S(6) 2.617(6) S(1)-M(1)-S(8) 173.9(2) S(7)-M(5)-S(4) 850(3)
M(3)-S(8) 2.734(4) x2 S(5)-M(l)-S(8) 93.7807) S(4)-M(5)-S(4) 94.1(4)
M(3)-8(7) 2.995(5) x2 S(9)-M(l)-S(8) 91.3(2) S(7)-M(5)-S(6) 796(3)
S(4)-M(5)-S(6) 163.9(5)
M(4)-8(3) 2.770(4) S(2)-M(2)-S(2) 8987(17) S(4)-M(5)-S(6) 89.17(16)
M(4)-S(5) 2.791(7) S(2)-M(2)-S(2) 91.56(19) S(6)-M(5)-S(6) 835(4)
M(4)-8(1) 2.795(5) x2 S(2)-M(2)-S(10) 9202(10)
M(4)-S(9) 2.828(5) x2 S(2)-M(2)-S(10) 178.0905) S(4)-M(55)-S(4) 97.5(16)
S(2)-M(2)-S(10) 88.72(10) S(4)-M(55)-S(7) 82.0(18)
M(5)-S(7) 2.609(13) S(lO)-M(2)-S(10) 9094(3)
M(5)-S(4) 2.749(9) x2 S(2)-M(2)-S(4) 178.3(2) S(4)-M(6)-S(8) 179.1(2)
M(5)-S(6) 3.020(11) x2 S(2)-M(2)-S(4) 91.3407) S(4)-M(6)-S(6) 95.1207)
S(10)-M(2)-S(4) 86.7700) S(8)-M(6)-S(6) 84.2807)
M(55)-S(4) 268(3) x2 S(6)-M(6)-S(6) 91.3809)
M(55)-S(7) 2.84(1 1) S(6)-M(3)-S(8) 85.99(18) S(4)-M(6)-S(7) 83.3607)
S(8)-M(3)-S(8) 948(2) S(8)-M(6)-S(7) 97.2407)
M(6)-S(4) 2.616(8) S(6)-M(3)-S(7) 7994(16) S(6)-M(6)-S(7) 178.5(2)
M(6)-S(8) 2.624(7) S(8)-M(3)-S(7) 165.2209) S(6)-M(6)-S(7) 88.86(13)
M(6)—S(6) 2.811(5) x2 S(8)-M(3)-S(7) 88.7503) S(7)-M(6)-S(7) 90.85(l9)
M(6)-S(7) 2.824(5) x2 S(6)-M(3)-S(7) 7994(16)
S(8)-M(3)-S(7) 165.2209) S(3)-M(7)-S(3) 938(3)
S(7)-M(3)-S(7) 84.40(17) S(3)-M(7)-S(9) 797(4)

 

90

Table 4-6. Anisotropic displacement parameters (A2 x 103) for CdBi487. The
anisotropic displacement factor exponent takes the form: -21'1:2 [ h2 a*2U11 + + 2 h k
3* I)" U12 ].

 

 

011 022 U33 023 013 012
Bi(l) 17(1) 17(1) 21(1) 0 7(1) 0
Bi(2) 25(1) 22(1) 22(1) 0 11(1) 0
Cd(l) 22(1) 23(1) 24(1) 0 14(1) 0
8(1) 15(1) 20(2) 17(2) 0 0(1) 0
S(2) 15(1) 16(2) 16(2) 0 6(1) 0
8(3) 48(4) 15(2) 89(6) 0 57(4) 0
S(4) 16(2) 22(2) 14(2) 0 6(1) 0

 

Table 4-7. Anisotropic displacement parameters (A2 x 103) for Cdogngongissg. The

anisotropic displacement factor exponent takes the form: -2rt2 [ h2 a*2U11 + + 2 h k
3* b* U12 ].

 

011 022 U33 023 013 012
M(l) 16(1) 18(1) 21(1) -10) 0 0
M(2) 14(1) 17(1) 22(1) -2(1) 0 0
M(3) 23(1) 24(1) 27(1) -1(1) 0 0
M(4) 16(1) 19(1) 26(1) 0(1) 0 0
M(5) 22(2) 22(3) 20(4) -3(1) 0 0
M(55) 15(4) 38(5) 60(20) 0(9) 0 0
M(6)/Cd(6) 21(1) 20(1) 28(1) -30) 0 0
M(7) 20(1) 34(1) 66(1 1) -6(2) 0 0
80) 20(3) 18(3) 14(3) -2(2) 0 0
S(2) 17(3) 17(3) 18(3) -2(2) 0 0
8(3) 22(5) 25(5) 15(4) 0 0 0
S(4) 21(4) 17(3) 25(4) 7(3) 0 0
S(5) 22(3) 19(3) 13(3) -3(2) 0 0
S(6) 14(3) 16(3) 13(3) -1(2) 0 0
8(7) 13(3) 17(3) 14(3) 1(2) 0 0
S(8) 17(3) 16(3) 24(3) 8(3) 0 0
S(9) 18(4) 24(4) 50(5) 43(4) 0 0
s00) 18(5) 36(7) 79(10) -36(7) 0 0

 

91

4. Results and Discussion

Synthesis, thermal analysis and crystal growth. CchS7 and CdngngongisSg
were synthesized by reacting the pelletized elemental mixtures (Cd: Bi: S = 1: 4: 7; Cd:
Pb: Bi: S = 0.86: 0.37: 5.18: 9) at a temperature lower than the melting point. Direct solid
state reactions involving melting of the mixture such as flame reaction and high
temperature reaction were not successful due to incongruent melting and the high phase
stability of BiZS3, PbS and CdS.

The CdBi4S7 compound was already known as a member of Cd-Bi-S homologous
series with CdzBi6S”, CdBi2S4, and Cd2_3Blg_]S|5 all of which were synthesized by alkali
metal halides flux method.

In addition, CdBi4S7 phases doped by Bi283 with at 5% and 10% (based on the
reaction molar ratio of Bi2S3) were also successfully synthesized, however the 15% Bi2S3
-containing composition were mixed phases of CdBi4S7, CdngigIS” and BiZS3.
Compositions of the type Cd(beBi4-x)S7 and CdBi4(S7-xSex) with several x values were
also investigated but they were just mixtures of CdBi4S7 with binaries such as CdS, Sb2S3
or Bi2Se3. Cd/Bi/Q (Q = Se, Te) systems were also probed but produced mainly simple
binary CdQ and B12Q3. In addition, Cd1-bexBi4S7 system was examined with several x
values because the analogous structure of PbBi4S72'. From the various x values (x = 0.5,
0.7), however, the Cdogngongi5S9 phase was produced almost pure instead of the solid
solution phase of CdMPbeiaS-r. When the x = 0.2, and 0.9 mixtures were obtained of the
ternary CdBi487 or PbBi4S7 and binary CdS and Bi283. The purity and homogeneity of
samples were verified by comparing the X-ray powder diffraction patterns to those

calculated by the crystallographically determined atomic coordinates. One of the

92

Cdogngongissg phase (x = 0.7 for CdHPbeiaS7) appears to melt congruently at 768 °C
see Figure 4-1.

For charge transport properties we grew large crystals of CdBias7 and
Cdo_63Pbo_ngi589 phase compounds using the Bridgman technique. The obtained ingots
show well-grown, highly oriented characteristics, Figure 4-2. The long axis
(crystallographic b-axis for CchS7 and a-axis for CdogngnngisSo) in the ingots lies
parallel to the Bridgman translation axis. These ingots were out along the direction
parallel and perpendicular to the crystal grth using a wire-saw.

Structure Description.

The ternary bismuth sulfide CdBi4S7 crystallizes in a monoclinic space group C2/m,
while the quaternary bismuth sulfide Cd0_53Pb0ggBi5S9 crystallizes in the orthorhombic
space group Cmcm. Both are members of a tropochemical cell-twinning series generated
by galena slabs (NaCl-type)21 cut perpendicular to the [311] direction with a mirror as
twinning operation. This tropochemical cell twinning is a twinning on the cell scale
which is reported for Pb/Bi/S system as PbBi4S7 and Pb4,65Bi20,9S36.

To understand tropochemical cell twinning for CdBios7 and Cdogngongissg
structures, here we derive the galena slabs (N aCl3 I 1-type) and use them for composing the
unit structures, see Figure 4-3. The slab A and its mirror image slab A’ are joined to each
other by sharing one anion atom, shown in Figure 4-3 a). The metal sites in the dotted
circles are too close to be together in a restricted area. Therefore, only one metal site can
be possible with shifting to the center line (boundary plane or mirror plane) in a unit
structure. When the metal sites are located at the boundary plane they sit at the center of a

trigonal prism formed by anions, and a new type slab (denoted B) can be generated with

93

 

O _ Recrystallizing 747 °C

-10 _ CXO

 

Melting 768 °C
0 200 400 600 800 1 000
Temperature (°C)

 

 

 

T

Figure 4-1. Differential thermogram of the Cdogngongissg phase showing melting

and recrystallization events. Heating/cooling rate was 10 °C/min.

94

 

l

l i
. ‘ 1
. 9' 49.999.49.99
' i
l
......_...T_—
l t
I
.$_...-

 

H.111),

b)

ilillllili'uimt

 

....

 

C)

 

I . -
a-tlxis ‘ ' 1
-

Figure 4-2. a) SEM images of CdBi4S7 (left) and Cdogngongissg (right). b) Ingot
of CdBi4S7 grown in a Bridgman furnace. c) SEM image of well grown ingot of
CdogngongiSSgi phase prepared from CdHPbei4S7 (x = 0.7).

95

mirror plane on the center, shown in Figure 4-3 c). If the metal sites, however, are placed
outside of the boundary plane then the unit structure can have two different kinds of slabs,
such as distorted NaCl3 “ type (denoted C type) and Cde, type (denoted D) slabs, shown
in Figure 4-3 (1). The CdIz slab is composed of two octahedral units in a stepwise fashion
by sharing the edge of terminal octahedra. In addition, the two slabs A and A’ can also be
displaced by around half an octahedron length by forming a complete metal-based
octahedron on one side slab(A’) while using terminal anions fi'om the galena unit on the
other slab(A) thereby destroying the twin plane in it, shown in Figure 4-3 b). In Figure 4-
3, the several type binding modes of galena type slabs are depicted and are observed in
CdBizso, Cd3BigS|5, and Cdogngongissg (case of Figure 4-3 b)), Cdogngongi589 (case
of Figure 4-3 c)), and CdBi4S7, Cd3Bi3815, and Cdogngongi5S9 (case of Figure 4-3 d)).
CdBi487. The CdBi4S7 is isostructural to the corresponding PbBi4S7, which is a
strongly anisotropic three-dimensional framework composed of two types of slabs which
can be described as CdIz and NaCl3 1' structure type and alternatively stacked to c-axis,
Figure 4-4. The slab D (Cdlz type) is composed of two Bng octahedra units generated by
screw axis from a single Bi(1)-S octahedron and propagated along the b-axis. The other
slab C (NaCl311 type), distorted galena-type structure, is composed of single [Cng]
octahedron sandwiched by two square pyramids of [BiSs] with sharing edges. The two
slabs (C and D) are interconnected through the atom S( 1), which is a corner of two
octahedra from each slab.
The structure has two crystallographically independent Bi atoms. Bi(l) is in a slightly
distorted octahedral site with distances from 2.712(3) to 2.926(3) A to the coordinated S

atoms and composes Cdlz-type slab D. Bi(2) is in slab C and has five normal covalent

96

- ‘5‘
mirror plane
Ce.

boundary plane
0)

 

E trigonal prism .

Galena ‘ Galena Galena” CdIz

Figure 4-3. Derivation of the unit structures fi'om the galena type slabs(A and A’) based
on tropochemical cell twinning. (Dark gray circles bismuth atoms and gray circles
chalcogen atoms) a) A and A’ are joined with sharing one anion atom. b) A and A’ are
displaced around half octahedron difference. c) It is modified from a) and has a metal
ion in the center of trigonal prism. d) It is also modified from a) and metal atom reside

inside of trigonal prism.

97

,. _ 1’43 ,9 [’4‘ w/
‘1 ,e ‘ . 1.33:8 ‘ AMEE
A 8138’
/ ‘ S4 Bi2 . ' 3
one 1.1, "‘1 /,1‘151,1 ‘
L’b/ \ « w / \ SZCd)‘-4*

 

 

 

 

04-1 05* ,,.o*- Rfi y“
° \ a ' Uri-1 - ,\.
\ (La 2:5» :3.» Lieu/in,»

Figure 4-4. Projection of the structure of CdBi4S7 down the b-axis. The structure is
composed of two types of slab described as slab C (NaClm-type) and slab D (Cdlz-

type).

98

bonds with neighboring S atoms in a square pyramidal coordination (Sb28e3-type) and
two additional longer interactions with S(4) atoms in slab D at 3.392(2) A; namely Bi(2)
has one bond with S(2) at 2.614(2) A, two bonds with S(3) atoms at 2.996(5) A and two
with S(4) at 2.726(2) A. Cd(l) is in a warped octahedral site with four Cd(1)-S(2) bonds
at 2.815(2) A and two short Cd(1)-S(1) bonds at 2.553(4) A, Table 4-4. Since the
equivalent isotropic displacement parameters of Cd atoms are similar to the other
elements, see Table 4-2, the formally Cd2+ ions fully occupy an octahedral environment
of S atoms, which is a rather unusual coordination environment for this ion. Generally,
Cd2+ ions prefer to have a four-coordinated tetrahedral pocket. Only a few compounds in
the Cd/Bi/S homologous series show fully occupied six-coordinated Cd2+ sites in the
structure. The S(3), however, has very large anisotropic displacement parameters in Table
4-6, which could be split at low temperature due to the more ionic bonding characters of
Bi-S and be on the twofold axes at high temperature due to the more covalent bonding
character of Bi-S. This structural behavior of S is consistent with the lead bismuth sulfide
PbBi487.

Cdo,gsto.32Bissg. In contrast to CdBi487, the quaternary bismuth sulfide
Cd0_63Pbo.32Bi589, in accordance with the known phase Pb4_6Bi20.9S3620, is mainly
assembled by three different building slabs B, C, and D type, see Figure 4-3 and Figure
4-5. The B, C and D type slabs (or B’, C’ and D’ type slabs) are related to each other by a
twin-like operation. In particular, the slab B composed of a pair of galena type lattices
has a mirror plane placed in parallel with b-axis and bicapped trigonal prismatic
coordination located on the mirror plane in which Pb metal ions may possess. It is

impossible to assign the position of the Pb or Bi atoms from X-ray diffraction because of

99

their similar atomic weights and scattering power. Nominally, the 8-coordinated sites
could be assigned as Pb(7) and the 6-coordinate metal sites in the slab C could be also
assigned as mixed Pb(6)/Cd(6) because in the corresponding slabs from CdBi487, PbBi487
and Pb4,6Bi20.9S36 the M2+ ions mainly occupy similar sites. Actually the M(7) sites stand
aside to the mirror plane and have 50% occupancy because the bicapped S(9) atoms to
trigonal prismatic coordination by 8(1) and 8(3) have strong interaction, which make it
split and 033(5) A distance apart.

In addition, the distorted galena type C and CdIz type D slabs and their twinned C’
and D’ slabs are arranged on either side of the slab B or B’ in order of the sequence
[D’C’BCDCB’C’D’]. Specifically, the B and C slabs are connected by shifting passed
each other by half octahedron while other slabs are linked by sharing anion sites such as
S(4). They all stack along the c-axis and make long sequences with twin like boundaries,
which generate large unit cell parameter ~60 A (Table 4-1).

The structure has seven crystallographically independent metal ion sites that from
M(l) to M(5) can be denoted as Bi( 1) to Bi(5) while M(6) and M(7) as Pb(6) and Pb(7),
respectively. M(l) and M(4) in slab B are in slightly distorted octahedral site with
distances from 2.706(7) to 2.962(7) A to the coordinated S atoms. Moreover, M(7) is a
split site and nearly lies in the center of the bicapped trigonal prism with a strong
interaction to 8(3) ((1 = 2.754(7) A) while the other metal sites have longer M-S distances
of 309(3) A, 3.399(1) A for two S(9) and 3.154(0) A, 3.335(0) A for two S(l), see Table
4-5.

The anisotropic thermal parameters of M(7) and S(9) are very large along the c-

axis direction because they have relatively strong interactions, which is consistent with

100

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

mirror plane mirror plane

Figure 4-5. Projection of the structure of Cdo_6ngo,32Bi589 down the a—axis. The
structure is composed of two types of slab described as slab C (NaClm—type) and slab
D (CdIz-type).

101

the long bond length between M(7) and S(9), Table 4-7. M(2) in slab D is in the least
distorted octahedral site with M—S distances at 2.713(7) — 2.910(7) A and S—M—S angles
at 86.77(10)° — 92.02(10)°. S(10) in slab D, however, is almost identical to the
corresponding the S atoms of CdBi4S7, PthS7 and Pb4_6Bi20,9836 structures on which S
atoms have large anisotropic thermal parameters in a certain direction (c-axis in here) due
to their bonding characters based on the temperature. M(3) and M(5) in distorted galena
type slab C have five normal covalent bonds with neighboring S atoms at a square
pyramidal coordination (Sb28e3-type) and two additional longer interactions. In addition,
they are bound to each other by sharing the lateral edge of each one and connected on
either side of the flattish and distorted octahedra in which M(6) and Cd(6) occupy 31 %
and 69%, respectively. In practice, M(3) has one short bond with S(6) at 2.617(6) A, two
with S(8) at 2.734(4) A, two with 3(7) at 2.995(5) A and longer interaction with S(5) at
3.392(1) A while M(5) has one short bond with S(7) at 2.609(13) A, two with S(4) at
2.749(9) A, two with S(6) at 3.020(11) A and a longer interaction with S(2) at 3.266(0) A.
M(6)/Cd(6) has two bonds with S(6) at 2.811(5) A, two bonds with S(7) at 2.824(5) A
and two short bonds with S(4) and S(8) at 2.616(8) A and 2.624(7) A, respectively.
Charge Transport Properties and Energy Gaps. The CchS7 and
CdObngongisSo compounds are valence precise and are narrow gap semiconductors. The
infrared absorption spectra of CdBi4S-; and CdnengnngisSo were measured at room
temperature and showed intense absorptions for CdBi4S7 and Cd0,6ngo,32Bi5$9 around 0.5
eV and 0.1 eV, respectively, See Figure 4-6. This is in agreement with the fact that the
replacing the alkali metals with other less electropositive metals, for example AgBi3S5

. described earlier”, in the alkali metal bismuth sulfide compounds is expected to lower

102

the band gap. Surprisingly, addition of 5% and 10% of BiZS3 to Cch87 cause the band
gap to be around 0.1 eV because of the high doping level. The Cdo,63Pbo_ngi5$9 type
compound showed much smaller band gap because Pb atom is much heavier than Cd
atom, which is consistent with the general behavior of semiconductors.

Preliminary thermopower and electrical conductivity measurements were carried
out on the well oriented polycrystalline ingots along the crystal grown direction (i.e.,
crystallographic b-axis and a-axis for CdBi4S7 and Cd0_63Pbo_32Bi5S9, respectively). The
electrical conductivities of the ternary or quaternary ingots were found to be strongly
influenced by the ingot preparation conditions such as crystal orientation, various degrees
of doping and sample quality. For example, the CdBi4S7 type ingots showed differing
electrical conductivities 356 S/cm for CdBi4S7, 448 S/cm for CdBi4S7 (5% Bi283 doped)
and 235 S/cm for CdBi4S7 (10% BiZS3 doped) at room temperature. The Cdo_6ngo,32Bi589
type ingots showed electrical conductivity of 15 S/cm and 1381 S/cm for the compounds
prepared from Cd1-bexBi4$7 (x = 0.5) and Cd;-be,,Bi4Sy (x = 0.7), respectively. Much
difference of the electrical conductivities between two Pb included compounds must be
caused of sample quality. In addition, the high electrical conductivities from CchS7 and
CdogngongisSg prepared from Cd1-bexBi487 (x = 0.7) are well in accordance with a
heavily doped state and their narrow band gaps.

Both CdBi487 and CdnbngonzBisSo type compounds possessed n-type behavior
and narrow gap semiconductors indicating that electrons are the dominant charge carriers.
The thermopower of CdBi4S7 type and Cdo,63Pbo,32Bi589 type compounds were measured
in the temperature range 300-700 K. The thermopower of CdBi4S7 increases steadily up

from -128 uV/K at 300 K to -265 pV/K at 700 K while 5% and 10% doped CdBi487 start

103

 

Absorption coefficient (a/S)

 

 

 

0:0 ' 0:1 ' 0:2 ' 0:3 ' 0:4 I 0:5 ' 0:6 I 0:7 I 0.3
Energy (eV)

Figure 4-6. Solid-state infrared absorption spectra showing band gap transitions for a)
CdofingongisSo and b) CdBi4S7. The band gaps in each case are estimated from the

crossing point of the solid lines shown in each spectrum.

104

A 50
g .
> 0-
:3, -50-
H ‘ A
a q C . ‘ A ‘
,2-100- ' . O ‘ A 1
2 . . . ' o : A : .
Ld::-1so- ' . ' ‘ n
0 ' I . I
o i I 0
Q-ZOO~ ' u ‘ l g g
M I l -
0-250. I ' ' I I
B . I I I I
8-300-
{/3 .
'350 I I I fi I
300 400 500 600 700
b Temperature (K)
) A 100
5 0-
ar .
.2 '50“ :.. ..... .
O - I I . I . 0 I
E-100- ...-II I. I .....
8 i I ' I .
U '150“ I I . .
'24) -200-
Q)
'8 2 i
o - 50-
m .
'300 I I I I I T
300 400 500 600 700
Temperature (K)
Figure 4-7. Variable-temperature thermoelectric power data for a) I CdBi4S7, o

CdBi4S7 (5% Bi283 doping) and A CdBi4S7 (10% Bi283 doping), b) I
Cdo,6ngo_3zBi589 phase prepared from Cd1-bexBi4S7 (x = 0.5) and o Cdo,6ngo_ngi589

 

 

 

 

 

 

phase prepared from Cd1-bexBi4S-, (x = 0.7).

105

 

 

from -73 and -60 pV/K at 300 K to -202 and -211 pV/K at 700 K, respectively, see
Figure 4-7 a). Both Bi2S3-doped materials (5% and 10%) showed slightly lower
thermopower and the 5% Bizsg-doped one has higher electrical conductivity than
CdBi4S7, implying that doping with Bi283 increase number of carriers.

The thermopower of Cd0_68Pb0_82BiSS9 type compounds start from -57 uV/K at
300 K to -157 pV/K at 700 K for the compound prepared from Cd1-bexBi4S7 (x = 0.5)
and -40 uV/K at 300 K to -95 uV/K at 700 K for the compound prepared from Cd]-
bexBi4S7 (x = 0.7), see Figure 4-7 b). The ingot samples of these compounds were
prepared with a different composition than that for single crystal preparation. Since Cd
and Pb are both divalent and Cd2+ is found in a certain site (M6) with mixed occupancy,

various proportions of Cd to Pb can be possible.

Concluding Remarks

New synthetic investigations of the systems Cd/Bi/S and Cd/Pb/Bi/S lead to the
ternary and quaternary narrow band gap semiconductors, CdBi4S7 and CdofingngBisSo.
The crystal structures of both compounds exhibit a tropochemical cell-twinning feature
generated by modified galena slabs (NaCl'm-type) with a mirror as twinning operation.
Both type compounds have n-type semiconducting character with electrons as the charge
carries and relatively high electrical conductivity. Surprisingly, as the result of Bi2S3
doping of CdBi4S7 and Pb incorporation in the structure as typified by Cd0.68Pb0_8zBiSS9,
the band gaps can become very narrow ~ 0.1 eV which is not very common among
bismuth sulfide compounds. The electrical conductivities as a result are increased more

than that of CdBi4S7 but the thermopowers are relatively reduced. The substitution with

106

other elements and modification of the CdBi4S7 and Cdo,63Pbo,82Bi5$9 structure by
partially or totally replacing divalent metal ions in it, such as Sn or alkaline earth metals,
could expand further the scope of investigations of the thermoelectric properties in this

class of compounds.

107

Reference

1. ZT = SZO'T/K, where S is the Seebeck coefficient, 0 is the electrical conductivity, T is
the temperature and K is the thermal conductivity, which includes electron and phonon
contributions.

2. CRC Handbook of Thermoelectric Materials. Rowe, D.M..Ed., CRC Press, Inc.: Boca
Raton, FL, 1995.

3 Slack, G. A. “New materials and performance limits for thermoelectric cooling” in
CRC handbook of thermoelectrics” Edited by Rowe, D. M> CRC press, Boca Raton,
1995, P 407-440, b) Slack, G. A. in “ solid state Physices”, eds, Ehrenreich, H. ; seitz, F. ;
Tumbull, D. Academic, New York. 1997, Vol. 34, 1.

4. Chung, D-. Y.; Hogan, T.; Brazis, P. W.; Kannewurf, C. R.; Bastea, M.; Uher, C.;
Kanatzidis, M. G. Science 2000, 287, 1024-1027.

5 . Chung, D. -Y.; Choi, K. —S; Iordanidis, L; Schindler, J.L. ; Brazis, P. W.; Kannewurf,
C. R.; Chen, B.; Hu, 8.; Uher, C.; Kanatzidis, M. G. Chem. Mater., 1997, 9, 3060-3071.

6 . Chung, D.-Y.; Jobic, S.; Hogan, T.; Kannewurf, C. R.; Brec, R.; Rouxel, J .; Kanatzidis,
M. G. J. Am. Chem. Soc. 1997, 119, 2505-2515.

7 . Mrotzek, A; Chung, D. —Y.; Hogan, T; Kanatzidis, M. G. J. Mater. Chem, 2000, 10,
1667-1672.

8 . Choi, K. —S.; Chung, D. —Y.; Mrotzek, A.; Brazis, P.; Kannewurf, C. R.; Uher, C.;
Chen, W.; Hogan, T.; Kanatzidis, M. G. Chem. Mater. 2001, 13, 756-764.

9 . Iordanidis, L.; Brazis, P. W.; Kyratsi, T.; Ireland, J .; Lane, M.; Kannewurf, C. R. Chen,
W.; Dyck, J. S.; Uher, C.; Ghelani, N. A.; Hogan, T.; Kanatzidis, M. G. Chem. Mater.
2001, 13, 622-633.

10 . Hsu, K. —F.; Chung, D. —Y.; Lal, S.; Mrotzek, A.; Kyratsi, T.; Hogan, T.; Kanatzidis
M. G. J. Am. Chem. Soc., 2002, 124, 2410-2411.

11 . Hsu, K. -F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.;
Polychroniadis. E. K.; Kanatzidis, M. G. Science 2004, 303, 818-821.

12 . McCarthy, T. J.; Ngeyi, S.-P.; Liao, J .-H.; DeGroot, D.; Hogan, T.; Kannewurf, C.
R.; Kanatzidis, M. G. Chem. Mater. 1993, 5, 331-340.

13 . Iordanidis, L; Bilc, D; Mahanti, S. D. and Kanatzidis, M. G.; J. Am. Chem. Soc. 2003,
125, 13741-13752.

108

 

14. McCarthy, T. J.; Tanzer, T. A.; Kanatzidis, M. G. J. Am. Chem. Soc. 1995, 117,
1294-1301.

15 . (a) McCarthy, T. J .; Tanzer, T. A.; Chen, L.H.; Iordanidis, L.; Hogan, T; Kannewurf,
C. R.; Uher, C; Chen, B; Kanatzidis, M. G. Chem. Mater. 1996, 8, 1465-1474. (b) Chen,
B; Uher, C; Iordanidis, L; Kanatzidis, M. G. Chem. Mater. 1997, 9, 1655-1658.

16. a) Takagi, J .; Takeuchi, Y. Acta Crystallogr. 1972, 328, 369. b) Makovicky, E.
Neues Jahrb. Mineral. 1989, I60, 269.

17, Zakr'zewski, M. A.; Makovicky, E. Can. Mineral. 1986, 24, 7.
18. G. Ilinca, E. Makovicky, Eur. J. Mineral. 1999, 114, 691.

19. Kim, J.-H.; Chung, D. —Y.; Bilc, D.; Loo, 8.; Short, J.; Mahanti, S. D.; Hogan, M.;
Kanatzidis, M. G. Chem. Mater. 2005, 17, 3606-3614.

20 Takeuchi, Y; Ozawa, T.; Takagi, J. Z. Kristallogr. 1979, 150, 75.
21 Takeuchi, Y.; Takagi, J. and Yamanaka, T. Z. Kristallogr. Bd. 1974, 140, 249.
22. Choe, W.; Lee, S.; O’Connell, P.; Covey, A. Chem. Mater. 1997, 9, 2025-2030.

23 . Kyratsi, T.; Chung, D.-Y.; Choi, K.-S.; Dick, J. S.; Chen, W.; Uher, C. and
Kanatzidis, M. G. Mat. Res. Soc. Symp. Proc. 2000, 626, 28.8.1- 28.8.6.

24 . Kubelka-Munk function: oz/S =(1-R)2/2R, where a is the absorption coefficient, S is
the scattering coefficient, and R is the reflectance at a given wavenumber.

25 . SMART, SAINT, SHELXTL: Data Collection and Processing Software for the
SMART-CCD system; Siemens Analytical X-ray Instruments Inc.: Madison, WI, 1997.

26 Adolphe, C. Ann. Chim. 1965, 10, 271-297.

109

CHAPTER 5

Structural diversity in the Quaternary Bismuth Selenides
AM6Se9 (A= Rb, Cs ; M= Bi, Ag or Cd)

1. Introduction

There has been a strong interest in developing new thermoelectric materials in
bismuth chalcogenide chemistry over the past decade since the excellent thermoelectric
properties of BizTe3 near room temperature were reported.1 Furthermore, several attempts
to improve ZT2 values have been made with an effort by including various concepts such
as quantum confinement (QC)3 and phonon glass electron crystal (PGEC)4. As a result of
these efforts, for example, the nanostructured thin-film superlattices of BizTe3/Sb2Te35,
the quantum dot superlattices of PbSeoogTeonz/PbTe 6 and the skutterudites
CeFe3,__<,Coo,5Sb127 were reported with very high ZT.

The previous compounds BaBiTe3,8 CsBi4Te6,9 fl-KzBi38e13,10, Ag1-be|ngTe20”
showed interesting and promising thermoelectric properties, as they possess low thermal
conductivity, high thermopower and high electrical conductivity (when appropriately
doped). An important motivation for this work is the potential of complex bismuth
chalcogenides as useful thermoelectric materials.12 In addition, these compounds have
great structural and compositional diversity, with characteristics common to those in
natural and synthetic bismuth chalcogenide compounds such as sulfosalts: gustavite-

4

lillianite series, '3 the kobellite series,1 and pavonite '5 , and homologous series:

A..[M..,Se2.,]2,,,[M2,.,,Se2.3,.,,] (A = K, Rb, Cs, 8:, Ba; M = Sn, Pb, Eu, Bi, Sh),16

110

CstmBi3Te5+m,'7 (szTe3),,,-(Sb2),,‘8, A2[M5+,,Seo+,,] (A = Rb, Cs; M = Bi, Ag, Cd)”, and
(CdS),,(Bi2$3),,,20. The foremost structural feature in this class of compounds derives from
relatively few common building block motifs such as NaCl-(NaCl'OO), szseg-(NaCllOO),
BizTe3-(NaClm), Cdlz-(NaCll ' l) and galena types(NaCl3 I I) all of which are based on the
NaCl-type structure. Conceptually they derive from excising along different directions of
the NaCl structure type. When compounds can be identified in a simple way such as a
homologous series, it helps to understand large groups of materials, thereby allowing
useful generalizations and predictions.

An outstanding demonstration of structural diversity and complexity is found in
the three new quaternary compounds with the general formula AM6$e9 (A= Rb, Cs; M=
Bi, Ag or Cd) described here. We present the synthesis, physicochemical, spectroscopic,
and structural characterization of CsAgo,5Bi5,5Se9, RboostogsBisnsSeo, and RdeBisseo.
These compounds crystallize in pseudo two dimensional BizTe3(NaC1m) type layered
structures retaining distinct building blocks in a systematic way. They are in monoclinic
space groups (the first one in P21/m and last two of them in C2/m) and feature a
significantly different packing and bonding arrangement of the building blocks in their
different forms. These can be described as excised fragments from the NaCl structure.
These compounds are narrow energy gap semiconductors and are of interest as

thermoelectric materials.

111

2. Experimental Section

Reagents. Chemicals were used as obtained: bismuth chunks (99.999% Noranda,
Canada), Se shots (99.999% Noranda, Canada) Rb (99.8% purity, Alfa Aesar, Ward Hill,
MA), Cs (99.98% purity, Alfa Aesar, Ward Hill, MA), Cadmium powder (99.999%, -
200mesh Cerac).

Ag Powder. A silver coin (99.999%) was dissolved in nitric acid. The solution
was neutralized to a pH of 7 with ammonium hydroxide. Sodium borohydride was added
to reduce the Ag ions to a black precipitate of Ag metal powder. The precipitate of silver
was filtered and washed thoroughly with water and dried in a vacuum oven at 150 °C.
The obtained fine powder of Ag was identified by powder X-ray diffraction.

Synthesis. All manipulations were carried out under a dry nitrogen atmosphere in
a Vacuum Atmospheres Dri-Lab glovebox and in a Schlenk line. For all compounds the
yield was quantitative. The purity and homogeneity of the products were verified by
comparing the X-ray powder diffraction patterns to those calculated by the
crystallographically determined atomic coordinates. A28e (A = Rb, Cs) were obtained by
stoichiometric reactions of elemental alkali metals and selenium in liquid NH3. The
purity and homogeneity of the products were verified by comparing the X-ray powder
diffraction patterns to those calculated by the crystallographically determined atomic
coordinates.

CsAgo,5Bi5,5Se9. A mixture of 0.0517 g (0.15 mmol) of C8236, 0.0324 g (0.3
mmol) of Ag, 0.2508 g (1.2 mmol) of Bi and 0.2013 g (2.55 mmol) of Se was loaded in a

fused silica tube (carbon coated 9 mm diameter) and subsequently flame-sealed at a

112

residual pressure of <104 mbar. The tube was heated at 750 °C for 72 h, cooled to 550 °C

in 20 h, and then further cooled to 50 °C in 20 h. Isolation in degassed

dimethylformamide (DMF) and water gave silvery-black needles and Bi2863
(approximately 1:1 ratio) as evidenced from X-ray powder diffraction. A quantitative
microprobe analysis using Energy Dispersive Spectroscopy (EDS) was performed on a
Scanning Electron Microscope (SEM) on several needles gave an average composition of
C8148Ago.443i5.11369-

Rbo,95Cd035Bi5,458e9. A mixture of Rb2Se powder (0.0750 g, 0.3mmol) and
elemental Cd powder (0.0674 g, 0.6 mmol), Bi (0.5016 g, 2.4 mmol), and Se (6.771 g,
32.4 mmol) was loaded in a fused silica tube (9 mm diameter) and subsequently flame-
sealed at a residual pressure of <104 mbar. The tubes were heated at 750 °C for 72 h,
followed by cooling to 550 °C at a rate of 5 °C/h then to room temperature in 10 h. A
silvery-black needle type polycrystalline ingot of Rb0,95Cdo,35Bi5,4sseo (yield >90%) was
obtained after isolation in dimethylformamide (DMF) and washing with methanol and
diethyl ether. SEM/EDS analysis on several single crystals of Rbo,95Cdo.35Bi5_45Seo
showed the approximate composition of Rb1,43Cdo,47Bi5,OZSeo.

RdeBisseg. A mixture of RbZSe powder (0.0750 g, 0.3mmol) and elemental Cd
powder (0.1686 g, 1.5 mmol), Bi (0.3762 g, 1.8 mmol), and Se (0.3316 g, 4.2 mmol) was
loaded in a fused silica tube (9 mm diameter) and subsequently flame-sealed at a residual
pressure of <10'4 mbar. The tubes were heated at 750 °C for 72 h, followed by cooling to
550 °C at a rate of 5 °C/h then to room temperature in 10 h. after isolation in degassed
imethylformarnide (DMF) and water, a mixture of silvery-black needle type

polycrystalline ingot of RdeBi58e9 (~80%) and Rbo.95Cdo,3sBi5.4SSeo (~20%) were

113

obtained which were verified by comparing the X-ray powder diffraction patterns to
those calculated by the crystallographically determined atomic coordinates. SEM/EDS
analysis on several single crystals of RdeBi5Se9 showed the approximate composition
0be139CdL2Bi512369-

Bridgman growth for Rbo,95CdogsBi5,4sse9

In order to grow highly oriented crystal specimens for thermoelectric property
measurements, a mixture of szse powder (0.8746 g, 3.5 mmol) and elemental Cd
powder (0.3147 g, 2.8 mmol), Bi (8.0457 g, 38.5 mmol), and Se (4.6981 g, 59.5 mmol)
was loaded in a carbon coated fused silica tube (13 mm diameter) with a point end and
sealed under vacuum. The tubes were heated at 750 °C for 72 h, followed by cooling to
550 °C at a rate of 5 °C/h then to room temperature in 10 h, and then the tube was heated
again to 750 °C in a Bridgman fumace and descended at a rate of 3.25 mm/h through a
sharp (100 °C/cm) temperature gradient.21 A pure (> 95%) and well oriented ingot (32

mm long, 11 mm diameter) of Rbo,95Cdo_35Bi5.4SSeo was obtained shown in Figure 5-8.

3. Physical measurements

Electron Microscopy. Quantitative microprobe analysis for the compounds was
performed with a JEOL JSM-6400V Scanning Electron Microscope (SEM) equipped
with a Noran Vantage Energy Dispersive Spectroscopy (EDS) detector. Data were
collected for 30 sec using an accelerating voltage of 20kV. All reported results are an

average of measurements on at least three different crystals.

114

Differential Thermal Analysis. Differential thermal analysis (DTA) was
performed with a computer-controlled thermal analyzer (Shimadzu DTA-50). 20 mg of
ground crystals were sealed in silica ampoule under vacuum. A silica ampoule containing
the equal mass of alumina was placed on the reference side of the detector. The sample
was heated to the desired temperature a 10 °C/min, isothermed for 2 min and then cooled
at 10 °C/min. The heating program was recycled to check reproducibility of the thermal
behavior of the sample. The reported melting point is the peak temperature. After DTA,
the sample was examined by powder X-ray diffraction to identify if any decomposed
product formed during heating/cooling cycles.

Infrared Spectroscopy. Optical diffuse reflectance measurements were made on
the finely ground sample at room temperature. The spectrum was recorded in the infrared
region (6000-400 cm") with the use of a Nicolet MAGNA-IR 750 Spectrometer equipped
with a Collector Diffuse Reflectance of Spectra-Tech. Inc. Absorption data were
calculated from the reflectance data using the Kubelka-Munk function.22

Charge transport measurements. The Seebeck coeffiecient of polycrystalline
samples was measured between 300 and 800 K by using a SB-lOO Seebeck Effect
Measurement System, MMR Technologies. The electrical conductivity measurements
were performed in the usual four-probe geometry at room temperature.

Powder X—ray Diffraction. A calibrated CPS 120 INEL X-ray powder
diffractometer equipped with a position-sensitive detector, operating at 40kV/25mA with
a flat geometry and employing graphite monochromatized Cu Ka radiation, was used to

obtain powder patterns of starting materials and all products.

115

Single-crystal X-ray Crystallography. For the single crystal of Rbo,95Cdo_3sBi5.4SSe9,
intensity data were collected at 273 K using graphite-monochromatized Mo Ka radiation
(A = 0.71073 A), on a STOE IPDS-II diffractometer. A analytical absorption correction to
the data was applied with the program X-RED23 based on a crystal shape description
determined using equivalent reflections with X-SHAPE.24

For the single crystals of CsAgo,5Bi5,5Seo and RdeBisseo, X-ray diffraction
intensities were collected at room temperature on a Bruker SMART Platform CCD
difiractometer using a graphite-monochromatized Mo KG radiation. The individual
frames were measured with an omega angle rotation of 03° and an acquisition time of 30
sec for each crystal. The SMART25 software was used for the data acquisition and
SAINT software for data extraction and reduction. An analytical absorption correction
was performed using face indexing and the program XPREP in the SAINT software
package, followed by a semiempirical absorption correction based on symmetrically
equivalent reflections with the program SADABS. Structural solution and refinements
were successfully done using the SHELXTL package of crystallographic programs. The
structures were solved with direct methods.

The complete data collection parameters, details of the structure solution, and
refinement for CsAgo,5Bi5,5Se9, RboostogsBisASSeo, and RdeBi58e9 are given in Table
5-1. The fractional coordinates and temperature factors (Ueq) of all the atoms with

estimated standard deviations are given in Tables 5-2 ~ 5-10.

116

Mk. .0 $3“- use Swen
N036 u my?» dimmed u Lm
we _.o u NM? .Evod H La

Go;
mom \ o \ Vmov

.\. one ..oflwm
a 58 u $9: $9.
wwmm_
Suvrvfi

- .muvaVm- .Gnvaer
been. 9 we.
ms...

.....e 226
.582 9.8

w

.< 53st

< €9.08 u o
.38? Ha

... 2 :33... n o
< @303 u e
58
0820952

... $28

M £on
:32
696.08

.-< .o :24 2.. on}.
328 u 23 .88 n .2
e: S n 23 .886 n. .2

R:
z: E \ 8.3
u so mocmscm¢m8_ “Saginaw
.5 8e 3on
:88 n 9.9: 8%
an:
anvrva
- .onvaVm- .Sfivnuvflr
8on 2 e3
memm
..88 an. E
men: 23
e
.< GK. SM:
< 53.: u o
.63.; "a
4 $32... n o
.... @928 n e
58
03:00:02
4 $28
a. ASSN
3er.
oomnv.n_mnm.¢flmvma.c£y~

N

..._:.A....~Ew\r¢.n~. RENO u 33 .__.m__.d__...~_ t _...~__w u E.

aw. .o m—Vfi- v8 30%
omefio u NM? 4 god u LM
2116 n NM? 6536 u LM

mmod
mo \ o \ wNVN

.\. mg .028
888 u GEE mg
neon

mmuxuvfi

- .mnvxnvfl K Tvnuvmw T
.28 o. ...2
SS

78:. 89%
.632 53

N

...... 388

< $33.: u o
Osage: "a
< 8:83.... u o
< 523: u e
5:8
OWE—0952

< $28

.... Ema
3.2.8
£25550

Ben 98 6.8% HE. «moment—
Eee =3 .86... m
2382?: .86... m 1......
Nu co Enuohmocvooc
88888 \ $5862 \ Sea
8508 “8853””

82: 8 30883800
80838“ “83385
880:8 mcouoecem

momma x85

838:8 58 no.“ emcee 82E.
88E

820808 8qu9?.
8.3.3 .....eo

N

oEEo>

£86886 :8 E5
9.on 8QO

889$ Rambo
E8363
ogfiomEoH

Emma? 328.8”—
EBEQ RoEqfim

 

.aommE—envam ES .eommémmgumqfim aommnmmwetwflmu .8.— Eoancou 8.85.8 can 82. .835 .—.m 93:.

117

Table 5-2. Atomic coordinates ( x 10‘) and equivalent isotropic displacement
parameters (Azx 103) for CsAgo,58i55Se9. U(eq) is defined as one third of the trace of
the orthogonalized Uij tensor.

 

 

x y z U(eq) occupancy

Bi(l) 2061(1) 2500 1546(1) 15(1) 1
Bi(2)/Ag(2) 4904(1) -7500 3824(1) 18(1) 0.863(5)/0.137
Bi(3) 2375(3) -2500 3886(2) 13(1) 0.637
Ag(3) 2397(9) -2500 3770(7) 12(4) 0.363
Bi(4) 7324(1) -12500 3762(1) 26(1) 1

Bi(5) 4610(1) -2500 858(1) 23(1) 1

Bi(6) -135(1) 2500 3872(1) 28(1) 1

Cs(1) -1259(2) 2500 1133(1) 24(1) 1

86(1) 6008(2) -7500 2792(2) 19(1) 1

Se(2) 6232(2) -12500 4968(2) 16(1) 1

Se(3) 3602(2) -2500 2848(2) 17(1) 1

Se(4) 770(2) -2500 764(2) 17(1) 1

Se(5) 1303(2) -2500 5037(2) 17(1) 1

Se(6) 3026(2) 2500 476(2) 17(1) 1

Se(7) 6034(2) -7500 81 1(2) 17(1) 1

Se(8) l 1 12(2) 2500 2942(2) 17(1) 1

Se(9) 8610(2) -12500 2875(2) 18(1) 1

 

118

Table 5-3. Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for Rb0,95Cd035Bi5,45Se9. U(eq) is defined as one third of the
trace of the orthogonalized Uij tensor.

 

 

x y z U(eq) occupancy
Bi(l) 4402(1) 0 3987(1) 22(1) 1
Bi(2) 7100(3) -5000 2389(1) 19(1) O.63(5)
Bi(22) 7032(4) -5000 2372(2) 18(1) 0.37(5)
Bi(3) 5630(1) -5000 3546(1) 23(1) 0.917
Bi(4) 2986(1) -5000 4335(1) 24(1) 1
Bi(5) 4461(1) -5000 1530(1) 22( 1) 0.898
Bi(6) 5587(1) 0 999(1) 23(1) 0.653(9)
Cd(6) 5660(2) 0 1059(4) 17(1) 0.347(9)
Se(l) 5000 -5000 5000 20(1) 1
Se(2) 5051(1) 0 2542(1) 23(1) 1
Se(3) 3883(1) 0 5429(1) 21(1) 1
Se(4) 3981(1) -10000 581(1) 21(1) 1
Se(S) 6084(1) -5000 1986(1) 23(1) 1
Se(6) 5000 -5000 0 22(1) 1
Se(7) 3926(1) -5000 3006(1) 20(1) 1
Se(8) 7949(1) -5000 2910(1) 19(1) 1
Se(9) 7115(1) -10000 1105(1) 26(1) 1
Se(lO) 7134(1) -10000 4124(1) 19(1) 1
Rb(l) 3160(1) -15000 935(1) 31(1) 0.946

 

119

Table 5-4. Atomic coordinates ( x 104) and equivalent isotropic displacement

parameters (Azx 103) for RdeBi5Se9. U(eq) is defined as one third of the trace of

the orthogonalized Uij tensor.

 

 

x y z U(eq) occupancy
Bi(1)/Cd(1) -575(1) 5000 3217(1) 20(1) 0.736(5)/0.264
Bi(2) -547(1) 0 1984(1) 20(1) 1
Bi(3)/Cd(3) -562(1) 0 4435(1) 20(1) 0.683(5)/0.317
Bi(4)/Cd(4) 636(1) 0 3020(1) 15(1) 0.740(5)/0.260
Bi(5)/Cd(5) 585(1) -5000 4294(1) 20(1) 0.597(5)/0.403
Bi(6)/Cd(6) -516(1) -25000 10763( 1) 17(1) 0.799(5)/0.201
Bi(7) 2132(1) -15000 2843(1) 21(1) 1
Bi(8)/Cd(8) 618(1) -20000 10522(1) 18(1) 0.688(5)/0.312
Bi(9) 3039(1) -20000 221 1(1) 20(1) 1
Bi(10)/Cd(10) -681(1) -15000 8203(1) 16(1) 0.757(5)/0.243
Bi(ll) 2138(1) -25000 1189(1) 16(1) 1
Bi(12) 1727(1) -10000 4186(1) 16(1) 1
Rb(l) 3085(1) -15000 4237(1) 32(1) 1
Rb(2) -1834(1) -35000 10481(1) 25(1) 1
Se(l) 17(1) 0 3743(1) 22(1) 1
Se(2) -1050(1) 0 2688(1) 17(1) 1
Se(3) -1055(1) 5000 3943(1) 19(1) 1
Se(4) 11 12(1) -5000 3555(1) 19(1) 1
86(5) -1027(1) -5000 1495(1) 16(1) 1
Se(6) 64(1) 5000 2518(1) 17(1) 1
Se(7) -1037(1) 0 5186(1) 19(1) 1
Se(8) 0 -5000 5000 20(1) 1
Se(9) 1164(1) 0 2329(1) 17(1) 1
Se(lO) -1015(1) -30000 10284(1) 16(1) 1
Se(ll) 84(1) 0 1279(1) 18(1) 1
Se( 12) 2991(1) -25000 1454(1) 16(1) 1
Se(13) 2188(1) ~20000 2068(1) 15(1) 1
Se(14) 1114(1) -25000 11014(1) 18(1) 1
Se(15) 2137(1) -15000 4722(1) 17(1) 1
Se(16) 2122(1) -30000 538(1) 19(1) 1
Se(17) 2970(1) -15000 2934(1) 16(1) 1
Se(18) -2302(1) -10000 6442(1) 18(1) 1
Se(19) 0 -25000 10000 18(1) 1

 

120

 

53.5
55.8
55.8
58.5:
58.5
5:8
8:55:
8:38
52.8

53.5
55.5
8:852
58.;
58.8
53.5

55.5
55.8
8:22:
55.5
58.8
55.5

55-55-55
55-55-55
55-55-55
55-55-55
55-55-55
55-55-55
55-55-55
55-55-55
55-55-55

55-55-55
55-55-55
55-55-55
55-55-55
55-55-55
55-55-55

55-2.5-55
55-55-25
55-55-55
55-55-55
55-55-25
55-55-25

58.8
55.8
52.8

55.8
55.8

584.5
2 :88
2 :35
58.8
55...:
585:
2 :88
55.8
255.8

58.5
58.8
55.5
58.;
58.8
52.5:
55.8
55.8

55-55-55
25-55-25
55-55-55

55-53-55
55-55755

55-55-35
55-55-55
55-55-55
55-55-55
35-55-55
55-55-55
55-55-55
55-55-55
55-55-55

55-5555
55-55-55
55-35-25
55-55-55
55-35-55
55-25-55
55-55-55
55-25-55

333.8

58.8
55.8
58.8
5232
58.8
58.5
55.5
2 :58
58.8

N x 5me5
N x 58:
585.5
585.5
555.5
N x 5NN5.N

N x 583
528N
N x 553
5o5N

N x Gamma-N

AnvomAmzmACom

55-25-55
55-25-55
55-25-55
55-55-55
55-55-55
55-55-55
55-55-55
55-55-55
55-25-55

5556
55-56
55-56
55-25
55-25
55-25

55-55
55-55
55-55
55-55

55-55

N x 583
583

558.5
N x 5595
N x 555
555

N x 55>.N
552

N x 558.5
5:2
552

N x 583

5NNZ
N x 582
N x 5N$.N
553

N x 583
5:3
N x 553
522

55-55
55-55

55-55
55-55
25-55
55-55

55-53
55-5»...

55-55
55-55
55-55
55-55

55-55
55-55
55-55
55-55

55-55
55-25
55-25
55-25

 

8555.559 .8 ... ...—ms. E... 3... ...—W.;. 585 .m-m 03.:-

121

 

55.38
3:88
3:88

5N8

58.8
58.8
53.8
58.5:
58.8
58.8
58.8:
58.8
58.8

58.8
58.8
58.8
58.8
58.8:
588:
58.8:
58.8
5N: .8
58.8

58.8
58.8
588:

55-38955
55-58-55
55-58-58
55-58-55

58-55-58
55-55-58
55-55-55
58-55-55
58-55-58
58-55-58
58-55-55
58-55-58
55-55-58

58-55-58
58-55-55
35-55-58
35-55-58
58-55-55
58-55-55
58-55-35
58-55-58
55-55-58
55-55-58

55-55-55

55-55-58
55-55-58

58.8
58.8
58.8

58.8
58.8
588:
58.8
58.8
588:
58.8
588
58.8

58.8
38.8
58.8

58.8

588

3:83:
588
53.8

3:88

3:88
3:88
588:
58.8
58.8

55-55-55
55-55-55
55-55-55

58-55-35
25-55-55
25-55-58
35-5535
58-55-55
35-55-35
55-55-55
55-55-58
58-55-55

55355-55
55-35-58
55335-58
55-35-58
55335.58

58355.58
55355.58
58355.55
55355.58

55-55-55
58-55-58
58-55-58

55-55-55

55-35-55

383588.383: .8 C 8.85 Ba .1 2.85. 285 8.5 2...:-

3:82:
3:88
5:8
388

58.8
588
58.8:
58.8
58.8
583:
53.8
58.8
55.8

N x 5383
N x 5833
3883
3883
55.3

5883
582
N x 58:3

m x $35205
Amvooa.m
N x 35333
Amvwwhd

55-55-58
55-35-58
58-35-55
55-55-55

35-25-25
35-25-55
35-25-35
55-25-25
25-55-58
25-55-55
58-55-55
58-55-55
55-25-55

55-5.5
55-5.5
55-55
55-55
55-55

@5onva
55-58
55-53

58-55
35-55
55-55
55-55

N x 3:883
2:883
583

N x 5883

N x 3:883
N x 3:883
3:883

2 :883

N x 3:583
N x 8:883
5883

N x 5883
5883
5883

N x 5383

5883
N 18:82.3
N x 3:883
5883

N x 3:88
3: _ 23

N x 3:283
3:883

55-55
58-55
55-55
58-55

55-55
58-55
55-55

55-55
55-55
AmvomAmzm
55-55

55-335
55-355
55-355
55-35

55-55
55-35
58-35
58-55

fivomA — v5
AmvomA _ 3m
AhvomA _ rm
55A _ :5

122

 

838.8: 55-55-55 5852 55-25-55 N x 5%; 835-: 35
83%.; 55-55-55 55.8 55-55-55 5NQ-N 835-: 35 N x 558.,” 55-55
83ng 55-55-55 832.8 55-55-55 838N§N 55-55
53.5 55-55-55 55.? 55-55-55 583 835-835 583 55-55
8325: 55-55-55 N x 5285 2 35-835 N x 5NowN 55-55
55.? 55-55-55 N x 585.». 835-55 N x 5N5.N 55835
52.8 55-55-55 N x 5855 835-55 583 835-835 58; 55-55
838.5 55-55-55 523 55-55 N x 5883 55-55
N x 553” 835-55 N x 5883 835-55 N x 553 55-55
58.5 2 35-85-55 585 835-55 N x 553N 835-55 58:3 55-55
55.3 2 35-85-55 5v:.N 835-55
8:3: 2 35-85-55 N x 58:.»- 835-55 N x 2 358.5 55-55
53.5 55-85-55 N x 58$.»- 55-55 N x 2 3285 835-55 532 55-55
833.5 55-55-55 585.8 835-55 583 2 35-55 532 55-55
55.5 55-55-55 522 835-55 N x 553 835-55 N x 58$.N 55-55
58.8 835835-55 N x 5.55 835-55 5893 835-55
838.3 55-85-55 552 55* 35 583 c 35835
5%.: 55835.55 N x 533 835-55 N x 53: 55.55
N x 5825 55-835 N x 5§N 835-55 N x 583 55-55
52.5 835A 35-55 553 55-835 58$.N 835-55 552 55-55
5N5; 55-55-55 552 835-835
8382: 55-55-55 N x 5%: 835-835 N x 53.3 c 3585 5825 55.55
52.5 55-55-55 8383N 835-55 N x 583 55-55
55.3 55-55-55 5N8N.m 835-: 35 532 55-55 N x 585N 50235
58.8 55-55-55 N x 583. 835-: 35 N x 553 835-55 552 55-55

 

.35-58.5 8.. E 8.55 ES :2 3552 2:5 .5 2...:-

123

 

58.8
52.8
55.8
58.8.
58.8
8388:
58.8
58.8
838.8

58.8
58.8
58.8:
58.8
58::
58.8
58.8
838.8
58.8

58.8
58.8
58.82

55835-55
5583555
55835835
55835835
55835835
55835835
55835-835
835835-835
835835835

835-: 35-835
835-: 35-835
835-: 35-835
835-: 35-835
835-: 35-835
835-: 35-835
835-: 35-835
835-: 35-835
835-: 35-835

835-835-: 35
55835-55
55835-835

53.8
58.8
58.8:
58.8
838.8
58.8

58.8
58.8
588:
52.8
52.8
58.8

58.8
58.8
58.8
58.8
58...:
58.8
58.8

SSOONS

838.8
58.8

: 35835-: 35

: 35835-55
: 35835-55

: 35-835835

55835.55
85835.55

835-55-835
835-55-835
83585835
83585835
835-55-835
835-55-835

835835-835
83585835
83585.: 35
83585-835
835835-835
835835-835
: 35-55835
: 35-55835
835835-835
835835-835

58.8
58.8
58.8
58.52
58.8
58.8
58.8

58.8
58.8
58.8
58.8
58.8
52.8:
58.8
58.8:
58.8
58.8
58.8
838.8

58.8
58.8

835-55-835
835-55-835
835-55-835
835-55-835
83555835
83585835
835-55-835

: 35-55-: 35
: 35-55-55
: 35-55835
: 35-55835
: 35-55-55
: 35-85835
: 35-55835
835835-55
835-55-835
83585835
5585835
835-55-835

Adam-ACE: Sow
235-55-55

58.8
58.8
58.8
5882
58.8
58.8:
58.8
55.8
: 38.8

8: :8
52.8
58.8
58.8

838.8:

58.8
58.8
58.8

838.8:

SSS-No
Gama-3

58.8
58.8

55-55-55
55-55-55
55-55-55
55-55-55
55-55-85
55-55-55
55-55-55
55-55-55
55-55-55

:5-55-55
55-55-55
55-55-55
55-55-55
55-55-55
55-55-55
55-55-55
55-55-55
55-55-55
55-55-55
55-55-55

55-55-55
55-55-55

 

Kum 035,—- 65.5.50

124

Table 5-8. Anisotropic displacement parameters (Azx 103 for CsAgo_5Bi5.5Se9. The

anisotropic displacement factor exponent takes the form: -21r2 [ h2 a*2U” + + 2 h k
a* b* U12 ].

 

 

U11 U22 U33 U23 U13 U12
31(1) 17(1) 10(1) 16(1) 0 5(1) 0
Bi(2) 18(1) 12(1) 23(1) 0 8(1) 0
Ag(2) 18(1) 12(1) 23(1) 0 8(1) 0
Bi(4) 27(1) 19(1) 33(1) 0 14(1) 0
Bi(5) 31(1) 19(1) 21(1) 0 12(1) 0
Bi(6) 33(1) 22(1) 31(1) 0 16(1) 0
Cs(1) 23(1) 24(1) 22(1) 0 6(1) 0
Se(l) 20(2) 17(2) 17(2) 0 3(1) 0
Se(2) 17(2) 14(2) 17(2) 0 6(1) 0
Se(3) 17(2) 10(1) 20(2) 0 3(1) 0
Se(4) 19(2) 1 1(1) 22(2) 0 7(1) 0
Se(S) 21(2) 12(1) 17(2) 0 7(1) 0
Se(6) 19(2) 13(1) 22(2) 0 12(1) 0
Se(7) 17(2) 1 1(1) 22(2) 0 8(1) 0
Se(8) 19(2) 17(2) 16(2) 0 8(1) 0
Se(9) 21(2) 15(2) 18(2) 0 9(1) 0

 

125

Table 5-9. Anisotropic displacement parameters (Azx 103) for RbogstgogsBlsasqu.
The anisotropic displacement factor exponent takes the form: -21r2 [ h2 a*2Un + + 2 h

 

 

k a* b* U12 ].
U11 U22 U33 U23 U13 U12

131(1) 27(1) 21(1) 18(1) 0 4(1) 0
Bi(3) 28(1) 22(1) 20(1) 0 4(1) 0
Bi(4) 27(1) 21(1) 25(1) 0 3(1) 0
Bi(5) 26(1) 21(1) 19(1) 0 4(1) 0
Se(l) 23(1) 18(1) 18(1) 0 1(1) 0
Se(2) 24(1) 24(1) 21(1) 0 2(1) 0
Se(3) 23(1) 22(1) 19(1) 0 5(1) 0
Se(4) 24(1) 21(1) 20(1) 0 5(1) 0
Se(5) 22(1) 25(1) 22(1) 0 4(1) 0
Se(6) 23(1) 23(1) 20(1) 0 6(1) 0
Se(7) 24(1) 19(1) 17(1) 0 0(1) 0
Se(8) 25(1) 18(1) 16(1) 0 5(1) 0
Se(9) 39(1) 21(1) 16(1) 0 3(1) 0
Se(lO) 25(1) 16(1) 15(1) 0 5(1) 0
Rb(l) 29(1) 36(1) 28(1) 0 5(1) 0

 

126

 

 

Table 5-10. Anisotropic displacement parameters (Azx 103) for RdeBi5Se9. The
anisotropic displacement factor exponent takes the form: -211:2 [ h2 a*2Un + + 2 h k
21* b* U12 ].
U11 U22 U33 U23 U13 U12

Bi(l)/Cd(1) 23(1) 17(1) 19(1) 0 4(1) 0
Bi(2) 23(1) 19(1) 19(1) 0 4(1) 0
Bi(3)/Cd(3) 22(1) 19(1) 20(1) 0 7(1) 0
Bi(4)/Cd(4) 18(1) 13(1) 14(1) 0 1(1) 0
Bi(5)/Cd(5) 21(1) 18(1) 22(1) 0 3(1) 0
Bi(6)/Cd(6) 18(1) 17(1) 17(1) 0 4(1) 0
Bi(7) 19(1) 18(1) 26(1) 0 6(1) 0
Bi(8)/Cd(8) 19(1) 18(1) 19(1) 0 6(1) 0
Bi(9) 17(1) 17(1) 26(1) 0 4(1) 0
Bi(lO)/Cd(10) 18(1) 15(1) 17(1) 0 4(1) 0
Bi(] 1) 18(1) 12(1) 18(1) 0 3(1) 0
Bi(12) 17(1) 12(1) 17(1) 0 1(1) 0
Rb(l) 27(2) 33(2) 32(2) 0 -1(2) 0
Rb(2) 24(2) 23(2) 26(2) 0 2(1) 0
Se(l) 21(2) 25(2) 20(2) 0 6(1) 0
Se(2) 15(2) 18(2) 19(2) 0 5( 1) O
Se(3) 19(2) 17(2) 23(2) 0 7(1) 0
Se(4) 24(2) 13(2) 18(2) 0 -2( 1) O
Se(S) 17(2) 14(1) 16(2) 0 1(1) 0
Se(6) 14(2) 15(1) 19(2) 0 -1(1) 0
Se(7) 22(2) 19(2) 17(2) 0 7( 1) O
Se(8) 19(2) 18(2) 22(2) 0 2(2) 0
Se(9) 14(2) 17(2) 21(2) 0 3(1) 0
Se(lO) 17(2) 14(1) 19(2) 0 3(1) 0
Se(] 1) 14(2) 16(2) 22(2) 0 2(1) 0
Se(12) 19(2) 15(1) 14(1) 0 3(1) 0
Se(13) 17(2) 12(1) 18(2) 0 5(1) 0
Se(14) 13(2) 19(2) 21(2) 0 3(1) 0
Se(15) 20(2) 11(1) 17(2) 0 0(1) 0
Se(16) 26(2) 14(1) 15(2) 0 -2(1) 0
Se(17) 16(2) 16(2) 17(2) 0 4(1) 0
Se(18) 25(2) 14(1) 15(2) 0 7(1) 0
Se( 1 9) 15(2) 18(2) 21 (2) 0 7(2) 0

 

127

4. Results and Discussion.

Synthesis and Crystal Growth. All three compounds were synthesized by the
molten salt (AzQx flux) method26 at high temperature. They crystallized as long thin
needles that are stable in air and water. The differential thermal analysis (DTA) studies
indicate that CsAgo,5Bi5,5Se9, RbogstogsBlsasSCg, and RdeBi5Se9 showed metastable
character. CsAgo,5Bi5_SSe9, for example, melts at 726 °C but transforms on cooling to
CsAgo,5Bi3_5Se6 and Bi28e3 while Rbo,95Cdo,35Bi5,45Se9 melts congruently at 661 °C and
RdeBlsSCg melts incongruently at ~694°C. For electrical conductivity measurements we
tried to grow large crystals of Rbo.95Cdo,3sBi5,458e9 with direct combination and using the
Bridgman technique. The obtained ingot has mixed phases of RbogstoasBisasSeg as a
major phase and RdeBiSSeg with little impurities (<10 %).

Structure Description.

Comparing with typical two dimensional layer compounds such as BizTe3, B-
CstBi3Se627 and A2[M5+,,Se9+,,] (A = Rb, Cs; M = Bi, Ag, Cd)“, the related quaternary
bismuth compounds AM6Se9 (A= Rb, Cs ; M= Bi, Ag or Cd) crystallize in a monoclinic
unique layer structures. The three phases in the AM6Se9 system are defined by modular
construction. Two slab types classified as SbZSe3-(NaCl'00) and BizTeg-(NaClm), build
up the structures by alternating stacking as a natural pseudo-superlattice, Figure 5-1. Two
Bi-Se octahedral units are interlinked between the BizTe3 type and szse3 type slabs in
four different ways such as two trans-mode and two cis-mode to SbZSe3 slab. In Figure 5-
1 the possible sites and the combinations are shown with their corresponding structures.

The structures of all compounds were confirmed by single crystal and powder X-ray

128

”“5
‘55:: if}

L
Alkali metal (A, 1”? O J“ 3,. o
. “L45? . $3.11“;

§\};
(8, C)
N}, x \K :21;

g. L k 5. s L L
1,11,1‘ . I
'. L ’, ‘ \ 5’ '
“1 J’ {‘3' p ‘, 1’,” trans formatlon
s x o. 'u ‘s k x u g. tea in ,L.\L\‘5-" '5
b) ‘~ /.‘\ I "‘x -’l\ /.\~.
9». 1 k. k. ’- ,5
\‘ \‘ ,.\~ ~ /‘\ .\ .\

 

 

 

 

Sb2363 / t; /‘
BiSe ‘ ‘ ‘3‘ ‘ \
‘3 ff??’
‘1’, d n“u>“1\‘1 "a"
L’ \‘L’ 1.1"; \ ”L (a,d) )H g VII?” )5 NET-'1’;
b sVLdmRKKK
( ,c) 1 1 1. am. 1. a
. . ‘ L x L’ r \
CIS forrnatlon 0,71.“
s37}. is
A.“
+ " O
t. x A L 1 1»
‘K 1, ,1, 1 \1 "
‘.\kk>JNk
i 1.. 1.1 ‘1 1 ‘1.
L L L K‘L“\.’\ \

Figure 5-1. Derivation of the structures; a) CSAgo.sBi3_5Se(5, b) RbogstogsBisasseg and
c) RdeBi5Se9, fiom the two BlzTC3 type slabs, two BiSe units and SbZSe3 type slabs
with difi‘erent arrays. a, b, c and d in the circles on the SbZSe3 type slab represent the

possible sites and the combinations in the parenthesis show four possible arrays of

assembly.

129

diffraction studies, and refined unit cell parameters and space groups are listed in Table
5-1. The space groups vary from P21/m to C2/m caused by doubling the size of the unit
cell based on the different arrangement of Sb28e3 slabs and Bi-Se octahedra.
CsAgo.sBi5,SSe9. The compound has a strongly anisotropic three dimensional
framework composed of two types of slabs that create parallel tunnels for the charge
balancing Cs+ ions, see Figure 5-2. Both BizTe3-type and SbZSe3-type slabs propagate
along the b-axis and are parallel to the a-axis. The Sb28e3-type slabs are composed of two
edge-sharing square pyramids. At the terminals of this slab, two bismuth selenide
octahedra are linked trans to each terminal by corner-sharing. In addition, these fragments
are connected to BizTe3-type slabs by sharing the edges of octahedra at every fourth
bismuth octahedron. All Bi atomic sites are fully occupied while two bismuth sites, the
Bi(2) and Bi(3) sites in the BizTeg-type layer, are co-occupied with Ag atoms at a rate of
bismuth atoms 86.3% and 63.7% respectively to preserve charge neutrality (Table 5-2).
In the SbZSeg-type slab, the Bi(5) atom is in vertically distorted square pyramidal
coordination with one short Bi-Se bond at 2.695(4) A and four long but almost equal Bi-
Se bonds between 2.937(2) and 2.958(2) A. Furthermore, Bi(5) has a long interaction to
the nearest Se(l) on the BizTeg-type layer with 3.809(7) A distance. This is too far to be
considered bond. The Bi( 1) atom is in a distorted octahedral coordination resembling a
trigonal pyramid, where the Bi centers move towards one octahedron face, with three
short bonds between 2.719(3) and 2.767(2) A trans to three longer ones between

3.1217(3) and 3.239(3) A. (Table 5-5)

130

BlzTe3
type slabs

$132363
type slabs

 

edge sharing

comer sharing

   
  
 
 
    

Bi-Se
octahedron
Trans-mode

of linking

Figure 5-2. Projection of the three dimensional structure (top) and polyhedral
representation (bottom) of CsAgasBi5.5Se9 down the b-axis. The Cs ions are in the
tricapped trigonal prismatic spaces.

~~: 11;.

131

All Bi atoms in the BizTe3-type layer are in distorted octahedral coordination with
Bi-Se distances between 2.821(3) and 3.122(3) A. Bi(3) is in the least distorted
octahedral coordination with Bi-Se varying between 2.830(3) and 3.038(3) A while most
others (Bi(2), Bi(4), and Bi(6)) are in trigonal pyramidal distorted octahedra. For example,
Bi(2)/Ag(2) has three short bonds between 2.821(3) and 2.872(2) A trans to three longer
ones between 2.999(2) and 3.122(3) A. The Ag(3) atom is found further towards one
octahedron face than any other atoms with three shorter bonds between 2.778(12) and
2.783(8) A, Figure 5-3. The Cs+ atoms are in tricapped trigonal prismatic coordination

with Cs—Se distances between 3.622(3) and 3.825(3) A.

Sel 56.4
2.821(3) A 975(3) °
2.714(4) A

 
 

2.778(12) A

  
  
  
   

2.872(2) A

   
  

9331(8) A
Se2

 

 

2.999(2) A 953(4) °

Bi2/AgZ 3.122(3) A

562

Figure 5-3. A scheme of local coordination environment of Bi(2)/Ag(2), and
Ag(3)/Bi(2) atoms in CsAgo,5Bi5.SSe9.

Rbo,95Cdo.35Bi5,458e9. The RbogstogsBii-tsSeg phase is also a strongly anisotropic
three dimensional framework composed of NaCl-(NaCllOO) and BizTe3-(NaC1m) type
slabs, that are piled up alternatively producing parallel tunnels for the charge balancing
Rb+ ions, see Figure 5-4. It is interesting to note that the Rbogstostlsasseg phase

crystallizes in the space group C2/m while previous CsAgo,58i5_SSe9 phase has PZI/m

132

Bi2T63 type

slabs
Se $132863

t l b
$12 ype s a s

 

comer sharing

   
   

Trans-mode

of linking
\

—- Bi-Se
octahedron

edge sharing

Figure 5-4. Projection of the three dimensional structure(top) and polyhedral
representation(bottom) of Rbo_95Cdo.35Bi5.45Se9 down the b—axis. The Rb ions are in the
bicapped trigonal prismatic space.

133

symmetry, nearly double the size of the c-axis of CsAgo.sBi5,5Seg phase. The symmetry
and unit cell change is caused by a slightly different formation of the NaCl-(NaCl'OO) slab
composed of a Sb2Se3-type slab and two Bi-Se octahedra.

Interestingly, Rbo.95Cdo,35Bi5_45Se9 is constructed in nearly the same way as the
sulfosalt minerals cannizzarite (Pb46Bi54S127)28 and PbsBi6Se1429 in which NaCl-(NaCllOO)
and BizTe3-(NaCll l I) type slabs build up the structures by alternate stacking in a 1:1 ratio.
Since the periodicity of each NaCl100 and NaCl”' sublattice is different, their internal
binding is “out of joint”, that is they are incommensurate. This affects the inner NaC]loo
slab, which is distorted. Therefore, in PbsBi6Se14, bicapped trigonal prismatic
coordination for the Bi atoms is created at every 5th Bi site on the NaCl100 type slab. All
Bi atoms on the NaCl100 type slab have either distorted square pyramidal (or augmented
trigonal prismatic coordination when bonded to two more Se atoms in the neighboring
slab.) or octahedral coordination, see Figure 5-5.

In Rbo_95Cdo,35Bi5,45Se9 Rb+ ions are in bicapped trigonal prismatic coordination sites
created at every 3“! bismuth site on the NaCl100 type slab. The Bi atoms in the NaCl'00
type slabs have distorted square pyramidal coordination and octahedral coordination.
These slabs interconnect to the BizTe3-(NaC1m) type slabs through the corners of the
terminal Bi octahedra.

All atomic sites are fully occupied while Bi(3), Bi(5) and Rb(l) are partially
occupied at the fraction of 92%, 90%, and 95%, respectively. Moreover, Bi(2) split with
Bi(22) and Bi(6) is co-occupied with Cd(6) in the ratio (x : y) (Table 5-3). The formally
Cd2+ ions are unusually situated in an octahedral environment of Se atoms instead a

tetrahedral environment.

134

} BizTe3 type slabs

} NaClloo type slabs

   

633 >3” "‘vvév-‘lb / 2%

’ ' ’ %fifim _- [A a
Mum-55.11:: 1mm. \

a // ,./; W/ , / /:/" a 100 e
@AXXS;>o%%xx% ......

M6 M7M8 M9M10M1

 

Figure 5-5. Projection of the structure of Pb5Bi5Se14 down the b—axis (top) and
polyhedral representation of PbsBi5Se14 down the b—axis (bottom). M5 in a circle is in

a bicapped trigonal prismatic space.

135

All the Bi atoms are in distorted octahedral coordination (toward a trigonal
pyramid) at a range between 2.728(9) and 3.235(9) A but Bi(4) on NaCl100 type slab is in
square pyramidal coordination with Bi—Se distances at 2.7139(19) — 3.0658(13) A and
two additional longer interactions with Se(3) atoms in NaCll“ type slabs at 3.709(36) A
(Table 5-6). Cd(6) mixed with Bi(6) is further toward a trigonal pyramid in the distorted
octahedral with three short bonds between 2.714(4) and 2.730(5) A trans to three longer
bonds between 3.058(6) and 3.185(3) A, see Figure 5-6. The Rb(1)-Se distances vary

from 3.400(3) to 3.562(2) A.

 

 

Se4
a2.730(5) A 2.778(2) A
a2.714(4) A

Cd6/Bi6

SeS
Se6

2.8588(19) A

3.0157(15) A

Figure 5-6. A scheme of local coordination environment of Cd(6)/Bi(6) atoms in
RbogstogsBisasseg. (a Cd-Se bonds).

RdeBisseg. This compound has common structural characteristics with the

above members but features no Bi-Se links between the slabs. The structural arrangement

136

of the SbZSeg-type slabs is such that it causes a doubling of the crystallographic c-axis in
Rbo.95Cdo,35Bi5,458e9. The trans-mode of linking of the Sb28e3-type slab to the BizTe3
slabs has already been described. In RdeBi58e9 this mode is different adopting a cis-
type of linking with sharing the comer and the edge of the octahedron between the
BizTe3-slab and the Sb28e3-type slab. The trans-mode of linking cross-links the infinite
B12T63 slabs into a three-dimensional framework. In RdeBi5Se9 the cis-mode of linking
does not cross—link the slabs and maintains the two-dimensional character of the
compound. This creates bonding to only one side of the BizTC3-(N3C1Hl) type slab which
generates a 21 screw operation. The Rb+ ions are in capped trigonal prismatic spaces with
distances at 3.556(5) — 3.775(4) A for Rb(l) and at 3.405(5) — 3.569(4) A for Rb(2), see
Figure 5-7 and Table 5-7.

The RdeBisSCg has Cd atoms mixed in seven bismuth sites at the fraction of
26%, 32%, 26%, 40%, 20%, 31% and 24%, which maintain charge neutrality (Table 5-4).
Bi(7) and Bi(9) atoms have square pyramidal coordination with Bi-Se distance at
2.676(3) — 3.044(2) A and longer interaction with Se(9) and Se(2) in BizTe3-(NaClm)
type slab at 3.825(5) and 3.661(4) A, respectively. All other Bi atoms are in distorted
octahedral coordination with varying angles and distances ranging between 82.78(7)° and

96.88(10)° for Se-Bi-Se angles, and 2.732(3) and 3.196(2) A for Bi-Se bond lengths.

Charge Transport Properties and Energy Gaps. The energy gaps of the
members of the AM6Se9 series were measured at room temperature using mid-infrared
spectroscopy. The presence of energy gaps clearly show that the compounds are narrow

gap semiconductors. The two rubidium compounds are almost the same due to their

137

Se7 Se3 Se2 Se5 SelOSel4Se9 Se4 Se6 SellSel9

«mum-gw we

Bi12 Bi7Sel3
Se15~ Sb2S63 type
"Se 8 ‘
L. slabs
Rb] 319 Rb2
”Q We ‘Q
Bis Q
l I. l '
Bi4Bi10 Bi8
. L

 

cis mode of
linking

 
  
 
  
  

edge sharing
comer sharing

_ Bi-Se
octahedron

Figure 5-7. Projection of the two dimensional structure(top) and polyhedral
representation(bottom) of RdeBIsSCg down the b-axis. Shaded rectangle area show cis
formation between two Bi-Se octahedra and Sb2S63 type slab.

138

 

 

 

 

b) I ' Energy(eV)

 

d
o
l A

a:
1

A

 

 

 

O)

IIIIIIIIIIIIIIII

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

C) Energy(eV)

 

_A .s N N
O) on O N
1 1 . l . 1

A

A
A l A
t

12‘

Absorption coefficient (a/S)

A
o
J A

 

 

/
070 Y 071 1 0:2 ' 0.3 V 0T4 - 075 . 0:6 ' 0:7 . 0.8
Energy(eV)

 

 

Figure 5-8. Solid-state infrared absorption spectra showing band gap transitions for a)
CsAgo_5Bi5,5Se9 at 0.30 eV, b) Rbo,95Cdo.3sBi5_45Se9 at 0.51 eV, and c) RdeBi5Se9 at
0.49 eV. The band gaps in each case are estimated from the crossing point of solid lines

shown in each spectrum.

139

5

3 4

2

HllHHjillllllll|HH||HHHH‘HH

-

1cm

lllljilli'll

I
(I)

 

Figure 5-9. Ingot of RbogstogsBisasseg grown in a Bridgman furnace. The ingot was
cut along the direction parallel and perpendicular to the crystal growth.

140

 

J

L. .

01 01

o o o

l J I L
I

I

Seebeck Coefficient (uV/K)

 

 

 

300 T 400 ' 500 600 ' 700 1 800
Temperature (K)

r

Figure 5-10. Temperature dependence of the Seebeck coefficient for a single crystal of
Rb0.95Cd0.3sBis.45$e9-

141

structural equivalence. CsAg0_5Bi5,5Se9 exhibits the narrowest energy band gap at ~ 0.30
eV, see Figure 5-8. Rbo.95Cdo.35Bi5_45Se9 and RdeBi5Se9 have band gaps of ~05] and
0.49 eV, respectively.

Preliminary charge transport measurements of RbogstogsBisASSeg among the
three phases in the AM6Se9 system were carried out. It was not possible to obtain a pure
phase of high purity due to the thermal behavior, which showed incongruent melting.
The electrical conductivity of Rbo_95Cdo,35Bi5,45Se9 was measured on a well oriented
polycrystalline ingot grown from a vertical Bridgman grth technique”, Figure 5-9.
We observed high electrical conductivity of ~870 S/cm along the direction of crystal
growth, b-axis, at room temperature, which is consistent with 3 doped narrow band gap
semiconductor. In addition, thermopower measurements were also performed in the
direction of crystal growth at the temperature between 300 and 800 K. The thermopower
of Rb0,95Cd0_35Bi5,45Se9 was negative and increased almost linearly from -60 to -157
,uV/K in between 300 and 700 K then decreased fast, see Figure 5-10. The decrease in
thermopower beyond 700 K may be due to decomposition.

Interestingly, the Pb5Bi6Se14 introduced above with structural similarity, has
electrical conductivity of 657 S/cm, thermopower of -l3l uV/K and extremely low

thermal conductivity of less than 1.0 W/m-K at room temperature.28b In comparison with

the Pb5Bi6Sel4, the Rbo_95Cdo,35Bi5.45Se9 has also n-type semiconductor character but
more electrons as the charge carriers consistent with the lower thermopower and the
higher electrical conductivity. This could be due to changes of a few metal sites on the
NaCl'00 slab with Rb+ ions, and compositional complexities such as mixed or empty

metal sites. Moreover, thermal conductivities in this structure or even the AM6Se9 series

142

may be expected to be lower due to their structural and compositional similarities to that

of PbsBi6Sel4 and the extra contributions of Rb+ ions in the tunnel as rattlers.

Concluding Remarks

An outstanding demonstration of structural diversity and complexity was
established in the new quaternary AM6Se9 (A= Rb, Cs; M= Bi, Ag or Cd), which
includes CsAgo.5Bi5,5Seg, Rbogstostisasng and RdeBi5Se9. These compounds

11 l I) type structures retaining distinct

crystallize in pseudo two dimensional BizTe3(NaC
NaCl'OO type building blocks in a systematic way. The small changes in structure and
formula affect the space group, cell parameters and even energy band gaps. Interestingly,
the crystal structure of Rbo,95Cdo,35Bi5,45Se9 showed similar structural features to
PbsBi6Se14, which can be considered a derivative of this series by substituting an alkali
metal ion for a metal ion in the tricapped trigonal pyramidal site. Comparing the
preliminary transport properties shows that RbogstonBisasSeg is heavily doped with
electron carriers and exhibits lower thermopower and high electrical conductivity. This
heavy doping could be caused by structural and compositional defects. In addition,
thermal conductivities in this family may be much lower due mainly to extra
contributions of Rb+ ions in the tunnel as a rattler. To better understand the AM68e9
series from the thermoelectric point of view with relation to structural and compositional
diversities, more investigations with both alkali metal ions and various mono or divalent

metal ions need to be conducted, and the synthetic methods such as crystal growth

conditions need to be optimized.

143

References

l a) CRC Handbook of Thermoelectric Materials. Rowe, D.M. Bd., CRC Press, Inc.:
Boca Raton, FL, 1995 b) Polvani, D. A.; Meng, J. F.; Shekar, N. V. C.; Sharp, J. and
Badding, J. V. Chem. Mater. 2001, 13, 2068. c) Kanatzidis, M.G.; Mahanti, S. D.; Hogan,
T. P. Chemistry, Physics, and Materials Science of Thermoelectric Materials: Beyond
Bismuth T elluride.; Kluwer Academic/Plenum Publishers: New York, 2003; p 35. d)
Shelimova, L. B.; Karpinskii, O. G.; Svechnikova, T. E.; Avilov, E. S.; Kretova M. A.
and Zemskov, V. S. Inorg. Mater. 2004, 40, 1264-1270.

2 ZT = SZGT/K, where S is the Seebeck coefficient, 0 is the electrical conductivity, T is
the temperature and K is the thermal conductivity, which includes electron and phonon
contributions.

3 a) Hicks, D.; Dresselhaus, M.S. Phys. Rev. B, 1993, 4 7, 12727-12731. b) Hicks L. D.;
Dresselhaus M. S. Phys. Rev. B 1993, 4 7, 16631-16634. c) Hicks L. D.; Harman T. C.;
Dresselhaus M. S. Appl. Phys. Lett. 1993, 63, 3230-3232.

4 a) Slack, G. A. “New materials and Performance Limits for Thermoelectric Cooling” in
CRC Handbook of Thermoelectrcis edited by Rowe, D. M. CRC Press, Boca Raton, 1995,
407-440. b) Slack, G. A. in “Solid State Physics”, eds. Ehrenreich, H.; Seitz, F. ;Turnbull,
D. Academic, New York. 1997, Vol. 34, 1.

5 Venkatasubramanian, R.; Siivola, B.; Colpitts, T.; O’Quinn, B. Nature 2001, 413, 597-
602.

6 Harman, T. C.; Taylor, P. J .; Walsh, M. P.; LaForge, B. E. Science 2002, 297, 2229-
2232.

7JP. Fleurial et. al. Proc. 15th Int. Conf on T hermoelectrics. IEEE, Piscataway, NJ,
1996.

8 Chung, D.-Y.; Jobic, S.; Hogan, T.; Kannewurf, C. R.; Brec, R.; Rouxel, J .; Kanatzidis,
M. G. J. Am. Chem. Soc. 1997, 119, 2505-2515.

9 a) Chung, D.-Y.; Hogan, T.; Brazis, P.; Rocci-Lane, M.; Kannewurf, C. R.; Bastea, M.;
Uher C.; Kanatzidis, M. G. Science 2000, 287, 1024-1027. b) Chung, D.-Y.; Hogan, T.;
Brazis, P.; Rocci-Lane, M Brazis, P.; Ireland, J. R.; Kannewurf, C. R.; Bastea, M.; Uher
C.; Kanatzidis, M. G. J. Am. Chem. Soc. 2004, 126, 6414-6428.

10 a) Chung, D.-Y.; Choi, K-.S.; Iordanidis, L.; Schindler, J. L.;Brazis, P. W.; Kannewurf,

c. R.; Chen, B.; Hu, 8.; Uher c.; Kanatzidis, M. G. Chem. Mater. 1997, 9, 3060-3071.
(b) Kanatzidis, M. G.; DiSalvo, F. J. Nav. Res. Rev. 1996, 4, 14-22.

144

 

“ a) Hsu, K. -F.; L00, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.;
Polychroniadis. E. K.; Kanatzidis, M. G. Science 2004, 303, 818-821 b) Quarez, B.; Hsu,
K.-F.; Pcionek, R. ; Frangis, N. ; Polychroniadis, E. K.; Kanatzidis, M. G. J. Am. Chem.
Soc. 2005, 127, 9177-9190.

'2 a) Kanatzidis, M. G. Semicond. Semimet. 2001, 69, 51400. b) Chung, D.-Y.;
Iordanidis, L.; Choi, K.-S.; Kanatzidis, M. G. Bull. Kor. Chem. Soc. 1998, 19, 1283-1293
13 a) Takagi, J. Takeuchi, Y. Acta Crystallogr. 1972, B28, 369 b) Makovicky, E.,Neues
Jahrb. Mineral. 1989, 160, 269.

‘4 Zakrzewski, M. A.; Makovicky, E. Can. Mineral. 1986, 24, 7.

'5 Ilinca, G.; Makovicky, E. Eur. J. Mineral. 1999, 114,691.

‘6 a) Mrotzek, A.; Kanatzidis, M. G. Acc. Chem. Res., 2003, 36, 111-119. b) Kanatzidis,
M. G. Acc. Chem. Res., 2005, 38, 359-368.

‘7 Hsu, K. F.; Lal, S.; Hogan, T.; Kanatzidis, M. G. Chem. Commun. 2002, 13, 1380-
1381.

18 Poudeu, P. F. P.; Kanatzidis, M. G. Chem. Commun., 2005, 21, 2672-2674.

‘9 Kim, J.—H.; Chung, D.-Y.; Kanatzidis, M. G. Chem. Commun., 2006,15, 1628-1630,
or See Chapter 3.

2" Choe W.; Lee 8.; O’Connell P.; Covey A. Chem. Mater. 1997, 9, 2025-2030.

2' Kyratsi, T.; Chung, D.-Y.; Choi, K.-S.; Dick, J. S.; Chen, W.; Uher, C. and Kanatzidis,
M. G. Mat. Res. Soc. Symp. proc. 2000, 626, Z8.8.l- 28.8.6.

22 . Kubelka-Munk function: or/S = (l-R)2/2R, where a is the absorption coefficient, S is
the scattering coefficient, and R is the reflectance at a given wavenumber.

23 X-RED 1.22, Program for data reduction; STOE & Cie: Darmstadt, Germany, 2001.

24 X-SHAPE 1.06, Program for crystal optimization for numerical absorption correction;
STOE & Cie: Darmstadt, Germany 1999.

25 SMART, SAINT, SHELXTL: Data Collection and Processing Software for the
SMART-CCD system; Siemens Analytical X-ray Instruments Inc.: Madison, WI, 1997.

26 Kanatzidis, M. G.; Sutorik, A. C. Prog. Inorg. Chem. 1995,43, 151-265.

27 a) Chung, D.-Y.; Iordanidis, L.; Rangan, K. K.; Brazis, P. W.; Kannewurf. C. R.;
Kanatzidis, M. G. Chem. Mater. 1999, 11, 1352-1362.

145

 

2" Matzat, E. Acta Cryst. 1979, B35, 133-136.

29 a) Zhang, Y.; Wilkinson, A. P.; Lee, P. L.; Shastri, S. D.; Shu, D.; Chung, D.-Y.;
Kanatzidis, M. G. J. Appl. Cryst. 2005. 38, 433-441. b) Chung, D.-Y.; Malliakas, C.;
Pcionek, R.; Park, S.-M.; Breshears, J. D.; Billing, S.; Kanatzidis, M. G. unpublished
data.

146

CHAPTER 6

Structural Diversity and Characterization of the Quaternary
Bismuth chalcogenide AM4Q6, A2M4Q6 and A2M6Q9 (A = K, Rb,
Cs ; M = Bi, Ag, Cu, Cd; Q = S, Se)

1. Introduction

Synthetic exploratory efforts, focused on complex ternary and quaternary bismuth
chalcogenide compounds', have resulted in many new bismuth chalcogenide
compoundsz'6 One of the notable features in these compounds is that they are built with
relatively few common structural motifs, such as NaCl-(NaCllOO), szSeg-(NaClloo),
BizTe3-(NaClm), CdIz-(NaCll 1 I) and galena types(NaCl3 ' 1), all of which are based on the
NaCl-type structure, but derived by excising along different directions of the NaCl
structure type. When compounds can be identified in a simple way such as homolog
series 7 , tropochemical cell-twinning 8 , and structural polymorphism 9 , it helps to
understand large classes of materials, thereby allowing useful generalizations and
predictions.

Interestingly, these multifonn building components are originated from
multifarious Bi-Q coordinations, such as square pyramidal and octahedral shape, due to
hybridization of the 682 pair of electrons with p orbitals. The degree of this hybridization
can cause either stereochemical distortion in the bismuth coordination (when sp3
hybridization is present), or the adoption of a symmetric octahedral coordination

geometry (caused by hybridizing with energetically adjacent p and d orbitals). Metals

147

such as Ag, Pb, Sn, Sb, and even alkali metal ions,2b’3b's‘i'c'f’a’l0 have been frequently
observed in mixed site occupancy with bismuth atoms, creating compositional
complexities. However, the alkali metal bismuth chacogenide compounds with Cd and
Cu atomsll have not been extensively studied.

Here, we will introduce several alkali metal chalcogenide compounds including
Ag, Cd, and Cu ions with their unique structural character following three distinct
formulae AM4Q6, A2M4Q6 and A2M6Q9. These are fl'CSAg05Bi3jse6, K1,36Ago,93Bi3,o7S(,,
K1.84Ag0.9zBi3.08566, Rb1.7Ag0.8sBi3.1556, Rb1.6A80.83i3.2Se6, CSr.7Ago.8sBi3.1536,
C81.5Ag0.7sBi3.25566, Rb134Cd134Bi26656, K1.22Cd1.2213i2.7856, Rb2CUBi3SG6, C82CUBi3S6,
and Rb2_76AgO.69Bi4_8SSC9. In addition, we present the synthesis, crystal growth, ion-
exchange feature, physicochemical, spectroscopic, and structural characterization.
Especially, the B—CsAngiysSeé compounds will be evaluated with their potential as
thermoelectric materials with various x. For the layered compounds (Rb1,7Ago_35Bi3,.5S6,
Rbl.6Ago_gBi3,ZSe6), ion exchange of large Rb+ ion with transition metal Ag and Pb2+ ions

in nitrate solution will be reported.

148

2. Experimental Section

Reagents. Chemicals were used as obtained: bismuth chunks (99.999% Noranda,
Canada), sulfur powder (sublimed, Spectrum Chemical Mfg. Corp., Gardena, CA), Se
shots (99.999% Noranda, Canada), K (rod 99.5% purity, Aldrich, Milwaukee, WI), Rb
(99.8% purity, Alfa Aesar, Ward Hill, MA), Cs (99.98% purity, Alfa Aesar, Ward Hill,
MA), Cadmium powder (99.999%, -200mesh Cerac). Copper powder (Fisher Scientific
Company, Fair Lawn, NJ .).

Ag Powder. A silver coin (99.999%) was dissolved in nitric acid. The solution
was neutralized to a pH of 7 with ammonium hydroxide. Sodium borohydride was added
to reduce the Ag ions to a black precipitate of Ag metal powder. The precipitate of silver
was filtered and washed thoroughly with water and dried in a vacuum oven at 150 °C.
The obtained fine powder of Ag was identified by powder X-ray diffraction.

Synthesis. All manipulations were carried out under a dry nitrogen atmosphere in
a Vacuum Atmospheres Dri-Lab glovebox and in a Schlenk line. For all compounds the
yield was quantitative. The purity and homogeneity of the products were verified by
comparing the X-ray powder diffraction patterns to those calculated by the
crystallographically determined atomic coordinates. A2Q (A = K, Rb, Cs; Q = S, Se)
were obtained by stoichiometric reactions of elemental alkali metals and sulfur(or
selenium) in liquid NH3. The purity and homogeneity of the products were verified by
comparing the X-ray powder diffraction patterns to those calculated by the

crystallographically determined atomic coordinates.

149

,B-CsAgo5Bi358e5. A mixture of 1.1868 g (8.93 mmol) of Cs, 0.4816 g (4.47
mmol) of Ag, 6.5317 g (31.26 mmol) of Bi and 4.2307 g (53.58 mmol) of Se was loaded
in a fused silica tube (carbon coated 13 mm diameter) and subsequently flame-sealed at a

residual pressure of <104 mbar. The tube was heated at 200 °C for 4 h then 750 °C for 48

h and cooled to 50 °C in 10 h. The product consisted of a silvery-black ingot made of

needles. A quantitative microprobe analysis using Energy Dispersive Spectroscopy
(EDS) performed on 3 Scanning Electron Microscope (SEM) on several needles gave an
average composition of Cso,9gAgo_43Bi3,5;Se6. . In order to grow highly oriented crystal
specimens for thermoelectric property measurements, the product was ground and loaded
in a silica tube (13 mm diameter carbon coated) with a point end and sealed under
vacuum. The tube was heated to 750 °C in a Bridgman furnace and descended at a rate of
3.25 mm/h through a sharp (100 °C/cm) temperature gradient.‘2 A pure and well oriented
ingot (25 mm long, 11 mm diameter) of ,B-CsAgo, 5Bi3,5Seé was obtained.
K1,36Ago,93Bi3,o-;Sg. A mixture of K2S powder (0.1323 g, 1.2 mmol) and elemental
Ag powder (0.0431 g, 0.4 mmol), Bi283 (0.8226 g, 1.6 mmol), and S (0.0641 g, 2 mmol)
was transferred to a silica tube which was flame-sealed under vacuum. The tube was
heated for 3days at 750 °C, then cooled to 550 °C in 20h, and subsequently cooled to
50 °C in 100h. Silvery-black plate type crystals of K;_36Ag0,93Bi3,o7S6 (> 95%) and
silvery-black needle type crystals of K4_35Bi7,0581313 were obtained after isolation in
dimethylformamide (DMF) and washing with methanol and diethyl ether. SEM/EDS
analysis on several single crystals of K1_36Ag0,93Bi3,07S6 showed the approximate

composition of K2,45Ag0_598i3_05S6.

150

K1,34Ago,92Bi3,osse6. A mixture of K2Se powder (0.943 g, 0.6 mmol) and
elemental Ag powder (0.0431 g, 0.4 mmol), Bi2$e3 (0.5239 g, 0.8 mmol), and Se (0.0790
g, 1 mmol) was prepared and treated in the same manner as in compound
K1,36Ago.93Bi3_o7S6. SEM/EDS analysis on several single crystals, silvery-black plate type,
of KLgaAgongiyogSe6 showed the approximate composition of K1_11Ago.57Bi3.|3Se6.

Rb1,7Ago,35Bi3,1586. A mixture of Rb2S powder (1.5225 g, 7.5 mmol) and
elemental Ag powder (0.5393 g, 5 mmol), Bi (4.1796 g, 20 mmol), and S (1.3626 g, 42.5
mmol) was loaded in a firsed silica tube (13 mm diameter) and subsequently flame-sealed
under vacuum. The tube was heated at 750 °C for 1h with rocking (30 min), then cooled
to 550 °C in 20h, furthermore cooled to 50 °C in 10h. Silvery-black plate type crystals of
Rb1_7Ag0,8sBi3,15S6 were obtained after isolation in dimethylformamide (DMF) and
washing with methanol and diethyl ether. SEM/EDS analysis on several single crystals of
Rb1.7Ago,35Bi3,15S6 showed the approximate composition of Rb2,o3Ago,gBi3,19$6.

Rb1,6Ago,38i3,2Se6. A mixture of Rb2Se powder (1.1245 g, 4.5 mmol) and
elemental Ag powder (0.3236 g, 3 mmol), Bi (2.5078 g, 12 mmol), and Se (2.0135 g,
25.5 mmol) was loaded in a fused silica tube (13 mm diameter) and subsequently flame-
sealed at a residual pressure of <104 mbar. The tube was heated at 750 °C for 72 h,
followed by cooling to 550 °C at a rate of 10 °C/h then to room temperature in 100 h. A
silvery-black plate type polycrystalline ingot of Rb1,6Ago,gBi3,2Se6 (yield >95%) and
unidentified impurities were obtained after isolation in dimethylformamide (DMF) and
washing with methanol and diethyl ether. SEM/EDS analysis on several single crystals of

Rb1,6Ago_3Bi3,2Se6 showed the approximate composition of Rb1,g3Ago,g6Bi3_63Se6.

151

Csr,7Ago,gsBi3,1585. A mixture of Cszs powder (0.0596 g, 0.2 mmol) and
elemental Ag powder (0.0431 g, 0.4 mmol), Bi (0.3344 g, 1.6 mmol), and S (0.1090 g,
3.4 mmol) was prepared and heated in the same manner as in compound
waAgonBiyzSeé. Silvery-black plate type crystals of Cs].7Ago,35Bi3,1586 (yield ~50%)
and needle type Bi283 were obtained after isolation in dimethylformamide (DMF) and
washing with methanol and diethyl ether. SEM/EDS analysis on several single crystals of
Csl,7Ago_35Bi3_.5S6 showed the approximate composition of Cs|,4Ago.53Bi3,57S6.

Cs1,5Ago,7sBi3,258e6. A mixture of C82Se powder (0.2586 g, 0.75 mmol) and
elemental Ag powder (0.0539 g, 0.5 mmol), Bi (0.4180 g, 2 mmol), and S (0.3356 g, 4.25
mmol) was prepared and heated in the same manner as in compound Rb1_6Ago.3Bi3.2Se6.
Silvery-black plate type crystals of Cs1,5Ago,75Bi3_258e6 were obtained after isolation in
dimethylformamide (DMF) and washing with methanol and diethyl ether. SEM/EDS
analysis on several single crystals of Cs.,5Ag0,75Bi3,25Se6 showed the approximate
composition of Cs1,43Ago,4gBi3,53Se6.

Rb134CdIMBiM686. A mixture of Rb2S powder (0.1827 g, 0.9 mmol) and
elemental Cd powder (0.0674 g, 0.6 mmol), Bi (0.5016 g, 2.4 mmol), and S (0.1635 g,
5.1 mmol) was loaded in a fused silica tube (9 mm diameter) and subsequently flame-
sealed at a residual pressure of <10'4 mbar. The tube was heated at 750 °C for 72 h,
followed by cooling to 550 °C at a rate of 5 °C/h then to room temperature in 10 h. A
silvery-black plate type polycrystalline ingot of RbL34CdL34Bi266S6 (60%) and needle
type Bi2S3 were obtained after isolation in dimethylformamide (DMF) and washing with
methanol and diethyl ether. SEM/EDS analysis on several single crystals of

Rb1,34Cd1,34Bi2_6686 showed the approximate composition of Rb.,54Cdo,9Bi2,g6S6.

152

K1,22Cd1,22Biz,-;3$5. A mixture of K28 powder (0.0882 g, 0.8 mmol) and elemental
Cd powder (0.0450 g, 0.4 mmol), Bi2S3 (0.8226 g, 1.6 mmol), and 8 (0.0641 g, 2 mmol)
was prepared and treated in the same manner as in compound Rb1,34Cd1,34Bi2.66S6. The
product consisted of silvery crystalline plates of KLZQCdLZZBiZJBSfi. SEM/EDS analysis
on several plates gave an average composition of K|_5Cdo_74Bi3.5;S6.

szCuBi3Se5. A mixture of Rb2Se powder (0.7497 g, 3 mmol) and elemental Cu
powder (0.1906 g, 3 mmol), Bi2Se3 (2.9468 g, 4.5 mmol), and Se (0.1184 g, 1.5 mmol)
was loaded in a fused silica tube (13 mm diameter) and subsequently flame-sealed at a
residual pressure of <104 mbar. The tubes were heated at 800 °C for 72 h, followed by
cooling to room temperature at a rate of 10 °C/h. A silvery-black needle type
polycrystalline ingot of Rb2CuBi3Se6 was obtained. SEM/EDS analysis on several single
crystals of Rb2CuBi3Se6 showed the approximate composition of Rb2_26Cu1_22Bi2,92Se6.

CszCuBi386. A mixture of Cszs powder (0.2383 g, 0.8 mmol) and elemental Cu
powder (0.0254 g, 0.4 mmol), Bi (0.3344 g, 1.6 mmol), and S (0.109 g, 3.4 mmol) was
loaded in a fused silica tube (9 mm diameter) and subsequently flame-sealed at a residual
pressure of <10'4 mbar. The tube was heated at 750 °C for 72 h, followed by cooling to
550 °C at a rate of 10 °C/h then to room temperature in 20 h. A silvery-black needle type
polycrystalline ingot of CszCuBi3S6 was obtained after isolation in dimethylformamide
(DMF) and washing with methanol and diethyl ether. SEM/EDS analysis on several
single crystals of CszCuBi3S6 showed the approximate composition of C8326Cu1,ogBiz,7.S6.

Rb2,76Ago,69Bi4358e9. A mixtures of Rb2Se powder (0.1999 g, 0.8 mmol) and
elemental Ag powder (0.0216 g, 0.2 mmol), Bi (0.3344 g, 1.6 mmol), and Se (0.2685 g,

3.4 mmol) was loaded in a fused silica tube (9 mm diameter) and subsequently flame-

153

sealed under vacuum. The tube was heated at 750 °C for 1h with rocking, followed by
cooling to 550 °C at a rate of 5 °C/h, furthermore cooled to 50 °C in 50h. Silvery-black
needle type crystals of Rb2.76Ago,69Bi4_35See (~60%) and unidentified impurities were
obtained after isolation in dimethylformamide (DMF) and washing with methanol and
diethyl ether. SEM/EDS analysis on several single crystals of Rb2,76Ag0.69Bi4,358e9

showed the approximate composition of Rb2,21Ag0,37Bi5_12Se9.

3. Physical measurements

Electron Microscopy. Quantitative microprobe analysis for the compounds was
performed with a JEOL JSM-6400V Scanning Electron Microscope (SEM) equipped
with a Noran Vantage Energy Dispersive Spectroscopy (EDS) detector. Data were
collected for 30 sec using an accelerating voltage of 20kV. All reported results are an
average of measurements on at least three different crystals.

Differential Thermal Analysis. Differential thermal analysis (DTA) was
performed with a computer-controlled thermal analyzer (Shimadzu DTA-50). 20 mg of
ground crystals were sealed in silica ampoule under vacuum. A silica ampoule containing
the equal mass of alumina was placed on the reference side of the detector. The sample
was heated to the desired temperature at 10 °C/min, isothermed for 2 min and then cooled
at 10 °C/min. The heating program was recycled to check reproducibility of the thermal
behavior of the sample. The reported melting point is the peak temperature. After DTA,
the sample was examined by powder X-ray diffraction to identify if any decomposed

product formed during heating/cooling cycles.

154

Solid-State UV/vis Spectroscopy. Optical diffuse reflectance measurement was
made at room temperature with a Shimazu UV-3101 PC double-beam, double-
monochromator spectrometer operating in the 200 ~ 2500 nm region. The instrument was
equipped with an integrating sphere and controlled by a personal computer. BaSO4
powder was used as reference (100% reflectance). Absorption data were calculated from
the reflectance data using the Kubelka-Munk function. '4

Infrared Spectroscopy. Optical diffuse reflectance measurements were made on
the finely ground sample at room temperature. The spectrum was recorded in the infrared
region (6000-400 cm") with the use of a Nicolet MAGNA-IR 750 Spectrometer equipped
with a Collector Diffuse Reflectance of Spectra-Tech. Inc. The reflectance versus
waventunber data were used to estimate a material’s band gap by converting reflectance
to absorption data as described previously.

Charge transport measurements. The Seebeck coeffiecient of polycrystalline
samples was measured between 300 and 700 K by using a SB-lOO Seebeck Effect
Measurement System, MMR Technologies. The electrical conductivity measurements
were performed in the usual four-probe geometry at room temperature.

Powder X-ray Diffraction. A calibrated CPS 120 INEL X-ray powder
diffractometer equipped with a position-sensitive detector, operating at 40kV/25mA with
a flat geometry and employing graphite monochromatized Cu Kor radiation, was used to
obtain powder patterns of starting materials and all products.

Single-crystal X—ray Crystallography. For the single crystal of [3-
CsAgo,5Bi3,5Se6, intensity data were collected at 298 K using graphite-monochromatized

Mo Ka radiation (A=0.71073 A), on a STOE IPDS-Il diffractometer. A numerical

155

absorption correction to the data was applied with the program X-RED15 based on a
crystal shape description determined using equivalent reflections with X-SHAPE.16

For the single crystals 0f K1.86A80.93Bi3.0756, K1.84Ago.923i3.085€6,
Rb1.7Ago.8sBi31536, Rb1.6Ag0.8Bi3.2Se6, CSL7Ago.8sBi3.1556, C51.5Ago.753i3.25566,
Rb1.34Cd1.34Bi2.6636, K1.22Cd1.22312.7336, Rb2CUBi3SB6, C82CUBi3S6, and
Rb2,76Ago,6gBi4,35Se9, X-ray diffraction intensities were collected at room temperature on
a Bruker SMART Platform CCD diffractometer using a graphite-monochromatized MoKa
radiation. The individual frames were measured with an omega angle rotation of 03° and
an acquisition time of 30 sec for each frame. The SMART17 software was used for the
data acquisition and SAINT sofiware for data extraction and reduction. An analytical
absorption correction was performed using face indexing and the program XPREP in the
SAINT software package, followed by a semiempirical absorption correction based on
symmetrically equivalent reflections with the program SADABS. Structural solution and
refinements were successfully done using the SHELXTL package of crystallographic
programs. The structures were solved with direct methods.

The complete data collection parameters, details of the structure solution, and
refinement for B-CSAgosBi35366, K1.86Ag0.933i3.07s6, K1.84Ag0.9zBi3.08366,
Rb1.7Ag0.8sBi3.1556, Rb1.6Ag0.8Bi3.2Se6, CSr.7Ag0.8sBi3.1sS6, C51.5Ag0.7sBi3.2SSe6,
Rb1.34Cd1.34Bi2.6656, K1.22Cd1.2zBiz.7856, Rb2CUBi3SC6, C82CUBi336, 311d
szqéAgofigBiagsSeg are given in Table 6-1. The fractional coordinates and temperature
factors (Ueq) of all the atoms with estimated standard deviations are given in Tables 6-2 ~

6-19.

156

8866

L. .e 82- 6.8 83
286 n 23 66666 n .2
826 u 23 6866 u .E
SE

2 \ 6 \ SN

mm :0 mosacm-am8_ xtfifiésm
.x. 6.8 $6.8

H826 n 95% SN

22

Suvrv:

- .euvaVm- .vuvnHVm-
.0663 a 86

:4

..88 an?

new: 4.26

_

2 s 566.62

61868.8 1 e

< 2 36266 n e

< 236266 1 e
o§$6n~

:28?on

4 mSKd

m Sam

mmd 2:
om8.~mmvm._fiovm._flm

3. .6 8667 28 mm?
8.26 n as .386 u .E
6:26 .1. 23 .886 n .E

662
m~\6\66_

Nu co 686266-88— SWEET—Em

ex. 36 .082
8866 n 65va 6M:
Nmmm

wanvrvmwm

- .muvxnvfi .muvnHVm-
.38 9 a:

woe

_.88 6:64

men: 36.6

_

.4. @363

< 628.2 1 e

< @8666 u e

4 @8666 u a
ofigmcm

Ecowmxo:

4 M836

6. $28

263
omwh.~_m-._fiUNN._V~

.6_:Nn.me\£~.m- ”£sz u 23 .__...e__w\__...~_1 _..k__w u E.

..< .e 83. as 36
62:6 u 23 .2: 86 u .E
886 u 23 .886 u .E
M: 2

8 \ 6 \ Nam

m co 68633-68— anEA—sm
.x. “8.16.2

8626 u 98a Nam

466mg

omnvrva

- .envxuvfl .mmuvnuvflu-
066m 2 666

~6-

.88 6866

as»: 366

a

mm 56. a:

< 63966.: n e

< 5326 u e

< $382.6. 1 a

SEQ

esaefieeeo

< 8:6

6. Sam

8.82

emaemewfio

N

EoBEooo nonocuxm
22 B... 6.8.. .86 8693
3% =3 8665 a
882.68%: 8665 m sea
mm :0 5.3885680
6820883 \ 856.568 \ «SD
3508 “EEoSMoM

805 8 30583800
“Beacon": 289839:
368:8 6:038qu

momma x02:

828:8 Sac co.“ emcee Sufi.

88E
626808 8:88.?

86866688 8889
N
0830>

323086 :3 “ED
9.on 3QO

8293 EEOC
595—363
ohzfloqfiuh

Emma? 638.8.”—
2258 RoEQEm

.0323. 65. 5.23 ace)? 86 866862 83638 69. see .880 .3 uses

157

...-< .e 686. 68 686
826 u 23 .886 u .2
6626 u 23 .6866 u .2

82

3 8 \ SN

em 20 8.823-500: 62222223.:
.x. 6.8 .088
2.686 n 652: EN
682

omnvrv:

- .mnvxuvfl .muv22HVm-
08.8 a 8.2

68

as: 5.8

.882 626

2

.2 82868

4 @888 u e

< @826 n e

2 @826 u e
0.52ch

Ecowmxom

4 8:6

2 58.6.

3.82
enma.em..ew<e._2

n2 .6 866. Be 86.2
6626 u. 23 6286 u .2
826 u 23 .886 n .2
82

2 8 \ EN

2.: co 8823-800: 256223.”:
.8 6.8 .088

8866 u 652: as

«com

anrv:

- ..mnvxnvN- .mHV2HVm-
08.8 2 86

8m

..88 866m

.882 866

2

.2 @268

< @2888 u e

< 528:. u e

< 35526 n 6
05.56%
220222.:

4 mnozmo

v: Amvmom

©.wwm:
commodmmmodw<vQ _v~

.SECEENEQK. «558 n 23 .__.m__w\__...~_1 _....~__w u 2.

3... .n 866- 28 826
6866 u 23 .886 n .2
6866 u 23 .886 .... .2

88.2

2 \ 6 \ 82

2.: co 602223.860— 236823":
.2. 6.8 .038
2886 n 652. 82
a:

emuvrv8

- .mnvaVm- .mnv:HVm-
08.8 3 e:

68

.88 23:3.

.882 82

2

.2 $38

2 2 268.8 1 e

< 9.2886 n e

< 32886 n 6
022568

2283on

.2. 86:6

2 :88

8.82
omhoammmo.cw<¢w._v~

22020808 2282st
22 as 286 .26 6683
:88 :3 .865 2
268.82%: 8265 2 22
am no 520.802.2000
8080888 \ 6220.282 \ 025
3508 28802.:0M

305 8 802803800
88:00:02 2202202025
380:8 6:280:06:

momqfi 202.:

20:00:00 302 8.: 0222 3025.
866:

22205000 20:22.8ng
8828288 885

N
082:5

62282022: :00 223
95.2 008m

808% 23960
5320.63
022809202.

2303 0388.:
638.5: 282.2925

 

.7: 030,—. 05.5.50

158

$266

.-.2. .0 266.2 2- 228 mmm...
8626 u 23 .6286 u .22
8626 n 23 .6686 u .22
82.2

m 2 \ o \ Rm

.2 :0 20202222222002 22:22:23.2
.x. 660 $22.22

26826 n c8222 SN

wwwm

mmuv2uv~m

- .mHV2HVm- . "$2qu
.0228 3 8.2

now

2-88 28.3

.882 226

2

.... 22 22262

2 586.8 u o

22. 32282.2. u .2

2 @282... u _.

ESSSK

20:02:20:

.2. 86286

2 2882

3.6822
comadmmmfiow<n2wo

N

..< .0 and- 26:8 Rho
wmmmd u N23 .336 n .22
22mm: n 23 .wmnod u .22

ommd

v2 \ o \ mom

..2 :0 82222222002 222.268-2222,»
$2.3 .omwsm
2886 n .8222 8a
~32

omuv2uvom

- .mHV2HVm- . uvnan-
omwsm 8 08.2

23.

.88 88.2.

.882 866

2

.2... 22262.22

2 6282.2 u ...

< $268... u .2

2 E268... u a
2:5on

228026203

.2. 22825.22

2 238“

22.202 2
emfldmmmmdw<fi2mo

N

.... 2 2.2...222222.2..m. .6235 u 2... .__.k__w\__.,.2_ I 3:2 u 2.

.2 .o 28.9 8... 22m...
2.626 u 23 .886 u .2
2.826 n 23 6286 u .2

62.2

22262662

.2 :0 20222322222002 22.508-2232
.2. 662 .8662

2886 u c8222 82
66mm

83258

- .mquTVm- .vuvnan-
.6662 2 8.2

68

..88 686m

.8222 22.6

2

.2 $226268

.2. 8282.8 n o

,2. 22 2288... n .2

... 22 :88... n ..
0523602

230262022

22. 2.25.0

2 2.8.28...

2.0.2.2222
cmfldwmmwdwxxfixfim

N

2220205000 :022o:22xm
02022 20:: 0200.2 2.22: 220202
28.8 22.2 .888 2
2222882.?22 .888 2 882

mm :o 22.50.805.000
802086.28 \ 3:22.220: \ 0225

2522208 2:080:60”—
0205 02 80:80—22:00
802802.202 2:020:0ch
2002002200 2:02:00c0y2

202:8 20225

:02200228 02220 .28 02:8 02022.2.

266622
25205000 20228082

28823282 828.:
N
0E=2o>

2:280:22: :00 22:3
22:22 006%

8028?. 282.220
5222020223
0528062802.
22203 «23:20.2
0258.28 BoEQEm

.Tc 03:.H 6:52:00

159

2 .0 a3- Ea 3?
82¢ n .23 633 u 2
as; u 23 .886 u a

33

o: \o \ 3%

Nu co moumsvm¢m8_ xEmEésm
x 25 .08:
5.86 u 95% SR
omoc

NNquVNN

- .muvxnvv. .Envnuvf-
032 2 :._

$2

:86 E3

men: 08.0

N

um 3:92 :
gocmtg an

< 2 can: u u

< 583 u a

< sin: n w
5x8

DEM—00:02

< 3:3

x 8%

8.28
«vomnwémmocdwfifimbm

LE 23.. as $3
83d u 23 $86 n 2
825 u 23 «:3 n E

33

t. \ S 82

«u so moumswmémo— x5243,»
$0.8 .oNSN
825d u 95”: 8M:
ovom

omnxuva

- .muvxuVm- .Snvanvmwm-
83$ 2 a:

22

was $0.?

as»: a?

v

$35.52
0382; : an

< Egg: u o

.4 8:23 n a

< G: :3 u a

58

08:00:02

SSE

v. Sam

8?:

$5530

.3“ $05.»??? ”.555 n 23 .__§_w\__§ u 35 u 3r

$3 92.? Ba 82
man; u 23 .286 u E
226 n 9:, .335 u E

83

mp \ o \ 3:

Nu no 883332 xEmEésm
$25 .058
535 u 95% a:
vomv

Suvwvom

- .muvaVm- .Suvnul m-
.05: 9 3:

VNNN

..88 83%

”an: :3

v

3:8me
0583: an

< 53$ u o

< :1er u a

< 351% u a

£8

0:500:32

$23

M 530.

2.2:

oompmsofix

286508 cozoczxm

2o: 98 xwoa Emu “mamas
3% =3 8065 m
gamma 8065 m :85

mm co “9.3.3030
flofififimm \ 3:?me \ Sun

@0508 “5:88qu
805 8 32833800
3262.?“ “53335
360:8 mcosoocom

mowcfl 5v:—

cosoo=oo 3% 8m omen“ SSE.
80on

520808 confining.
ABE—sofiov 3655

N

o§~o>

$5858? :3 “ED
gnaw 88m

829$ Embo
Ewan—05>?
oBEQQEoH

Emma}, «Bacon
£288 :33qu

.—.© «Bah. 6:52—00

160

Table 6-2. Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for ,B-CsAgo_5Bi3_5Se6. U(eq) is defined as one third of the trace

of the orthogonalized Uij tensor.

 

 

x y z U(eq) Occupancy
Bi(l) 1756(1) 2500 1640(1) 17(1) 1
Bi(2) 758(1) -2500 -497( 1) 15(1) 1
Bi(3) 244(1) -2500 2731(1) 16(1) 1
Bi(4) 2178(6) 2500 -1626(6) 22(1) 0 . 5
Ag(4) 2153(12) 2500 -1468(13) 22(1) 0.5
Cs(1) 1339(1) 2500 -4637(1) 26(1) 1
Se(l) 468(1) 2500 1007(2) 12(1) 1
Se(2) 939(1) -7500 -1930(2) 15(1) 1
Se(3) -5(1) -7500 4003(2) 15( 1) 1
Se(4) 1518(1) -2500 3113(2) 18(1) 1
Se(5) 2884(1) 2500 2126(2) 20( 1) 1
Se(6) 1970(1) -2500 99(2) 1 8(1) 1

 

Table 6-4. Atomic coordinates ( x 104) and equivalent isotropic displacement

parameters (A2x103)for A1+xCdl+xBi3-xS6. U(eq) is defined as one third of the trace of

 

the orthogonalized Uij tensor.

 

 

A=K A=Rb
x y :z U(eq) occupancy z U(eq) occupancy

Bi(l) 6667 3333 i5802(2) 18(2) 0.69 5759(1) 24(1) 0.67
Cd(l) 6667 3333 §5877(8) 6(4) 0.31 §5759(1) 24(1) 0.33
S(l) 10000 0 :5000 27(3) 1 g5000 22(2) 1
S(2) 3333 6667 56465(5) 24(2) 1 56365(4) 27(2) 1
A(1) 0 1000037500 42(9) O.61(7) 57500 67(6) 0.36
A(2) -3333 13333: i7500 79(8) 0.35

 

161

 

 

 

 

 

 

 

3 $8 28: Rd 6% 83 588 $8 83 22: 2mm- €<
as 68 83 8.0 :33; 28: 523 $8 83288 8 €<
_ 32 2:83 _ 52 653 2 38 2883 $8 2.8 50
_ 8: 88M _ 2va 88m 2 :2: 282 o 82: 30
2.8 8: 3va $8 $8 $3me 3 $2 @8me 8mm 88 E?
58 E: €322 8.8 32 :88? M8 38 €va 88 $8 35
2289600 333 N 559600 33: N 55958 93: N 2 x
omuowoui muowoufl omnoméui
8.8 2:8 28: 883 882 28: €88 8%: 832m: 82. a?
8.8 6% 285 $88 8882 282 683 3:2: 82888 c €<
_ $8 683: _ 38 2:83 _ $2 553 $8 2mm 50
2 5% 88M _ 38 88W _ 38 88W 8 82: So
So 6: 2882 m3 838 3:8me 88 2:3 8:8me 28 $8 53
8.8 ES 353 Ed 38 $52 Rd 8mm 58wa 28 $8 22m
2289500 Q55 N 2289500 Q65 N 5:338 333 N A x
muosmni umuomxui muomxui

 

.888 .25 Bszcowonto 05 mo comb 05 .«o BE“ 28 mm Eamon mm A35
.oOx+m_mx-_w<x~-~< “ammo—2&3 820682“ 88288328 oaobofl 503333 28 Q: 2 V 832688 288.2 .mé «3:.

162

Table 6-5. Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for szCuBi3Se6. U(eq) is defined as one third of the trace of
the orthogonalized Uij tensor.

 

 

x y z U(eq) occupancy
Bi(l) 2044(1) 5000 874(1) 18(1) 1
Bi(2) 4037(1) -5000 1705(1) 18(1) 1
Bi(3) 3143(1) O 3217(1) 16(1) 1
Rb( 1) 5000 -10000 0 33(1) 1
Rb(2) 1 124(1) 0 2864(1) 29(1) 1
Rb(3) 5198(2) -10000 4582(3) 36(1) 1
Se(l) 3022(1) 0 1170(1) 14(1) 1
Se(2) 2220(1) 5000 2794(1) 16(1) 1
Se(3) 1195(1) 0 490(1) 18(1) 1
Se(4) 4018(1) -5000 3507(1) 16(1) 1
Se(5) 4869(1) -10000 2099(1) 20(1) 1
Se(6) 3269(1) 0 5099(1) 15(1) 1
Cu(1) 2712(1) 5000 4725(2) 32(1) 1

 

Table 6-6. Atomic coordinates ( x 104) and equivalent isotropic displacement
parameters (Azx 103) for CSZCuBi3S6. U(eq) is defined as one third of the trace of the

orthogonalized Uij tensor.

 

 

x y z U(eq) occupancy
Bi(l) 2055(1) 5000 842(1) 20(1) 1
Bi(2) 4013(1) -5000 1721(1) 19(1) 1
Bi(3) 3135(1) 0 3186(1) 18(1) 1
Cs(1) 5000 -10000 0 60(1) 1
Cs(2) 1132(1) 0 2850(1) 30(1) 1
Cs(3) 5222(1) -10000 4571(2) 32(1) 1
Cu(1) 2703(1) 5000 4688(2) 39(1) 1
8(1) 3019(2) O 1180(3) 19(1) 1
S(2) 2256(2) 5000 2747(3) 18(1) 1
S(3) 1251(2) 0 475(4) 23(1) 1
S(4) 3983(2) -5000 3465(3) 19(1) 1
S(5) 4805(2) -10000 2104(4) 24(1) 1
S(6) 3213(2) O 5029(3) 20(1) 1

 

163

Table 6-7. Atomic coordinates ( x 104) and equivalent isotropic displacement

parameters (A2x103) for Rb2_76Ago.69Bi4.35869. U(eq) is defined as one third of the

trace of the orthogonalized Uij tensor.

 

 

x y z U(eq) occupancy
Bi(l) 4138(1) 7500 2342(1) 26(1) 0.911
Bi(2) 6383(1) 2500 3292(1) 24(1) 1
Bi(3) 2048(1) 2500 1210(1) 31(1) 1
Bi(4) 1514(1) -2500 3298(1) 24(1) 1
Bi(5) 3834(1) 2500 4509(1) 24(1) 0.938
Se(l) 5131(2) 7500 3985(1) 17(1) 1
Se(2) 2915(2) 2500 2936(2) 19(1) 1
Se(3) 7271(2) 2500 5014(2) 21 (1) 1
Se(4) 3283(2) -2500 871(2) 25(1) 1
Se(5) 383(2) -7500 3716(2) 24(1) 1
Se(6) 5268(2) 2500 1927(2) 21 (1) 1
Se(7) 7430(2) ~2500 2762(2) 27(1) 1
Se(8) 837(2) -2500 1778(2) 23(1) 1
Se(9) 1334(2) 2500 -295(2) 48(1) 1
Rb( 1) 8820(2) -2500 4371(2) 31(1) 1
Rb(2) -960(2) -7500 1952(2) 36(1) 0.930
Rb(3) 3534(5) 2500 -802(4) 67(2) 0.625
Rb(33) 4552(13) 2500 -248(13) 64(6) 0.203
Ag( 1) 462(5) -2500 86(5) 53(2) 0.417
Ag(2) 161(9) -4190(40) 54(9) 46(4) 0.278

 

164

Table 6—8. Bond lengths [A] and angles [°] for ,B-CsAgo.5Bi3,SSe(,.

 

Bi(1)-Se(5)
Bi(1)-Se(4)
Bi(1)-Se(6)
Bi(l)-Se(l)

Bi(2)-Se(2)
Bi(2)-Se(6)
Bi(2)-36(1)
Bi(2)-Se(1)

Bi(3)-Se(3)
Bi(3)-Se(2)
Bi(3)-Se(4)
Bi(3)-Se(1)

Bi(4)-Se(5)
Bi(4)-Se(2)
Bi(4)-Se(4)
Bi(4)-Se(6)

Ag(4)-Se(5)
Ag(4)-Se(2)
Ag(4)-Se(6)

Cs(1)-Se(3)
Cs(1)-Se(5)
Cs(1)-Se(4)
Cs(1)-Se(2)
Cs(1)-Se(3)
Cs(1)-Se(6)

Se(5)-Bi(1)-Se(4)
Se(4)-Bi(1)-Se(4)
Se(5)-Bi(1)-Se(6)
Se(4)-Bi(1)-Se(6)
Se(4)-Bi(1)-Se(6)
Se(6)-Bi(1)—Se(6)
Se(5)—Bi(1)-Se(l)

2.721(2)
2.9389(17) x 2
2.9933(18) x 2

3.131(2)

2.8805(17) X 2

2.947(2)
2.952(2)

2.9933(17) x 2

2.7653(15) x 2

2.973(2)
3.024(3)

3.1769(18) x 2

2.686(5) x 2
2.927(14)
3.071(13)
3.172(5) x 2

2.825(13) x 2
291(3)
3.007(13) x 2

3.647(3)
3.656(2) x 2
3.723(2) x 2
3.799(3)
3.847(2) x 2
3.971(3)

9101(6)
8995(7)
9045(6)
178.21(7)
9106(4)
8789(7)
178.1 1(7)

Se(4)-Bi(1)-Se(1)
Se(6)-Bi(1)-Se(1)

Se(2)-Bi(2)-Se(2)
Se(2)-Bi(2)-Se(6)
Se(2)-Bi(2)-Se(1)
Se(6)-Bi(2)-Se(1)
Se(2)-Bi(2)-Se(l)
Se(2)-Bi(2)-Se(1)
Se(6)-Bi(2)-Se(1)
Se(1)-Bi(2)-Se(1)
Se(1)-Bi(2)-Se(1)

Se(3)-Bi(3)-Se(3)
Se(3)-Bi(3)-Se(2)
Se(3)-Bi(3)-Se(4)
Se(2)-Bi(3)-Se(4)
Se(3)-Bi(3)-Se(1)
Se(3)-Bi(3)-Se(1)
Se(2)-Bi(3)-Se(1)
Se(4)-Bi(3)-Se(1)
Se(1)-Bi(3)-Se(1)

Se(5)-Bi(4)-Se(5)
Se(5)-Bi(4)-Se(2)
Se(5)-Bi(4)-Se(4)
Se(2)-Bi(4)-Se(4)
Se(5)-Bi(4)-Se(6)
Se(5)-Bi(4)-Se(6)
Se(2)-Bi(4)-Se(6)
Se(4)-Bi(4)-Se(6)
Se(6)-Bi(4)-Se(6)

Se(5)-Ag(4)-Se(5)
Se(5)-Ag(4)-Se(2)
Se(5)-Ag(4)-Se(6)
Se(5)-Ag(4)-Se(6)
Se(2)-Ag(4)-Se(6)
Se(6)-Ag(4)-Se(6)

9033(6)
8819(6)

9230(7)
9260(6)
8903(5)
177.65(7)
175.14(6)
8974(4)
9172(5)
8659(5)
8789(6)

9738(7)
91 .93(6)
9573(6)
168.37(7)
171.07(5)
9032(4)
8327(5)
8794(5)
8166(5)

101.3(3)
81.8(3)
88.9(3)
165.2(3)
165.0(4)
8730(7)
873(3)
103.8(3)
81.82(17)

94.7(6)
798(6)
88. 14(1 1)
169.5(10)
90.8(6)
87.4(5)

 

165

 

 

3:2: 368 322: 8:25 2 :28 30-25530

38.8 38.8 38.8 85.5 2:88 2:88 30-35-30

38.8 2:88 3:8 2:38 3:38: 8:88 30-35-30

38.8: 33: 388: 33: 38.8 33: 30-35-30

8:88 28.8 35.8 398 2:88 35.8 30-25-30

8x385 8x388.»- ox 8:888 ex 38% ex 383 ex 35% 30.2?

938.3 9383 mx 8:88.4- mx 383 mx 389m mx 354m 30-3<

9 3888

mx 3N8? Nx 3888 9 3:42 mx 3888 mx 3825 3<-3<

mx 3582 9 389m 9 32.2 mx 383 mx 3:3 9 388 30-3?

9 8:888 mx 3883 9 2:888 mx 2:888 9 382m 9 382 30-35

mx 382m 9 38: 9 8:823 mx 382 9 3288 9 35.8- 30-35
6muoaou< mu0won< 6mu0m§u< mn0xan< 6mu0£n< mu0£u<

 

80.35.253.68 5 E 8&5 Ba 7: 855:2 255 .3 83.5.

166

Table 6-10. Bond lengths [A] and angles [°] for A1+xCd1+xBi3-,-S6.

 

 

A = K A = Rb
Bi(1)—S(2) 2.726(7)x3 Bi(1)-S(2) 2.721(5)x3
Bi(1)-S(1) 2.901(3)x3 Bi(1)-S(1) 2.9259(9)x3
Cd(1)-S(2) 2.640(1 1) Rb(1)-Rb(2) 2.3184(6)
Cd(1)-S(2) 2.640(11) x2
Cd(1)-S(1) 3.006(12 )x3 Rb(1)-S(2) 3.536(7) x5
Rb(2)-S(2) 3.536(7) x5
K(1)-S(2) 3.241(8)x5
S(2)-Bi(1)-S(2) 95.1(2)
S(2)-Bi(1)-S(2) 94.0(3) S(l)-Bi(1)-S(2) 173.97(17)
S(2)-Bi(1)-S(1) 174.9(3) S(l)-Bi(1)-S(2) 8895(12)
S(2)-Bi(1)-S(1) 89.49(15) S(l)-Bi(1)-S(1) 86.66(3)
S(l)-Bi(1)-S(1) 86.79(10)
S(2)-Cd(l)-S(2) 98.0(5)
S(2)-Cd(1)-S(1) 169.3(6)
S(2)-Cd(1)—S(l) 88.93(18)
S(l)-Cd(1)-S(1) 83.1(4)

 

167

Table 6-11. Bond lengths [A] and angles [°] for RbZCuBi3866.

 

Bi(1)-Se(2)
Bi(1)-Se(3)
Bi(1)-Se(l)
Bi(1)-Se(l)

Bi(2)-Se(4)
Bi(2)-Se(6)
Bi(2)-86(1)
Bi(2)-Se(2)

Bi(3)-Se(6)
Bi(3)-Se(5)
Bi(3)-Se(4)
Bi(3)-86(1)

Rb(1)-Se(5)
Rb(1)-Se(3)

Rb(2)-Se(2)
Rb(2)-Se(5)
Rb(2)-Se(6)
Rb(2)-Se(3)

Rb(3)-Rb(3)
Rb(3)-Se(4)
Rb(3)-Se(4)
Rb(3)-Se(6)
Rb(3)-Se(5)

Cu(1)—Cu( 1)
Cu(1)-Se(6)

Cu(l )-Se(6)
Cu(l )-Se(2)

2.841(2)
2.8551(15) x2
3.0856(14) x2
3.101(2)

2.799(2)
2.8285(14) x2
3.1244(15) x2
3.233(2)

2.812(2)
2.9086(14) x2
2.9751(15) x2
3.068(2)

3.366(2) x2
3.4532(17) x4

3.429(2) x2
3.554(2) x2
3.628(2) x2
3.729(3)

1.865(9)
3.475(3) x2
3.571(4) x2
3.609(5)
3.629(5)

2.601(3)
2.4473(17) x2

2.509(3)
2.795(3)

Se(2)-Bi(l)-Se(3)
Se(3)-Bi(1)-Se(3)
Se(2)-Bi(1)-Se(1)
Se(3)-Bi(1)-Se(1)
Se(3)-Bi(1)-Se(1)
Se(1)-Bi(1)-Se(1)
Se(2)-Bi(1)-Se(l)
Se(3)-Bi(1)-Se(l)
Se(1)-Bi(1)-Se(1)

Se(4)-Bi(2)-Se(5)
Se(5)-Bi(2)-Se(5)
Se(4)-Bi(2)-Se(1)
Se(5)-Bi(2)-Se(1)
Se(5)-Bi(2)-Se(1)
Se( 1 )-Bi(2)-Se(1)
Se(4)-Bi(2)-Se(3)
Se(5)-Bi(2)-Se(3)
Se(1)-Bi(2)-Se(3)

Se(6)-Bi(3)-Se(4)
Se(4)-Bi(3)-Se(4)
Se(6)-Bi(3)-Se(2)
Se(4)-Bi(3)-Se(2)
Se(4)-Bi(3)-Se(2)
Se(2)-Bi(3)-Se(2)
Se(4)-Bi(3)-Se(l)
Se(2)-Bi(3)-Se(1)

Se(6)-Cu(1 )-Se(6)
Se(6)-Cu(1)-Se(6)
Se(6)-Cu(1)-Se(2)
Se(6)-Cu(1)-Se(2)

9279(5)
93.87(6)
91 .16(4)
174.04(4)
9039(4)
85.06(5)
174.63(5)
90.88(4)
84.89(4)

9277(5)
95.03(6)
8850(4)
174.18(4)
9059(4)
8376(5)
169.66(6)
9421(5)
83.81(4)

9190(4)
91.64(6)
91.48(4)
176.37(5)
8957(5)
8903(6)
8763(4)
89.00(4)

116.92(12)
116.70(7)
104.32(8)
9258(10)

 

168

Table 6-12. Bond lengths [A] and angles [°] for C82CuBi3S6.

 

Bi(1)-S(2)
Bi(1)-S(3)
31(1)-so)
31(1)-so)

Bi(2)-S(4)
Bi(2)-S(5)
Bi(2)-8(1)

Bi(3)-S(6)
Bi(3)-S(4)
Bi(3)-S(2)
Bi(3)-8(1)

Cs(1)-S(5)
Cs(1)-S(3)

Cs(2)-S(2)
Cs(2)-S(5)
Cs(2)-S(6)
Cs(2)-S(3)

Cs(3)-Cs(3)
Cs(3)-S(4)
Cs(3)—S(4)
Cs(3)-S(6)
Cs(3)-S(5)

Cu(1)-Cu( 1 )
Cu(1)-S(6)

Cu(1)-S(6)
Cu(1)-S(2)

2.736(3)
2.724(3) x2
2.995(3) x2
2.992(5)

2.660(5)
2.704(3) x2
3.026(3) x2

2.720(5)
2.805(3) x2
2.840(3) x2
2.937(5)

3.375(5) x2
3.503(4) x4

3.445(4) x2
3.624(4) x2
3.632(4)
3.698(6)

1.963(6)
3.510(4) x2
3.532(5) x2
3.631(5)
3.489(6)

2.569(4) x2
2.328(3) x2

2.413(6)
2.734(6)

S(2)-Bi(1)-S(3)
S(3)-Bi(1)-S (3)
S(2)-Bi(1)-S(1)
S(2)-Bi(1)-S(1)
S(3)-Bi(l)-S(1)
S(l)-Bi(l)-S(1)
S(3)-Bi(1)-S(1)
S(3)-Bi(1)-S(1)
S(l)-Bi(l)-S(1)

S(4)-Bi(2)-S(5)
S(5)-Bi(2)-S(5)
S(4)-Bi(2)-S(1)
S(5)-Bi(2)-S(1)
Se(5)-Bi(2)-Se(1)
Se(1)-Bi(2)-Se(1)
Se(4)-Bi(2)-Se(3)
Se(5)-Bi(2)-Se(3)
Se(1)-Bi(2)-Se(3)

Se(6)-Bi(3)-Se(4)
Se(4)-Bi(3)-Se(4)
Se(6)-Bi(3)-Se(2)
Se(4)-Bi(3)-Se(2)
Se(4)-Bi(3)-Se(2)
Se(2)-Bi(3)-Se(2)
Se(4)-Bi(3)-Se(1)
Se(2)-Bi(3)-Se(1)

Se(6)-Cu(1)-Se(6)
Se(6)-Cu(1)-Se(6)
Se(6)-Cu(1)-Se(2)
Se(6)-Cu(1 )-Se(2)

93.27(13)
95.63(16)
89.36(11)
173.66(13)
90.99(12)
85.96(11)
173.87(11)
8974(10)
84.76(11)

93.44(14)
9659(15)
87.84(1 1)
173.35(10)
9059(4)
83.76(5)
169.66(6)
9421(5)
8381(4)

9190(4)
9164(6)
91.48(4)
176.37(5)
8957(5)
89.03(6)
8763(4)
8900(4)

116.92(12)
116.70(7)
104.32(8)
9258(10)

 

169

 

38.8
32.8
38.8:
38.8
38.8
32 2.8
38.8.
8:88
388

38.8
38.8
388:
33.8
38.8
38.2.2
388
8:28
38.8

GCNovN
Atvmmmfi

Amvwnfl-
mx Atom—N
A994.—
Anvmwd

38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38

38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-25-38
38-35-38

38-38.
38-38.

38733
3873?
33-33
38138.

mx

mx

NX

Nx

NX

NX

mx

3888
3888
388.8

3:88

GSde
@330;
Amzwvwd

2388.8
8:288
8:888
8:88....

mx Amvmcd

N
X

X

N

mx
mx
mx

388.8
338.8
388.8
338.

3:;

Amvhmnd
A3806
$390M
Avvm 01m

38-38.
38-38.
38.33

38:33

38733
38:38.
38:38.

38-8355
3883.5
38-8355
38.833
233.835.

238.835
AvvomAOn—M
Abram-23m
gum-33.x
Sana-33M

38-355
3833
38-32
38-33.

NX
NX

NX

NX

mx
mx

mx
mx

NX
NX

Nx
mx

388.8
3838
33}.
38:8

3586
ACE-0N
Sumo-N
35ch

38:
388
382

338
388.8
32.8
388

38;
335.8
382
3:3

382
388.8
388.8
33.8

38.38
38-35.
38-355
38-355

38-35
38-35
38-35
38-35

Amvomszm
58-35
38-35

38-35
38-35
38-35
38-35

38-35
38-35
38-35
38-35

38-35
38-35
38-35
38-35

 

888.58.83.35 3 E 8.88 Ba 3: 2:82 2:5 .28 as:

170

 

A903
vadfl

388.3
38
8:22
38.2
382
332
8:88
33

38.8:
38.8
33:
38.2
33
3mm:
8::

8:28
8:32
332

38.8
8:3:

ASK-32

33-33-38
88-33-38

33-33-33
33-33-33
33-33-33
33-33-33
33-33-33
33-33-33
33-33-33
33-33-33

33-33-33
33-33-33
33-33-33
33-33-33
33-33-33
33-33-33
33-33-33

33-33-33
33-33-33
33-33-33

33-33-33
33-33-33
33-33-33

38.3.
3N8
8:32
8:88
28:282

332
8:882
38.2:

332
33.

332
38.2
8:32
8:2:

$5.2 :

32:

33
8:2:
8:3

32.2;
8:28:
8:2:

8:3

33-33-33
33-33-33
33-33-33
33-33-33
33-33-33

38-33-38
38-33-38
88-33-38

33-33-33
33-33-33

33-33-33
33-33-33
33-33-33
33-33-33

33-33-33

33-33-33
33-33-33
33-33-33
33-33-33

33-33-33
33-33-33
33-33-33
33-33-33

322

38.8
32.8
38.8
33.88
38.82
3832
38.8
38.8
8:88

38.88
38.8
38.8
3882
38.8
8:28
388

Ahvomdw

38.8
c :88:
3:88
32.88
38.82
388
8:88

38.8

33-33-33

38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38

38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38

38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-38
38-35-88
38-35-38
38-35-38

 

.nuue o—aah. 05.5.30

171

Table 6-14.

The anisotropic displacement factor exponent takes the form: -21c2[ h2 a*2U11 + + 2

Anisotropic displacement parameters (Azx 103) for fl-CsAgasBi358e6.

 

 

h k a* b* U12 ].
U11 U22 U33 U23 U13 U12

Bi(l) 14(1) 15(1) 22(1) 0 -3(1) 0
Bi(2) 14(1) 14(1) 16(1) 0 -1(1) 0
Bi(3) 24(1) 11(1) 14(1) 0 5(1) 0
Bi(4) 25(1) 23(1) 18(3) 0 3(2) 0
Ag(4) 25(1) 23(1) 18(3) 0 3(2) 0
Cs(1) 30(1) 25(1) 21(1) 0 -4(1) 0
56(1) 10(1) 14(1) 11(1) 0 -1(1) 0
Se(2) 16(1) 14(1) 15(1) 0 4(1) 0
Se(3) 23(1) 11(1) 11(1) 0 3(1) 0
Se(4) 21(1) 18(1) 14(1) 0 4(1) 0
Se(5) 12(1) 25(1) 25(1) 0 -2(1) 0
Se(6) 18(1) 23(1) 14(1) 0 0(1) 0

 

Table 6-16. Anisotropic displacement parameters (A2 x 103) for A1+xCd1+xBi3-xSG. The
anisotropic displacement factor exponent takes the form: -21t2 [ h2 a*2Un + + 2 h k
3* b* U12 ].

 

 

 

 

 

 

A = K
U11 U22 U33 U23 U13 U12
8(1) 26(5) 26(5) 29(7) 0 0 13(2)
8(2) 23(3) 23(3) 26(5) 0 0 12(2)
A(l) 51(12) 51(12) 23(12) 0 0 26(6)
A = Rb
U11 U22 U33 U23 U13 U12
131(1) 24(1) 24(1) 24(1) 0 0 12(1)
Cd(l) 24(1) 24(1) 24(1) 0 0 12(1)
3(1) 25(3) 25(3) 16(4) 0 0 13(1)
S(2) 26(2) 26(2) 29(4) 0 0 13(1)
A(l) 100(13) 100(13) 37(11) 0 0 50(6)
A(2) 92(9) 92(9) 17(6) 0 0 46(5)

 

172

 

 

 

 

 

 

 

 

 

53 o o Es. even 62% Ea o o 82? 82% 8E €<
$3 o o E: 8:3 £wa 82% o o 82% £25 5:: €<
32 o o 5m :28 58 a; o o $8 5: 5: So
36 o o a: 62 52 So o o $2 $2 2%: :20
32 o o 5: 58 58 5:2
:2 :2 82 22 .82 :2 :2 :2 82 22 22 :2
omnowou< mnoaona.
2 2% o o 823 828: 838: a?
22% o o 62% $3. $3 €<
E: o o $2. :22 58 Q: o o 62 58 58 30
So 0 o S: :22 :22 5: o o 58 5mm 58 So
22 :2 82 22 S2 :2 :2 :2 82 22 S2 :2
052154 mnodfiTat
E: o 0 $2 2va =va E: o 0 $3 SS. SR 50
52 o o 53 3: ER 35 o 9 6mm 5: 5M: 30
822 o o 828: a 2% 32% 5: c o snow 82mm 82mm 5?
E: o o 3% 58 2me 52 o o 53 $8 58 SE
:2 :2 32 22 82 :2 :2 :2 82 22 NS :2
emuoiua‘ muoin<

 

 

 

.: N.2 z 46 a: a N + + :21“ N: 2 New- ace 65 683
25:0me 88$ EoEoomEflt 3205225 2:. .oOx+m_mx._w<x~.~< 8m Ame x N<v mbuofifimm EoEoqumE oioboflé .m—ne 935,—.

173

Table 6—17. Anisotropic displacement parameters (Azx 103) for Rb2CuBi38e6 The
anisotropic displacement factor exponent takes the form: -2n2[ h2 a*2U11 + + 2 h k
a* b* U12 ].

 

 

 

U11 U22 U33 U23 U13 U12
Bi(l) 20(1) 17(1) 15(1) 0 5(1) 0
Bi(2) 20(1) 17(1) 19(1) 0 9(1) 0
Bi(3) 20(1) 17(1) 13(1) 0 7(1) 0
Rb(l) 28(2) 44(2) 30(2) 0 13(1) 0
Rb(2) 25(1) 26(1) 36(1) 0 10(1) 0
Rb(3) 22(2) 57(3) 25(2) 0 3(2) 0
Se(l) 16(1) 16(1) 10(1) 0 5(1) 0
Se(2) 17(1) 19(1) 13(1) 0 6(1) 0
Se(3) 16(1) 17(1) 21(1) 0 6(1) 0
Se(4) 17(1) 17(1) 14(1) 0 6(1) 0
Se(5) 16(1) 16(1) 27(1) 0 8(1) 0
Se(6) 18(1) 18(1) 11(1) 0 6(1) 0
Cu(1) 34(1) 22(1) 47(2) 0 21(1) 0
Table 6-18. Anisotropic displacement parameters (Azx 103) for CSZCuBi3S6 The

anisotropic displacement factor exponent takes the form: -21[2[ h2 a*2U11 + + 2 h k

a* b* U12].

 

 

U11 1122 U33 U23 U13 U12
Bi(l) 20(1) 17(1) 15(1) 0 5(1) 0
Bi(2) 20(1) 17(1) 19(1) 0 9(1) 0
Bi(3) 20(1) 17(1) 13(1) 0 -7(1) 0
Rb(l) 28(2) 44(2) 30(2) 0 13(1) 0
Rb(2) 25(1) 26(1) 36(1) 0 10(1) 0
Rb(3) 22(2) 57(3) 25(2) 0 3(2) 0
Se(l) 16(1) 16(1) 10(1) 0 5(1) 0
Se(2) 17(1) 19(1) 13(1) 0 6(1) 0
Se(3) 16(1) 17(1) 21(1) 0 6(1) 0
Se(4) 17(1) 17(1) 14(1) 0 6(1) 0
Se(5) 16(1) 16(1) 27(1) 0 8(1) 0
Se(6) 18(1) 18(1) 11(1) 0 6(1) 0
Cu(1) 34(1) 22(1) 47(2) 0 21(1) 0

 

174

Table 6-19. Anisotropic displacement parameters (A2 x 103) for Rb2,76Ago.69Bi4.358e9.

The anisotropic displacement factor exponent takes the form: -21r2 [ h2 a*2Un + + 2

 

 

h k a* b* U12 ].
U11 U22 U33 U23 U13 U12
Bi(l) 29(1) 24(1) 24(1) 0 -3(1) 0
Bi(2) 28(1) 19(1) 25(1) 0 0(1) 0
Bi(3) 36(1) 28(1) 28(1) 0 -7(1) 0
Bi(4) 31(1) 21(1) 22(1) 0 4(1) 0
Bi(5) 31(1) 20(1) 21(1) 0 2(1) 0
36(1) 20(1) 21(2) 12(1) 0 2(1) 0
Se(2) 22(1) 17(2) 17(1) 0 -1(1) 0
Se(3) 21(1) 20(2) 23(1) 0 4(1) 0
Se(4) 34(2) 20(2) 20(1) 0 -3(1) 0
Se(5) 27(2) 21(2) 24(1) 0 5(1) 0
Se(6) 26(1) 21(2) 16(1) 0 0(1) 0
Se(7) 25(2) 23(2) 34(2) 0 7(1) 0
Se(8) 30(2) 21(2) 18(1) 0 1(1) 0
Se(9) 51(2) 67(3) 24(2) 0 -9(2) 0
Rb(l) 32(2) 35(2) 27(1) 0 4(1) 0
Rb(2) 38(2) 43(2) 28(2) 0 6(1) 0
Rb(3) 81(5) 84(6) 41(3) 0 30(3) 0
Rb(33) 59(12) 70(15) 67(13) 0 36(10) 0

 

175

4. Results and Discussion.

Synthesis and Crystal Growth. fl-CsAgojBi358e6 was synthesized by reacting
the direct combination of elemental mixtures (Cs: Ag: Bi: Se = 1: 0.5: 3.5: 6).
Furthermore, the ,B-CsAngi3_5Se6 group of compounds with several x values (up to x =
0.9) were investigated. The CsAngi358e6 with x = 0.1, 0.3, and 0.5 produced pure solid
solutions with compositions intermediate between CsBi3,67Se63a and ,B-CsAgo_5Bi3,5Se6.
When x = 0.7 and 0.9 the reactions provided a mixture of the ,B-CsAngBilg-Seg, and
CsLsAgostiggsSef, phases. fl'CSAg05Bi3jsefi appears to melt congruently at 672 °C. For
thermopower and electrical conductivity measurements we grew large crystals of ,B-
CsAngi3_5Se6 (x = 0.1, 0.3, 0.5) using the Bridgman technique. The obtained ingots
show well grown highly oriented characteristics in which the long axis (crystallographic
b-axis) lies parallel to the Bridgman translation axis, Figure 6-1.

All the following compounds were synthesized by the molten polychalcogenide
salt2| method with excess AzQ and Q (Q = S, Se) at relatively high temperature (750 ~
800 °C). The hexagonal phases, A2-2xAg1-xBi3+xQ6 (A = K, Rb, Cs; Q = S, Se), formed
mostly by reacting (AzQ: Ag: Bi2Q3: Q = 3: 2: 4: 5) combinations at high temperature
(750 °C) which is almost similar to the conditions used to prepare the isostructural
compounds APbBi3Q6 (A = K, Rb, Cs; Q = S, Se)”. The differential thermal analysis
(DTA) studies indicate that all the hexagonal phases melt congruently at 702 °C
(K1.86Ago.93Bi3.07S6), 683 °C (K1.84Ago.9zBi3.08566), 698 °C (Rb1.7Ago.8sBi3.1536), 668 °C

(Rb1.6Ago.8Bi3.2S€6), 691 °C (C81.7Ago.8sBi3.1536), and 668 °C (C81.5Ago.7sBi3.2SSe6),

176

 

 

Figure 6-1. Ingot of CsAgo_5Bi3.5Se6 grown in a Bridgman furnace.

177

respectively. For all compounds a comparison of the X-ray powder diffraction patterns
before and after the DTA experiments showed no significant phase change.

The other hexagonal phases, A1+xCd1+xBi3-xS(, (A = K, Rb), were found from a
molar ratio 2(3)/1(2)/4/5 of K28(Rb28)/Cd/BiZS3/S at 750 °C. In addition, their selenide
analogs A1+xCdl+xBi3-xSe6 were identified by X-ray powder diffraction patterns to be
isostructural. They were produced from various molar ratios such as 2~3/1~2/4/5 of
Azse/Cd/Bi28e3/Se at 750 °C. Single crystal structural refinements for these compounds
were not done. K1,22Cd1,22Bi2_7gS6 and Rb1.34Cd._34Bi2,66S6 melt congruently at 796 °C and
790 °C, respectively.

RbZCuBi3Se6 and CS2CuBi386 were discovered initially in low yield from
combining Rb28e(Cszs), Cu, BiZSe3, Se(S) in a 4:2:4:5 molar ratio at 850 °C and 750 °C,
respectively. Later RbZCuBi3Se6 was synthesized purely in a stoichiometric molar ratio at
800 °C. DTA studies indicated that szCuBi3Se6 melts at 592 °C while CszCuBi386 melts
at 600 °C.

Rb2.76Ago,69Bi4,gSSe9 was prepared from a molar ratio 4/1/4/5 of
RbZSe/Ag/BiZSe3/Se at 750 °C. DTA indicated that the compound melts incongruently at

645 °C and undergoes a phase change to the hexagonal structure of Rb;.¢&.gBi3_2Se6.

Structure Description.

fi-CsAgo,sBi3,5Se6. fl-CsAgo.sBi3.5Se(s adopts the structure of a—CstBi3Se63b and
CsBi3,67Se63a. The three-dimensional “Ago,5Bi3,SSe6” framework is assembled from NaCl-
type building blocks, (2x2) long, and infinite along the b axis, which share edges on the

slant and have one dimensional tunnels parallel to b axis, see Figure 6-2. The Cs+ ions

178

 

 

 

L

_ V-
. fl, . Se A
T—v a Bi4/Ag4

Figure 6-2. Projection of the structure of a-CstBi38e6 down the b-axis. The shaded
area is NaCl type (2X2) block.

179

reside in tricapped trigonal prismatic coordination with Cs—Se distances fiom 3.647(3) to
3.971(3) A (Table 6-8). Most metal sites are fully occupied with Bi atoms while the Bi(4)
site is occupied with Ag atoms at a rate of 50% to preserve charge neutrality (Table 6-2).
Interestingly, the Bi(4) sites in isostructural CsBi3,67Se6 and a-CstBi3Se6 are only 2/3
occupied with Bi3+ and suggested to be occupied with Pb2+, respectively.

All Bi atoms in the NaCl-type building blocks are in distorted octahedral
coordinations with various distances and angles. Bi(l) is in the square pyramidal like
octahedral site with four similar bonds between 2.9389(17) and 2.9933(18) A, one short
bond at 2.721(2) A, and long one at 3.131(2) A. Bi(2) is in the least distorted octahedral
coordination with Bi-Se bonds between 2.8805(17) and 2.9933(17) A, and Se—Bi-Se
angles between 8659(5) and 92.60(6)°. Both Bi(3) and Bi(4) have almost the same
environment, which is a more distorted octahedral coordination, approaching a seesaw
shape. Bi(3), for example, has two short Bi-Se bonds at 2.7653(15) A, two bonds
between 2.973(2) and 3.024(3) A, and two long bonds at 3.1769(18) A while Bi(4) sites
has two short Bi-Se bonds at 2.686(5) A, two bonds between 2.927(14) and 3.071(13) A,
and two long bonds at 3.172(5) A. In addition, the Ag(4), occupying in the Bi(4) sites, is
shifted to two nearest Se(6) atomic sites with having shorter bonds to Se(6) atoms
3.007( 13) A and longer bonds to Se(5) 2.825(13) A comparing to Bi(4)-Se distances.

Hexagonal phases A2-2,Ag1-xBi3+xQ6 (A = K, Rb, Cs; Q = S, Se) and
A1+xCd1+xBi3-,S(s (A = K, Rb). Both A2.2xAgl-xBi3+xQ6 (A = K, Rb, Cs; Q = S, Se) and
A1+xCd1+xBi3-xS6 (A = K, Rb) compounds are lamellar, composed of BizTe3-type layers
propagated along the ab-plane and perpendicular to c-axis, with alkali metal ions residing

in the space between the layers, see Figure 6-3 a). These hexagonal phases are

180

isostructural to the compounds APbBi3Q6 (A = K, Rb, Cs; Q = S, Se)”. If we compare
the current systems with the known compounds, the only difference is that the
monovalent Ag+ or divalent metal Cd2+ ions share sites with the Bi atoms instead of Pb2+
ions. The Ag+ containing structure can be represented by the simple formula A2M4Q6 (M
= Bi3+, Ag+) whereas the M2+ containing structures (M = Bi“, Pb”, Cdzl) can be
represented by AM4Q6.

Since the A2-2xAg.-xBi3+xQ6 (A = K, Rb, Cs; Q = S, Se) and A1+xCd1+xBig-xS6 (A =
K, Rb) compounds are isomorphous, we will describe in detail only K1,36Ag0,93Bi3_07S6
and Rb1,34Cd),34Bi2,66S6. The Bi atom sites in BizTeg-type layers on both compounds are
partially occupied with Ag and Cd atoms at the fraction of 77% and 67%, respectively, to
preserve charge neutrality (Table 6-3 and 6-4). Therefore, the formally Ag+ and Cd2+ ions
are situated in an octahedral environment of S atoms (Table 6-9 and 6-10). All the Bi
atoms from both structures are in distorted octahedral coordination (toward a trigonal
pyramid) with three short equal Bi-S bonds at 2.757(6) A and 2.721(5) A, and long one at
2.929(4) A and 2.9259(9) A for K1,36Ag0.93Bi3,07S6 and Rb.,34Cd1,34Big_66S6, respectively.

The K and Rb atoms in both structures occupy, partially, two crystallographically
distinct sites between the layers, see Figure 6-3 b), both of which have trigonal prismatic
S coordination. The occupancy of Cs+ ions in the previous known compounds, for
example ,B-CstBi3Se6, is only one-half of the total capacity of the interlayer space for
such ions but the occupancy of K+ and Rb+ ions are 94% and 61% of the total capacity,
respectively. In addition, long-range ordering of the structures was probed with electron

diffraction performed on crystals of Rb1_6Ag0,gBi3.2Se6 and Rb..7Ag0.35Bi3.15S6 using a

181

[AgBi3Q6]2'
or
[CdBi3Q6ll-

 

 

 

 

 

 

 

 

 

\
l‘
’\
~ow
\ \
--\
‘\
\
I
."‘
’\
I ..\ ’ \
‘9‘ 'C.‘
\” \
K ‘s

 

Figure 6-3. Projection of a) the structure of A2-2xAg1-xBi3+xQ6 (A = K, Rb, Cs; Q = S,
Se) and A1+xCd1+xBi3-xS6 (A = K, Rb) and b) A+ arrangements in A2-2xAg1-xBi3+xQ6 (A
= K, Rb, Cs; Q = S, Se) and A1+xCd1+xBi3-xS6 (A = K, Rb) with inter layer distances of
Rb1.7Ago.8sBi3.1536-

182

transmission electron microscope. No evidence of a superlattice was observed, see Figure
6-4.

AzCuBi3Q6 (A = K, Rb,Cs; Q = S, Se).

The A2CuBi3Q6 compound adopts a new two-dimensional layered structure
composed of three bismuth octahedra wide of BizTe3-type fragments which are linked to
each other by two Cu tetrahedral units sharing their edges and propagated along the c-
axis creating stepwise corrugated layers, see Figure 6-5. The akali metals reside in the
space formed between the layers.

RbZCuBi3Ses, for example, will be described in detail among the compounds. In
the BizTe3-type framework, there are three crystallographically different Bi atoms. Bi(l)
and Bi(2) are in a distorted octahedral coordination analogous to trigonal pyramid, where
the bismuth metal ions move to one octahedron face, with three short bonds between
2.841(2) and 2.8551(15) A trans to three longer ones between 3.0856(14) and 3.101(2) A
for Bi(l), and with three short ones between 2.799(2) and 2.8285(14) A trans to three
longer ones between 3.1244(15) and 3.233(2) A for Bi(2) (Table 6-11). Bi(3) is in the
least distorted octahedron (approximately a square pyramid) with one short bond at
2.812(2)) A, four bonds between 2.9086(14) and 2.9751(15) A, and one long bond at
3.068(2) A, which faces trans to the short bond. In addition, the Cu(1), located side of the
terminal of BizTe3-type, is in a distorted tetrahedral coordination with Cu—Se distances at
2.4473(17) — 2.795(2) A, Se—Cu—Se angles at 92.58(10)° — 116.92(12)°, and additional
Cu(1)-Cu(1) distance at 2.601(3) A.

There are three types of Rb atomic sites between the layers. Interestingly, Rb(l) is

in the distorted octahedral pocket composed of six Se anions with various Rb-Se

183

 

Figure 6—4. Selected area electron diffraction patterns revealing a hexagonal P lattice
viewed along crystallographic [001] direction for a) RbL7Ag085Bl3JSS6 and b)
Rbl.6A80.8Bi3.2SC6-

184

   

Tetrahedral
coordination

Figure 6-5. a) Projection of A2CuB13Q6 (A = K, Rb, Cs; Q = S, Se) down the b-axis
and the BizTe3-type building block is shown within shaded parallelograms. b) The
coordination geometry of two distinct Rb(3) atoms in Rb2CuBi3Seg. c) Polyhedral
representation of the structure of A2CuBi3Q6 (A = K, Rb, Cs; Q = S, Se).

185

distances (two short ones at 3.366(2) A and four long ones at 3453207) A), while Rb(2)
is in the monocapped trigonal prismatic pocket parallel to b-axis with Rb-Se distances
between 3.429(2) and 3.729(3) A. However Rb(3), half occupied and close to each other
(Rb(3)- Rb(3) at 1.865(9) A), is in the center of trigonal prismatic pockets with Rb-Se
distance at 3.429(2) — 3.729(3) A, see Figure 6-5 b).

sz.76Ago,59Bi4,358e9. Rb2.76Ago.6gBi4,35$eg has a layered structure with slabs
assembled from modified BizTe3—type fragments, see Figure 6-6. The fi'agments are three
and four “MSe6” octahedra wide and three octahedra thick, which are propagated along
the diagonal direction on the ac—plane by linking with identical neighboring fragments
through sharing an edge of the Ag(1)-Se tetrahedron, located on the each side of BizTe3—
type fragments, to form a stepwise slab.

All metal sites are fully occupied while Bi(l), Bi(5), Ag(l) and Ag(2) are partially
occupied at the fraction of 91%, 94%, 42% and 28%, respectively, to preserve charge
balance (Table 6-7). The structure has five crystallographically different Bi atoms. Bi(l)
octahedron is distorted along a pseudo three-fold axis forming three short bonds at
2.778(3) A to Se(4), and 2.850(2) A to two Se(6) atoms, and three long bonds at 3.045(2)
A and 3.128(3) A to Se(2) and Se(l), respectively (Table 6-13). The Bi(2) octahedron is
similar to that of Bi( 1) with short bonds between 2.814(3) and 2.838(2) A and three long
ones between 3.144(2) A and 3.187(3) A. Both Bi(l) and Bi(2) have Se—Bi—Se angles of
8269(7) — 95.71(10)°. Bi(3) is in a distorted octahedral coordination (almost a square
pyramid) with one short bond at 2.753(4) A, four long ones between 2.917(3) and
3.016(3) A and a longer one trans to the short one at 3.180(3) A. The Se—Bi—Se angles

vary from 8530(7) to 95.58(9)°. Interestingly, Bi(4) atom is in a vertically distorted

186

 

 

. ’ Tetrahedral
0. '5 coordination

Figure 6-6. a) Projection of Rb2.76Ago,69Bi4.85Se9 down the b—axis. The shaded area
indicates the BizTe3-type building blocks. b) Polyhedral representation of the structure of
of Rb2.76Ago,69Bi4.353e9. c) The arrangements of Ag(l) and Ag(2) atoms in the

tetrahedral coordination.

187

 

Absorption coefficient (or/.5)

 

 

 

a) J
b /
c} A
’l
d) 1
0.0 0T2 074 ' 0:6 ' 0:8 ' 1:0 . 1:2 ' 1.4
Energy(eV)

Figure 6-7. Solid-state UV/vis and infrared absorption spectra showing band gap
transitions for a) B-CsAg0.sBi3.sSe(5 at 0.51 eV, b) Rb2CuBi3Se6 at 0.56 eV, c)
CSZCuBi3Se6 at 0.62 eV, and d) Rb2.76Ago_69Bi4,358e9 at 0.51 eV. The band gaps in each

case are estimated from the crossing point of the solid lines shown in each spectrum.

188

square pyramidal like octahedral coordination with one short Bi-Se bond at 2.759(3) A,
four almost equal Bi-Se ones between 2.852(2) and 3.102(2) A and a long bond to the
nearest Se(3) with 3.350(4) A distance, which, however, is much longer than other
regular Bi-Se bond lengths. Bi(5) is in the least distorted octahedral site with Bi—Se
distances at 2.860(2) — 3.076(2) A and Se—Bi—Se angles at 86.34(8)° — 94.75(10)°.

Interestingly, in the structural diversity of Ag coordination (linear”°, square
planar”, triangle planar”, tetrahedrall l°’ 24, octahedral”), the Ag(l) atom is in a vertically
distorted tetrahedral coordination where it lies closer to one triangular face composed of
three Se(9) atoms with two short and one long Ag(1)-Se(9) bonds at 2.602(5) A and
2.800(9) A, respectively, and one long Ag(1)-Se(8) at 2.946(8) A and Se-Ag-Se angles at
100.7(2) — 124.03(17)°. In addition, Ag(2) is found between Ag(l) atoms on the same
plane with Ag(2)-Se(9) bonds at 2.385(17) and 2.462(16) A and Se(9)-Ag(2)-Se(9)
angles at 159.8(6)°. The shortest distance between Ag(l) and Ag(2) is 0.848(13) A while
that of Ag(2) and Ag(2) is 085(3) A. This can be understood by the two Ag metal ions
moving about in the two tetrahedral sites like rattlers in a cage. We expect that the two
rattling Ag metal ions could reduce the thermal conductivity.

All the Rb atoms except Rb(l) reside in the tunnels between the layers, and are
partially occupied at the fraction of 93%, 63%, and 20% for Rb(2), Rb(3), and Rb(33),
respectively. Rb(l) is located in the smaller tunnels between the layers having a
octahedral coordination with Rb-Se distances varying between 3.383(5) and 3.446(5) A.
Rb(2) has monocapped trigonal prismatic coordination while Rb(3) and Rb(33) reside in
the center of six coordinated trigonal prismatic pockets, located in a wide tunnel, which

are uniquely along with (101) plane with Rb-Se distance at 3.481(5) — 3.847(23) A like

189

previous szCuBi3Se6 compound, see Table 13. Both Rb(3) and Rb(33) have large disk
shaped temperature factors because they have partial occupancy and are sitting in
oversized tunnels. This implies a rattling motion and suggest that they could be ion
exchangeable (Table 6—19).

Charge Transport Properties and Energy Gaps.

The optical and infrared absorption properties of ,B-CsAgo_5Bi3_5Se6, hexagonal
phases A2-2xAg1-xBl3+xQ6 (A = K, Rb, Cs; Q = S, Se) and A1+xCd1+xBi3-xS6 (A = K, Rb),
A2CuBi3Q6 (A = K, Rb,Cs; Q = S, Se), and szqugosgBiagsSeg were examined and
indicated that all compounds are semiconductors.

fl-CsAgo,5Bi3,5Se6 compound show a clear band gap of 0.51 eV whereas the band
gap is similar to those of the isostructural CsBi3,67Se(, (0.53 eV) and a-CstBi3Se6 (0.55
eV), and the quaternary compounds CsAgo_5Bi3_5Se6 (0.54 eV), see Figure 6-7 a) and
Table 6-20.

The hexagonal phases A2-2xAgl-xBl3+xQ6 (A = K, Rb, Cs; Q = S, Se),
Kl.86Ago.93Bi3.07S6, K184Ag0923i308566, Rb1.7Ago.8sBi3.lSS6, Rbl.6A80.8Bi3.2Se6,
Csl_7Ago.35Bi3.15S6, and Csl_5Ago,75Bi3,25Se6 show well-defined electronic transitions
associated with a band gap of 1.17, 0.95, 1.03, 0.72, 1.01, and 0.57 eV, see Figure 6-8 a)
~ c) and Table 6-21. As expected, the sulfide compounds exhibit wider energy band gap
than the selenides due to higher electronegativity of the S atom. By comparison the
cadmium containing hexagonal sulfides, K1,22Cd1.22Bi2.73S6 and Rb1_34Cd1,34Bi2,56S6,
possess band gaps at 1.22 and 1.33 eV, respectively, see Figure 6-8 (1).

The RbZCuBi3Se6 and CszCuBi3Se6, have narrow band gaps of 0.56 eV and 0.62

eV, respectively, while their analogue expanded layer structure, Rb2.76Ag0.69Bi4,35Seg, has

190

 

 

 

Absorption coefficrent (a/S) v
Absorptron coeffiClent (a/S) 0"
v

\

 

 

 

 

 

 

Y Y T Y I v V I ‘ I V v I v I

2 3 4
Energflev)

 

 

 

0

Absorption coefficient (a/S) V

 

 

 

Absorption coefficient (a/S)

j_

 

 

 

 

 

 

 

 

T ' T V T ' T V I

3 4 5
Energy(eV)

 

Ad

 

Figure 6-8. Solid-state UV/vis absorption spectra showing band gap transitions for a)
K1.86A80.9313i3.0736 and Kl.84A80.9zBi3.08366 at 1-17 311d 0.59 CV, 9) Rbl.7Ago.ssBi3.lsS6
and Rb1.6Ag0.8Bi3.2Se6 at 1-03 and 0-72 CV, 0) CSlJAgossBistSts, CSl.5Ago.7sBi3.25386,
at 1.01 and 0.57 eV, and d) Rb(,34Cd1_34Bi2_66S6, K1,22Cd._22Bi2,73S6 at 1.33 and 1.22
eV. The band gaps in each case are estimated from the crossing point of the solid lines

shown in each spectrum.

191

a lower band gap of 0.51 eV, see Figure 6-7 b) ~ 6-7 d) and Table 6-21. This is in
agreement with the fact that the incorporation of the heavier and less electronegative Ag
atoms into a denser selenide compound is expected to lower the band gap.26

Electrical conductivity and thermoelectric power measurements were carried out

on polycrystalline ingots along the needle growth direction (i.e., crystallographic b-axis)
for ,B-CsAngi35Se6 with x = 0.1, 0.3 and x = 0.5 which were grown from a vertical
Bridgman grth technique, shown in Figure 6-1. The preliminary thermopower values
are negative and increase almost linearly from -91 uV/K at 300 K to -158 uV/K at 700 K
for x = 0.1, -70 uV/K at 300 K to -130 uV/K at 700 K for x = 0.3, and -l39 uV/K at 300
K to -225 uV/K at 620 K for x = 0.5, see Figure 6-9. The negative value indicates that the
predominant carriers are electrons (n-type). In general, a decrease of thermoelectric
power with composition x indicates an increase of the carrier concentration because the
Seebeck coefficient depends on the carrier concentration. Electrical conductivity
measurements were also carried out along the needle direction at room temperature and
values are ~35 S/cm for x = 0.1, ~343 S/cm for x = 0.3 and ~106 S/cm for x = 0.5. The x
= 0.3 sample had the highest electrical conductivity, which is consistent with the
concomitant the minimum value of the Seebeck coefficient.

By comparison the isostructuarl compounds, CsBi3,67Se6 and a—CstBi3Se6, have
equal or greater thermopower of -l60 and -730 uV/K at room temperature and
considerably lower electrical conductivity ~1 and 0.6 S/cm.3b In contrast to the charge
transport properties of CsBi3.67Se6 (x = 0), the fl-CsAngi35Se6 (x = 0.1, 0.3, 0.5) solid
solutions have reduced band gaps and increased charge carriers resulting from higher

electrical conductivity and lower thermopower. This may be the reason that mixing the

192

01
CD

 

lent (uV/K)
. I? <.=.

Seebeck Coeffic
’8
‘P .
b
b
P
I
p
D
p

 

 

 

300 400 500 600 700 800
Temperature (K)

Figure 6-9. Temperature dependence of the Seebeck coefficient for an ingot of ,6-
CsAngi35Se6 with various x values (0 for x = 0.1, I for x = 0.3, A for x = 0.5).

193

283 we: .

 

 

8 mm: SEES 3o: 35:? 2238.4 2282:. 2 625?: smsemzseuxsem
e S: €82: 38: 6:83 8533 sagas _ seamen emsemnseoai
a one €25». 38: 32.8 824 €22. _ 65534 smears:
... :8 63%. So: @222 53:. €33 _ nausea smegma.
s 23 322% 38: 83.3 683. 683. _ 65.364 £22932.
2. 23 532 so: 52.2% 53.3 €48.24 2 6.5364 smememmo
a :3 53% So: €32 5:3. 5:3 _ 6:538: 692.568:
Menage one 52mm: on $32: 928:4. 933.8 4 85$ 862286
manage «no 8222: 8 6.83: 92:22. 22:38 4 85¢ £32336
.... who 5%: 8 632: $23 @808 e 65:4 83224268
.. me $53.: 8 6222 €53. 55.3 4 seem £5,226
e :2 SEN: om 3298.: 53.2.4 £24522 4 seem semfimswfiofi
s 52E om 682... 53va €223 e 2652 seessmmsémssnc
s 52% 8 Esme? €393 vammme e 5622 shimmeoéaso
s :7 35.8% $5332 5:8: 82884.4 588.: M: $8 86326
82832 Amwwmm £43 0829 802: .8 b. 3: o 39 a Q: a N 228w 8QO £28.28

 

8582.5 e043? 1838 .5.“ mop—32> new £80883 :8 :23 date 03.;

194

8222228 283252 92 222 282.2228: ...2 682222 .2 .28; 22¢ .

 

 

e 222 22222 22 8.2 8252.3 2 2822 5228.2. 82:32 N 228 emaemeefissem
2 8.22 2.2222: 8.2 €2.22: 2.23.: 6222.2. 5224.2 2V ES 26222222680
.2 2222.222: 82 2.22222 22 $822.: 82:82. 822 23.8. 2. $8 $222682
22 e22 22322 8.2 @822: 535 23:22.2. 22524.: 2. she 2822226322
.2 6223.22: 8.2 22 22.8.2222 8222232 $22.22. 22232.8 4 2&8 8222692
e 3.22 2 2226: 22 82 62222.2 82282.2. 82282.2. 2 65526.2 288222322326
6 222.2 2222.23“ 3 82 22232.8 €382. €682. 2 65526.22 easieefiao
e 2.22 822233 3 22.2 632$ 32282.2. 62222.2. 2 6582382 32228342222
e 8. 2 2 22222222 3 82 852.8 2 2223.2. 2 2233.2. 2 3252662 62222822322222
e 3.22 52: S 82 2222238 22222222. 22222222. 2 e823; 28822282232822
e :2 $22.42 3 o: 2 2322.8 2288.4 2288.2. 2 6553.2 .mss2m2semfiea2
0 new
8:20.22 wawoco A223 0523 Row: 20 Q 922 9 Q42 2 23 e N 92on 08% £28.28

 

2.0.2232 22222 mohdozbm e0§~< 2.833. 20.2 8829» can $808232 :8 :23 Jane 032;.

195

Ag d states with the Se p states can be increased to have a rather narrow hybridized

valence band.

Solution Ion-Exchange Properties of Rb1,7Ago,gsBi3,1586 and Rb1,5Ag03Bi3_2Se6.

The previous ion-exchange study of the hexagonal phase APbBi3Se6 (A = Rb, Cs)
with NaI and Lil was successfully done in both solid and solution states.3b Herein, we
examined the ability to undergo ion-exchange reaction with Ag+ and Pb2+ ions with the
isostructural Rb1_7Ago,35Bi3,|5S6 and Rb1_6Ago,gBi3,2Se6. The Ag+ and Pb2+ ions were
prepared as 1 M nitrate solutions and mixed with the compounds at a molar ratio 1:1
then stirred for one day at room temperature. Surprisingly, an ion-exchange process
seems to occur at interparticle interfaces via an unknown mechanism.

stiring

 

 

Rb2-2xAg1-xBi3+xQ6(s) (Q = S, Se) + 2AgNO3(aq) > 3Ag.-xBil+x/3Q2(s) +
2RbN03(aq) 99(1)
Rb2-2xAg1-xBi3+xQ6(s) (Q = 8, Se) + PbNO3(aq) "mg 7‘ Pb1-xAg1-xBi3+xQ6(s) +
2RbNO3(aq) 99(2)

SEM/EDS analysis show the low content (<8%) of Rb+ ion versus high content of
Ag ion in these products. The X-ray diffraction patterns of the products obtained from
reaction of eq(1) showed an interlayer contraction as a result of ion exchange, see
Figure 6-10 and Table 6-22 a). The Ag+-exchanged hexagonal sulfide compound
however showed mainly cubic AgBiSz phase with only minor hexagonal phase with Ag+
exchanged with the Rb+ ions. The 002 reflection shifts from 11.64 A in the pristine
material to 9.94 A in the Ag exchanged material. Decomposition however seems to be

the dominant process leading to AgBiSz as the main phase. The hexagonal selenide

196

compound displayed ion-exchange as indicated by the interlayer spacing of 10.06 A, of
the (002) plane. The interlayer contraction of 1.70 A for Ag—exchanged
Rb1_7Ag0,3sBi3,1586 and 2.11 A for Ag-exchanged Rb1.6Ago.8Bi3,2Se6 accounts for regular
AgQ octahedra between the host layers. This is comparable to the c-axis contraction
observed in going from the hexagonal phases (c = 23.175(9) A for sulfide and c =
24.525(5) A for selenide) to the trigonal phases of AgBiSz (c = 19.78 A) and AgBiSez (c
= 19.67 A)”.

In the second reaction shown in eq (2), both Pb2+-exchanged hexagonal
compounds show an interlayer contraction as a result of ion exchange observed by X-
ray diffraction, see Figure 6-11 and Table 6-22 b). SEM/EDS analysis evidence that
both the amount of Pb2+ ion reach almost all possible Rb+ ion sites at a molar rate 2:1
(Rb+ : Pb2+). The Pb2+-exchanged hexagonal sulfide compound shows clearly shifts in
the spacings of the (001) planes as a evidence of forming an ion-exchange Pb1-xAg1-
xBi3,+,,S(5 phase. The Pb2+-exchanged hexagonal selenide compound showed a similar d-
spacing contraction of the spacings of the (001) plane. We also observed an extra strong
peak, which is of unknown origin and may be due to decomposition. Moreover, both
interlayer contractions of 1.74 A (exchanged with Rb1_7Ago.gsBi3,1586) and 2.34 A
(exchanged with Rb1.6Ago_3Bi3,2Se6), are similar to those of Ag.

Comparison of the AM4Q6, A2M4Q6 and A2M6Q9.

It is fascinating that the general formulae AM4Q6, A2M4Q6 and A2M6Q9 exhibit

such as a wide structural diversity with five different forms for AM4Q6, two different
designs for A2M4Q6 and a unique layered structure for A2M6Q9. They can be classified

roughly into two main groups, NaCl-type and BizTe3-type based anionic frameworks.

197

 

Intensity

 

 

 

 

 

20 (deg)

Figure 6-10. Comparison of diffraction patterns of a) original Rb1,7Ago.35Bi3,15S5 and
the products obtained from the ion-exchange reaction with b) Pb(NO3)2 and c) AgNO3.

 

 

Intensity

 

 

 

 

 

Figure 6-11. Comparison of diffraction patterns of a) original RbmAgogBimSeg and
the products obtained from the ion-exchange reaction with b) Pb(N03)2 and c) AgNO3.

198

Table 6-22. Comparison of specific interlayer distances (001) and EDS data of the
products obtained from the ion-exchange reaction of a) Rb1,7Ago_35Bi3,1586 and b)
Rb1_6Ago,3Bi3_2Se6 With AgN03 and Pb(NO3)2.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

a) Rbl.7AgossBi3.lsso
origin Ag)r Pb2+
(001) 002 004 002 004 002 004
20 (deg) 7.59 15.30 8.89 18.81 8.92 18.62
d (A) 11.64 5.79 9.94 4.71 9.90 4.76
difference‘il 1.70 1 .08 1.74 1 .03
EDS Rb2.03Ago.8Bi3.l9S6 Rb0.l4Agz.7Bi3.2S6 Rb0.31A80.7Pbl.l8Bi3.33S6
a the difference of d spacing between original Rb1,7Ago,35Bi3,15S6 and ion-exchanged
products
b) Rbl.6Ago.83i3.2Seo
origin Ag+ Pb2+
(001) 002 004 002 004 002 004
20 (deg) 7.26 14.64 8.79 17.95 8.99 18.31
(1 (A) 12.17 6.05 10.06 4.94 9.83 4.84
differenceal 2.1 1 1.1 l 2.34 1.20
EDS Rbl.83Ago.86Bi3.68366 Rb0.l4Ags.4Bi2.533€6 Rb0.07Ago.8Pbl.ozBi3.3Se6

 

 

 

 

 

“ the difference of d spacing between original Rb;_7Ago_35Bi3,1586 and ion-exchanged

products

199

 

 

They are mostly layered except for three dimensional tunneling in AM4Q6 (CsBi3,67Se6,
a—CstBi3Se6, ,B-CsAgo_5Bi3_5Se(,), see Figure 6-12. The impressive structural diversity
come into existence basically because of the different orientations of each building block
in the anionic framework and is affected by the size differences of anions and additional
metal ions. An extraordinary analogous example of such diversity is encountered in
ABi3Q5 (A = Rb, Cs; Q = S, Se, Te)26 where the various members have closely related
structures and differ only by simple displacements or changes in stacking sequence.

The AM4Q6 (group I) formula has five metal atoms including the alkali metal ions
and six chalcogen atoms and it comes in five different structure types. In the NaCl-type
groups, CsBi4Te623 and CsMBi3Te6 (M = Sn, Pb)“ show two different layers and infinite
NaCl-type anionic slabs (Figure 6-12). The structures of CsBi3.67Se6, a—CstB13Se6, ,6-
CsAgo,5Bi3,SSe6, show size limited (2x2 octahedra) and edge-sharing NaCl-type anionic
slabs, which may be caused by smaller size of Se. In the BizTeg-type groups, the
isostructural compounds, CsAgo_5Bi3_5Se6 and CstBi3Se6, have stepwise and infinite
BizTe3-type anionic frameworks in which Cs+ atoms have a bicapped trigonal prismatic
coordination Figure 6-12. In particular, the CsAgo,5Bi3.5Se6 is the polymorphic quaternary
bismuth selenide with the phase described as fl'CSAg05Bl3jse6 previously. On the other
hand, the hexagonal AMBng6 (A = K, Rb, Cs; M = Pb, Cd; Q = S, Se) show typical
infinite BizTe3-type layers and all alkali metal ions occupy only one-half of the total
capacity of the interlayer space (trigonal prismatic coordination).

Interestingly, A2M4Q6 (group II) have the same structure with the hexagonal
AM4Q6. The substitution of the Ag ions for the divalent atoms, such as Pb2+ and Cd”, in

the same anionic framework, [M4Q6], induces the amount of alkali metal ion increasing.

200

In addition, the smaller but similarly charged Cu+ ion creates a new layered phase,
AzCuBi3Q6. It consists of stepwise and finite BizTe3-type anionic frameworks (2x3
octahedra) binding to each other by Cu+ ions, see Figures 6-5.

Furthermore, an intriguing group III compound, is A2M6Q9 (Rb2.76Ag0_69Bi4,35Se6),
which conceptually derives from AzCuBi3Q6 by adding Bi2Q3. It too composed of finite
size BizTe3-type blocks (2 and 3 octahedra wide and 3 octahedra thick) but the Ag ions
are tetrahedra and serve to bind the [Bi58e9]-blocks in the framework instead of Cu+.

When the size of chalcogen atoms is decreased the fundamental building units
change from infinite NaCl-type to finite BizTe3-type block and the corresponding spaces
for the alkali metal ions are also decreased from large tricapped trigonal prismatic

(group I) to the smaller octahedral coordination (group III).

201

.222 2508 8:22.22 2: we £23222 5 memes—222 v.22 2808 052802 282,8 288.222 22 .922 cameo—22:0 98
222 282.22 222222322 .8 2208 £52.25, 523 oOe2~< 252 6024—2322 602.—>722 02222.8.2 05 222 2082 32822 goshm .N—te 95me

202.222.. .2 2225.6
>

6:28 .2: 2228.20

A

oomnimmmxnéw<um+nnm

P. 0.04% 0..
”we...“ . a
K»: .. Kama...

222-82.22.22

2.0.23. .22 2.22:0
1?

9m .m n o n 5222263

 

222-8922

 

\
.2222. .26 u :2 "£322.26 3. .2: .222 1.2 2 28322.26

------------------------------------

.- J . I J

-214 2.21.1443.
.Cr.» finflfr A...
. .7? tr. .5

w< u 2 2e0n+nmmr22nfiu<
5 .22.. u :2 30.422.22.45.

8882,

222-8222

 

 

j

 

 

 

 

wintry- 1.7%....er
.3...» -.2 e . . 2-4.24.2
”Hr.” fer/H. (Htlrelw
. . V/KIIF/J . I .
J “\Tlflll+. “/02 «J J'lJ
4 x, l . trqu \
”aka 0 . airwk
axe»? ... o 4-3.14.1;
if. ff? «if. ..
I o H #941 4 o
O i 1 O
a o
:m 2r— u 2 . eobnmmEmU
o .2
allOlJll loll lJlAVla
1 2 e
H
I;
O
H

 

 

#— u E ” ooth—U

20,...

r

O
.2. O

.olhloiuLo .04040.
84,22,829?

I. 'r h
.../.....21 41?er
J + '

d
a . O . I .

. t . O o
H.042- Luffinfiwnfl

It 2 hi . rr2 rtF
.tflttfi fr» 214.1132 -.-T

22.22-20.272

202

Concluding Remarks

A class of new quaternary compounds AM4Q6, A2M4Q6 and A2M6Q9 (A = K, Rb,
Cs ; M = Bi, Ag, Cu or Cd; Q = 8, Se) with three dimensional and layered structures has
been identified by the reactions of Bi, Ag, Cu, and Cd in corresponding AzQx fluxes or
stoichiometric method. All the layered compounds contain BizTe3-type layers but build
up in different ways depending on the incorporated metal ions and their coordination,
while the polymorphic three-dimensional fl-CsAgo,5Bi3_5Se6 consists of a NaCl-type
framework. They are all narrow gap semiconductors with band gaps in the range of 0.51
to 1.33 eV. It is remarkable that relatively small changes in experimental conditions
induce such multi structural arrangements, as ,B-CsAgo5Bi35866, CsAg(),5Bi3_5Se(<,27 and
Cs1,5Ag0_75Bi3.25Se6. In addition, preliminary thermoelectric measurements of ,8-
CsAg0_5Bi3,5Se6 evaluated with various Ag content showed n-type promising
thermopower and relatively high electrical conductivity which hopefully be optimized by
fine tuning. These phases are expected to have low thermal conductivity due to rattling
heavy alkali metal ion and solid solution.2b The two layered compounds,
Rb1,7Ago,35Bi3,15S6 and Rb1_6Ago,gBi3,ZSe(5, demonstrate ion-exchange properties with
Ag+/Pb2+ ions, however all results were not successful due to phase decomposition and

instability in water.

203

References

' a) Chung, D.-Y.; Iordanidis, L.; Choi, K.-S; Kanatzidis, M. G. Bull Korean Chem. Soc.
1998, 19, 1283-1293. b) Kanatzidis, M. G. Semicond. Semimet. 2001, 69, 51-100.

2 a) Kanatzidis, M. G.; McCarthy, T. J .; Tanzer, T. A.; Chen, L.-H.; Iordanidis, L.; Hogan,
T.; Kannewurf, C. R.; Uher, C.; Chen, B. Chem. Mater. 1996, 8, 1465-1474. b) Chung,
D.-Y.; Choi, K-.S.; Iordanidis, L.; Schindler, J. L.;Brazis, P. W.; Kannewurf, C. R.; Chen,
B.; Hu, 8.; Uher C.; Kanatzidis, M. G. Chem. Mater. 1997, 9, 3060-3071. c) McCarthy, T.
J .; Tanzer, T. A.; Kanatzidis, M. G. J. Am. Chem.Soc. 1995, 117, 1294-1301.

3 a) Iordanidis, L.; Brazis, P. W.; Kyratsi, T.; Ireland, 1.; Lane, M.; Kannewurf, C. R.;
Chen, W.; Dyck, J. S.; Uher, C.; Ghelani, N. A.; Hogan, T.; Kanatzidis, M. G. Chem.
Mater. 2001, 13, 622-633. b) Chung, D.-Y.; Iordanidis, L.; Rangan, K. K.; Brazis, P. W.;
Kannewurf, C. R.; Kanatzidis, M. G. Chem. Mater. 1999, 11, 1352-1362.

4 a) Mrotzek, A. Kanatzidis, M. G. Inorg. Chem. 2003, 42(22), 7200-7206. b) Iordanidis,
L.; Kanatzidis, M. G. Angew. Chem. Int. Ed. 2000, 39, No.11, 1927-1930. c) Iordanidis,
L.; Kanatzidis, M. G. J. Am. Chem. Soc., 2000, 122, 8319-8320.

5 a) Mrotzek A.; Kanatzidis M. G. Acc. Chem. Res., 2003, 36, 111. b) Kanatzidis M. G.
Acc. Chem. Res., 2005, 38, 359. b) Kanatzidis, M. G. Acc. Chem. Res., 2005, 38, 359. c)
Mrotzek, A.; Iordanidis, L.; Kanatzidis, M. G. Inorg. Chem. 2001, 40, 6204-6211. (1)
Choi, K.-S.; Chung, D-.Y.; Mroztek, A.; Brazis, P. W.; Kannewurf, C. R.; Kanatzidis, M.
G. Chem. Mater. 2001, 13 (3): 756-764. e) Mrotzek, A.; Iordanidis, L. Kanatzidis, M. G.
Chem. Commun. 2001, 1648-1649.

6 a) Hsu, K. F.; Chung, D.-Y.; Lal, S.; Mrotzek, A.; Kyratsi, T.; Hogan, T.; Kanatzidis, M.
G. J. Am. Chem. Soc., 2002, 124, 2410-2411. b) Hsu, K.-F.; Lal, S.; Hogan, T.;
Kanatzidis, M. G. Chem. Commun. 2002, 1380-1381.

7 a) Mrotzek, A.; Kanatzidis, M. G. Acc. Chem. Res., 2003, 36, 111. b) Kanatzidis, M. G.
Acc. Chem. Res., 2005, 38, 359. c) Hsu, K. F.; Lal, S.; Hogan, T.; Kanatzidis, M. G.
Chem. Commun. 2002, 13, 1380. d) Poudeu, P. F. P.; Kanatzidis, M. G. Chem. Commun.,
2005, 21, 2672. e) Takagi, J.; Takeuchi, Y. Acta Crystallogr. 1972, B28, 369. f)
Makovicky, E. Neues Jahrb. Mineral. 1989, I60, 269. g) Zakrzewski, M. A.; Makovicky,
E. Can. Mineral. 1986, 24, 7.

8 a) Anderson, S.; Hyde, B. G. J. Solid State Chem. 1974, 9, 92-101. b) Takuchi, Y.;
Takagi, .1. Proc. Jpn. Acad. 1974, 50, 843-847. c) Takuchi, Y. Recent Prog. Nat. Sci. Jpn.
1978, 3, 153-181. (1) Takuchi, Y. Z. Kristallogr. 1979, 150, 75-84. e) Takuchi, Y. T ropo-
Chemical Cell-TwinningsA Structure Building Mechanism in Crystalline Solids; Terra
Scientific Publishing Company: Tokyo, 1997.

204

 

9 Iordanidis, L.; Bile, D.; Mahanti, s. D.; Kanatzidis, M. G. JAm Chem Soc 2003, 125
(45), 13741-13752.

10 . a) Kim, J. -H.; Chung. D.-Y.; Kanatzidis. M. G. Chem. Comm. 2006, 15, 1618-1630.
b) See Chapter 5. c) Mrotzek, A.; Chung, D.-Y.; Hogan, T.; Kanatzidis, M. G J.Mater.
Chem. 2000, 10, 1. (1) Wang, Y. C.; DiSalvo, F.J. Chem. Mater: 2000, 12, 1011-1017.

” a) Choe, W.; Lee, S.; O’Connell, P.; Covey, A. Chem. Mater. 1997, 9, 2025-2030. b)
Huang, F. Q.; Somers, R. C.; McFarland, A. D.; Van Duyne, R. P.; Ibers, J. A. J. Solid
State Chem, 2003, 174, 334-341.c) Huang, F.Q.;Mitchell, K.;Ibers, J. Alloys Compd.
2001, 325, 84-90. (1) Yang, Y.-T.;Brazis, P.;Kannewurf, C.R.;Ibers, J .A. J. Solid State
Chem. 2000,-155, 243-249.

'2 a) Kyratsi, T.; Chung, D.-Y.; Choi, K.-S.; Dick, J. S.; Chen, W.; Uher, C. and
Kanatzidis, M. G. Mat. Res. Soc. Symp. proc. 2000, 626, Z8.8.1- Z8.8.6. b) Kim, J .-H.;
Chung, D.-Y.; Bilc, D.; Loo, S.; Short, J.; Mahanti, S. D.; Hogan, T.; Kanatzidis, M. G.
Chem. Mater. 2005, 17, 3606-3614.

'3 unpublished data: Monoclinic (C2/m), a = 30.866(16) A, b = 4.109(2) A, c =
25.828(13) A, ,6: 122.278(7)°.

'4 . Kubelka-Munk function: or/S = (1-R)2/2R, where on is the absorption coefficient, S is
the scattering coefficient, and R is the reflectance at a given wavenumber.

‘5 X-RED 1.22, Program for data reduction; STOE & Cie: Darmstadt, Germany, 2001.

'6 X-SHAPE 1.06, Program for crystal optimization for numerical absorption correction;
STOE & Cie: Darmstadt, Germany 1999.

'7 SMART, SAINT, SHELXT L: Data Collection and Processing Software for the
SMART-CCD system; Siemens Analytical X-ray Instruments Inc.: Madison, WI, 1997.

2‘ Kanatzidis, M. G.; Sutorik, A. C. Prog. Inorg. Chem. 1995,43, 151-265.

22 Tillinski, R.; Rurnpf, C.;Naether, C.; Duerichen, P.; Jess, 1.; Schunk, S.A.; Bensch, W.
Z. Anorg. Allg. Chem.l998, 624, 1285-1290.

23 Bronger, W.;Eyck, J.;Schils, H. J. Less-Common Met. 1978, 60, 5-13.

24 Emirdag, M.; Schimek, G.L.; Pennington, W.T.; Kolis, J .W. J. Solid State Chem. 1999,
144, 287-296.

25 most abundant coordination in the mineral chacogenide compounds.

205

 

26 Kyratsi, T.; Dyck, J. S.; Chen, W.; Chung, D.-Y.; Uher, C.; Paraskevopoulos K. M.;
Kanatzidis, and M. G. J. Applied Physics, 2002, 92, 965-975.

24 a) Chung, D.-Y.; Hogan, T.; Brazis, P.; Rocci-Lane, M.; Kannewurf, C. R.; Bastea, M.;
Uher C.; Kanatzidis, M. G. Science 2000, 287, 1024-1027. b) Chung, D.-Y.; Hogan, T.;
Brazis, P.; Rocci-Lane, M Brazis, P.; Ireland, J. R.; Kannewurf, C. R.; Bastea, M.; Uher
C.; Kanatzidis, M. G. J. Am. Chem. Soc. 2004, 126, 6414-6428

25 Geller, S.;Wemick, J. H. Acta Crys. 1959, 12, 46-54.

26 Iordanidis, L.; Bilc, 1).; Mahanti, s. 1).; Kanatzidis, M. G. J. Am. Chem. Soc. 2003, 125,
13741-13752.

27 Chapter 3

206

CHAPTER 7

Conclusions and Future work

The excellent thermoelectric properties of BizTe3 near room temperature have
motivated extensive studies in bismuth chalcogenide chemistry over the past decade. This
work has largely concentrated on the formation of new ternary and quaternary bismuth
chalcogenide compounds with diverse structures and compositions. To do so we have
investigated the system A/M/Bi/Q (A = K, Rb, Cs; M = Cu, Ag, Cd, Pb, Sb; Q = S, Se)
using several synthetic methods such as polychalcogenide flux, direct combination and
pellet, which have proven to be excellent tools in the synthesis of ternary and quaternary
bismuth chalcogenide compounds. The polychalcogendie flux (A2Qx flux) method has
provided many new and interesting compounds including a new chalcogenide
homologous series A2[M5+,.Se9+,,] (A = Rb, Cs; M = Bi, Ag, Cd; n = 1, 2, 3, 4) and
diverse structures in AM4Q5, A2M4Q5, AM6Se9 and A2M5Q9 (A = K, Rb, Cs ; M = Bi, Ag,
Cu, Cd; Q = S, Se). Direct combination and pellet methods were successfully adopted to
give ternary bismuth sulfide systems and AgBi3-bexS5 (x = 0~0.5), CchS7 and
CdofingongisSo. Furthermore, we have applied the Bridgman growth method to prepare
large, high quality samples for the measurement of TE properties.

In this structural diversity a new homologous series A2[M5+,,Se9+,,] (A = Rb, Cs;
M = Bi, Ag, Cd; n = 1~4) was established by ,B-CsBi3Se5, Rb2CdBi6Sen, and
szAg1,5Bi7,5Sel3 and new members of CsAgosBi3,5Se6, CstBi3Se6 and

CszAg1,5Bi7,5Se13, were produced by utilizing the predictive ability of the homology.

207

Only ,B-CsBi3Se5 and CstBi3Se6 crystals were successfully grown by Bridgman method
but showed lower than estimated electrical conductivities caused by poor crystal quality.
The common feature of this series is that the structures based on a single evolving
module NaCl1 1 1-type block possess mono and divalent metal ions in their bismuth atomic
sites. It would be of great interest to investigate the partial substitution of bismuth atomic
sites with additional mono or divalent metal ions, such as Cu, Tl, Sn, Hg, and Pb, as they
would have different electronic band contributions near the Fermi energy level. In the
K/Sb/T e system, an additional member found by an other student presents the possibility
of more phases to be found. The investigation with both modifications of the main frames
and alkali metals may need proper condition for producing large enough crystals for TB
measurements.

In the cases of AMAQ6, A2M4Q6, AM6Seo and A2M6Q9 (A = K, Rb, Cs ; M = Bi,
Ag, Cu, Cd; Q = S, Se), we have shown various structures depending on alkali metal ions,
incorporated metal ions and their coordination. Given the solid-state absorption spectra
that they exhibited they can be considered narrow band gap semiconducting materials.
Interestingly, it is remarkable that relatively small changes in experimental conditions
induced such structural diversity. The new quatemaries AM6Se9, CsAgo,5Bi5_5Se9,
Rbo,95Cd0,35Bis_45Se9 and RdeBi5Se9, crystallized in pseudo two dimensional
BizTe3(NaC1m) type structures retaining distinct NaCl100 type building blocks in a
systematic way. The preliminary transport properties of Rb(),ostogsBisAsSeo showed that
it is heavily doped with electron carriers and exhibits lower thermopower and high
electrical conductivity. This heavy doping could be caused by structural and

compositional defects. In addition, thermal conductivities in this family may be much

208

lower due mainly to extra contributions of Rb}r ions in the tunnel as rattlers. To better
understand the AM6Se9 series fiom a thermoelectric point of view with relation to
structural and compositional diversities, more investigations with both alkali metal ions
and various mono or divalent metal ions and more optimized synthetic method for high
quality crystals should be conducted.

Of all the quaternary AM4Q6 and A2M4Q6 (A = K, Rb, Cs ; M = Bi, Ag, Cu, Cd;
Q = S, Se), Ag-inserted ones showed interesting structural correlation. fl-CsAgo,5Bi3.5Se6,
CsAgo,5Bi3.5Se6 and Cs;_5Ago,75Bi3,25Se6 have appealed the structural diversity with
polymorphic concept consisting of three-dimensional NaCl'OO-type, modified
BizTe3(NaClm) type and BizTe3(NaClm) type frameworks, respectively. In addition, ,6-
CsAngi3ISSe6 (x = 0.1, 0.3, 0.5) phases, carried out with various amounts of Ag,
suggested promising TE properties with the solid solutions. In the fl-CsAngi3,5Se6
structure, the additional metal ions, here Ag, occupied the Bi4 site in which inserting
metal ions may strongly induce a phonon damping effect due to the mass and charge
difference. To understand more about this phase, other monovalent metal ions or even
divalent ones such as T1, Cu, Cd, and Sn with much different masses would be worth
exploring.

Interestingly, A2M4Q6 demonstrated the same structure as the hexagonal AM4Q6.
The substitution of the Ag+ ions for the divalent atoms, such as Pb2+ and Cd”, in the
same anionic framework, [MAQ6], induced an increasing amount of alkali metal ion. In
addition, the smaller but similarly charged Cu+ ion created a new layered phase,
A2CuBi3Q6, which consisted of stepwise and finite BizTe3-type anionic frameworks (2x3

octahedra) binding to each other by Cu+ ions. The two layered compounds in the A2M4Q6,

209

Rb(,7Ag035Bi3JSS6 and Rb1,6Ago,gBi3,ZSe6, displayed ion-exchange properties with
Ag+/Pb2+ ions in the solution state; however all results were not successful due to phase
decomposition and instability in water. A following A2M6Q9 (A = Rb; M = Bi, Ag; Q =
Se) related to A2M4Q6 showed that the building frames with Ag were constructed with
the same manner of the previous Cu-inserted phase, A2M4Q6 (A2CuBi3Q6). Both two-
dimensional layered structures can also be good candidates for ion exchange reactions,
which could be useful for designing new complex structures and available for treatment
for environmentally hazardous heavy metal ion waste.

In the ternary transition metal bismuth sulfide systems, the synthesis, crystal
growth, and thermoelectric properties of AgBi3S5 and its solid solution Ango,3Bi2,7S5, as
well as CdBi4S7 and CdofingongisSo were studied for the first time. This study
emphasized the importance of crystal grth because the common feature of these
compounds is that they have a strongly anisotropic, three-dimensional framework
composed of galena slabs (N aCl-type) cut perpendicular to the [311] direction. Especially,
the crystal structures of CdBi4S7 and Cdo,63Pbo,32Blssg exhibited a tropochemical cell-
twinning feature with a mirror as the twinning operation.

Interestingly, all the compounds showed n-type semiconducting behavior with
relatively high electrical conductivities. In the Ag systems, the electronic band structure
calculation of AgBi3S5 suggested that the electron and hole transports took place in
different slabs and the silver d-states were involved near the Fermi level, which
influenced the charge transport properties. To further improve the thermoelectric
properties, p-type doping and partially replacing Ag with monovalent alkali metal, copper,

or thallium should. As the result of B12S3 doping of CdBi4S7 and Pb incorporation in the

210

structure as typified by Cdo_63Pbo_32B15$9 the band gaps could become very narrow, ~ 0.1
eV, which is not common among bismuth sulfide compounds. Substitution with other
elements and modification of the CdBi487 and Cdo_6ngo,ngi5S9 structure by partially or
totally replacing its divalent metal ions, such as Sn or alkaline earth metals, could expand

further the scope of investigations of the thermoelectric properties in this class of

compounds.

211

    
 

 

MMMMMMMMM

1.111111111111111Ellfllllllllj

3 1293 0

AR ES

”fill!

97