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This is to certify that the

dissertation entitled

Exploratory Synthesis

Of Transition Metal Binary, Ternary, and Quaternary Nitrides

presented by

Xianzhong Chen

has been accepted towards fulfillment
of the requirements for

Ph.D. Chemistry

degree in

 

 

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EXPIDRAIORY SYNTHESIS OF TRANSITION METAL
BINARY, TERNARY, AND QUATERNARY NIT RIDES

By

Xianzhong Chen

A DISSERTATION

submitted to
Michigan State University
as partial fulfillment of the requirements

for the degree of

DOCTORAL OF PHILOSOPHY

Department of Chemistry

1996

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ABSTRACT

EXPLORATORY SYNTHESIS OF TRANSITION METAL
BINARY, TERNARY, AND QUATERNARY NITRIDES

By

Xianzhong Chen

A method for synthesizing transition metal nitrides by heating nanoscale metal
particles under flowing N2(g) or NH3(g) has been explored. Homogeneous reduction of
metal chloride(s) by an alkalide in dimethyl ether solution produces highly reactive
nanoscale metal particles. These particles have been used as precursors for the synthesis
of transition metal nitrides. Several previously known transition metal nitrides were
prepared by heating under N,(g) or NH3(g) at the temperatures : 'y—MozN (800°C, N2), MozN
(720°C, N2), Ta3N5 (650°C, NH3), FegMogN (>650°C, N2), and NbN”, (800°C, N,). In
addition, a previously unreported phase in the Ba-Nb-N system has been prepared. The
synthesis temperatures for y—MozN and M02N, Ta3N5, and NbN”, are significantly lower
than those required by conventional methods.

Three ternary nitrides, Ba,NbN,, SerbN3, and BaThN2, were synthesized at high
temperature by heating a mixture of Sr2N (or BazN) + NbN and Ba,N + Th,N,, respectively,
under flowing N2. Their structures were determined by Rietveld X-ray powder diffraction
techniques. M2MN3 M = Ba, Sr) crystallizes is0typically with Ba,,TaN3 and Bazan, in
the monoclinic system with space group C2/c(#15). Ba'I‘hN2 is isostructural with BaCeN2

and RbScO2 in the hexagonal system with space group P6,/mmc(#194).

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Two isostructural quaternary nitrides, Li,Ba,_h_/IN, M = Nb, Ta), were synthesized
from Li, Ba, and Nb or Ta metals under flowing N2 at 850°C. The structures, as
determined by single crystal X-ray diffraction, are monoclinic, space group C2./c with Z =
4. The pure nitrides can be synthesized as powders by heating a mixture of Li3N and
BaszN3 or Ba,TaN3 in a closed Nb or Ta tube at high temperature.

Two 1J3Ba2MN4-related M = Nb, Ta) new isostructural quaternary nitrides,
Li,Sr21\_d_N, M = Nb, Ta), were synthesized from Li, Sr, and Nb or Ta metals in flowing
Ar + NH3 at 800°C under ambient pressure. The structures, as determined by single crystal
X-ray diffraction, are orthorhombic, space group Pnnm (#58) with Z = 4. A new procedure
for growing nitride crystals under flowing NH,(g) at ambient pressure is expected to be
promising for crystal growth of lithium and alkaline earth metal-containing ternary and
quaternary nitrides.

A new Nb-containing oxidenitride, Li,6Nb2N,O, was found in the Li-Ba-Nb-N
system. The oxidenitride can be synthesized by heating a mixture of UN, Nb205, and NbN
in the molar ratio of 16/3:1/5:8/5 under flowing Ar+N2 at high temperature. The structure
as determined by single crystal X-ray diffraction is rhombohedral(hexagonal axes), space
group R3(h) with Z = 3. LimszNsO crystallizes isotypically with Li,6Ta,N,O with an anti-

fluorite type superstructure.

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ACKNOWLEDGEMENTS

I would like to express my deep gratitude to Professor Harry A. Eick, my research
advisor, for his guidance, encouragement, support, and patience throughout the course of
this work.

I would like to thank Professor Mercouri G. Kanatzidis for serving as second reader,
for his inspiration, encouragement, and many helpful suggestions and discussions during
this work.

I also would like to thank Professor James L. Dye for serving as my committee
member, for giving me the opportunity to work in his lab for the synthesis of nanoscale
metal particles, and for many helpful suggestions for this work.

Thanks are extended to Professor Eugene LeGoff for his being my committee
member, to Dr. Donald L. Ward, Dr. Wieslaw Lasocha, Dr. Kou-Lih Tsai, Dr. Guo Liu, Dr.
Xiang Zhang, and Mr. Reza Loloee for their various helps during this work.

I also thank the Department of Chemistry and the Center for Fundamental Materials
Research at Michigan State University for financial support.

Special thanks are given to my wonderful wife, Laiping Chen, for her continuous

love, understanding, and encouragement. This work could not be done without her support.

iv

Ch

TABLE OF CONTENTS

PAGE

LIST OF TABLES ............................................. viii
LIST OF FIGURES ............................................. xi
Chapter 1 Introduction .......................................... 1
Chapter 2 General Background ..................................... 2
1. Synthesis, structure, and prOperties of transition metal nitrides ........ 2

l. Binary Nitrides .................................... 2

2. Ternary Nitrides ................................... 7

II. X-ray Crystallography .................................... l4

1. X-ray Single Crystal Diffraction ........................ 15

2. X-ray Powder Diffraction and the Rietveld Technique ........ 16

III. Basic theory of Magnetochemistry .......................... 18

1. Several common magnetic effects ....................... l8

2. Some basic definitions and laws ........................ 19

Chapter 3 Synthesis of Transition Metal Nitrides by Using Nanoscale Metal

Particle Precursors ......................................... 22
I. Introduction ........................................... 23
H. Experimental .......................................... 24
1. List of Chemicals and Instruments ...................... 24
2. Purification of some reagents prior to use ................. 25
3. Preparation of Nanoscale Metal Particles .................. 26
4. Synthesis of Nitrides ................................ 33
III. Results and Discussion .................................. 34
1. Metal Particles .................................... 34
2. Nitrides ......................................... 42

Ch.

Chapter 4 Synthesis and Rietveld Structural Refinement of Ternary Nitrides

BaszN3, SerbN3, and BaThN2 ............................... 55
I. Introduction ........................................... 55
11. Experimental .......................................... 56
1. List of chemicals and equipment ....................... 56
2. Synthesis of Ba2NbN3, SerbN3 and BaThN2 ............... 56

3. Preliminary phase analysis by Guinier powder X-ray diffraction
method and powder pattern indexing .................. 59
4. Powder Diffraction Data Collection for Structural Refinement . . . 59
5. The Rietveld Structural Refinement ..................... 60
6. Magnetic Susceptibility Measurement .................... 61
III. Results and Discussion .................................. 61

1. X-ray powder examinations for the precursors Ser, NbN, and

Th3N, ....................................... 61

2. X-ray powder diffraction results and structural refinement for
BaszNa, SerbN3, and BaThN2 ..................... 65

Chapter 5 Synthesis and Single Crystal Structures of Li3BaMN4M = Nb, Ta),
LimszNso, and Li,Sr,_MN,M = Nb, Ta) ......................... 88
I. Introduction ........................................... 89
11. List of Chemicals and Equipments ........................... 89
III. Li,Ba,NbN, .......................................... 90
1. Experimental ..................................... 90
2. Results and Discussion .............................. 93
VI. Li._,Ba,TaN4 .......................................... 106
1. Experimental ..................................... 106
2. Results and Discussion .............................. 111
V. Li,,Nb2N,O ........................................... 116
1. Experimental ..................................... 116
2. Results and Discussion .............................. 118
VI. Li,Sr,NbN, ........................................... 130
1. Experimental ..................................... 130

vi

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Cha
Ref:

2. Results and Discussion .............................. 133

VII. Li,Sr2TaN, .......................................... 145
1. Experimental ..................................... 145
2. Results and Discussion .............................. 147

Chapter 6 Some Unsuccessful Transition Metal Ternary Nitride Synthesis

Experiments -- Reference Information for Future Work ............... 153
Chapter 7 Conclusion ........................................... 159
References ................................................... 161

vii

LIST OF TABLES

PAGES

Table 1 Transition metal binary nitrides .............................. 3

Table 2 Superconducting critical temperatures of binary nitrides in groups 3-6 . . . . 6

Table 3 Recently reported transition metal ternary and quaternary nitrides ....... 9
Table 4 Solubilities of some alkali metal halides in liquid ammonia at 0°C (g/100

g soln.) ................................................. 40

Table 5 Guinier X-ray powder diffraction data for the mixed phases of NbN, . . . . 45

Table 6 Miller indices and observed and calculated d-spacings and intensities for

Ta3N5 .................................................. 46
Table 7 Guinier powder data for the Fe-Nb particles heated at 600°C under flowing
N2 .................................................... 49

Table 8 Guinier X-ray powder diffraction data for Fe-Nb particles heated at 800°C
in N2 and the reported X-ray powder diffraction data for NbN”, and a-Fe . 50

Table 9 d-spacings and intensities for product obtained at 700°C and for Cu ..... 52
Table 10 Indexing result for the new phase found in Ba-Nb-N system ......... 54
Table 11 Miller indices and observed and calculated d-spacings and intensities for

Th3N, .................................................. 62
Table 12 Miller indices and observed and calculated d-spacings and intensities for

Ser ................................................... 63
Table 13 Rietveld refinement results for BaszN3 and SerbN3 .............. 66
Table 14 Atomic Positions and B(eq) for BaszN3 ....................... 69
Table 15 Atomic Positions and B(eq) for SerbN3 ....................... 69
Table 16 Selected Bond Distances (A) and Angles (deg) for Ba2NbN, ......... 70
Table 17 Selected Bond Distances (A) and Angles (deg) for SerbN3 .......... 71

Table 18 Miller indices and observed and calculated interplanar distances and

viii

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Ta

Ta

Ta

Ta'

Ta]

Tat
Tat

Tab
Tab

intensities for BaszN3 ..................................... 72
Table 19 Miller indices and observed and calculated interplanar distances and

intensities for SerbN, ...................................... 73
Table 20 Atomic positions for BaThN2 ............................... 79
Table 21 Selected Bond Distances (A) and Angles (deg) for BaThN2 .......... 79
Table 22 Data collection parameters and the Rietveld refinement results for
83'1th ................................................ 81
Table 23 Miller indices and observed and calculated interplanar distances and
intensities for BaThN2 ...................................... 82
Table 24 Summary of Crystal and Diffraction Data for Li;,Ba,NbN4 ........... 101
Table 25 Atomic Positions and B(eq) for Li,Ba,NbN, ..................... 102
Table 26 Anisotropic thermal parameters, U”, for Li3B a,NbN4 ............... 102
Table 27 Selected Bond Distances (A) and Angles (deg) in Li3Ba2NbN, ........ 103
Table 28 Miller indices and observed and calculated interplanar distances and
intensities for Li,Ba,NbN, ................................... 104
Table 29 Summary of Crystal and Diffraction Data for LiaBaQTaN, ........... 109
Table 30 Atomic Positions and B(eq) for Lia,Ba,TaN4 ..................... 110
Table 31 Anisotropic thermal parameters, U”, for Li,Ba,TaN4 ............... 110
Table 32 Selected Bond Distances (A) and Angles (deg) in Li3Ba2TaN, ........ 113
Table 33 Miller indices and observed and calculated interplanar distances and
intensifies for Li3Ba2TaN, .................................... 114
Table 34 Summary of Crystal and Diffraction Data for Li,6Nb2N,O ........... 120
Table 35 Atomic Positions and B(eq) for LimszNso ..................... 121
Table 36 Anisotropic thermal parameters, U”, for LimszNso ............... 121
Table 37 Selected Bond Distances (A) and Angles (deg) in Li,,Nb,N,o ........ 122
Table 38 Miller indices and observed and calculated d-spacings and intensities for
Li,,Nb,N,O .............................................. 128
Table 39 Summary of Crystal and Diffraction Data for Li,Sr,NbN, ........... 135
Table 40 Atomic Positions and B(eq) for Li,Sr,NbN, ..................... 136
Table 41 Anisotropic thermal parameters, U“, for Li3Sr2NbN, ............... 136
Table 42 Selected Bond Distances (A) and Angles (deg) in Li,st,NbN, ........ 137

ix

Table 43 Miller indices and observed and calculated interplanar distances and

intensities for Li,Sr,NbN4 .................................... 143
Table 44 Summary of Crystal and Diffraction Data for Li38r2TaN4 ............ 148
Table 45 Atomic Positions and B(eq) for Li,Sr2TaN,, ..................... 149
Table 46 Anisotropic thermal parameters, U“, for Li3Sr2TaN, ................ 149
Table 47 Selected Bond Distances (A) and Angles (deg) in Li38r2'l‘aN4 ......... 150

Fig
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Fi g1
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Figu
Figu
Figll:

LIST OF FIGURES

PAGES
Figure 1 NaCl-type structure ...................................... 5
Figure 2 anti-NiAs structure ....................................... 5
Figure 3 WC structure ........................................... 5
Figure 4 The anti-fluorite type superstructure of Li7NbN, .................. 12
Figure 5 lS-crown-S ............................................ 24
Figure 6 H-cell with a frit ........................................ 27
Figure 7 The X-ray photoelectron spectroscopy of Nb particles .............. 36
Figure 8 The X-ray photoelectron spectroscopy of Mo particles .............. 37
Figure 9 EUR spectrum of Mo particles before heat treatment ............... 38
Figure 10 FI‘IR spectrum of Mo particles after heat treatment ............... 39
Figure 11 X-ray powder pattern of Fe3Mo,N ........................... 48
Figure 12 Powder diffraction pattern of the new phase in Ba—Nb-N system ...... 53
Figure 13 X-ray powder pattern of NbN .............................. 64

Figme 14 Observed and calculated powder pattern with a difference plot for
Ba,NbN3 ................................................ 67

Figure 15 Observed and calculated powder pattern with a difference plot for
SerbN3 ................................................ 68
Figure 16 The structure of MZNbN3M = Ba, Sr) ........................ 74
Figure 17 Coordination of Ba(l) or Sr(l) by N atoms ..................... 75
Figure 18 Coordination of Ba(2) or Sr(2) by N atoms ..................... 75
Figure 19 Coordination of N(l) .................................... 76
Figure 20 Coordination of N(2) .................................... 76
Figure 21 Coordination of N(3) .................................... 76
Figure 22 Coordination of N(4) .................................... 76

xi

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Figure 23 A plot of the molar magnetic susceptibility vs. temperature for

Ba,NbN, ................................................ 77
Figure 24 A plot of the molar magnetic susceptibility vs. temperature for

SerbN3 ................................................ 78
Figure 25 Observed and calculated powder pattern with a difference plot for

BaThN, ................................................ 80
Figure 26 The packing diagram for Ba'I'hN2 ............................ 84
Figure 27 Th atoms octahedrally coordinated by N atoms .................. 85
Figure 28 The coordination of the Ba atom ............................ 86
Figlue 29 The coordination of the N atom ............................. 86
Figure 30 The molar magnetic susceptibility data for Ba'l‘hN2 shows the presence

of a paramagnetic impurity, probably ThN ........................ 87
Figure 31 The edge-sharing tetrahedra NbN, and LiN, .................... 95
Figure 32 A view down the b direction. Two Nb-Li(1) chains are linked together

by Li(2) atoms. ........................................... 96
Figure 33 The three dimensional structure of Lia,Ba2NbN4 .................. 97
Figure 34 A view of the Ba atom distorted dodecahedral coordination ......... 99
Figure 35 The coordination environment of the atom N(l) .................. 100
Figure 36 The coordination environment of the atom N(2) ................. 100
Figme 37 Plot of magnetic susceptibility (emu/mol) of Li3Ba2NbN, against T

(K) ................................................... 105
Figure 38 A view of Li3Ba2TaN4 down the b axis ........................ 111
Figure 39 Plot of magnetic susceptibility (emu/mol) of Li3Ba2TaN4 against

temperature (K) ........................................... 115
Figure 40 Unit cell of Li,6Nb2N,O with Li(l) and Li(2) omitted for clarity ...... 119
Figure 41 The environment of Li(l) ................................. 123
Figure 42 The environment of Li(2) ................................. 123
Figure 43 The environment of Li(3) ................................. 123
Figure 44 The coordination of Li(4) ................................. 123
Figure 45 Oxygen atom coordination environment ....................... 124
Figure 46 N(l) coordination environment ............................. 125

xii

Fi
Fl;
Fig
Fig
Fig
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Fig

Figure 47 N(2) coordination environment ............................. 126
Figure 48 X-ray powder diffraction pattern for Li,6Nb2N,O ................. 127
Figure 49 Plot of magnetic susceptibility (emu/mol) of Li,6Nb2N,O against

temperature (K) ........................................... 129
Figure 50 The structure of Li3Sr2NbN4. Li(l) atoms link the layers formed by

NbN4 and Li(2)N4 terrahedra .................................. 138
Figure 51 The Li,Sr2NbN,, layer structure. NbN, and Li(2)N, tetrahedra connect

each other form layers in the be plane ........................... 139
Figure 52 Coordination environment of Sr(l) ........................... 140
Figure 53 Coordination environment of Sr(2) ........................... 140

Figure 54 Observed and calculated powder pattern with a difference plot for
Li,Sr,NbN, .............................................. 142
Figure 55 Plot of magnetic susceptibility (emu/mol) of Li,Sr,NbN, against

temperature (K) ........................................... 144
Figure 56 The structure of Li3Sr2TaN4. Li(l) atoms link the layers formed by TaN4

and Li(2)N, tetrahedra ...................................... 151
Figure 57 Extended view of the Li.,Sr,TaN4 structure ..................... 152

xiii

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CHAPTER 1 INTRODUCTION

Research on the transition metal ternary and quaternary nitrides has received an
increased level of attention during the last several years because these compounds have
potentially interesting magnetic and electrical properties. Many transition metal binary
nitrides are technologically important materials. Some examples of uses include cutting
tools, high temperature-resistant materials, and catalysts. Although the ternary and
quaternary niuides are less explored in comparison to the binary nitrides, a considerable
number of these have been reported since 1989. Since "N3" is similar to "0”" with respect
to size (for CN=4, N3'=1.32 A and 0221.24 Au», polarizability, and electronegativity,
it seems probable that families of nitrides will exhibit high temperature superconductivity
comparable to that found in oxide families. Many transition metal binary nitrides have long
been known as low T, superconductors (See Table 2). All these features make nitride
research very attractive.

On the other hand, since ternary and quaternary nitrides are normally less stable than
the corresponding oxides and are very sensitive to moisture and oxygen, the N2(g) or NH3
gas used for synthesis must be very pure. Oxygen atoms are very difficult to remove once
they get into nitrides and this is the major difficulty encountered in the synthesis of
transition metal nitrides. In the 19508, ternary nitrides such as Li7VN,, Li7MnN4Q), and
Li9CrN,(_3_) formed from lithium metal and various transition metals were studied
extensively by Juza and his coworkers. But very few ternary nitrides were reported
between then and 1989. Around 1989, the discovery of high Tc superconductors was

probably the stimulus that rejuvenated this research area.

the
ear
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2
Most of the ternary and quaternary nitrides reported to date can be represented by
the formulas M’,M,N, and M’,M"pM,N,, where M’ and M" represent alkali and/or alkaline
earth metals and M transition metals, the latter of which usually belong to the first
transition series and those in the groups 4, 5, and 6 of the second and third series. The
work introduced here mainly includes nitrides prepared by using groups 5 and 6 metals,

principally Nb, Ta, and Mo and the actinide metal Th.

1.5

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Penta

CHAPTER 2 GENERAL BACKGROUND

1. Synthesis, structure, and properties of transition metal nitrides

l. Binary Nitrides

Transition Metal binary nitrides have been well studied. Table 1 lists most of the
transition metal binary nitrides.

Synthesis

(1) Nitriding the metal powder under flowing N2(g) or NH,(g)Q):

2M0 + NH3(g) or N2(g)—) MON
(2) Nitriding metal oxide, chloride, or hydride powders in NH3(g)(g):
2 MoO3 + 4 NH,(g) —-> Mo,N + 6 H20 + 3/2 N2(g)
3 TaCls + 20 NH,(g) -) Ta3N, + 15 NH4C1
I-IfH2 + NH,(g) —) HfN + 5/2 H,(g)

(3) Electrochemical Synthesis

Crook and his coworkersQ) reported that anodic dissolution of Nb foil in an
electrolyte solution containing liquid NH3 and NH,,Br produces an insoluble precursor that
can be calcined to yield phase-pure NbN.

(4) Thermal decomposition from an organic precursor(§).

Duyne reported that ammonolysis of CBuCH2)3Ta=CI-I'Bu produced an intermediate
pentamer, [(‘BuCI-lngaNL, and then generated an orange precipitate upon further reaction.
Thermolysis of the latter at 400°C and then at 820°C produced cubic TaN.

Structure

Transition metal binary nitrides have structures in which the metal atoms are nearly

 

 

 

 

 

 

 

 

 

 

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close-packed and the nitrogen atoms are inserted into interstitial sites. Metal atoms in these
nitrides often form simple fcc, hcp, or simple hexagonal substructures. If nitrogen atoms
fill all of the octahedral interstitial sites in the fee structure as shown in Figure l, the
nitrides will have the NaCl-type structure. In the NaCl-type structure, both metal and
nitrogen atoms are octahedrally coordinated. Most common transition metal binary

mononitrides have the NaCl-type structure. The following are some of the examples:

ScN TiN VN CTN
YN ZrN NbN
LnN HfN TaN

where Ln = lanthanides.

There are several other structural types for the binary nitrides as listed below.

 

Nitrides Structure
8’-NbN anti—NiAs type
WN WC type
B—szN, y-Ta,N, FezN w,c type

 

These three type of structures are illustrated in Figures 2 and 3.

Figure 2 can be seen as the NiAs structure if the open circles represent As atoms
and the x’s represent Ni atoms. In the anti-NiAs structure the metal and nonmetal atoms
exchange relative positions. So for 8’-NbN with an anti-NiAs type structure in Figure 2,
the Nb metal atoms form an hcp array and the nitrogen atoms form a simple hexagonal
array. Therefore, Nb is coordinated in a trigonal prismatic arrangement by nitrogen atoms

while N is coordinated octahedrally by Nb.

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Figure 1 NaCl-type structure Figure 2 anti-NiAs structure Figure 3 WC structure

The WC type structure is shown by Figure 3, where both metal (open circle) and
nonmetal atoms (x) form simple hexagonal substructures and so both metal and nonmetal
atoms are in trigonal prismatic coordination about each other.

The W2C type structure can be demonstrated by Figure 2. In the W2C type
structure, the metal atoms (open circles) have an hcp arrangement, but only half of the
interstitial sites (x) are occupied by nonmetal atoms (nitrogen in this case) in a random
manner. Thus the W2C type structure is closely related to the NiAs-type structure.

Properties

Transition metal binary nitrides are usually chemically stable at room temperature
and are metallic. Nitrogen atoms in these nitrides are interstitially located. Many transition
metal binary nitrides have long been known to possess technologically useful properties
such as hardness, high melting point, and superconductivityQ). The compounds, 7-
MozN, TaN, and B-WzN are refractory metallic solids with mechanical and thermal
properties suitable for use in cutting tools, wear-resistant parts, and high-temperature
structural applications. As elecu'onic conductors, the nitrides possess attractive electrical

and magnetic properties. For instance, thin-film resistors of WzN-W and Mo,N-Mo exhibit

 

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7

high resistivities and low temperature coefficients of resistance(§). Some transition metal
nitrides have catalytic activity both for ammonia synthesis(_9_) and for some reactions
traditionally catalyzed by noble metalsMI). Many transition metal binary nitrides are

low temperature superconductors. Table 2(1) lists some of these nitrides.

Table 2 Superconducting critical temperatures of binary nitrides in groups 3-6.

 

 

111 VI V VI
ScN <1.4 K TiN 5.49 K VN 8.5 K CrN <1.28 K
YN <1.4 K ZrN 10.0 K NbN 17.3 K Mo,N 5.0 K
MoN 12.0 K
LaN <l.8 K HfN 8.83 K TaN <1.2O K W,N <1.28 K

 

NbN is a critical component in dc-superconducting quantum interference devices (SQUIDS)
and it is also used in many other technologically important devices including Josephson
junctions, switches for pulsed power applications, and fusion reactor magnetsQ_l).

2. Ternary Nitrides

Numerous transition metal-containing ternary nitrides have been reported during the
last several years. Table 3 lists most of the ternary nitrides reported since 1989. Most of
them were synthesized by using conventional solid-liquid-gas methods. The structural
chemistry of these new nitrides has been studied extensively. Their physical properties,

however, are relatively less explored. With the exception of some of the elements in the

 

 

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first transition series the central transition metal elements in the nitrides usually are in their

highest oxidation states.

Preparation
(1) Heating a mixture of UN or alkaline earth metal nitride and transition metal or
transition metal nitride under flowing N2 or NH,.

up to 870°C, N,

 

 

Li,N + Mo > LigMoN,Qz)
680°C, N,
Ca,N2 + Zn > Ca,ZnN,@

850°C, N, or NH,
BagN2 + NbN > Ba,NbN,(this work)

 

The nitrides M3FeN, M = Ba, Sr)(l_4_) and Ba,_l\_'lN,M = M0, W)Q_5_) were
prepared by heating the alkaline earth metal in Fe, Mo, and W crucibles, respectively. To
obtain single crystals, the reaction mixture is usually first heated under flowing Ar and then
under N,(g). M3MnN3M = Ba, Sr, Ca)(l§) and M,TaN,M = Ba, Sr)(_ll) were
synthesized by reaction of Li, alkaline earth metal and Mn in either tantalum or tungsten
crucibles. In this case Li metal and its reaction product, Li,N, served as flux. Special
conditions are required to prepare Ca,CrN,(_l_§) and Ca3VN302). Ca,CrN,® was
synthesized by heating a mixture of Ca,N, and CrN/Cr,N in a sealed stainless steel tube
under an argon atmosphere at 1350°C and Ca,VN,(1_9) was prepared under similar

conditions but in a Mo tube.

 

 

 

 

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(2) High pressure synthesis: using alkali metal amides as a nitrogen source

Most of the alkali metal-containing transition metal ternary nitrides were synthesized
by this method. Only the alkali metal, Li, form a thermodynamically stable nitride. When
an appropriate transition metal is involved, however, they can form ternary nitrides both at
high and ambient ammonia pressure. Numerous of these nitrides have been prepared by
reaction of transition metals, transition metal binary nitrides, transition metal oxides, or
transition metal chlorides with alkali metal amides at high pressure by using autoclaves.
Examples include MNbN2M = Na, K, Cs)@), MTaN2M = Na, K, Rb, and
Cs)(2_1), Na,_MN,M = M0, W)(2_2),(._2§), and K,,W,N,6NH@. Some of
the reaction equations are as follows:

Ta,N,+3MNH,—)3MTaN,+2NH,

Ta,O,+7MNH,—)2MTaN,+5MOH+3NH,

TaCl, + 6 MNH2 -) MTaN2 + 5MC1 + 4 NH,

3NaNH,+W-)Na,WN,+3H,
In all of these reactions, alkali metal amides undergo a thermal decomposition reaction:

2MNH,—>2M+N,+2H,,
causing the final products to be embedded in solid alkali metal. A typical procedure for
the high pressure preparation of Na,WN,(2;), for example, involved heating a mixture of
tungsten powder and excess NaNH2 in an autoclave to 900°C. The product was embedded
in solid sodium formed by thermal decomposition of the excess amide. Single crystal
Na,WN, was obtained when the sodium metal was washed out by liquid ammonia.

M’MN2M’ = Na, K and M = Nb, Ta)(;5) and NaMN,M = M0, W)L2§)
were also prepared at ambient pressure by using alkali metal amides. We tried to use a

sealed Nb metal tube in place of the autoclave and found that the sealed Nb tube can resist

 

 

the

(3)

synt
Fe,.\

temp

Some

includt
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11

the high pressure generated and does not leak at temperatures below 800°C if the alkali

metal amide used is not in to great an excess.

(3) Ammonolysis of an oxide precursor

An important method for the synthesis of transition metal nitrides is direct
arnmonolysis of a ternary oxide precursor. The fu'st metallic, layered lithium ternary nitride
synthesized from oxide precursors was LiMoN,(2_7). Other recent examples include
Fe,Mo,N(_2_8_), MWN, M = Mn, Fe, Co, Ni)@, and LiWN,(§Q). The reaction
temperature is usually between 700 and 800°C and water is one of the reaction products.

Li,MoO, + 7/3 NH,(g) —> LiMoN, + 1/2 Li,O + 1/6 N, + 7/2 H,O

3 FeMoO, + 8 NH,(g) —-) Fe,Mo,N + 7/2 N, + 12 H,O

FeWO4 + 8/3 NH,(g) —-) FeWN, + 1/3 N, + 4 H,O

A limitation of this method is that it can only be applied to selected nitride systems.
Some oxides form only oxidenitride upon being heated in flowing NH,(g). Examples
include BaMoO,(_fl), SrMoO,(_3_2_), BaCeO,(§), and Na,WO,'2H,O(y_).
DiSalvo and his coworkers® recently analyzed the thermodynamics of ternary nitride
formation by arnrnonolysis. They concluded that ternary oxides containing the most
electropositive metals (alkali, alkaline earth, rare earth) will not form ternary nitrides by
arnmonolysis. But when the metals in the ternary oxide are from group 5 or greater,

ternary nitride formation by reaction with ammonia is likely.

 

 

 

Li
Li;
“'il
Hell

[no

12

Structure
Transition metal ternary nitrides have various structural types. The number of N
atoms coordinated to the transition metals varies from two (linear) to six (ocrahedral) as

exemplified the following examples:

linear Ca,ZnN, and Ca,FeN2
trigonal planar Ca,VN,, Sr,MnN,
tetrahedral Ba,MoN,, Ba,TaN,
octahedral NaTaN,

Most Li—containing ternary nitrides have an anti-fluorite type superstructure. For
example, Li,_MN,,M = V and Mn(3_6), Nbfl), TaCi8_), Li6MN4M = Cr, Mo,
MM), Li,FeN,@), all crystallize with an anti-fluorite type superstructure. Figure 4
shows the Li7NbN4 structure. The superstructure doubles the basic fluorit axes. At each
corner of the cubic, one Nb and seven Li atoms occupy F sites and four N atoms occupy
Ca sites. N atoms are omitted for clarity.

The formulas for the nitrides

 

 

LigMN, M = Cr, Mo, W) can be
considered as Li,,l_vl_ClN4 = A,B and
they all have a structure similar to

that shown in the Figure 4 with one

 

 

 

 

 

 

Li site empty. The structure of

 

 

 

 

 

Li,FeN, can be described similarly

 

 

 

with iron atoms occupying the
neighboring tetrahedral holes along Figure 4 The anti-fluorite type superstructure of Li.,NbN,

[001] and forming an ordered structure.

 

 

Ca,
(nit

nitrj

compo

haVC nc

SCI'craI ,

13

Other alkali metal-containing transition metal nitrides are those prepared mainly
from Ta or Nb with the formula A_M_N,, where A represents Na, K, Rb, or Cs and M is Ta

or Nb. Their reported structure types are summarized below:

 

Nitrides NaNbN,, NaTaN, KTaN,, RbTaN, CsNbN,, CsTaN,

 

Structure a-NaFeO, KGaO, B-cristobalite

 

Some other nitrides and their related Structures are:

Ba,TaN, -- Ba,ZnO,

Ba,MN,], _M_ = M0, W --- Na,As

cazanz NaaHgoa

LiMo(W)N,, CuTaN,, FeWN, ---- CuFeO,

Ca,V(Cr)N,, Ba,FeN,, Ba,MnN,---new structure described below
The compounds, Ca,CrN,(_l§), Ca,VN,Q2), M,FeN,M = Ba, snug), and M,MnN,M =
Ca, Ba, Sr)(1_6), all have trigonal planar MmN,]‘* anions with either 0,,
(nitridovanadate(lII) and niuidochromateOID) or D,h symmetries (nitridoferrate(III) and

nitridomanganate(III)).

Properties

Most of the transition metal ternary nitrides are air-sensitive yellow or nearly yellow
compounds. They react vigorously with water to release NH,(g). Their physical properties
have not been well explored. Magnetic and electrical properties have been reported for

several of these nitrides. LiMoN,(_21) was the first layered lithium-containing transition

 

 

de

St:
Wa
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14

metal nitride reported to be metallic. Magnetic susceptibility data were reported for the
nitrides, Ca,ctN,(1_8), Ca,VN,(_19_), Ca,ZnN,(_l_3), LiMoN,(2_7), Li,FeN,(§2), and
Ba,[CrN,]N&0_). In Ca,CrN,, Cr’*(d3) has been determined to be in a low spin state
(S=1/2). As stated before, trigonal planar [CrmN,]° anions in Ca,CrN, have (1,, symmetry.
Since the Cl" site symmetry is so low none of the Cr" (1 states is degenerate. The d'

configuration of Cr” in Ba,[CrN,]N was also confirmed by magnetic susceptibility dataQQ).

3. Quaternary Nitrides

There is much less quaternary niuide data to report. Most quaternary transition
metal-containing nitrides involve Li-M-Ni-N systems, where M = Ca, Sr, or Ba(4_l).
In LiSr,CoN,@;) and M,LiFe,N,M = Ba or Sr)@ the [Co‘N,]’° and [Fe,N,]"
anions, respectively, have been characterized. Two new quaternary nitrides Li,Ba,MN,M

= Nb, Ta) have been reported recently based on work reported hereinL4A).

II. X-ray Crystallography

X-ray diffraction has become one of the most important methods for the
determination of the structures of solids. In a manner similar to that of light diffracted by
an optical grating, X-rays can be diffracted by crystals which have regularly repeating
structures with interatornic layer separation of about 1 A -~ distances similar to the
wavelength of X-rays. The X-rays are diffracted by atoms or ions in successive planes in
the crystal and the resultant diffracted beams can be detected by films or other devices.
The diffracted beams cancel unless they are in phase, i.e. they must obey the Bragg law:

n1 = 2d sin 0

where A is the wavelength of the X-rays and dis the distance between crystal planes. The

ft

 

 

to

C01-

C01]

15

angle 0 is the angle of incidence of the X-rays to the plane. In an X-ray diffraction
experiment, 0 can be measured directly and so d-spacings can be easily calculated. From
the d,,,, h,k,l values, and lattice constants can be calculated and space group(s) can be
derived based upon systematic extinctions. By measuring the intensity of each diffracted
X-ray beam the crystal structure can be detemlined as demonstrated by the following two
equations. The observed intensities are directly related with the structure factor F, which
is directly related with the atomic coordinates.

10(th = K2 (2(th) |F°(hkl)|2

N
1mm: a,expti2n (In, +ky,+zz,)l
_ 1.1

where C(th) contains several geometric and physical factors which depend upon both hkl
and the experimental conditions such as Lorentz, polarization, and absorption corrections.
In these equations, K is a scale factor, gj is the atomic scattering factor, and x], y,, and z,
are the atomic coordinates of jth atom. Numerous textbooks provide detailed discussions
of X-ray crystallographic procedures(;4_5_). Two experimental methods are widely used
for crystal structure analysis by X-ray diffraction.

1. X-ray Single Crystal Diffraction

This method uses a single crystal which is mounted on a single crystal
diffractometer. The crystal rotates about a crystallographic axis and a monochromatic X-ray
beam is incident on the crystal. The rotation of the crystal brings successive crystal planes
to the reflection positions which obey the Bragg law. The positions (i.e. 0 angles) and
correspondent intensities for each reflection are thus recorded and in theory allow a

complete structure to be determined. Details for this method can be found elsewhere(15_).

 

 

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Entire d

16
The X-ray single crystal diffraction method provides more accurate structural

information than does the powder method, which will be discussed next. This method

requires good techniques for single crystal growth and great care for crystal selection.

2. X—ray Powder Diffraction and the Rietveld Technique

(1) X-ray Powder diffraction

There are many instances in which single crystals are not available or are difficult
to obtain. In such cases X-ray powder diffraction is the only available way to study the
crystallographic properties of solid compounds. In this method, the powder sample is
placed in a beam of monochromatic X-radiation. As the beam travels through the powder
sample, it meets thousands of tiny crystals or assemblages of smaller crystals which are
oriented randomly with respect to the incident beam. When the Bragg law is obeyed each
and every set of lattice planes can make reflections and thus the whole powder sample acts
as a single crystal rotated not about one axis, but about all possible axes. The diffraction
pattern can be recorded by various counters if a diffractometer is used or by film when a
focussing (Guinier) camera is used. Numerous books describe the details of the powder
methodM).

(2) The Rietveld method and the program DBWS-9411(_41)

(a) The Rietveld method is now the most widely used powder refinement method.
It was first introduced by Rietveld(i8) for the refinement of neutron data. It is a
whole-pattem-fitting (profile-fitting) method that matches the calculated to the observed
powder pattern through least-squares refinement of model(s) for the structure(s), diffraction
optics effects, and instrumental factors. Overlapping peaks are not decomposed since the

entire diffraction pattern is calculated and fitted to the experimental profile. Thus, this

17

method makes maximum use of all the information present in the observed powder pattern.

The Rietveld method is a refinement method and needs a structural model. In this
method, the following quantity is minimized with a least-squares procedure:

Sy = 2w, [Y,(obs.) - 1/c Y,(calc.)]2
where c is a scale factor, Y,(obs.) the intensity at the ith step of the diffraction pattern and
w, = Y,(obs.)'m. The Y,(calc.) is the sum of the calculated contributions of all overlapping
peaks at the ith step. For this calculation, a peak shape function that describes the
distribution of the intensity of a single Bragg reflection is needed. The distribution function
for X-ray diffraction peaks is a convolution of different Gaussian and Lorentzian
contributions. Consequently, it can be approximated by a Voigt functionefi) which is
a convolution of a Gaussian and Lorentzian. Because the calculation of this function is
time-consuming and expensive, various approximation have been proposed and used. These
range from modified Lorentzians to Pearson VIIQQ) and Pseudo-Voigt(5_l). These
and other functions are included in the Rietveld program DBWS-9411 which will be
introduced in the next section. The Pseudo-Voigt function is used most often for X—ray
powder diffraction.

(b) The program DBWS-9411 is the latest version in the series of DBWS program
and can be run on a PC. It is designed to carry out Rietveld structural refinement by using
digitized X-ray or neutron powder diffraction data collected with fixed wavelength radiation
at equal increments of the angle 20. The program uses the Newton-Raphson algorithm to
minimize the quantity:

Sy = E wao't - yea?
where wi = l/y,, and yi and ye, are observed and calculated intensities at the ith step. The

sum is over all data points. The calculated intensities (in counts) yci are determined by

18

using the following equation:

ya = 8 Sn A Ex [lFttl2 W29. - 29x) Lx PK] + ya:
where

s is a scale factor.

S, is a function to model the effects of surface roughness. Four choices are
available.

A is an absorption factor, here left at 1.00.

FK is the structure factor.

(D is a reflection profile function. Eight choices are available.

LK contains the Lorentz, Polarization and multiplicity factors.

PK is a prefen'ed orientation function. Two models are available.

ybi is the background contribution.
Among the parameters above, 3, 8,, FR, PK, ybi are all refinable. <l> is sometimes also a
refinable parameter depending on the particular profile function used.

Up to eight phases can be refined simultaneously by the program. Pattern

calculation only and phase quantitative analysis are two other features of the program.

III. Basic theory of Magnetochemistry

The behavior of inorganic solids in a magnetic field depends on whether they have
unpaired electrons and how the unpaired electrons are arranged.

1. Several common magnetic effects

A diamaggetic substance has no unpaired electrons. Under an applied external field,
the induced internal field Opposes the external magnetic field. Magnetic susceptibility x

is both temperature and field independent.

19
A paramaggetic substance has unpaired electrons oriented randomly. Under an

applied external field the induced internal field aligns in parallel direction to the external
magnetic field. Magnetic susceptibility x is temperature dependent (x decreases with
increasing of temperature) but field independent.

Ferromagnetic behavior is observed when the unpaired electrons in the substance are
aligned in a parallel manner and the aligned parallel moments give rise to magnetization
in the absence of the external field. Below the Curie temperature T,, x is both temperature
(x decreases with increasing temperature) and field dependent. Above T,, the substance
shows simple paramagnetism and follows the Curie-Weiss law with a positive Weiss
constant.

An antiferromaggetic phenomenon occurs when unpaired electrons in the substance
are aligned in antiparallel fashion with zero overall magnetic moment. Below the Neel
temperature T... the magnetic susceptibility is both temperature (x increases with increasing
temperature) and field dependent. Above TN, it shows simple paramagnetism and follows
Curie-Weiss behavior with a negative Weiss constant.

An ferrimaggetic substance is one in which the alignment of the spins is antiparallel
with unequal magnitude. The temperature dependence of x is similar to that of a
ferromagnetic substance. But in the paramagnetic range it does not strictly follow the

Curie-Weiss law.

2. Some basic definitions and laws
(1) Magnetic induction B for a substance placed in a magnetic field
B=H+4nl

where H = the magnitude of the magnetic field

20

I = the intensity of magnetization
(2) Permeability B/H and the volume susceptibility lt(emu/cm3)
B/H = l + 4 it K
where K = W
(3) Gram x‘(emu/g) and molar susceptibility xM(emu/mol):
x, = x/d
where d = density g/cm3
XM = X. ' M
where M = molecular weight
(4) Curie and Curie-Weiss Laws
Curie law: xM = Cfl‘
Curie-Weiss law: x” = C/(T-G)
where C = Curie constant
T = absolute temperature
0 = Weiss constant
(5) Magnetic moment it for spin-only situations:
em
where n = number of unpaired electrons
If the Curie law holds, the magnetic moment can be calculated by using the following

equation:

u=2.84 xu- T

The unit for magnetic moment is the Bohr magneton(BM or us)

1 u, = eh/41tm = 9.27 x 10'24 III (T = Tesla)

where e = electronic charge (C)
m = electron mass (kg)

h = Planck’s constant (J s)

21

CHAPTER 3 SYNTHESIS OF TRANSITION METAL NITRIDES BY USING

NANOSCALE METAL PARTICLE PRECURSORS

ABSTRACT

A method for synthesizing transition metal nitrides by heating nanoscale metal
particles under flowing N,(g) or NH,(g) is reported. In some cases the reaction
temperatures are significantly lower than those required by conventional heating procedures.
Nanoscale metal particles of Mo, Nb, Ta, Fe-Mo, Fe-Nb, and Cu-Nb were prepared by
reduction of the respective metal chlorides or mixtures of chlorides with the sodide, [K*(15-
crown-5),]Na', in dimethyl ether solution. Several previously characterized transition metal
nitrides, y—Mo,N, Mo,N, Ta,N,, Fe,Mo,N, and a mixed phases of niobium nitrides were

prepared. A previously unreported phase has been found in the Ba-Nb-N system.

22

23
I. Introduction

Nanomcter-size metal particles are scientificaly and technologically important
because of their high reactivity which is due to their high surface area. Numerous methods
have been reported for the preparation of nanoscale metal particles of l - 20 nm
diameter(_5_2). Preparation procedures include (1) chemical reductions with borohydride
ions(5_3), alkali metal or aromatic radical anions(5_4), hydrogen(_5_5_), and some
other mild reducing agentsQQ); (2) matrix isolation (solvated metal atom dispersion);
(3) evaporation of metals(fl); (4) chemical vapor deposition(_5§); (5) thermal
decomposition of a precursorQQ); and (6) sol-gel processes@). These reactive
metals have been used for organometalic synthesesfig), for degrading chlorocarbons in
contaminated water suppliesL61), and for catalysis(_6;).

Recently Dye and Tsai(6_3) reported that transition metal salts dissolved in
dimethyl ether (Me,O(I)) or tetrahydrofuran (THF) are reduced rapidly to metal particles
with diameters from <3 to 15 nm by dissolved alkalides or electrides. Alkalides and
electrides(6_A) are crystalline salts which contain alkali-metal anions or trapped
electrons. These two unique classes of compounds have been extensively studied by
numerous experimental methods(§§). A major advantage of using alkalides or
electrides is that the reduction reactions occur rapidly with homogeneous solution to
produce intermetallic compounds or alloysQQ). The strong reducing power (-3 V) of
alkalides and electrides means that they can reduce any soluble metal salt to the metallic
state. Numerous transition metal and main group metal particles and alloys have been
synthesized by this new method. It is still a challenge to synthesize extremely oxophyllic
metal particles such as Nb, Mo, Ta without surface oxidation. However, less oxophilic

metallic particles (for example, Au, Cu, Te, and Pt) have been synthesized with little or no

24

oxidation and should be readily usable for either solid state or organometallic synthesis(ii).
In this work, several extremely 0x0phyllic metal particles such as Nb, Mo, Ta were used
for nitride synthesis.

Conventional preparative procedures for the synthesis of transition metal nitrides not
only require severe conditions (e.g., T 2 1200°C for NbN and TaN), but also produce
specimens with very low surface areas, a detriment for their potential use as catalysts and
for their use as solid state synthesis precursors. Although some transition metal nitrides can
be prepared at relatively low temperature by reacting a chloride@) or oxidew with
ammonia gas, preparations directly from the metal require high temperatureQ). We thought
that the small size and high reactivity of nanometer-size metal particles might enable them
to react with N,(g) or NH,(g) at lower temperatures. The synthesis of some transition metal
nitrides from nanoscale metal particles prepared by chemical reduction with alkalides will
now be discussed. The ligand 15-crown-5 has the structure shown in Figure 5. K+ can be

sandwiched between two 15-crown-5 molecules to form [K(15-crown-5),]*.

II. Experimental o/—\
1. List of Chemicals and Instruments [— 3
Chemicals 0 o
lS-crown-S (Aldrich, 98%), potassium metal K/OV

(M.S.A. Research Corp.), sodium metal, dimethyl ether,
liquid ammonia (Matheson), MoCl5 (Aldrich, 99.99%), TaCl,, Figure 5 15-crown-5
FeCl, (Fisher, purified), NbCl, (Johnson Matthey, grade 1),

CuCl, (Aldrich, 99.99%), MoBr, (Pfaltz & Bauer, 99.8%), WCl, (Aldrich, 99.99%).

25

Instrumentation

X-ray powder diffraction (Philips XRG-3000 powder X-ray gemerator) was used for
examining for the presence of crystalline materials of appropriate size in the reduction
products.

A Perkin-Elmer PHI 5400 ESCA/XPS spectrometer was used to perform X-ray
photoelecuon spectroscopy (XPS) for detecting the presence of metallic elements with zero
oxidation state.

A Nicolet 740 FT-IR spectrometer was used to detect the presence of organic

impurities in the metal products.

2. Purification of some reagents prior to use

lS-crown-S was purified by vacuum distillation.

Dimethyl ether was distilled from solutions of excess Na-K and benzophenone into
a stainless-steel storage vessel.

Liquid ammonia was purified by condensing the ammonia over sodium metal and
was stored in a stainless-steel storage vessel.

TaCls was purified by dissolving it in purified dimethyl ether solvent in an H-cell
with a medium fiit(_6_8). A cloudy solution became very clear after filtration through
the frit. The tantalum chloride was regenerated by evaporating the dimethyl ether solvent.

FeCl, and MoBr, were purified by sublimation.

26
3. Preparation of Nanoscale Metal Particles

All of the nanoscale metal particles were prepared in Prof. Dye’s laboratory by a
modification of the procedure of Dye and Tsai(6_3_). The basic reactions can be expressed

by the following equations:

M900)
2 K + 2 (lS-crown-S) -------------- > [K*(15-crown-5),]K' ( 1)

M900)
2MC1, +5[K*(15-crown-5),]K’ ----------- > 2M + 5[K”(15-crown-5),]Cl' + SKCl (2)

01'

M4500)
K-Na + 2 (lS-crown-S) ----------- > [K*(15-crown-5),]Na' ( 1)

M6700)
2MC1, +5[K”(15-crown-5),]Na‘ --—-----> 2M + 5[K*(15-crown-5),]Cl’ + 5NaCl (2)

Reaction byproducts can be washed out by using Me,O(I) and NH,(I). Preparations of each
kind of metal particle described below were usually repeated several times to find optimum

reaction conditions and/or the most satisfactory product.

Mo metal particles

(1) Pretreatment of Potassium Metal

In an Ar-filled glove box, typically 0.685g (17.5 mmol) of K metal and a small
stirring bar were inserted into a pre-evacuated H-cell fitted with a medium frit(§§) (Figure

6).

27
The H-cell was removed from the glove

box and evacuated to 10" Torr. About 2 ' 2:;

ml of dry liquid ammonia was condensed

. —- I! III——
over the metal. The blue solution was It | ‘

 

stirred to ensure that all the metal I!

. \
dissolved completely. Very fine K metal .
particles with a high surface area remained / \
after the liquid ammonia was removed - 4 [- \ 85
from the H-cell and it was then evacuated
to 105 Torr. The H-cell was Figure 6 H-cell with a flit

put into the glove box as soon as possible to prevent oxidation of the K metal.

(2) Reduction Reaction

The reduction was carried out in the H-cell. In the Ar-filled glove box, typically
3.865g (17.5 mmol) 15-crown-5 was added into the side of the H-cell containing K metal
and 0.678g (2.48 mmol) MoCl, was added into the other side of the cell. Another small
stirring bar was inserted into the MoCl, side of the cell. This sample loading procedure
was finished as quickly as possible because some of the K metal and 15-crown—5 react
immediately upon contact and the formed alkalide decomposes equally quickly at room
temperature. To minimize alkalide decomposition, the H-cell was quickly removed from
the glove box, chilled to -60°C with an isopropyl alcohol-dry ice bath and maintained at
that temperature. The cell was connected to a high vacuum line and evacuated to 10‘ Torr.
Purified Me,O(I) was next distilled into both sides of the cell as quickly as possible. K
metal reacts immediately with 15-crown-5 in the dimethyl ether solvent to form the

alkalide, [K*(15-crown-5),]K‘, which dissolves easily in Me,O(I) to form a blue solution.

28
The compound MoCl, also dissolves easily in Me,O(I) to form a brown solution. The H-

cell then was normally maintained in dry ice overnight to ensure that the K metal and 15-
crown-S reacted completely. The next day, while keeping the H-cell cold, the alkalide
solution was poured, in several steps through the fiit into the metal salt solution. A
homogeneous reduction reaction occurred immediately to form the black powder product.
In the initial steps of the reaction the blue color of the alkalide solution disappeared
immediately upon contact with the salt solution. Slow disappearance of the blue color in
later steps indicated that the reduction was nearly complete and that the alkalide should be
added in smaller quantities. The reaction system was stined rapidly while the reduction
was underway.

(3) Product Washing

The byproducts, [K*(15-crown-5),]Cl' and KCl, can be removed by washing first
with Me,O(I) and then with NH,(I). The washing procedure is as follows:

(a) Let the product settle in side A of the H-cell after the reduction reaction is
complete.

(b) Decant the top clear Me,O solution through the frit into the other side (side B)
by inserting side B into liquid N,.

(c) Switch the liquid N, from side B to side A and let Me,O(I) distill back to the
product side so that the byproducts dissolved in the Me,O(I) remain in the side B.

((1) Stir the solution in the side A and repeat steps (a) to (c). Each time additional
byproducts are removed from the metal product contained in side A.

(c) When most of the byproducts have been removed, the Me,O(I) is distilled and
NH,(I) condensed into side A. The metal product is then washed with liquid NH, by

following the procedures detailed above. During the washing the large amount of white

29
substance initially in the product could be seen to disappear. Eventually, only a small

portion of black powder remained in side A.
Intermediate grinding of the powder product during washing is necessary to break

up cluster aggregates and to remove the byproducts more completely. A typical washing

 

 

procedure is described below.
Solvent Volume used Number of washing times
Me,O(I) 35-40 ml 3 times
NH,(I) 35-40 ml 15 times

Intermediate grinding of the powder product in glove box

NH,(I) 35-40 ml 6 times

 

Product washing was considered finished when the black metal particles remain suspended
in the solution and settled very slowly.

A mixture of Me,O(I) and NH,(I) was used in an attempt to improve product
washing. But it did not make a big difference.

(4) Heat Treatment of the Metal Particles under High Vacuum

The reactive nature of the nanoscale Mo particles causes some reduction of the
organic complexant and/or solvent so that traces of ammonia-insoluble organic compounds
remained in the product@§_). IR spectraL63) confirmed their presence. Because we were
unable to find a solvent that would wash out these organic impurities, we developed a heat
treatment procedure in anticipation that at an appropriate temperature under high vacuum

we might be able to remove at least part of the organic and some other volatile impurities.

30

Three heating approaches were used:

(a) The product was heated from room temperature to 150°C in one hour and held
at 150°C for one hour.

(b) The product was heated from room temperature to 300°C in three hours.

(c) The product was heated from room temperature to 500°C in three hours.
Glass tubes containing the metal powder products were all evacuated to < l x 10" Torr
prior to initiating the heating cycle and the evacuation was continued during the heating
process.

The final product was analyzed by X-ray powder diffraction and IR again.

Attempts to use MoBr, to prepare Mo particles was unsuccessful due to its low

solubility in both dimethyl ether and THF.

Nb metal particles

Nb metal particles were prepared by two slightly different methods. The first
method is the same as that described above(6_3) for preparation of Mo metal particles.
Typically 0.500g (12.8 mmol) K metal, 2.016g (9.15 mmol) 15-crown-5, and 0.485g (1.80
mmol) NbCls were used for the preparation. NbCl, is very soluble in the Me,O(I) and

forms a yellow solution. The amount of solvents used for washing is shown below

 

 

Solvent Volume used Number of times of washing
Me,O(I) 3540 ml 2 times
NH,(I) 35-40 ml 15 times
Intermediate grinding of the powder product in glove box
NH,(I) 35-40 ml 4 times

 

21 total washing treatments.

31
In the second method Na-K liquid alloy(with Na:K = 1:1 molar ratio) was used to prepare

the sodide [K*(15-crown-5),]Na', which served as reducing reagent in place of K metal and
the potasside [K*(15-crown-5),JK'. Typically 0.588g Na-K alloy (9.47 mmol Na, 9.47
mmol K), 4.250g (19.3 mmol) 15-crown-5, and 0.831 g (3.08 mmol) NbCl, were used for
the reaction. It was not necessary to pretreat the alkali metals by using liquid ammonia
because of the liquid state of the Na-K alloy. The rest of the procedure, including
reduction, product washing, and heat treatment was similar to those used for Mo particle
preparation. But the time spent on product washing was significantly decreased because
of the much larger solubility of NaCl in liquid ammonia. A typical product washing was

as follows when Na-K alloy was used.

 

 

Solvent Volume used Number of times of washing
M6200) 60 ml 3 times
NFL“) 60 ml 4 times

Intermediate grinding of the powder product in glove box

NH,(I) 60 ml 3 times

 

Tatal washing treatments number 10. Larger volumes of solvents were used here because

a larger quantity of NbCl, was involved.

Ta metal particles
Ta metal particles were prepared by using Na-K alloy; 0.344g Na-K alloy (5.54

mmol Na, 5.54 mmol K), 2.493g (11.3 mmol) 15—crown-5, and 0.693g (1.93 mmol) TaCl,

32

were used for the reduction. The entire procedure was the same as that used for Nb particle

preparation.

Mixed metal particles
The mixed particle preparation procedure was essentially the same as that used for
Nb and Ta metal particles with the exception that two chlorides instead of one were added

together into the H-cell in 1:1 molar ratio.

Fe-Mo metal particles
0.581 g (9.36 mmol Na, 9.36 mmol K) Na-K alloy, 4.259g (19.3 mmol) 15-crown-5, 0.249g
(1.54 mmol) FeCl,, and 0.419g MoCl, (1.53 mmol) were used for the preparation of the Fe-

Mo metal particles.

Fe-Nb metal particles
0.596g (9.60 mmol Na, 9.60 mmol K) Na-K alloy, 4.357g (19.8 mmol) 15-crown-5,

0.258g (1.59 mmol) FeCl,, and 0.435g NbCl, (1.61 mmol) were used

Cu-Nb metal particles
0.623g (10.0 mmol Na, 10.0 mmol K) Na-K alloy, 4.461 g (20.3 mmol) 15-crown-5,

0.258 (1.92 mmol) CuCl,, and 0.516g (1.92 mmol) NbCls were used.

W metal particles
Since WC],5 does not dissolve in either Me,O or THF, W metal particles could not

be prepared.

33
4. Synthesis of Nitrides

N,(g) was purified by molecular sieves (Aldrich) to remove moisture and by De-Ox
catalyst (Johnson-Matthey) to remove 02 before it flowed into the reaction system.
Ammonia was purified by condensing the gas over sodium metal and was stored in a
stainless steel vessel. Alundum (Al,O,) containers used in the nitride preparation were
heated at 900°C under vacuum for several hours to eliminate H,O and other possible
contaminants. All preparatory manipulations were performed in an Ar-filled glove box.

Typical contents of O, and H,O in the box are <1 ppm and 0.5 ppm, respectively.

(1) Mo,N

Mo particles confined in an Al,O, boat were heated in flowing N,(g) at two different
temperatures. In one case, the metal particles were heated slowly to 800°C and held at
800°C for 5 hours. In another case, they were heated at 720°C for 20 hours.

(2) NbN,

Nb particles were heated first in flowing NH,(g) at 700°C for 4 hours and then at
860°C for 8 hours.

(3) Ta,N5

Ta particles were heated in flowing NH,(g) at 650°C for 8 hours.

(4) Fe,Mo,N

Fe+Mo particles were heated in either flowing N,(g) or NH,(g) at various
temperatures that ranged between 400 and 980°C.

(5) Ba-Nb-N system

The Nb particles were mixed with Ba,N, in the approximate molar ratio Ba:Nb=2:1.

The mixture was then heated in flowing N,(g) at 720°C for 20 hours.

34
(6) Fe-Nb system

The Fe-Nb metal particles were heated at 500°C and 800°C in NH3(8). 600°C and
800°C in N2(g). A fresh sample was used at each different temperature.

(‘7) Cu-Nb system

Different samples of the Cu-Nb metal particles were heated under flowing N,(g) or
NH,(g) at four temperatures: 355°C, 420°C, 600°C, and 700°C.
6. X-ray Powder Diffraction Analysis of the Nitrides

All reaction products were examined by Guinier X-ray powder diffraction with
monochromatic Cu Kotl radiation; NBS certified Si (a = 5.430826) A) served as internal
standard. Guinier X-ray powder patterns of some of the products were indexed by using
the programs VISSER@), TREORSCE) and DICVOL92@. The program

LAZY PULVERIXQZ) was used for intensity calculations.

III. Results and Discussion

1. Metal Particles

All six of the metal particles prepared as described above are black powders that
catch fire upon being exposed to air. The X-ray photoelectron spectra (XPS) of the Nb and
Mo particles shown in Figures 7 and 8, respectively, indicate that Nb° and Mo" are present.
The spectra do not show any peaks assignable to Cl', K“, or Na‘, suggestive that these
elements were effectively removed from the product by the washing procedure. The Ta,
Fe-Mo, Fe-Nb, and Cu-Nb metal particles were prepared by the same procedure and were
all presumed to be at zero oxidation states based on their flammability upon exposure to
air. The spectra also show evidence of surface oxidation.

Guinier X-ray powder patterns did not show any reflections for these metal products

35

after the washing procedures and before the heat treatment, suggestive that the particles are
very small. For powder X-ray diffraction, as the crystallite size decreases, the normally
sharp diffraction maxima first broaden at their base, then broaden uniformly throughout
until, finally, they become so broad and diffuse that they are no longer clearly visible.
Crystallites much smaller than 100 A in their average length are too small to yield normal

diffi'action maximaQQ).

36

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40
The IR spectra(6_3) (Figures 9 and 10) show that some organic impurities can not

be removed by washing with Me,O(I), NH,(I), and the mixture of Me20(l) and NH,(I).
Most of the IR peaks, however, disappeared after the sample was subject to heat treatment
at 500°C.

Originally, potassium metal and 15-crown-5 were used to prepare [K*(15-crown-
5);]K' which was then used as the reducing alkalide and the reduction reaction is expressed
as:

M
2MC1, +5[K*(15-crown-5)2]K‘ «-5399» 2M + 5[K+(15-crown-5)2]Cl' + SKCI

The byproducts, [K(15-crown-5),JC1 and KCl, can be removed by washing met with
dimethyl ether and then with liquid ammonia. But since the solubility of KCl in liquid
ammonia, 0.13 g/ 100 g solufion(0°C)@, is very small, it takes a considerable
amount of time to wash out KCl with liquid ammonia. We noticed that the solubilities of

many iodides and bromides are much higher than that of KCl as shown in the Table 4:

Table 4 Solubilities of some alkali metal halides in liquid ammonia at 0°C (g/100 g soln.)

(7_4).

 

Cation Cl‘ Br’ I‘
Li+ 1.43 - -
Na“ 1 1.37 29.00 56.88
K+ 0.132 21.18 64.81
Rb“ 0.289 18.23 68.15
Cs+ 0.381 4.38 60.28

 

41

So from the product point of view, use of a transition metal iodide or bromide would seem
to facilitate product washing. On the other hand, a homogeneous reduction reaction
requires a high or moderately high solubility of the transition metal iodides or bromides in
M90 solution. Dye and Tsai(_6_6) reported that the solubility of MoI2 in MezO is very low
(<0.005 mol dm”). Since we could not locate any other iodide from chemical companies,
we bought some MoBr3 and tried to use it to prepare Mo particles. But it does not dissolve
in either MezO or TI-IF even after purification by sublimation. So we turned our attention
to NaCl which is still much more soluble than is KCl in NH,(I). The sodide K”(15—crown-
5),]Na' was considered as a reducing agent since it would form NaCl instead of KCl as by
product. In this case, Na-K alloy instead of K metal would be used to prepare K"(15~

crown-5),]Na‘. The reduction reaction then becomes:

2MC1, +5[K*(15-crown-5)2]Na' «Eff-(2» 2M + 5[K(15—crown-5),]Cl + 5NaCl

NaCl was expected to be more easily washed out by liquid ammonia because of its much
larger solubility and this was confirmed experimentally as is evident from the two Nb metal
particle preparations. Another advantage is that the liquid state of the Na-K alloy precludes
the need for the NH,(I) pretreatment and reduces the chances of oxidation of the alkali
metal because of the shortened preparative time. In addition, the Na-K liquid alloy is easier
to handle than the sticky K metal.

Vigorous stirring is critical during the reduction reaction. The reduction reaction
proceeds very rapidly and the reduced fine metal particles and byproducts may be wrapped
together to form very hard aggregates. Even with vigorous stirring during reduction, the
metal particles still surround some byproducts and form aggregates, thus an intermediate

grinding is necessary to break the aggregates during washing.

42

Heat treatment for the metal product up to 500°C is essential to remove most of the
volatile impurities. However, even after this heat treatment, the metal particles still release
volatile impurities when they are heated between 700-800°C under flowing N,(g) during
the preparations of nitrides. Since the nanoscale particles were expected to lose their high
reactivity if heated at too high temperatures, they were not heated higher than 510°C.

The unsuccessful preparations of W metal particles from WC16 and Mo particles
from MoBr3 indicate that effective reduction reactions require a high solubility of the metal

halides in the dimethyl ether or THF.

2. Nitrides

(1) Mo,N

X-ray powder diffraction data of two single phase modifications of molybdenum
niuide, y-MozN and MozN, obtained by heating nanoscale Mo particles in flowing N2 at

800°C for 5h and 720°C for 20h, respectively, are listed below.

(i) y—Mo,N(cubic)

 

 

This work Literaturefi)
do I; d 1
2.4003 vs 2.404 100
2.0852 m 2.081 48
1.4729 m 1.4720 30
1.2570 m 1.2550 40
1.2014 w 1.2020 12

 

 

* vs = very strong; m = medium; w = weak.

43

(ii) Mo,N (tetragonal)

 

 

This work Li teratureQ6_)

do 10‘ d 1
2.3894 vs 2.3850 100
2.0937 m 2.0940 33
2.0252 w 2.0120 14
1.4815 w 1.4810 10
1.4582 m 1.4510 20
1.2596 m 1.2580 27
1.1971 m 1.1930 12

 

 

* vs = very strong; m = medium; w = weak.

In fcc Mo,N the metal atoms are arranged in a face-centered cubic array with
nitrogen atoms occupying half of the octahedral sites randomlyC7_7_).

The reflections of both nitride forms are rather broad, indicative of small particle
sizes. MoN was not observed under the reaction conditions employed, consistent with some
previous reports. Lyutaya(2_8) reported that nitriding activated Mo metal with NH, at
450°C yielded MoN, while nitriding at 700°C produced Mo,N. Jaggers and
coworkersCQ) also reported MozN to be the favored modification at higher

temperatures.

(2) NbN,

After the Nb particles were heated in flowing NH3 at 700°C for 4h, the X-ray
pattern showed several very broad reflections which could be assigned to an oxidenitride
phase with formula NbN“ 0.3(_8_(_)_). The oxygen may result from surface oxidation of

the extremely oxophilic metal particles. To get a better crystallized product, the obtained

44

nitride was reheated in flowing NH3 at 860°C for 8h. The Guinier X-ray pattern shows that
the oxidenitride was converted into mixed Nb nitride phases. All reflections with one very
weak exception can be assigned to the three nitride phases, NbNo_,,(_8;),
Nb4N3.92(82_), and NbN(8_3). Conversion of oxidenitrides to nitrides has also been
observed by Lyutaya7L8) and Stacy, et al.(l9_). They observed an oxidenitride intermediate
MoO,N,_, during the reaction of MoO3 with NH3. DiSalvo and coworkersw reported
that the free energy of formation of NbN by ammonolysis of Nb205 is -80.8 Kj/mol at 1000
K. Lyutaya found that the activation energy for formation of MoO,N,,, is lower than that
for Mo,N, an indication that at lower temperatures formation of an oxidenitride intermediate
should be favored over the nitride.

Observed interplanar d-spacings and intensities for the mixed nitride phases are

listed in table 5.

(3) TaaNs
The d-spacings obtained by a Guinier camera for Ta3Ns are presented in table 6.
The observed pattern matches that calculated by the program LAZY PULVERIXQQ) with

the reported atomic positions of Ta3N5 84 .

Table 5 Guinier X-ray powder diffraction data for the mixed phases of NbN,

 

The roduct .N_bNaas(.8_1) M83.) MAE)
A. A“ A A

(1., I0 d I d I d I
2.8012 s - - 2.817 100 - -
2.7756 m 2.767 27 - - - -
2.5679 3 2.570 1 1 2.564 40 - -
2.5379 s - - - - 2.5364 100
2.4989 111 - - 2.499 100 - -
2.3341 8 2.331 100 2.335 40 - -
2.1999 s - - - - 2.1965 81
2.1341 vw - - 2. 120 60 - -
1.8919 8 1.883 26 1.896 60 - -
- — - - 1.690 20 - -
1.5561 w - - - - 1.5533 43
1.5023 vw 1.499 24 - - - -
1.4839 s 1.4840 18 1.480 40 - -
1.4057 vw - - 1.409 40 - -
- - 1.3840 5 - - - -

- - - - 1.364 20 - -
1.3279 vw - - - - 1.3249 29
1.3105 s 1.3080 17 1.310 40 - -
1.2862 vw 1.2850 1 1.279 40 - -
1.2728 vw - - - - 1.2685 12
1.2540 vw 1.2520 14 1.252 10 - -
- - - - 1.234 10 - -

- - - - 1.214 10 - -
1.1673 vw 1.1660 6 1.167 20 - -
1.0558 w 1.0550 13 - - - -

 

Table 6 Miller indices and observed and calculated d-spacings and intensities for Ta,N,.

 

h k 1 do dc 1.. 1c
0 2 0 5.1088 5.1059 111 43
1 1 0 3.6321 98
0 2 2} 3°38" { 3.6193 5 13
1 1 1 3.4327 3.4240 vw 9
1 l 2 2.9759 2.9646 vw 14
0 2 3 2.8415 2. 8419 s 100
0 0 4 2. 5656 7
1 3 0 } 2.5596 { 2.5606 s 65
0 4 0 2. 5529 15
1 1 3} { 2.4902 100
1 3 1 24930 24844 S 17
0 4 2 2.2857 2. 2857 m 27
1 3 3} { 2.0499 4
0 4 3 20452 2.0460 w 17
2 0 0 1.9480 1.9431 m 31
2 0 2 1.8172 1
2 2 0 } 1.8191 { 1.8160 vw 5
1 3 4 1.8124 10
0 0 6 1.7104 11
1 5 2 } 1.7068 { 1.7052 w 23
0 6 0 1.7020 2
2 2 3 } {1.6040 35
1 3 5 ”5039 1.6015 m 17
2 0 4 1.5490 3
1 1 6 } 1.5486 { 1.5474 w 7
2 4 0 1.5462 7
2 2 4} { 1.4823 4
2 4 2 ”826 1.4804 w 13
1 3 6 1.4223 13
0 4 6 } 1.4228 { 1.4210 vw 2
0 6 4 1.4183 <1
0 2 7} { 1.4091 5
2 4 3 L40” 1.4089 "“’ 9
1 7 2} { 1.3198 14
0 6 5 13170 1. 3101 ”w 10
3 1 0} { 1.2851 4
2 0 . 6 12859 1.2839 WV 8

 

s = strong; m = medium; w = weak; vw = very weak.

47

(4) Fe,MoaN

The black powder compound is air stable. The X-ray powder pattern shown in
Figure 11 is almost exactly the same as that reported(_2§). There are two very weak
reflections at about 20 = 52.0° and 813° which are probably due to MOZC. The indexing
result also agrees with that reported by Loye and coworkers(;_8). The compound has a
cubic cell (a = 11.0848(5) A).

The reaction between Fe+Mo particles and N2 or NH3 gas was studied at various
temperatures. Based on the highly reactive nature of the metal particles, the nitride
Fe,M03N might be synthesized at significantly low temperature(< 500°C). But Guinier X-
ray powder diffraction shows that no crystalline compound forms below 600°C under
either flowing NH3(g) or N,(g). At 650°C under an NH3 atmosphere crystalline Fe,Mo,N
is formed. The X-ray powder diffraction data suggest that a temperature higher than 700°C
is necessary to get a well crystallized niuide Fe,Mo3N by the nanoscale metal particle
procedure. Although 700°C is not significantly lower than the temperature reported in the
published work (preparation of Fe3M03N at 800° by reaction of FeMoO4 with NH3 gas) the
new reaction route used in this work confirms that nanoscale metal particles can be used
effectively for solid state syntheses. If an appropriate nitriding agent and solvent could be
found, there is a possibility that nitrides could be prepared in solution at room temperature

by using these highly reactive particles.

(5) Fe-Nb system
This system showed the most complexity when the metal particles were heated under
flowing N2 or NH3. When the metal particles were heated at 600°C under flowing N2, the

Guinier X-ray data show only a few reflections that can not be identified (see Table 7).

48

zfioznon— .«0 5230 .8038 apex S Esmfi

 

 

 

 

 

 

 

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49

Literature powder data for all possible iron nitrides and oxides, niobium nitrides and oxides,
iron niobium carbides and oxides, and Fe-Nb alloys were checked in an effort to identify
the powder pattern. But none of these data matches the reflections listed in Table 7. It
remains unknown if it is one or more new niuide phases. When the metal particles were
heated at 800°C under N2, a different powder pattern was observed (Table 8). In this case,
NbN”, and a-Fe metal phases were identified. The reported powder data for NbNo.”L8_1)
and a-Fe(8_5_) are also listed in Table 8 for comparison. There is a third phase in the
product which has not been identified. Due to the co-existence of Fe metal and NbNo_,,,
the possibility that the third phase is iron niobium ternary nitride is small. It might be a
new modification of niobium nitride. The program TREORSQQ) was used to try to index
the data. Among several possible solutions, the best one is based on an orthorhombic cell
with a = 12.7638 A, g = 6.2248 A, c = 5.0112 A, and FM = 17. One out of 13 reflections

is not indexed.

Table 7 Guinier powder data for the Fe-Nb particles heated at 600°C under flowing N2

 

do 1o
4.0275 3
2.8201 s
1.9865 W
1.7738 w
1.6154 vw

1.3952 vvw

 

50

Table 8 Guinier X-ray powder diffraction data for Fe-Nb particles heated at 800°C in N2
and the reported X-ray powder diffraction data for NbNm and a-Fe

 

T116 “CI M9563 91:26.5)
PM F—N—fl r—A-fi

d, I, d I d 1
3.9406 s - - - -
3.9025 vw - - - -
3.5146 vw - - - -
2.7962 vw - - - -
2.7723 8 2.767 27 - -
2.5704 m+ 2.570 1 1 - -
2.3353 8 2.331 100 - -
2.2268 111 - - - -

2.0293 vs - - 2.0268 100
1.9713 w - - - -
1.9512 vw - - - -
1.8869 w 1.883 26 - -
1.7572 vw - - - -
1.7494 vw - - - -
1.6103 vw - - - -
1 .6007 vw - - - -
1 .573 1 w - - - -
l .5001 w 1.4990 24 - -
l .487 1 w 1.4840 18 - -

1 .4346 w - 1.4322 20
l .3867 w 1.3840 5 - -
l .34 l 1 w - - - -
l .3099 w 1.3080 17 - -
1.2850 1 - -
l .2540 w 1.2520 14 - -
l- 1709 m+ - - 1.1702 30

 

5'1
(6) Ctr-Nb system
CuTaN2 was reported previously by Zachwieja and Jacobs(&). It was prepared
by heating pressed pellets of CuI + NaTaN2 under flowing N2 at 400°C. Cu3N was reported
to decompose to Cu and N2 at 470°C with an enthalpy of -83.7 KJ/mol(8_7). Thus we
expected a ternary copper nitride to be preparable at a relatively low temperature. Based
on this consideration, nanoscale metal particles of Cu-Nb were prepared and were used in
an attempt to synthesize the niuide CuNbN2 which is expected to be similar to CuTaNz.
Unfortunately, the synthesis was not successful. Two different samples of Cu-Nb metal
particles were heated under flowing NH3 at 355°C for 24 hours and 600°C for 43 hours,
respectively. In both cases powder X-ray diffraction indicate the reaction products were
Cu metal and an unidentified phase. Since copper compounds are easily reduced by 112
which can be formed from NH3 during the reaction, two different samples were heated
under flowing N2 at 420°C for 3 days and 700°C for 15 hours, respectively. At 420°C,
only a single phase, Cu, was observed on the Guinier X-ray pattern, indicative that Cu had
been sintered to large enough particles for detection by powder X-ray diffraction. Neither
Nb metal nor any possible Nb-containing product had particle sizes suitable for detection
at this temperature. At 700°C, in addition to Cu phase, another phase, not yet identified,
was observed. These reactions suggest that Cu does nor form a ternary niuide under the

selected conditions. The Guinier powder X-ray data for the reaction at 700°C are shown

in Table 9 together with the reported powder data for Cu.

52
Table9 d-spacings and intensities for product obtained at 700°C and for CuL8_8_)

 

 

the product Cu
doll lobe dobs loll
3.9731 m - -
3.9283 m - -
2.7938 s - -
2.5576 vw - -
2.2094 vw - -
2.0886 vs 2.088 100
1.9883 w - -
1.9664 w - -
1.8085 s 1 .808 46
1.7750 w - -
1.7622 w - -
1.6201 w - -
1.6132 w - -
1.5580 vw - -
1.3979 w - -
1.3198 w - -
1.2787 8 1.278 20
1.0901 s 1.0900 17
1.0435 w 1.0436 5
(7) Ba-Nb—N

A green colored compound (hereafter called Ba—Nb—N phase) with trace unknown

impurities was obtained by heating the mixture of Ba,N2 and Nb (nanoscale particles) under
flowing N2 at 720°C. The compound releases NH, upon contact with air. It does not
dissolve in the (CH,),O, THF, CH,NI—l,, CH,CN, CH,OH, and NH,(I), but most of the trace
impurities can be removed by washing it with CH,OH. The Guinier X-ray powder pattern
was indexed by using programs TREORSQQ), VISSERQQ), and DICVOL92Q1_) based on
a monoclinic cell with lattice parameters a = 8.137(2)A, g = 7.883(1)A, s = 5.672(2)A, a
= 92.54(2)°, M(20) = 44, F 20 = 70. Table 10 lists the indexed result. Since we have not

been able to find a structural model for the new compound, its structure has not been

53

determined. The powder diffraction pattern of the new phase is shown in Figure 12.

 

 

 

 

 

 

 

100
‘f
4
so -» I
I
I
n i o
u 1 i
01 1 4 :5
O 60 .. ' i‘
H ‘. t g 1* I
. 5 I . ‘ 1,
h 1 .i 1* ‘9
u ‘ . .1; i 11
'3 ‘0 ' 5'55 i J:
11- " t 11'
g . a
l : :
‘3 9 ii
H '- .2. :93 :
20 --
0.0
20.0 40.0 60.0
2-theta

 

 

 

 

 

 

Figure 12 Powder diffraction pattern of the new phase in Ba-Nb-N system

Table 10 Indexing result for the new phase found in Ba-Nb«N system

 

 

No. 29. d, dc h it 1 I.
1 10.85566 8.14320 8.1306 1 0 0 44
2 15.64770 5.65850 5.6577 0 0 1 24
3 21.83744 4.06660 4.0653 2 0 0 26
4 22.52616 3.94380 3.9401 0 2 0 21
5 24.62714 3.61190 3.6129 2 1 0 44
6 25.08286 3.54730 3.5457 1 2 0 79
7 26.38114 3.37560 3.3740 -2 0 1 21
8 27.56547 3.23320 3.2333 0 2 1 100
9 28.75498 3.10210 3.1016 -2 1 1 48
10 29.44853 3.03060 3.0313 -1 2 1 57
11 29.83517 2.99220 2.9913 2 1 1 50
12 29.96429 2.97960 2.9783 1 2 1 95
13 31.59425 2.82950 2.8288 0 0 2 44
14 33.02139 2.71040 2.7098 -1 0 2 27
15 34.97427 2.56340 2.5628 -2 2 1 27
16 40.85246 2.20710 2.2063 2 3 0 18
17 46.06380 1.96880 1.9682 4 1 0 20
18 47.44648 1.91460 1.9145 3 0 2 52
19 48.20581 1.88620 1.8859 0 0 3 22
20 49.64896 1.83470 1.8344 4 1 1 22

 

CHAPTER 4 SYNTHESIS AND RIETVELD STRUCTURAL REFINEMENT OF

TERNARY NITRIDES BA,NBN,, 3112191319,, AND 13.4111112

ABSTRACT

Three ternary nitrides, Ba,NbN,, Sr,NbN,, and BaThN,, were synthesized at high
temperature by heating a mixture of Sr2N (or BazN) + NbN and Ba2N + Th,N,, respectively,
under flowing N,. Their structures have been determined by X-ray powder diffraction by
the Rietveld method. M2NbN, (M. = Ba, Sr) crystallizes isotypically with BazTaN, and
Ba,ZnO3 in the monoclinic system with space group C2/c(#15). BaThN2 is isostructural
with BaCeN, and RbScO, in the hexagonal system with a = 3.7523(6) A, g = 12.8580) A,

Z = 3, and space group P6,/mmc(#194).

 

I. Introduction

L17NbN4Q‘D was the only known Nb-containing ternary nitride when we
synthesized Ba,NbN,(§2) but soon thereafter the compound was reported by O. Seeger,
M. Hofmann and I. StrtihleQO). Some Nb-containing oxidenitrides were known, for
example, BaNbO,N and SrNbOZNQD, suggestive that nitrides of Ba-Nb and Sr-Nb

might be preparable. Isostructural compounds Ba2TaN, and Sr,TaN,(_1_Z), which have been

55

56

reported recently, were synthesized by reacting the metals with nitrogen gas at around
1000°C. LiThN2 and Be'I'hN2 are the only known examples of tetravalent Th-containing
ternary nitrides. The chemistry of Th nitride has not been well explored. Another
tetravalent compound BaCeN2(9_2) was reported recently. Since T11“ is similar to Ce“

in size, we undertook the synthesis of BaThN2~

11. Experimental
1. List of chemicals and equipment
Chemicals
Ba,N2 (Cerac, 99.7%), Ba metal (Cerac, 99.7%), Sr metal (Cerac, 99%), Nb metal

(99%, E. H. Sargent & Co., 80 mesh), Th metal, NbCl, (Johnson Matthey, grade 1),

ammonia (Matheson)

Euipment
Philips APD 3720 PDP Micro 11 controlled X-ray powder diffractometer system

Guinier Camera with a diameter of 114.6 mm
Microdensitometer (for obtaining quantitative intensities from films)

Furnaces with temperature controllers and indicators

2. Synthesis of Ba,NbN,, SerbN, and BaThN2

(1) Preparation of precursors

M18125) was prepared by heating NbCl, under flowing NH,(g) (Na purified) to
700°C in 3 hours and then at that temperature for 8 hours. The black powder product was

examined by X-ray powder diffraction.

57

swig): Sr metal was washed with hexane in the air to remove oil and then
quickly put into the Ar-filled glove box. The oxide surface was cut off in the glove box
with a knife. Sr,N was prepared by heating Sr metal in flowing N,(g) at 800°C for 36
hours. The Guinier X-ray powder diffraction of the black powder product agreed with that
calculated by using the program LAZY PULVERIXQQ).

M: The compound was prepared by heating Ba metal under flowing N,(g) at
870°C for 30 h.

Sr mwder: Sr metal was washed with hexane in the air to remove oil and then
quickly put into the Ar-filled glove box. The oxide surface was cut off in the glove box
with a knife. The shining Sr metal was then treated by liquid ammonia (Na purified) in a
H-cell to produce a powder form. Sr metal dissolves in liquid ammonia to form a blue
solution. Sr powder is obtained after the liquid ammonia is distilled from the H-cell.

Mg) was prepared by heating Th metal under flowing NH, at 850°C for

24 hours.

(2) Synthesis of ternary nitrides
Ba,NbN,: This nitride was synthesized by two different methods.

Method 1: A mixture of Ba,N,(or Ba,N) and Nb powder in a Basz = 2:1 molar ratio
was confined in either a Nb or an A120, boat which was then placed in a quartz reaction
tube. The mixture was heated under flowing NH,(g) (Na purified) to 850°C in 2 hours.
There is always a sharp NH,(g) pressure drop at about 360°C, indicative that reaction
begins. The sample was held at 850°C for 2 days and then cooled to room temperature by
quenching. Traces of unreacted Nb metal particles could be seen when the product of this

direct single step reaction, in which commercial Ba,N2 + Nb were the reactants, was

58

examined under the microscope. Since Ba,N2 is volatile at high temperature, it was kept
slightly in excess during sample preparation. Method 2, described below, produces
Ba2NbN, product with higher purity.

Method 2: The procedure is the same as that used in method 1 except that NbN is
used instead of Nb metal. There is always a sharp NH,(g) pressure drop at around 320°C

when the temperature is increasing, indicative of the beginning of the reaction.

Sr,NbN,: This nitride was also synthesized by two different methods.

Method 1: A mixture of Sr and NbN powders in 3 Serb = 2:1 molar ratio was
confined in either a Nb or an A120, boat which was then placed in a quartz reaction tube.
The mixture was heated quickly under flowing NH,(g) (Na purified) to 700°C and held at
that temperature for 2 hours. The temperature was then increased quickly to 900°C, held
there for 3 days, then lowered to room temperature by turning off the furnace.

Method 2: The 8er was mixed with NbN in a Serb = 2:1 molar ratio and the
mixture was heated to 900°C in one hour, held at that temperature for 1 day, then
quenched. During the heating procedure no NH, pressure drop was observed for either of
the methods.

Ser and NbN do not react to form Sr2NbN, under flowing N,(g) at temperature up
to and including 900°C. At 1100°C, SerbN, forms, but there is a significant loss of Sr,N

due to its volatility.

BaThN,: A mixture of Ba,N2 or Ba2N and 'I'l1,N4 in a BazTh = 1:1 molar ratio was
heated slowly to 900°C under flowing N2, held at the temperature for 40 hours, and then

quenched.

59
3. Preliminary phase analysis by Guinier powder X-ray diffraction method and

powder pattern indexing

All products, including even the binary nitride precursors, were examined first by
Guinier powder X-ray diffraction by using monochromatic Cu Ka, radiation and NBS
certified Si @ = 5.43082(3) A) as internal standard. The powder patterns of the two Nb-
containing nitrides were indexed by comparing the observed patterns (BaszN, and
Sr,NbN,) with those of Ba2TaN, and Sr,TaN,(fl). The powder pattern of BaThN2 was
indexed by the program DICVOL92m). The derived lattice parameters were refined by
the program APPLEMANQS). Intensity calculations were performed by using the

program LAZY PULVERIX(_7_2).

4. Powder Diffraction Data Collection for Structural Refinement
Sr,NbN,: The niuide sample mixed with standard Si was coated with mineral oil and
further protected by two pieces of Scotch Tape. Powder diffraction data were recorded on
film by using the Guinier camera mounted on a Philips XRG-3000 X-ray generator.
Accurate d-spacings and intensities were determined from a microdensitometric scan of the

Guinier film.

Bg,NbN,: The d-spacings and intensities were obtained in the same way as that for

SerbN,.

BaThN,: After the well-ground nitride sample has been scattered on a glass backing coated
with double-sided Scotch tape it was coated with a small amount of mineral oil to provide

oxidation protection. Intensity data were collected with the Philips APD 3720 powder

60
diffractometer with graphite monochromatized CuKa radiation, sample spinner, and an
automatic divergence slit (ADS). The diffractometer chamber was flushed continuously
with nitrogen gas vaporized from liquid nitrogen. The CuK0t2 radiation component was
stripped with the APD software. Before the data could be used for the Rietveld refinement,
they had to be converted from automatic divergence slit (ADS) values to fixed slit values
because intensities obtained from ADS vary with angle 20 as the slit width changes. In this

work, the intensities were corrected to values for a 1.0 degree divergence slit(9_6).

5. The Rietveld Structural Refinement

Structures were refined by the Rietveld method with the program DBWS-9411. The
refinement procedure was begun with the zero correction, sample displacement,
transparency factor, and scale factor parameters. Next, cell parameters, mixing parameters
NA and NB, asymmetry factor, and half width parameters U, V, W were refined. After
that, atomic position and isotropic temperature factors were refined. Finally, all of the
parameters listed above except for sample displacement were refined together. Background
was fixed by using the program DMPLOT(9_7) and not refined for any of the nitrides.
No obvious preferred orientation effect was observed. For the SerbN, compound, three
phases were refined together: Sr,NbN,, the Si standard, and SrO which was present in a
trace amount. For Ba,NbN,, two phases, BaszN, and the Si standard, were refined
together. For both Ba2NbN, and Sr,NbN,, the isotropic temperature factors for the N atoms
were refined on the assumption that they had the same values. A similar treatment can be
found in the literatureQQ). The pseudo-Voigt (PV) profile function was selected for
the refinement: 11L + (1-n)G, where L and G are Lorentzian and Gaussian functions,

respectively and n = NA + NB x 20. NA and NB are refinable parameters.

61

The Gaussian and Lorentzian functions are:

Gaussian function

MC, 4.09.4002
exp 2
11,7; H,

 

 

1°11, 1

7111,

01001-2902
4.
H3

Lorentzian function

 

 

 

whereCo=4ln2,C,=4andez=Utan20+Vtan0+W. U,V,Warerefinable

parameters.

6. Magnetic Susceptibility Measurement
Magnetic susceptibilities were measured with a Quantum Design SQUID

magnetometer at temperatures between 5 and 300 K.

111. Results and Discussion

1. X-ray powder examinations for the precursors Ser, NbN, and 'I'h,N4

NbN(2_5), Sr,N(9_3), and Th,N,(_94) are all phase-pure according to the powder X-ray
diffraction data. NbN and Sr2N are black compounds while Th,N4 is a brown solid. NbN
is air stable, but Sr2N and Th,N4 are air sensitive and become white in air within minutes.
The powder X-ray pattern of cubic NbN is shown in Figure 13. The Guinier powder
diffraction data for 'I'h,N4 and Sr2N are shown in Tables 11 and 12, respectively. The
calculated intensity data Ic listed in the table were obtained by using the parameters
published by O’Keeffe and Brese®. Sr,N crystallizes with the layered CdC12-type

structure in space group R3m with the following hexagonal parameters:

a = 3.8566(1) A, s; = 20.6958(4) A, 2:3.

 

If

Table 11 Miller indices and observed and calculated d—spacings and intensities for Th,N,

62

 

 

No. h k 1 d, de 1, 1c
1 1 0 1 3.3308 3.3275 m-s 39
2 o 1 2 3.2562 2.2562 w 16
3 o 0 9 3.0477 3.0428 m-s 48
4 1 0 4 3.0113 3.0108 vs 87
5 0 1 5 2.8633 2.8593 s 100
6 1 o 7 2.5488 2.5456 vw 7
7 o 1 8 2.3961 2.3951 w 21
8 1 0 10 2.1256 2.1208 w 12
9 1 1 0 1.9376 1.9355 111 49

10 1 0 13 1.7842 1.7836 w 25
11 0 1 14 1.6927 1.6895 vw 20
12 1 1 9 1.6338 1.6331 m 16
13 2 0 1.6041 1.6028 w 17
14 1 0 16 1.5244

15 0 0 18 15246 1.5214 W

16 2 0 8 1.5066 1.5054 vw

 

vs=very strong, s=strong, m=medium, w=weak, vw=very weak

Table 12 Miller indices and observed and calculated d-spacings and intensities for Sr2N

63

 

 

No. h k 1 d, d, 1, 1c
1 0 0 3 6.9126 6.8986 m 48
2 0 0 6 3.4564 3.4493 w 26
3 0 1 2 3.1797 3.1785 vs 100
4 1 0 4 2.8099 2.8061 vs 86
5 o 1 5 2.6024 2.5993 w 14
6 0 0 9 2.3065 2.2995 vw 10
7 1 0 7 2.2182 2.2138 w 26
8 0 1 8 2.0507 2.0452 w 18
9 1 1 0 1.9295 1.9283 111 34

10 1 1 :3 1.8583 1.8571 w 4
11 1 1 :6 1.6860 1.6831 w 11
12 2 0 2 1.6497 1.6486 in 16

0 1 11 1.6392 18
13 0 2 4 1.5901 1.5892 w 15
14 1 1 :9 1.4779 1.4776 vvw 8
15 0 2 7 1.4551 1.4540 vvw 7
16 1 0 13 1.4151 1.4371 w 11

2 0 8 1.4030 6
17 1 2 -4 1.2267 1.2264 w 8

2 1 1.2264 8
18 3 0 1.1131 1.1133 w 9

 

vs=very strong, s=strong, m=medium, w=weak, vw=very weak, vvw=very very weak

732 .«0 5280 $0300 3.2-x m— ”Sufi

 

 

 

 

 

 

 

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65
2. X-ray powder diffraction results and structural refinement for Ba,NbN,, Sr,NbN,,

and BaThN,

(1) Ba2NbN, and SerbN,

Yellow Ba2NbN, and grayish-white Sr,NbN, are both air sensitive. They
completely decompose to white substances within minutes upon exposure to the atmosphere
and release NH, which can easily be detected by moist pHydrion paper or by smell.

Preliminary examination of their powder patterns shows that BaszN, and SerbN,
are isostructural with Ba2TaN, and Sr,TaN,(_1_Z), respectively. The indexing results of the

two new Nb-containing nitrides are as follows:

 

BaszN, SerbN,
a(A) 6.1413(4) 5.997(1)
h(A) 11.8001(7) 11.246(2)
_c__(A) 13.249(1) 12.565(3)
B(°) 91.637(5) 9257(2)

 

The lattice parameters listed in Table 13 (see next page) are different from those
shown above because they were refined during the Rietveld procedure.

The refinement results for the two ternary nitrides are shown in Table 13. The
atomic positions for BaszN, and SerbN, are listed in Tables 14 and 15 (page 69),
respectively. The observed and calculated diffraction patterns for the two nitrides with
difference plots are shown in the Figures 14 and 15, respectively. Selected bond distances
and angles are listed in the Tables 16 and 17 (page 70 and 71). Some standard deviations
of the bond distances and angles are relatively large for both compounds. Miller indices
and observed and calculated interplanar distances and intensities for BaszN, and Sr,NbN,

are listed in Tables 18 and 19 (page 72 and 74), respectively.

66
Table 13 Rietveld refinement results for BaszN, and SerbN,

 

 

BaszN, SerbN,
Pattern 20 range(deg) 18-90 15-90
Step scan increment(20 deg) 0.015 0.015
Formula weight 409.61 310.2
Space group C2/c(#15) C2/c(#15)
a(A) 6.1400(3) 5.9864(2)
p(A) 11.7920(6) 11.2271(3)
9(A) 13.24490) 12.5465(4)
B(deg) 91.620(3) 92.587(2)
Z 8 8
Volume(A3) 958.60(9) 842.39(4)
Density(ca1)(g/cm3) 5.676 4.891
Scale factor 1.90x107 4.40x10’5
FWHM parameters(Half-width parameters)
U 009(2) 0.010)
V -0.05(2) 0.006(8)
w 0.017(3) 0.006(2)
Peak profile function Pseudo Voigt Pseudo Voigt
Mixing parameters NA, NB 0.24, 0.9 -0.17, 0.019
Asymmetry parameter 4.1(4) 0.2(2)
R... = 100 (2 ways-yam: wryorim 7.87 12.06
Rp = 100 Z lyoi-ch/Zlyoil 5.58 7.79

 

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Table 14 Atomic Positions and B(eq) for BaszN,

69

 

 

 

Atom 3 r .2 B(eqXA’)
Ba(l) 0.257(2) 0.3863(5) 0.0756(6) 3.4(2)
Ba(2) 0.244(2) 0.1980(7) 0.3564(6) 3.9(2)
Nb 0.238(3) 0.503(1) 0.333(1) 3.6(3)
N(l) 0.28(1) 0.160(7) 0.127(5) 4(1)
N(2) 0 0.432(8) 0.25 4(1)
N(3) 0.5 0.487(8) 0.25 4(1)
N(4) 0.27(2) 0.412(8) 0.448(6) 4(1)
Table 15 Atomic Positions and B(eq) for SerbN,

Atom 25 x 2 B(eqXA’)
Sr(l) 0.2726(8) 0.3858(4) 0.0771(4) 0.2(1)
Sr(2) 0.242(1) 0.2014(4) 0.3520(5) 0.5(1)
Nb 0.233(1) 0.4977(4) 0.3399(5) 1.4(1)
N(l) 0.334(5) 0.159(3) 0.146(3) 2.4(5)
N(2) 0 0.438(4) 0.25 2.4(5)
N(3) 0.5 0.509(4) 0.25 2.4(5)
N(4) 0.272(6) 0.406(3) 0.464(3) 2.4(5)

 

Table 16 Selected Bond Distances (A) and Angles (deg) for Ba2NbN,

70

 

Nb-N(1)
Nb-N(2)
Nb-N(3)
Nb-N(4)

Ba(l)-N(1)

Ba(l)-N(2)
Ba( 1)-N(3)

N(1)-Nb-N(2)
N(1)-Nb-N(3)
N(1)-Nb-(4)
N(1)-Ba(1)-N(1)
N(1)-Ba(1)-N(2)
N(1)-Ba(1)-N(2)
N(1)-Ba(1)-N(3)
N(l)—Ba(1)-N(3)
N(1)-Ba(1 )-N(4)
N(1)-Ba(1)-N(4)
N(1)-Ba(1)-N(4)
N ( l )-Ba(1 )-N (4)
N(1)-Ba(1)-N(4)
N(1)-Ba(1)-N(4)
N (2)-Ba( 1 )-N(3)
N (2)-Ba( 1 )-N (4)
N(2)-Ba(1)-N(4)
N(2)-Ba(1)-N(4)
N(3)-Ba(1)-N(4)
N(3)-Ba(1)-N(4)
N(3)-Ba(1)-N(4)
N(4)-Ba(1)-N(4)
N(4)-Ba(1)-N(4)
N(4)-Ba(1)-N(4)

193(8)
199(4)
198(2)
1.86(9)

2.76(8)
2.74(7)

2.89(2)
296(4)

120(3)
108(3)
1 10(3)
93(2)
91(2)
141(1)
100(2)
151(2)
100(2)
158(2)
94(2)
82(2)
66(2)
89(2)
6490)
60(1)
109(3)
129(2)
121(2)
96(2)
64(2)
84(3)
164(3)
80(3)

Ba(1)-N(4)

Ba(2)-N( 1)

Ba(2)-N(2)
Ba(2)-N(3)

Ba(2)-N(4)

N(2)-Nb—N(3)

N(2)-Nb-N(4)

N(3)-Nb-N(4)

N(1)-Ba(2)-N(1)
N( 1)-Ba(2)-N( 1)
N( l)-Ba(2)-N(1)
N( 1 )-B a(2)-N(2)
N( 1 )-B a(2)-N(2)
N( 1 )-B a(2)-N(2)
N ( l )-B a(2)-N(3)
N( 1)-Ba(2)-N(3)
N( l )-B a(2)-N(3)
N( 1 )-B a(2)-N(4)
N( 1)-Ba(2)-N(4)
N(1)-Ba(2)-N(4)
N( 1)-Ba(2)-N(4)
N( l )-B a(2)-N(4)
N( 1)-Ba(2)-N(4)
N(2)-B a(2)-N(3)
N(2)-B a(2)-N(4)
N (2)-B a(2)-N(4)
N(3)-B a(2)-N(4)
N(3)-B a(2)-N(4)
N(4)-Ba(2)-N(4)

{
{

3.2(1)
292(9)
3.0(1)

3.09(7)
329(6)
293(6)

343(8)
3.21(7)
2.80(9)
290(8)

104(1)
105(4)
1 10(4)
99(1)
87(2)
161(3)
76(2)
74(2)
125(2)
60(1)
58(1)
1 1 1(2)
124(2)
145(2)
97(3)
80(3)
94(3)
85(3)
105(1)
58(2)
135(2)
155(3)
92(2)
91(3)

 

Table 17 Selected Bond Distances (A) and Angles (deg) for SerbN,

 

Nb'Nm 137(3) Sr(1)-N(4) { 2.22242
Nb-N(2) 188(2) 281(4)
Nb-N(3) 2.002(7) 271(4)
Nb-N(4) 187(4) 5’9”“) { 3:38;
Sr(1)-N(1) 3388; Sr(2)-N(2) 326(4)
Sr(1)-N(2) 183(1) Sr(2)-N(3) 287(3)
Sr(1)-N(3) 286(2) Sr(2)-N(4) { 299(4)
261(4)
N(1)-Nb-N(2) 104(2) N(2)-Nb-N(3) 105.9(5)
N(1)-Nb-N(3) 100(2) N(2)-Nb-N(4) 111(1)
N(1)-Nb-N(4) 118(2) N(3)-Nb—N(4) 116(1)
N(1)-Sr(1)-N(1) 100(1) N(l)-Sr(2)-N(1) 103.1(8)
N(1)-Sr(1)-N(2) 91(1) N(1)-Sr(2)-N(1) 75(1)
N(1)-Sr(1)-N(2) 131.9(6) N(1)-Sr(2)-N(1) 161(1)
N(l)-Sr(1)-N(3) 99(1) N(1)-Sr(2)-N(2) 82.8(7)
N(1)-Sr(l)-N(3) 153.5(9) N(1)-Sr(2)-N(2) 72.0(7)
N(1)-Sr(l)-N(4) 103.6(9) N(l)—Sr(2)-N(2) 125.0(8)
N(1)-Sr(1)-N(4) 165(1) N(1)-Sr(2)-N(3) 64.1(7)
N(l)-Sr(1)-N(4) 91(1) N(1)-Sr(2)-N(3) 54.6(7)
N(1)-Sr(1)-N(4) 71.7(9) N(1)-Sr(2)-N(3) 110.0(9)
N(1)-Sr(1)-N(4) 70(1) N(1)-Sr(2)-N(4) 129(1)
N(1)-Sr(1)-N(4) 91(1) N(1)-Sr(2)-N(4) 141(1)
N(2)-Sr(1)-N(3) 65.8(4) N(1)-Sr(2)-N(4) 99(1)
N(2)-Sr(1)-N(4) 60.3(7) N(1)-Sr(2)-N(4) 82(1)
N(2)-Sr(1)-N(4) 103(1) N(1)-Sr(2)-N(4) 96(1)
N(2)-Sr(1)-N(4) 136.2(8) N(1)-Sr(2)-N(4) 89(1)
N(3)-Sr(l)-N(4) 121.3(7) N(2)-Sr(2)-N(3) 103.4(8)
N(3)-Sr(1)-N(4) 88(1) N(2)-Sr(2)-N(4) 61.8(9)
N(3)-Sr(1)-N(4) 70.6(8) N(2)-Sr(2)-N(4) 133.8(8)
N(4)-Sr(1)-N(4) 83(1) N(3)-Sr(2)-N(4) 153.5(9)
N(4)-Sr(1)-N(4) 159(1) N(3)-Sr(2)-N(4) 90.6(9)
N(4)-Sr(1)-N(4) 79(1) N(4)-Sr(2)-N(4) 86(1)

 

72

Table 18 Miller indices and observed and calculated interplanar distances and intensities
for Ba2NbN3

 

 

111:111o dclolchkldo (1,1,1:
0 2 0 5.9304 5.8960 vw 3 0 4 5 1.9709 1.9699 vw 2
1 1 2 4.1667 4.1541 WV 3 0 6 1 1.9442 1.9440 w 7
0 2 3 3.5364 3.5331 111 18 2 2 -5 1.9314 1.9223 vw 11
1 1-3 3.4724 3.4711 111+ 28 2 4 3 1.9014 1.9008 WW 3
1 1 3 3.3893 3.3870 111 21 0 6 2 1.8855 1.8841 vvw 3
0 0 4 3.3101 3.3099 m+ 38 2 2 5 1.8759 1.8747 w 9
1 3 -1 3.2238 3.2228 111 31 3 1 -3 1.8551 1.8531 vw 4
1 3 1 3.2013 3.1997 111 29 1 5 4 1.8267 1.8245 vw 1
2 0 0 3.0701 3.0688 vs 100 3 1 3 1.8159 1.8145 WV 3
1 3 -2 2.9796 2.9788 s 70 3 3 -1 1.8037 1.8040 vw 6
1 3 2 2.9430 2.9427 s 70 0 6 3 1.7963 1.7954 vw 7
0 4 1 2.8787 2.8775 In 21 1 1 7 1.7732 1.7729 w 8
1 14 2.8609 2.8602 m 24 0 4 6 1.7671 1.7665 w 12
1 1 4 2.7980 2.7973 111+ 25 3 3 -2 1.7619 1.7615 w 13
0 4 2 2.6943 2.6930 m+ 20 3 3 2 1.7394 1.7391 w 10
1 3 -3 2.6729 2.6676 vw 2 3 1 4 1.7013 1.7007 WV 4
1 3 3 2.6321 2.6288 vw 5 2 4 -5 1.6748 1.6738 vw 1
0 4 3 2.4526 2.4514 7 2 6 0 1.6567 1.6550 vw 1
0 2 5 2.4163 2.4155 m+ 27 1 7 1 1.6117 1.6139 WV 3
1 3 -4 2.3616 2.3585 vw 2 3 1 5 1.5830 1.5828 vw <1
1 3 4 2.3263 2.3229 vw 3 1 7 -2 1.5814 1.5804 vw 3
2 2 3 2.2919 2.2913 vw 8 1 7 2 1.5760 1.5750 WV 3
2 0 -4 2.2841 2.2828 m- 16 2 6 -3 1.5587 1.5575 vw 4
2 0 4 2.2204 2.2193 m- 12 2 4 -6 1.5471 1.5462 w 8
2 4.1 2.1064 2.1055 vw 4 4 0 0 1.5344 1.5344 w 9
2 4 1 2.1064 2.0927 vw 6 2 4 6 1.5164 1.5162 w 7
1 3 -5 2.0857 2.0833 vw 4 1 7 4 1.4542 1.4540 vw 3
1 3 5 2.0555 2.0525 vw 3 0 2 9 1.4287 1.4273 WV 4
2 4 -2 2.0383 2.0358 w 6 3 1 -7 1.3994 1.3988 vw 3
2 4 2 2.0140 2.0127 w 6

 

Table 19 Miller indices and observed and calculated interplanar distances and intensities

73

 

 

for SerbN3

h k 1 (10 (1c I0 Ic h k 1 (10 (1c I0 1..
0 0 2 6.2964 6.2669 w 4 2 4 1 2.0136 2.0099 w 8

0 2 0 5.6243 5.6136 m 15 1 5 -2 2.0081 2.0026 vvw 2

0 2 2 4.1976 4.1813 w 2 1 3 -5 1.9930 1.9901 vw 4

1 1 -2 4.1248 4.1187 w 5 1 5 2 1.9867 1.9835 w 4

0 2 3 3.3539 3.3515 111 18 2 4 -2 1.9669 1.9633 w 8

1 1 -3 3.3441 3.3413 111 26 1 3 5 1.9485 1.9443 vw 3

1 1 3 3.2178 3.2141 w 12 2 4 2 1.9304 1.9279 WV 6

0 0 4 3.1360 3.1334 In 37 0 6 0 1.8738 1.8712 vw 2

1 3 -1 3.0979 3.0931 111 21 2 4 -3 1.8633 1.8607 vw 4

1 3 1 3.0623 3.0581 111 20 2 2 -5 1.8567 1.8548 vw 12
2 0 0 2.9946 2.9901 vs 100 0 6 1 1.8529 1.8507 vw 7

1 3 -2 2.8606 2.8581 vs 73 2 4 3 1.8241 1.8158 w 1

1 3 2 2.8047 2.8035 vs 77 3 1 -3 1.8103 1.8077 w 7

1 1 -4 2.7457 2.7428 s 25 0 6 2 1.7955 1.7930 w 4

0 4 1 2.7389 20 2 2 5 1.7856 1.7824 w 6

1 1 4 2.6524 2.6485 m+ 28 3 3 -1 1.7555 1.7520 w 7

O 4 2 2.5652 2.5616 m+ 21 1 1 -7 f 1.7196 1.7166 w 10
1 3 3 2.5017 2.4979 w 8 3 3 -2 1.7148 1.7117 111 18
O 4 3 2.3331 2.3298 w 4 0 6 3 1.7108 1.7077 vw 7

0 2 5 2.2926 2.2889 m+ 22 3 3 2 1.6786 1.6766 s 16
2 2 -3 2.2769 2.2725 vw 5 0 4 6 1.6758 17
1 3 -4 2.2614 2.2565 vw 3 3 1 4 1.6376 1.6311 vw 6

2 0 -4 2.2175 2.2137 111 18 2 4 -5 1.6096 1.6098 vw 2

1 3 4 2.2027 2.2031 w 5 O 6 4 1.6015 1.6065 vw 2

2 2 3 2.1958 2.1921 w 9 2 6 0 1.5887 1.5862 vw 2

2 O 4 2.1201 2.1161 m- 14 2 6 -1 1.5806 1.5784 vw 3

O 4 4 2.0948 2.0907 vw 2 2 6 1 1.5719 1.5690 w 7

O O 6 2.0825 2.0890 vw 2 2 4 5 1.5638 1.5618 vw 3

l 5 -1 2.0719 2.0786 vw 4

2 4 -1 2.0344 2.0297 vw 4

 

74
The structure of MszN3(M = Ba, Sr) is shown in Figure 16 with Ba(Sr) atoms

omitted for clarity. They are isostructural with BazTaN3(_ll). In this structure, Nb atoms
are tetrahedrally coordinated by N atoms with average Nb-N distances of 1.94 A in
Ba¢NbN3 and 1.91 A in SerbN3 (Tables 16 and 17, respectively). The NbN4 tetrahedral are
connected to each other by sharing two corners and form a one dimensional chain
l[NbNm]‘* along the a direction. In Li7NbN4@) and Li3Ba2NbN4, which will be discussed
in the next chapter, Nb atoms are also tetrahedrally coordinated by N atoms. The average
NbrN bond distances in BangN3 and SerbN3 obtained here are close to those in
Li7NbN4(3_7) and Li3BaszN4, 1.95 A. The average Ta-N distance in Bafl‘aNJiZ) is 1.96
A.

 

 

 

 

 

 

 

 

a
4, 6
9'-

9 , M2NbN3

9- Nb-N 1.94 A
b .._-o

(M = Ba)
c Nb-N 1.91 A

\0 (M = Sr)

0 C 0 g. 0
N(1) N(2) N(3) N(4) Nb

Figure 16 The structure of MszNBM = Ba, Sr)

75

The coordination of the two Ba(Sr) atoms by N atoms is shown in Figures 17 and 18,
respectively. The average Ba—N distance is 2.99 A and the Sr-N distance is 2.88 A.
Figures 19-22 show the N atom coordination.
N
For M = Ba

(136-1~1),,,,,.,e = 2.92 A

°\/*9

3.0) .b For M = Sr
(Sr-Mm,” = 2.87 A

Figure 17 Coordination of Ba(l) or Sr(l) by N atoms

N
For M = Ba
(Ba-151),...” = 3.09 A

For M = Sr

70% (Sr-mm... = 2.89 A

Figure 18 Coordination of Ba(2) or Sr(2) by N atoms

76

 

 

 

M = Ba, Sr
M(Z)
T M a Ba, Sr
: ~01 .
\ ‘ N(2)
\

1 ’8(2) \\

4 ' M(l)
D Nb

Figure 19 Coordination of N(l) Figure 20 Coordination of N(2)

0 Ma)

 

   

Figure 21 Coordination of N(3) Figure 22 Coordination of N(4)

77
The magnetic susceptibility data for BaszN3 are shown in Figure 23. In the

temperature range 50 - 300 K, the change in the susceptibility is marginal. A paramagnetic
upturn at low temperature is indicative of low-level paramagnetic impurities. O. Seeger,
M. Hofmann and J. StréihleQQ) reported that BaszN3 exhibits a weak temperature

independent paramagnetism between 20 to 300 K.

 

 

 

 

6.0 bj'V'IYIVV‘IWTUI‘UVUUI‘IIIVI’TI’TT‘
:‘ z
5.5 E4 ..
E :. 1
E 4.5 E; '3
.2 E e 3
5 4.0? ‘ '5:
: A :
XE 3.5 :- 9. 1
1- “. q
R : A :
3'0 1' ‘ ‘ ‘ ‘ o A a A 11
25 :LllllLlllllLlllllljllllllllll:
O 50 100 150 200 250 300
T(")

Figure 23 A plot of the molar magnetic susceptibility vs. temperature for BaszN3

78

The magnetic susceptibility data for SerbN3, Figure 24, show an almost temperature
independent paramagnetism between 30 and 300 K. At low temperature the Curie tail
indicates the presence of paramagnetic impurities. Since the sample contains a small
amount of SrO, it may also contain a small amount of oxygen that is substituted for
nitrogen. A small amount of Nbll+ with n < 5 may contribute to the paramagnetism. Traces

of a niobium binary nitride could be another possible source of paramagnetic impurity.

 

 

5.0 VVITIITVIllT’FIIVUtIITIrYIIIU d

.1

4.5 -+

E l

2 4.0 ‘

q

3 3 .

12) I. 3
..A

.4 3.5 _ 1

x 1- “ q

E : ‘ ‘ ‘ 9 A . :

>< 3.0 L- ‘ j)

" -1

25 .llllllLllllljllllllllllillLAI‘

 

 

O 50 100 150 200 250 300
T(K)

Figure 24 A plot of the molar magnetic susceptibility vs. temperature for SerbN3

79
(2) BaThN2

The powder pattern of BaThN2 was indexed based on a hexagonal cell with lattice
parameters a = 3.7591 A and g = 12.8749 A, suggestive that it may be isostructural with
the niuide BaCeN2(_9_2) (hexagonal cell with a = 3.6506 A and g = 12.6603 A) and the
oxide B-RbScOfifl). Intensity calculations confinned this suggestion.

The Rietveld refinement results are listed in the Table 22. The atomic positions for
Ba'I'hN2 are listed in Table 20, and the observed and calculated diffraction patterns with the
difference plot is shown in Figure 25. Selected bond distances and angles are listed in
Table 21. Miller indices and observed and calculated interplanar distances and intensities

for BaThN2 are listed in Table 23.

Table 20 Atomic positions for BaThN2

 

 

atom x y Z W”)
Ba 2/3 1/3 1/4 036(9)
Th 0 0 0 031(4)
N 1/ 3 2/3 0.097(2) 1.9(8)

 

Table 21 Selected Bond Distances (A) and Angles (deg) for BaThN2

 

Ba-N 293(2) 6x

Ba-Th 3.8764(4) 6x

Th-N 250(1) 6x 330(4) 3x
Ba—Ba 3.7523(6) 6x N-N { 393(5)

Th-Th 3.7523(6) 6x ”523(6) 6"

84.5(7) 3x 82.7(7) 6x

N-Ba-N { 79.8(6) 6x N-Th-N { 97 .30) 6x

136.6(3) 6x 180.00 3x

 

80

«ZS—hm .5.“ SE 355% a .23 50:3 3952. 32:23 93 32830 mm Baum"—

 

 

 

 

 

 

 

uuonatw
0.00 0.09 c.0v 0.0N
r. 0.0
t O
I
u
a
2 .. OOH n
. 8
. . n.
.A
.. can
I
[
.. con
.2 00v
Eb_nb_l n...PF F b
Nada .QUAHUHZ nan—«Hog Edna

 

 

81

Table 22 Data collection parameters and the Rietveld refinement results for Ba’I'hN2

 

Step scan increment(20 deg) 0.02
Pattern 20 range(deg) 10 - 100
Count time(sec/step) 14
Divergence slit correction(deg) 1.0
Formula weight 397.4
Space group P6,/mmc
a(A) 3.7523(6)
C(A) 12.858(1)
Z 3
Volume(A3) 156.79(4)
Density(ca1)(g/cm3) 8.42

Scale factor 5.53 x 10‘7
FWHM parameters U, V, W 0.296, -0.147, 0.048
(Half-width parameters)

Asymmetry parameter -0.441

R... = 100 [2 wi(Yoi'yci)2/£ “(10.92)": 14.31

R? = 100 2 lyoi-ch/Zlyml 11.39

R,3 = 100 {(N-P+C)/2 w‘ymP’2 19.21

(N, tOtal # of data points; P, # of parameters adjusted; C, # of constraints applied)

 

Table 23 Miller indices and observed and calculated interplanar distances and intensities

82

 

 

for Ba’l‘hN2
No. h k 1 do do Io lo
1 0 0 2 6.4466 6.4292 w 13
2 1 0 0 3.2554 3.2496 m- 16
3 0 0 4 3.2182 3.2146 m+ 30
4 1 0 1 3.1551 3.1505 w 16
5 1 0 2 2.9023 2.9002 vs 100
6 1 0 3 2.5913 2.5895 m 17
7 1 0 4 2.2888 2.2853 m 18
8 1 0 5 2.0212 2.0166 w 6
9 1 1 0 1.8786 1.8762 m+ 27
10 1 0 6 1.7909 1.7890 m+ 30
11 1 1 4 1.6218 1.6204 m+ 29
12 0 0 8 1.6091 1.6073 vw 5
13 2 0 2 1.5768 1.5753 m 19
14 2 0 3 1.5216 1.5193 vw 3
15 2 0 4 1.4511 1.4501 vw
16 1 0 8 1.4417 1.4407 vw
17 2 0 5 1.3749 1.3736 vvw
18 2 0 6 1.2961 1.2947 m 13
19 1 1 8 1.2215 1.2206 m 18
20 2 1 2 1.2074 1.2064 m 23
21 1 0 10 1.1965 1.1956 w 11

 

vs=very strong, s=strong, m=medium, w=weak, vw=very weak, vvw=very very weak

83
The structure of BaThN2 is shown in Figure 26. BaThN2 crystallizes with the anti-

TiPLlfl) type structure in the hexagonal space group P63/mmc. In the structure
depicted on the next page, between two Th atom layers, N atoms form a simple hexagonal
packed structure and Ba atoms occupy half of the trigonal prismatic holes formed by the
N atoms. These trigonal prismatic layers are separated by Th atom layers. Note that the
trigonal prismatic units in two adjacent layers are oriented differently. The structure can
also be understood by considering the two N atom layers just above and below the Th atom
layer. These two N atom layers form a closest hexagonal packing structure and Th atoms
occupy the octahedral holes. These octahedral layers are separated by the Ba atom layers.
The octahedra in two adjacent layers are in different orientations. There are three Th layers
along c direction in the unit cell.

Figure 27 shows that Th atoms are octahedrally coordinated by N atoms with a Th-
N distance of 2.502 A, very close to the average Th-N distance of 2.52 A in a-
Th3N4Qfl). The coordination of Ba by N atoms is shown in Figure 28. The Ba—N
distance is 2.923 A, compatible to the 2.88 A value observed in BaceN,(9_2). Figure 29
shows that N atoms are octahedrally coordinated by three T11 and three Ba atoms in a facial

arrangement.

84

 

 

 

 

 

 

 

 

 

 

F' .
igure 26 The packing diagram for Ba‘I‘hN2

 

 

85

 

 

 

 

a

Figure 27 Th atoms octahedrally coordinated by N atoms

86

 

 

Figure 28 The coordination of the Ba atom

b

s A

 

 

V.) Th

 

.41;
‘1’

 

 

 

 

 

Figure 29 The coordination of the N atom

87

The molar magnetic susceptibility data for BaThN2 are shown in Figure 30. The
data were corrected for ferromagnetic impurities. The data show the presence of a
paramagnetic impurity, probably ThN, although ThN can not be observed in the Guinier X-
ray powder diffraction pattern of BaTth. If the weight percent of ThN in the sample is
less than 4%, it probably can not be observed by X-ray powder diffraction. The same

phenomenon was observed for the nitride BaCeN2 which contained CeN as an impuritng).

 

j‘TI'UTVIUUVTTTVTTIV‘TVIVYT

Xm ' 10‘ (emu/mol)

UdlllllllLlllLlllALllllllllll“

 

O

llLJlAlLlllILJIJJLAIJLLLJMIA

50 100 150 200 250 300
THO

v

 

 

Figure 30 The molar magnetic susceptibility data for BaThN2 shows the presence

of a paramagnetic impurity, probably ThN

CHAPTER 5 SYNTHESIS AND SINGLE CRYSTAL STRUCTURES OF

LI,BA21~_/1N,(_M = NB, TA), L1,,NB,N,0, AND LI,SR2MN,(M = NB, TA)

ABSTRACT

Two isostructural quaternary nitrides, Li:,Ba2_MN4 (M = Nb, Ta), were synthesized
from Li, Ba, and Nb or Ta metals under flowing N2 at 850°C. The structures, as
determined by single crystal X-ray diffraction, are monoclinic, space group C2/c with Z =
4.

A new Nb-containing oxidenitride, Li,oNb2NoO, was found in Li-Ba-Nb-N system.
The oxidenitride can be synthesized by heating a mixture of Li3N, szOs, and NbN in the
molar ratio of 16/3:1/5:8/5 under flowing Ar+N2 at high temperature. The structure as
determined by single crystal X-ray diffraction is rhombohedral(hexagonal axes), space group
R3(h) with Z = 3. LiloszNSO crystallizes isotypically with Li,oTa2NoO in an anti-fluorite
type superstructure.

The isostructmal quaternary nitrides, Li35r2MN4 (M = Nb, Ta), were also synthesized
from Li, Sr, and Nb or Ta metals in flowing Ar + NH3 at 800°C under ambient pressure.
The structure, as determined by single crystal X-ray diffraction, is orthorhombic, space
group Pnnm (#58) with Z = 4. Growth of nitride crystals under flowing NH,(g) under
ambient pressure is new and the method seems to be very promising for the crystal growth

of lithium and alkaline earth metal-containing nitrides.

88

89
I. Introduction

By using a lithium-barium melt, single crystals of two new quaternary nitrides were
grown under flowing nitrogen. No Nb or Ta-containing quaternary nitrides had been
reported previously. Single crystals of alkaline earth metal-containing nitrides have usually
been grown by reaction of an alkaline earth metal or metal nitride and a transition metal
or metal nitride under flowing N2(g). For example, crystals for Ca3VN,(1_9), Ba3_M_N4 M
= M0, W)L15_), BaoNbN3QQ), and Ba,o[TioN,2](_192) were grown in this way.
Sometimes Li or Li3N is added as flux. For example, Li metal was used to grow crystals
of BazTaN,(11), M.,MnN3 M = Ba, soup), Ba5[CrN4]N(_1_03), and Li3BaMN4 M =
Nb, Ta)(this work). Other methods of crystal growth include (1) synthesis of Ca3CrN3Q8)
by heating a mixture of CrN/Cer and Ca3N2 in a stainless-steel tube under an Ar
atmosphere, and (ii) use of a non-reactive Na metal flux to grow crystals of
SeriNzw. Single crystals of many alkali metal-containing nitrides have been
grown under high pressure by using alkali metal amides as nitrogen soureesQQ - 24).

We attempted to grow single crystals of a new quaternary nitride, Li,Sr,NbN4, under
flowing NH,(g) at ambient pressure. To our knowledge, no crystals of transition metal
nitrides have been grown under flowing ammonia at ambient pressure. Ammonia is a more
reactive nitriding reagent than is N,(g) and thus reactions that might lead to crystal growth
might be more favorable at lower temperature than in an atmosphere of N,(g).

Crystals of LiloszNBO were obtained in a separate experiment under reaction
conditions similar to those used for preparing LioBaQNbNo. The oxygen probably came

from the quartz reaction tube.

11. List of Chemicals and Equipments

Chemicals:

Li metal, Ba metal (Cerac, 99.7%), Nb metal (E. H. Sargent & Co., 99%, 80 mesh),
Ta metal (>99.5%, 325 mesh, Fansteel Metallurgical Corporation), NbCl, (Johnson Matthey,
99.9%), nitrogen and argon gas (AGA Gas, Inc.), Ta and Nb tubes (Fansteel Metallurgical
Corporation)

Equipment:

Rigaku 4-circ1e single crystal diffractometer, Glove box filled with Ar; Glove bag
equipped with microscope; Furnace with temperature programming controller; Thermonic

induction generator and associated oil pump and high vacuum line

111. LioBaQNbN4

1. Experimental

(1) Synthesis of L13B31NbN4:

Single c_rystal gowth: A mixture of elemental Li, Ba, and Nb powder in a 2:1:1
molar ratio was confined in a Nb boat which was then placed in a quartz reaction tube.
The mixture was first heated under flowing Ar to 850°C at a rate of 97°C/h, and then held
at this temperature under flowing nitrogen for 24 h, and finally cooled to 150°C under Ar
at a rate of 10.8°C/h. Pale yellow single crystals of LioBaQNbN4 were isolated from the
crushed product which was both air and moisture sensitive. The 10 rmn long Nb boat was
made from 9 mm diameter Nb tube. The tube was sealed on one side by arc welding and
then cleaned by heating it at 1500°C under high vacuum(< 3 x 10’ Torr) by induction.

Both the Ar and N2 gases were purified by molecular sieves (4—8 mesh, Aldrich) and
De-Ox catalyst (Johnson Matthey). Typical moisture and oxygen contents of the gases were

(0.5 ppm and <1 ppm, respectively.

91

Powder sample preparation: Several precursors have to be synthesized first.

L13N was prepared by heating Li metal under flowing N2(g) at 450°C for 36 hours.

BaQN was prepared by heating Ba metal under flowing N2(g) at 870°C for 30 h.

NbN was prepared by heating NbCl, under flowing NH,(g) (Na purified) to 700°C
in 3 hours and then at that temperature for 8 hours.

Ba2NbN3 was prepared by heating a mixture of Ba2N + NbN (Basz=2:1 molar
ratio) under flowing N2(g) at 850°C for 30 hours.

All of these precursors were phase-pure based on Guinier X-ray powder diffraction
data.

Finally, to synthesize LiaBaQNbNo, a mixture of Li3N + BaoNbN3 (1:1 molar ratio
with about 15% excess of LioN) was confined in a Nb tube, heated under flowing Ar at
720°C for 30 hours and then cooled to 380°C within 14 hours. One end of the Nb tube

was sealed by arc-welding and the other one was simply crimped.

(2) X-ray Single Crystal Diffraction and Structural Solution

An irregular shaped crystal of approximate dimensions 0.120 x 0.080 x 0.080 mm3
was selected and sealed in a 0.1 mm glass capillary in a Nz-filled glove bag. Lattice
parameters were obtained by least-squares refinement of the angle settings of 23 carefully-
centered reflections in the range 22.26 < 20 < 29.00°. The choice of space groups was
reduced to C2/c and Cc by systematic absences (p01, 1 46 2p; _h_lgl, h + g aé 211), and the
structure was solved based on space group C2/c (#15). The integral relationship between
the p and p lattice parameters was of particular concern, but cell reduction programs did not
identify a reasonable higher symmetry cell.

Data were collected at 23:1:1°C by the 02-20 scan technique to 20 S 65°. 0f the

92

1079 reflections which were collected, 1038 were unique. The intensities of three
representative reflections, which were measured after every 150 reflections declined by
0.45%. A linear correction factor was applied to account for this phenomenon. The linear
absorption coefficient for Mo Kat radiation, 142.0 cm“, was used to make an empirical .
absorption correction, which resulted in transmission factors that ranged from 0.81 to 1.00.
The data were corrected for Lorentz and polarization effects and for secondary extinction
(coefficient = 0.27:1:0.05 x 10“).
The structure was solved by direct methods with the program SHELX886M15).
All atoms were refined anisotropically with the refinement based upon F”. The final cycle
of full-matrix least-squares refinementMOg), which was based on 764 observed
reflections (I > 30(1)) and 48 variable parameters, converged with unweighted and weighted
agreement factors of:
R2 = ZlFoz-FfI/ZF} = 0.073
wR2 = [Zw( |Fo2-Fo’|)2/}.“.(wFo2)2]"2 = 0.105
For comparison, a refinement of the overall scale factor on F with the final parameters and
764 observed reflections (I > 30(1)) led to
R1 = XI |Fo|-|Fo| I/ZlFo|.-. 0.043.

The maximum and minimum peaks in the final difference Fourier map, 4.25 and
-3.07 e'lA’, respectively, are close to either Ba or Nb atoms.

Data collection and atomic positional parameters are listed in Tables 24 and 25,
respectively. Thermal parameters (Uij) are presented in Table 26.

All measurements were made on a Rigaku AFC6S 4 circle diffractometer with
graphite monochromated MoKor radiation (M = 0.71069 A). Neutral atom scattering

factors for both nitrides were taken from Cromer and Waber( 107). All calculations

93
were performed by using the TEXSAN( 108) crystallographic software package.

(3) Magnetic susceptibility measurement
Magnetic susceptibilities were measured with a Quantum Design SQUID

magnetometer at temperatures between 5 and 300 K.

2. Results and Discussion

LioBaszN4 is an air and moisture sensitive pale yellow nitride. The color suggests
that the Nb is in the highest oxidation state, +5. Crystals of the nitride were obtained from
the crushed product, suggestive that they grew from the melts. Here Li and Ba metal serve
as both flux and reactants. It is essential to heat the starting material to reaction
temperature firSt under flowing Ar in order to create a Li-Ba melt. Recently, Hubberstey
and Roberts studied the Li-Ba-N system(&) and showed that Ba metal is soluble in
liquid Li metal and nitrogen gas can initially dissolve in the melt Li-Ba to form a
homogeneous solution. This miscibility may be critical for the crystal nucleation of
LioBaszNo. Heating first under Ar can avoid early reaction between Li, Ba and N,. The
ideal temperature range for crystal growth is between 820 to 870°C. The initial heating rate
under flowing Ar has no significant effect on crystal formation.

In the structure, the ions, Nb5+ and Li(l), one of the two independent Li atoms, are
tetrahedrally coordinated by N ions. The Li tetrahedra are distorted (see bond lengths in
Table 27). The NbN4 and UN., tetrahedra are connected to each other by sharing two edges
to form parallel chains along p as shown in Figures 31 and 32. As mentioned earlier, in
the compounds MszN3 (M = Ba, Sr), Nb atoms are also tetrahedrally coordinated by N

atoms and the NbN4 teuahedra are connected to each other by sharing corners. These

94

chains are then linked together by Li(2) atoms to form a three dimensional network as
shown in Figures 32 and 33. The Nb-N(1) and Nb-N(2) bond lengths in LioBaszNo are
1.948(9) A and 195(1) A, respectively, the same as those in Li,NbN, (1.95 A) (3_7) where
Nb atoms are also tetrahedrally coordinated by N atoms. In LiaBazTaN4 comparable Ta-N
bond lengths are 1.962(6) A and 1.942(7) A.

Since the anisotropic temperature factor along p axis for Li(l) atom seems much
larger than those in other directions, it is possible that the Li(l) atoms may occupy at
general positions on both sides of currently determined position with 50% occupancy on
each side along 9 direction. A least-squares refinement based upon this assumption was
performed. But this did not improve R factors and a message of "non-positive definition"
was resulted for the Li(l) atom when it was refined anisotropically. Thus, the current

model is probably better.

95

e.) um

  
      
  

 

  

(Lo
N(2) ,
1 .95 2.38 l 1‘
e “I ‘
Nb a} '
N(2) 11(1)
- . \1 I
N(1) N11)

Figure 31 The edge-sharing tetrahedra NbN4 and LiNo

96

 

 

 

 

 

o O 99 0 ‘39
Nb Li(1) N(1) N(2) Li(2)

Figure 32 A view down the 1; direction. Two Nb-Li(1) chains are linked together by Li(2)

atoms .

97

 

Figure 33 The three dimensional structure of Li3Ba2NbNo.

98

The environments of the two kinds of Li atoms differ as we can see from Figure 32.
The Li(l) atoms are coordinated by two N(1) atoms at 223(3) A and two N(2) atoms at
238(3) A in a severely distorted tetrahedral arrangement. The Li(2) atoms are in the center
of a slightly-distorted trigonal plane comprised of one N( 1) atom (at 2.12(2) A) and two
N(2) atoms (at 199(2) and 2.05(3) A) such that each Li(2) atom links three Li(1)-Nb
chains. The Li(2)-N distances are comparable to the threefold coordinated Li-N distance
in Li3N, 2.106 Aw. Corresponding distances in LigBazTaN. (shown in the next
section) are identical within error limits. A layered fragment that shows the Li(2)-N(1)
bond that is perpendicular to the 3 plane, but devoid of Ba atoms, can be viewed in the
E plane as shown in Figure 32.

The edge-sharing of the [MN] and [LiN,,] tetrahedra is similar to that observed in
the molecular tetrathiometalates, Li,[MS,]-2 TMEDA (M = V, Nb, Ta; TMEDA =
N,N,N’,N’-tetramethylethylenediamine) (m), where [MSJ’ ions interact with Li
atoms by edge sharing to form a linear chain. In LioBaQNbN4 the bond angles N(2)-Nb-
N(2) (106.3°) and N(1)-Nb-N(1) (108.9°) are smaller than the N(1)-Nb-N(2) angles (avg.
110.4°), suggestive that the smaller angles result because of the Li(1)-N interactions. The
interactions of the three-coordinated Li(2) atoms (which have short Li(2)-N(1) and Li(2)-
N(2) distances) with the N(1) atoms probably cause the N(1)-Nb-N( 1) angle to be larger
than the N(2)-Nb-N(2) angle. The bond angle variations N(2)—Li(1)-N(2) (82°) < N(1)-
Li(1)-N(l) (90°) < N(1)-Li(1)-N(2) (avg. 122.2°) can be explained in the same way.
Comparable angular relationships were observed in the tetrathiometalates. The major
difference between this nitride structure and those of the tetrathiometalates is that in the
latter the linear chains are not linked together by Li(2) atoms. Instead, the chains are

connected to the TMEDAs through Li(2) atoms which are located in tetrahedral sites

99

formed by two sulfur and two nitrogen atoms.

The Ba atoms in Li3Ba2NbN4 are located between the Li(1)-Nb chains and serve to
balance charges. Each Ba2+ ion has four nearest N(1) and four nearest N(2) neighbors
whose distances range from 2.82 A to 3.39 A. Comparable values in BaQTaN3 (11) range
from 2.771 to 3.462 A and in Ba3MoN, (15) from 2.654 to 3.566 A. Two sets of two N(1)
and two N(2) atoms describe trapezoids that comprise the arcs of a distorted dodecahedron
as shown in Figure 34. The coordination environments for N(1) and N(2) are shown in

Figures 35 and 36.

 

Figure 34 A view of the Ba atom distorted dodecahedral coordination

100

Figure 35 The coordination environment of the atom N(1)

" Li(l)

:-

‘ 0
N(2) so 014(1)

Nb

D
‘4 Ba

‘.
\'

Figure 36 The coordination environment of the atom N(2)

Table

 

Table 24 Summary of Crystal and Diffraction Data for LioBaszNo

 

 

Chemical formula Li:,Ba2NbN4
Formula weight 444.42
Space group C2/c
a, b, c (A) 11.296(2), 5.673(1), 11.347(2)
fl (degree) 121.456(8)
v (A3) 620.3(2)
Z 4
Do,lo (g/cm’) 4.759
T(°C) 23 :1: 1
Crystal color, Habit pale yellow, irregular
Crystal dimensions (mm) 0.120 x 0.080 x 0.080
20 max (deg) 65.0
Scan type (1)-20
X-ray radiation (2) Mom (71:0.71069 A)
Monochromator Graphite
Octants collected hkl; hfl
Absorption coeff 11 (cm") 142.0
Measured reflections 1079
Observed reflections‘ 764
Unique reflections 1038
F000 760
No. of variables 48
Max (min) peak in final diff. map 4.25 (-3.07) e'lA3
R2”, wR2° 0.073, 0.105
‘ I > 3.000(1)

b R2 = zIl::oz"l:¢:2|/}:Fo2

c wR2 = [2W( |F02-Fc2 |)2/Z(WF02)2] 1/2

102
Table 25 Atomic Positions and B(eq) for LioBaszN4

 

 

 

 

Atom .8 2 .z. 804an
Nb 0 0.1 1 18(3) 0.25 0.49(4)
Ba 0.209330) -0.0359(1) 0.1 1209(7) 0.84(3)
N(1) 0.164(1) -0.088(2) 0.337(1) 0.8(3)
N(2) 0.001(1) 0.318(2) 0.113(1) 1.2(3)
Li(l) 0 -0.365(6) 0.25 3(1)
Li(2) 0.384(2) -0.052(5) 0.458(2) 1.6(8)
The equivalent isotropic temperature factor is defined as (112):
81:2 3 3 t t ..
B(eq) =— 2 2: U0“! 6 are
3 1,-1 1-1
Table 26 Anisotropic thermal parameters, Uij, for LioBaszN4
Atom U11 U22 U33 U12 U13 U23
Nb 0.0062(6) 0.0046(6) 0.0071(6) 0 0.0030(5) 0
Ba 0.0091(3) 0.01 10(4) 0.0108(4) 0.0002(2) 0.0045(2) 0.(D10(2)
N ( 1) 0.006(4) 0.014(5) 0.01 1(4) 0.006(3) 0.003(3) 0.003(3)
N(2) 0.016(5) 0.015(5) 0.015(5) -0.002(4) 0.009(4) 0.003(4)
Li(l) 001(2) 001(2) 007(3) 0 001(2) 0
Li(2) 001(1) 003(1) 002(1) 0.010(9) 0.002(8) 0.01(1)

 

The anisotropic temperature factor coefficients Uij are defined as:

exp(-2rt2(a‘2U,,h2 + 5202,18 + 9915,)2 + zg’p‘unhk + 2g‘g‘uohl + Zp'p‘Unkl»

Tabl

103

Table 27 Selected Bond Distances (A) and Angles (deg) in Li,Ba,NbN,

 

Bond
Nb-N(1)
Nb-N(2)
Li(1)-N(1)
Li(l)-N(2)
Li(2)-N(1)

Li(2)-N(2)

Ba-N(1)

Ba-N(2)

Nb-Li(1)

Nb-Ba

Li(1)-Ba

Li(2)-Ba

Li(1)-Li(2)

Li(2)-Li(2)

Ba—Ba

distances
1.948(9)
1.95(1)
223(3)
238(3)
2.12(2)

199(2)
205(2)

2.87(1)
2.97(1)
2.82(1)

3.10(1)
2.90(1)
323(1)

270(4)
2.97(4)

3.5503(9)
3.5381(9)

3.444(1)

2.97(1)
3.92(2)

335(2)
290(3)
329(2)
3.07 (2)

290(2)
3.41(3)

235(4)
4.056(2)

3.960(1)
3.956(2)

Bond
N(1)-Nb-N(1)

N(1)-Nb-N(2)

N (2)-Nb—N (2)
N(1)-Li(1)-N(1)
Li(l)—Nb-Li(l)
Nb-Li(1)-Nb

N(1)-Li(2)-N(2)

N (2)-Li(2)-N(2)

. N(2)-Li(1)-N(2)

Nb-N(1)-Li(l)
Nb-N(2)-Li( 1)

Li(1)-N(2)-Li(2)

Li(2)-N(2)-Li(2)
Nb—N( 1)—Li(2)
Li(1)-N(1)-Li(2)
Nb- N(2)-Li(2) 2x
N(1)-Li(1)-N(2) 2x

angles
108.9(6)

1 10.7(4)
1 10.1(4)

106.3(7)

90(1).
1 8000(0)
1 80.00(0)

129( 1).
1 19(1).
109(1).
82(1).
80.30)
85.90)
102(1).
81(1).
71(1).
133.9(5)
145.2(8)
144.3(9)
122.1(4)

 

Table 28 Miller indices and observed and calculated interplanar distances and intensities

104

 

 

for Li3Ba2NbN4
h k 1 do do Io Io h k 1 do do Io lo
1 1 -1 4.9083 4.9191 m 33 1 3 0 1.8547 1.8556 vvw <1
1 l 0 4.8791 4.8887 s 61 0 2 4 1.8410 10
2 0 0 4.8127 4.8180 w 21 6 0 —2 1.8384 1.8376 vw 3
1 1 -2 4.0020 4.0111 s 49 4 2 0 1.8362 3
3 1 2 3.1013 3.1075 vs 88 5 1 0 1.8222 1.8248 111 25
1 1 2 3.0524 3.0583 vs 100 2 0 4 1.8131 1.8156 w 7
0 2 0 2.8306 2.8365 vs 30 4 0 2 1.8121 2
2 0 -4 2.8360 64 1 3 -2 1.7917 1.7939 m 14
4 0 -2 2.8177 2.8229 m 35 3 1 -6 1.7937 5
3 l 0 2.7918 2.7951 m- 17 4 2 -5 1.7137 1.7159 w 10
2 0 2 2.7635 2.7678 m 24 5 1 -6 1.6902 1.6931 w 10
2 2 -1 2.5311 2.5346 m 27 4 2 1 1.6854 3
3 1 -4 2.4616 2.4656 m 25 3 3 -2 1.6817 1.6852 w 3
2 2 0 2.4404 2.4443 m 27 1 3 -3 1.6833 6
0 0 4 2.4150 2.4199 w 10 3 3 -1 1.6815 <1
1 1 3 2.4149 <1 1 1 -6 1.6665 1.6691 m 19
3 l 1 2.4095 13 6 0 -6 1.6424 1.6460 vw 9
2.4058 m
4 0 0 2.4090 17 3 3 0 1.6277 1.6296 vw 4
2 2 1 2.2306 2.2337 w 7 0 0 6 1.6109 1.6132 vw 4
0 2 3 2.1277 2.1303 m 19 0 2 5 1.5976 1.5990 vw 4
5 1 -3 2.0893 2.0918 vw 5 6 2 -3 1.5659 1.5682 vw 4
3 1 2 2.0494 2.0505 vw 4 7 1 -4 1.5474 1.5500 m 16
2 2 -4 2.0030 2.0055 m 26 6 2 -2 1.5397 1.5423 m- 6
4 2 -2 2.0009 12 3 3 1 1.5415 9
2 2 2 1.9784 1.9810 w 8 2 2 4 1.5252 1.5292 vw 4
5 1 -1 1.9801 <1 4 2 2 1.5271 5
4 2 -3 1.9649 1.9673 vw 5 3 1 4 1.5173 1.5196 m— 13
4 2 -4 1.8623 6 5 1 2 1.4826 2
1.4801 vw
1 3 -1 1.8600 1.8573 vvw 5 6 2 -1 1.4816 3
6 0 -4 1.8572 3 5 3 -3 1.4454 1.4477 w 7

 

105

The magnetic susceptibility data shown in Figure 37 indicate almost temperature
independent paramagnetism between 90 and 300 K. The data in the low temperature
region, however, show a complicated behavior. It is not clear if some impurities in the

sample cause this.

 

 

 

 

 

2.0 #Tl I I111! I111 11‘ tits]! I til! I cud
p‘ -(
. ‘ ‘ -1
1.8 - e
" '1
e .. .
E - o
3 1.6 e
E d
8 .
'3 1.4 ~ -
H .- .1
. 9 4
x o .
E _ ..
a 1.2 . . . . . . n
)- '1
r a
1.0 llLlLllllljllllllllll+lllll41
0 50 100 150 200 250 300
T(K)

Figure 37 Plot of magnetic susceptibility (emu/mol) of Li3Ba2NbN4 against T (K)

106
VI. LioBazTaN4

1. Experimental

(1) Synthesis of LioBazTaNo:

Single crystal growth: A 2:1:1 molar ratio mixture of elemental Li, Ba, and Ta
powder confined in a Ta boat was placed in a quartz reaction tube. The mixture was first
heated under flowing Ar to 860°C at a rate of 91°C/h. held at this temperature under
flowing nitrogen for 34 h, and then cooled to 150°C under Ar at a rate of 11.5°Clh. Pale
yellow single crystals of Li:,Ba¢TaN4 were isolated from the crushed product which was
b0th air and moisture sensitive. The Ta boat was made from 5 mm diameter Ta tube 8mm
in length. One side of the tube was sealed by arc-welding; it was then cleaned by induction
heating at 1500°C under high vacuum(< 3 x 10’ Torr).

Both the Ar and N2 gases were purified by molecular sieves and Der catalyst.
Typical moisture and oxygen contents of the gases were <0.5 ppm and <1 ppm,
respectively.

Powder sample preparation: Similar to that for LioBaszNo, except that TaoN, was
used instead of NbN and that the mixture of Li3N + BazTaN3 was heated at 750°C instead
of 720°C.

(2) XO-ray Single Crystal Diffraction and Structural Solution

An irregular crystal of approximate dimensions 0.090 x 0.100 x 0.140 mm’ was
selected and sealed in a 0.2 mm glass capillary in a Nz-filled glove bag. Lattice parameters
were obtained from a least-squares refinement by using the angle settings of 19 carefully-
centered reflections in the range 38.60 < 20 < 39.70°. Based on the systematic extinctions,
packing considerations, a statistical analysis of the intensity distribution, and the successful

solution and refinement of the structure, the space group was determined to be C2/c (#15).

107

Data were collected at 25 :1: 1°C by the (1)—20 scan technique to a maximum 20 value
of 80°. Of the 1830 reflections which were collected, 1759 were unique. The linear
absorption coefficient for Mo K01 radiation, 298.1 cm", was used to make an empirical
absorption correction which resulted in transmission factors that ranged from 0.76 to 1.00.
The data were corrected for Lorentz and polarization effects. A correction for secondary
extinction was applied with the coefficient = 0.83(2) x 10‘.

The structure was solved by direct methods with the programs MW
and DIRDIFM. All atoms were refined anisotropically with the refinement based
upon 1'3. The final cycle of full—matrix least-squares refinementMOfi), which was based on
1644 observed reflections (I > 0.000(1)) and 48 variable parameters, converged with
unweighted and weighted agreement factors of:

R2=2 |Fo2-Fo2 IIZZFo2 = 0.052

wR2=[Z(w|F.2-Fo’|)2/2(w F521"z = 0.074
For comparison, a refinement of the overall scale factor on F with the final parameters and
1455 observed reflections (I > 30(1)) led to

R: 2| |F.|- |l=,| |/2:|1=. =0.032

 

The maximum and minimum peaks on the final difference Fourier map corresponded to
3.45 and -3.27 e‘lA’, respectively. These residue peaks are too close to either Ta or Ba
atoms to be considered missed atoms.
Data collection and atomic positional parameters are listed in Tables 29 and 30,
respectively. Thermal parameters (Uij) are compiled in Table 31.
All measurements were made on a Rigaku AFC6S 4 circle diffractometer with
graphite monochromated MoKa radiation 0.01 = 0.71069 A). Neutral atom scattering

factors for both nitrides were taken from Cromer and Waber(107). All calculations were

108
performed by using the TEXSAN(108) crystallographic software package.

Table

‘0'... -

G

(

F.

a.RlvVZETC(2

‘
§“
1

Table 29 Summary of Crystal and Diffraction Data for LioBazTaN4

 

 

Chemical formula LioBazTaN4
Formula weight 532.46
Space group C2/c
a,b,c,(A) 11.294(2), 5.678(1), 11.350(2)
B(dcgmc) 121.407(7)
V(A3) 621.2(2)
Z 4
Doolo(g/cm3) 5.692
T(°C) 25 :1: 1
Crystal color, Habit pale yellow, irregular
Crystal dimensions(mm) 0.090 x 0.100 x 0.140
20 max(deg) 80.0
Scan type 00-20
X-ray radiationo.) MoKot().=0.71069 A)
Monochromator Graphite
Octants collected hkl;hla
Absorption coeff p(cm") 298.08
Measured reflections 1830
Observed reflections‘ 1644
Unique reflections 1769
F000 888
No. of variables 48
Max peak in final diff. map 3.45 e°/A3
R", R.“(%) 0.052, 0.074
' I > 0.000(1)

1’ 112:2 mgr: |/r:1=,,2
c WR2=[Z(W |F°2_F62 |)2/Z(W F02)2]1/2

Tabl

81011

Ta
Ba
N(l
N(2
Li(i
Li(:

The

Ta'

Atl

Ta
Ba
N(
N(
Li
Li

1 10
Table 30 Atomic Positions and B(eq) for L13B azTaN4

 

 

 

 

atom at 1 2 B(OQ)
Ta 0 0.1 1256(7) 0.25 0.24(1)
Ba 0.20933(4) -0.03379(8) 0.1 1 183(4) 0.64( 1)
N(1) 0.1648(6) -0.090(1) 0.3376(6) 0.8(2)
N(2) 0.0013(7) 0.312(1) 0.1116(7) 1.0(2)
Li(l) 0 -0.362(4) 0.25 2.4(8)
Li(2) 0.385(2) -0.044(3) 0.459(2) 1.8(5)
The equivalent isotropic temperature factor is defined as(112):
81:2 3 3 e e
3(99)=— 2 ”u“! “I 51.5}
3 1-1 1-1
Table 31 Anisotropic thermal parameters, U“, for LioBaQTaNo
Atom U11 U22 U33 U12 U13 U21
Ta 0.0026(1) 0.0016(1) 0.0047(1) 0 0.0017 0
Ba 0.0069(2) 0.0079(2) 0.0086(2) 0.0002(1) 0.0035(1) -0.0006(1)
N(1) 0.006(2) 0.009(3) 0.01 1(2) 0.004(2) 0.001(2) 0.000(2)
N(2) 0.013(3) 0.013(3) 0.013(3) -0.006(2) 0.006(2) 0.002(2)
Li(l) 002(1) 000(1) 004(2) 0 000(1) 0
Li(2) 0.014(7) 0.01 1(7) 0.030(9) -0.001(6) 0.002(6) 00] 1(7)

 

The anisotropic temperature factor coefficients Uij are defined as:

exp(-21r2(a"U,,h2 + p'zUnkz + 9911,312 + 2g‘p‘unhlt + 2g‘_c_’U,,hl + 2p‘g‘ugltl»

38. In
by N ‘
Li let:
These

In B:

reSpt
and

angl

111

2. Results and Discussion

LiaBazTaNg crystallizes isotypically to Li3Ba¢NbN4. The structure is shown in Figure

38. In the crystal structure of LioBazTaNo, Ta5+ and Li(l) ions are tetrahedrally coordinated

by N ions. The Li tetrahedra are distorted (see bond lengths in Table 32). These Ta and

Li tetrahedra are connected together by sharing two edges to form parallel chains along 9.

These chains are then linked together by Li(2) ions to form a three dimensional network.

In BazTaN3 (1_7) and Li7TaN4 (3_8) Ta ions are also tetrahedrally coordinated by N ions.

The Ta-N(1) and Ta-N(2) bond lengths in Li3Ba2TaN4 are 1.962 A and 1.942 A,

respectively, very close to the Ta-N distances in the nitrides Ba.,TaN3 (1.922, 1.950, 1.966,

and 1.988 A) and Li,TaN, (1.967 and 1.955 A). A list of selected bond distances and

angles is given in Table 32.

\'

 

0 Ta 0 Be an N(1) 0 N(2) O L10) 0 Li(2)

Figure 38 A view of Li:,Ba2TaN4 down the b axis

The av
found
compa

SUUCH

tetraht
tetrah
chain
reSpe
there
of N1
com]
C001

CODE

Cor

C01

rat

112
The average Ta-N bond length reported here is equal to the average length of Nb-N bonds

found in Li-,NbN4 (31), where Nb is also tetrahedrally coordinated by N ions. These
comparable distances suggest that Nb and Ta should form nitrides with the same or similar
structure types.

There are two kind of Li ions in the structure, Li(l) and Li(2). Li(l) ions are
tetrahedrally coordinated by N} ions in a distorted arrangement. The Li(l) and Ta
tetrahedra are connected to each other by sharing two edges to form Ta-Li(1)-Ta-Li(1)—...
chains. The Li(1)-N(1) and Li(1)-N(2) bond lengths are 221(2) and 2.43(2) A,
respectively, indicative that the Li tetrahedron is distorted. Li(2) bridges three W ions and
therefore each Li(2) ion links three Ta-Li chains (as in Figures 32 and 33 with Ta in place
of Nb). The Li(2)-N distances, Li(2)-N(1) 214(2) A, Li(2)-N(2) 201(2) and 203(2) A, are
comparable to those in 0mm: 1.938 (twofold coordinated Li) and 2.106 A (threefold
coordinated Li), respectively. If the Li(2)-N(1) bond (perpendicular to the _b_c plane) is not
considered, a layered fragment can be viewed in the pp plane as shown in Figure 32 for the
Nb compound.

The coordination environment of Ba2+ is similar to that in LioBaszN4 (Figure 34).
The shortest Ba—N bond is 2.800(6) A and the longest 3.257(6) A (Table 32), values
comparable to those in BaQTaN3 (shortest 2.771 A, longest 3.462 A).

It is interesting to compare the synthesis of LioBaQTaN4 to that of BazTaN, Both
compounds were prepared by using Li+Ba+Ta metals as reactants with N,. A LizBa molar
ratio of 2:1 was used to prepare LioBazTaNo; a 1:1 molar ratio was used for preparation of
lBaZTaNg. A more important factor that determines which compound forms may be the
reaction temperature. Li3Ba2TaNo was prepared at 860°C while BaQTaN3 was prepared at
1000°C. At the more elevated temperature Li3N may escape from the Li-Ba—Ta-N system

favoring formation of BaQTaN3.

Table
Bont

Ta-lN
Ta-IN'
Li(l]
Li(lj
Li(2:
Li(2j

Ba-I‘

Ba-l‘

Ta~L

Ta-E

Li(lj

Li(2)

Li(1 )
Li(2)

Ba~B

Table 32 Selected Bond Distances (A) and Angles (deg) in LiaBazTaN,

 

Bond

Ta-N( 1)
Ta-N(2)
Li(1)-N(1)
Li(l)-N(2)
Li(2)-N(1)
Li(2)-N(2) (2x)

Ba-N(1)

Ba-N(2)

Ta-Li( 1 )

Ta-Ba

Li(l)-Ba

Li(2)-Ba

Li(l)-Li(2)
Li(2)-Li(2)

Ba-Ba

F—A'fiflf'fiqf—M

distances

1.962(6)
1.942(7)
2.21 (2)
243(2)
2. 14(2)
2.03(2)

2.874(7)
2.800(6)
2.968(6)

3.062(8)
2.881(7)
3.257(6)

2.69(2)
299(2)

3.5502(6)
3.5406(7)
3.4502(6)

2.97(1)

336(2)
293(2)

327(2)
3.05(2)

2.90(2)
2.32(3)

4.484(1)
4.055(1)

3.9684(10)
3.9672(8)

Bond

N-Ta-N av.
N(1)-Li(1)-N(1)
N(1)-Li(1)-N(2) av.
Li(1)-Ta-Li(1)
Ta-Li(1)-Ta

N(1)-Li(2)-N(2)

N(2)-Li(2)-N(2)
N(2)-Li(1)-N(2)
Ta-N(1)-Li(l)
Ta-N(2)-Li(1)

Li(1)-N(2)-Li(2)

Li(2)-N(2)-Li(2)
Ta-N(1)-Li(2)
Li(1)-N(1)-Li(2)

Ta-N(2)-Li(2)

N(1)-Li(1)-N(2) 2x

angles

109.2(4)

91.8(9)
122.0(2)
180.00(0)
180.00(0)

130.6(9)
1 17.7(8)

109.8(8)
80.8(8)
80.1(5)
85.3(4)

99.60)
80.70)

70.2(8)
100.6(6)
142.9(7)

146.6(6)
142.8(6)

122.0(2)

 

Table

for Li3

b 211.01 1. 1
— — .13 3 024223213 205 4.
4 2 25\

Table 33 Miller indices and observed and calculated interplanar distances and intensities

114

 

 

forLi,Ba,TaN,
h k 1 do do Io Ic h k 1 do de lo 1e
2 0 -2 4.9369 13 4 2 -3 1.9646 1.9878 vw 10
1 1-1 4.9130 4.9222 s 53 4 2 -1 1.9537 1.9526 vw 2
1 1 0 4.8850 4.8923 s 90 1 3 -1 1.8588 8
0 0 2 4.8340 4.8435 w 24 1 3 0 1.8574 1.8572 w <1
1 12 4.0114 4.0134 w 49 6 0 -4 1.8567 5
1 l 1 3.9641 3.9653 vw 10 5 l 0 1.8221 1.8255 w 25
3 12 3.1017 3.1078 s 91 2 0 4 1.8157 1.8171 vw 9
1 13 3.0978 11 1 3 -2 1.7954 14
1.7918 w

3 1-1 3.0853 14 3 1 -6 1.7943 1
1 1 2 3.0554 3.0610 s 100 4 2 -5 1.7130 1.7164 w 12
0 2 0 2.8315 2.8390 s 22 5 1 -6 1.6900 1.6931 vw 12
2 0 -4 2.8368 65 4 2 1 1.6865 6
4 0 -2 2.8190 2.8224 m 40 3 3 -2 1.6863 <1
2 0 2 2.7643 2.7701 m 31 1 3 -3 “5838 1.6847 W 8
2 2 -1 2.5335 2.5363 in 35 3 3 -1 1.6827 3
3 14 2.4632 2.4660 m 29 1 1 -6 1.6674 1.6703 w 17
2 2 0 2.4399 2.4462 m 17 7 1 -4 1.5498 16
1 14 2.4424 11 2 2 -6 1.5489 <1
3 1 1 2.4111 16 1 1 3 1.5443 2
4 0 0 14064 2.4098 m 21 7 1 -3 15474 1.5432 w <1
2 2 1 2.2346 2.2355 w 14 3 3 1 1.5427 10
0 2 3 2.1299 2.1321 m 24 6 2 -2 1.5427 3
5 1.3 2.0881 2.0917 vw 7 3 1 4 1.5181 1.5209 vw 13
2 2 -4 2.0022 2.0067 m 19 6 2 -1 1.4797 1.4822 vw 5
4 2 -2 2.0016 9

2 2 2 1.9797 1.9827 W 6

5 1.1 1.9806 3

 

contai

W35 0

115

The magnetic susceptibility data, presented in Figure 39, show that the sample

contained paramagnetic impurities. Above 50 K, temperature independent paramagnetism

was observed.

 

 

 

 

5.2 111I[rrrrrtrr11.',,',,fiitv,,f

q

.1

.(

5.1 -

A at
'8 1
E 50 L J

3 ' l.

e 4 2
O I- .-
v P‘ d
g 4.9 - ..
h. I

fl 1- d
X : A ‘ 1
RE 408 L- . ‘ ‘ ‘ ‘ ‘ 1}
" d

I 1

4.7 4111lljjlllllLLLllLlellLLUl

0 50 100 150 200 250 300
T(K)

Figure 39 Plot of magnetic susceptibility (emu/mol) of Li3Ba2TaN4 against

temperature (K)

V. Li(.

(301113111

nkfide

inthe

LimTa

miXtu
mesh)
heatu
under
a rate
the Cr
WCrc

Matlh

quart.

1 1 6

The synthesis, structure, and selected properties of numerous transition metal-
containing oxidenitrides prepared by reacting an oxide with either ammonia gas or a metal
nitride have been reported recently: Na,WO,N(fi), SrMo(O,N)3(§2), BaCe_I_._rl(O,N)4 M =
La, Ce)(_3_3), VzoMomOmNuag), 122503N (A = V, Nb, Ti, A1)(Ll_6_), Li2_
,Nb2,,0 a, (OSxS0.31; 0.46Sy51.46)(1_11), and ngafiOsN2 M = Nd-Yb,
rams). Several other oxidenitrides synthesized by diverse methods, N11,,M02N2 M
= M0, WKM. Na5W04NQQ), LimTaZNSOQA), and K6W2N403
(12), have also been characterized structurally.

During our studies of the quaternary nitrides Li,B aLMN, (M = Nb, Ta) we identified

in the Nb system the phase LimszNsO which we later found to isostructural with

Li,,Ta,N,0(121).

1. Experimental

( 1) Synthesis: Single crystals of LimszNso were originally obtained when a
mixture in the molar ratio Li:Ba (99.7%, Cerac, Inc.):Nb (99%, E. H. Sargent & Co., 80
mesh) = 2:1:1, confined in a Nb boat, which was placed in a quartz reaction tube, was
heated under flowing Ar (AGA Gas, Inc.) to 850°C at a rate of 97°C/h, held at 850° C
under flowing nitrogen (AGA Gas, Inc.) for 24 h, and then cooled to 150°C under Ar at
a rate of 10.8°C/h. Colorless transparent single crystals of LimszNso were isolated from
the crushed product which was both air and moisture sensitive. Both the Ar and N2 gases
were purified by molecular sieves (48 mesh, Aldrich) and De-Ox catalyst (Johnson
Matthey). The oxygen in the compound probably came from moisture that adhered to the

quartz reaction tube.

meas
Cher.

then

yello
patte
N2 3'

1) ur

COPE
with
and

PUI
we“

Mo

C010

oft}

ref"

Of6

117

Samples of LimszNsO for X-ray powder diffraction and magnetic susceptibility
measurements were prepared by heating a mixture of the molar ratio Li3Nsz205 (Apache
Chemicals, 99.9%):NbN = 16/3:1/5:8/5. The mixture, confined in a Nb boat which was
then placed in a quartz reaction tube, was heated from room temperature to 900°C within
two hours under flowing Ar/Nz, kept at 900°C for 24 hours, then quenched. The light
yellow, air-sensitive powder was almost phase-pure based on the X-ray powder diffraction
pattern (see Fig. 48). The reactant Li3N was prepared by heating Li metal under flowing
N2 at 450°C for 36 hours. NbN was prepared by heating NbCl, (Johnson Matthey, grade
1) under flowing NH,(g) at 700°C for 5 hours.

X-ray diffraction: Powder X-ray diffraction data were collected with both a 114.6
mm Guinier camera (internal standard: Si) and a Philips APD diffraction system with
copper Kat radiation. The powder diffractometer data were stripped of the K012 component
with the APD software, corrected for the 0-compensating slit as described previously(_9_§),
and plotted by using the program GRAPHER(1_23). The program LAZY
PULVERIX(_7_2_) was used for powder intensity calculations. Single crystal diffraction data
were collected on a Rigaku AFC6S 4 circle diffractometer with graphite monochromated
Mo Kat radiation.

Structure determination: Lattice parameters of the irregular-shaped transparent
colorless crystal mounted on a glass fiber were obtained from a least-squares refinement
of the setting angles of 16 carefully-centered reflections in the range 37.89° < 20 < 39.28°.
Based on the systematic absences of hkifl: -h+k+c it 3n, the successful solution, and the
refinement of the structure, the space group was determined to be R3.

Data were collected at -100°C by the 00-20 scan technique to a maximum 20 value

of 60.0°. Of the 2224 reflections which were collected, 509 were unique (Rm = 0.036);

equh
rneas
appb
app]:
in 01

and

lieu
(Zak
JAM;

Of f:

CTys

calc

mag
susc

Cth

the l

118

equivalent reflections were merged. The intensities of three representative reflections
measured after every 150 reflections declined by 0.60%; a linear correction factor was
applied to compensate for this intensity decrease. An empirical absorption correction
applied by using the linear absorption coefficient for Mo Ka radiation, 20.7 cm", resulted
in transmission factors that ranged from 0.91 to 1.00. The data were corrected for Lorentz
and polarization effects. A correction for secondary extinction was applied (coefficient =
0.44(8) x 10“).

The structure was solved by direct methods with the program SHELXS86(1_05_).
Neutral atom scattering factors for both nitrides were taken from Cromer and Waber(jfl).
Calculations were performed by using the TEXSANL08) crystallographic software package.
All atoms were refined anisotropically with the refinement based upon F2. The final cycle
of full-matrix least-squares refinementm), based on 508 observed reflections with (I >
0.000(1)) and 43 variable parameters, converged with R2 = 0.024 and wR2 = 0.047. The
crystallographic data, atomic positions, uij values, bond distances, and observed and
calculated power patterns are shown in Tables 34-38, respectively.

Magnetic susceptibility: Data were measured with a Quantum Design SQUID
magnetometer at temperatures between 2 to 300 K. At each temperature magnetic
susceptibilities were also measured at various magnetic fields between 200 and 800 G and

extrapolated to zero reciprocal field to eliminate ferromagnetic impurity contributions.

2. Results and Discussion
Although the single crystals are colorless, the powder is light yellow. The air and
moisture sensitive compound crystallizes isotypically to LimTaQNsO<121 ). Figure 40 shows

the unit cell with the Li(l) and Li(2) atoms omitted for clarity. The unit cell comers are

occu
atom
11 gur

Nb-l‘

thic

119

occupied by one 0 atom; the remaining two 0 atoms are indicated by arrows. The Nb
atoms are tetrahedrally coordinated by N atoms. The Nb-N bonds are darkened in the
figure to facilitate viewing. The average Nb-N distance of 1.960 A is close to the average

Nb-N distance of 1.95 A in Li,Ba,NbN, and in Li,NbN,(3_7).

     
       

   

19—
II

1.1!]
I.

n . .

.
_ 0. .

  

  

.A __
\f-

t:
9 ,2.

III
1|

   

 

Figure 40 Unit cell of LimszNsO with Li( 1) and Li(2) omitted for clarity. The
thicker bonds denote NbN4 tetrahedra. The eight comers are occupied by O atoms. The

two positions indicated by arrows are also occupied by O atoms

Tabl

--floc

(Its.

a ‘

r‘

A}.

(u

)hrklthcslrrbhchh

-§
“-‘

Table 34 Summary of Crystal and Diffraction Data for LimszNgO

 

Chemical formula
Formula weight

Space group

a. c (A)

v (A3)

Z

Dub (glem’)

T(°C)

Crystal color, Habit
Crystal dimensions (mm)
20 max (deg)

Scan type

X-ray radiation (A)
Monochromator

Octants collected
Absorption coeff p (cm")
Measured reflections
Observed reflection s‘
Unique reflections

F000

No. of variables

Max (min) peak in final diff. map

Goodness of Fit
R2”, wR2c

LimszNao

424.92

R301) (# 148)
5.984(4), 25.484(8)
790.1(8)

3

2.678

-100 :t 1

colorless, irregular
0.080 x 0.120 x 0.150
60.0

01-26

MoKa (A=0.71069 A)
Graphite

hkl; hfi

20.67

2224

508

509

582

43

0.74 (-0.69) e/AB
1.03

0.024, 0.047

 

' I > 0.006(I)

b R2 = XIFoz-FCZIIZFOZ

° sz = [2w<IF.2-F.2I)2/2<Wf.2)21m

Table 3

Atom

Nb
N(1)
N(2)

Li(l)
Li(2)
Li(3)
Li(4)

Thee

Tab].

-5...‘

AlOr

121

Table 35 Atomic Positions and B(eq) for LimszNsO

 

 

 

 

Atom 5 x z. B(eqXA’)
Nb 0 0 0.249440) 0.270)
N( 1) 0.3089(4) 0.31 10(4) 0.22491 (7) 047(5)
N(2) 0 0 0.3265(1) 050(7)
0 0 0 0 0.5(1)
Li(l) 0.357(1) 0.303(1) 0.4747(1) 0.8(1)
Li(2) 0.032(1) 0.340(1) 0.0233(2) 0.9(1)
Li(3) 0 0 0.0769(3) 0.7(2)
Li(4) 0 0 0.4165(3) 1.0(2)
The equivalent isotropic temperature factor is defined as (_11_2):
811'2 3 3 e .
13(94):? 2: 2: U14“! “1 afar]
1-1 1-1
Table 36 Anisotropic thermal parameters, Uij, for LimszNaO
Atom U11 U22 U33 U12 U13 U23
Nb 0.0034(1) 0.0034 0.0034(2) 0.0017 0
N (1) 0.0064(9) 0.0060(9) 0.0054(7) 0.0031(8) 0.0007(7) 0.0003(7)
N(2) 0.007(1) 0.007 0.005(2) 0.003 0
0 0.006(1) 0.006 0.006(2) 0.003 0
Li(l) 0.012(2) 0.010(2) 0.010(2) 0.006(2) 0.001(1) -0.000(1)
Li(2) 0.012(2) 0.012(2) 0.012(2) 0.007(2) 0.003(1) -0.001(1)
Li(3) 0.010(2) 0.010 0.007(4) 0.005 0
Li(4) 0.013(2) 0.013 0.01 1(3) 0.007 0

 

The anisotropic temperature factor coefficients Uij are defined as:
exp(-21t2(_a"Uuh2 + ifUnk’ + g"’U;,,,l2 + 2§.Q.U12hk + Za‘g’Unhl + ZQ'Q‘Uzale

kam S

”——

Bond

 

lfirbul'
lQDJQCI

14(1))

Li(2)-
11(3)
Li(4)
Li(l‘
Li(d
Li(.
Li(
1.0
I.i
I.

122

Table 37 Selected Bond Distances (A) and Angles (deg) in LimszN30

 

 

Bond distances

Nb-N(1) 1.957(2) (3x)

Nb-N(2) 1.964(4)
2.164(5)

. 2.135(4)

Ll(1)-N(l) 2.066(6)

2.324(7)
Li(2)-N(1) 2.192(5)
Li(3)-N(1) 2.227(3)
Li(4)-N(1) 2.157(3)

, 2.033(4) (3x)
L‘(2)'N(2) 2.209(7) (3x)
Li(4)-N(2) 2.295(8)
Li(2)-O 2.034(6) (6x)
Li(3)-O 1.959(7)

. . 2.376(5) (2x)
L1(1)—L1(1) 2.469(8)
Li(1)-Li(3) 2.414(7)
Li(1)-Li(4) 2.487(6)

. . 2.279(7) (2x)
L1(2)-L1(2) 2. 44 (1 )
Li(2)-Li(3) 2.376(7)
Li(2)-Li(4) 2.423(6)

 

Bond angles
N(1)-Nb-N(l) 110.31(5)(3x)
N(1)-Nb-N(2) 108.62(5)(3x)

91.4(2)

l 16.5(3)

N(1)-Li(1)-N(1) 183$;
108.2(3)

1 18.6(3)

N(1)-Li(2)—N(2) { 151338;
N(2)—Li(2)-N(2) 109.8(3)
N(l)-Li(2)-O 109.7(2)
N(2)-Li(2)-O { {(32:28

N(1)-Li(3)-N(1)
N(1)-Li(3)-O

N(1)-Li(4)-N(l)
N(1)-Li(4)-N(2)

Nb-N( 1 )-Li( 1)

Nb-N(1)-Li(2)
Nb-N(1)-Li(3)
Nb-N(1)-Li(4)

Nb-N(2)-Li(2)

Nb-N(2)-Li(4)

107.7(2) (3x)
111.1(2) (3x)
1 l 1.5(2) (3x)
107.3(2) (3x)

77.2(1)
81.2(2)

106.1(2)
173.2(2)

79.6(2)
111.4(1)
114.1(1)

79.1(1) (3x)
112.2(1) (3x)

1 80.00

 

(Li(l)
0f 2.]
comp:

Figurt

Figu

F1811

123
The four independent Li atoms are tetrahedrally coordinated by either N atoms only

(Li(l) and Li(4)) or by both N and O atoms (Li(2) and Li(3)) with an average Li-N distance
of 2.18 A and an average Li-O distance of 2.00 A, the same as those reported for
comparable atoms in LimTaQN,O(121). Their coordination environments are shown in

Figures 41-44.

 

 

 

 

‘n
r . —II" ’0
.._, N(2) ’9 N(1)
Figure 41 The environment of Li(l) Figure 42 The environment of Li(2)

I: N(1)

 

 

 

9L)

 

 

 

 

Figure 43 The environment of Li(3) Figure 44 The coordination of Li(4)

and
it c2
spht
[CUE

fluo

and

124

As is shown in Figures 45, 46 and 47 , the coordination environments for 0, N(1),
and N(2) indicate that Linszso is a superstructure of the antifluorite type. In Figure 45
it can be seen that the O atom is coordinated by eight Li” cations in the first coordination
sphere and twelve N3’ anions in the second coordination sphere with all cations occupying
tetrahedral holes formed by the anions. The same situation prevails for the Ca atoms in the

fluorite structure.

 

 

 

 

Figure 45 Oxygen atom coordination environment. The first coordination sphere
is composed of six Li(2) and two Li(3) atoms in a distorted cubic arrangement. Six N(1)

and six N(2) atoms comprise the second coordination sphere.

similar
coordi
occup
(diSIOl

an am

OCC

125

The coordination environments shown in Figures 46 and 47 for N(1) and N(2) are
similar to that of 0. These anions are all coordinated by eight cations in the first
coordination sphere and 12 anions in the second coordination sphere with the cations
occupying the tetrahedral holes formed by the anions. The N and O atoms form a
(distorted) face centered cubic structure and Li and Nb atoms occupy the tetrahedral holes--

an antifluorite type superstructure.

N(2)

v"

    

 

\ l

“— -" Li(2)

\o/

 

 

 

Figure 46 N(1) coordination environment. It is coordinated first by eight atoms that
occupy the corners of a distorted cube (four Li(l), one Li(2), one Li(3), and one Li(4), and

one Nb atom) and then by twelve anions, one 0, nine N(1) two N(2) atoms.

COOTC

311d 1

126

 

N(1)

 

 

 

Figure 47 N (2) coordination environment. Eight atoms also provide distorted cubic
coordination for atom N(2): one Nb, one Li(4), and six Li(2) atoms. Six N(1), three N(2),

and three 0 atoms comprise the second coordination sphere.

Many Li—containing ternary nitrides have an anti-fluorite superstructure: Li7MN4CM
= V and an, Nb(_31), Ta@), and L16M_N(_=Cr,Mo,W)L1_). The tendency of the
small Li” ion to occupy tetrahedral holes formed by N3“ ions is probably the main reason

these compounds form the anti-fluorite type superstructure.

vent

(Phfl

xtmlcoLE

127
The X-ray powder diffraction pattern of Li ,6Nb2N 80 is shown in Figure 48. The one

very weak impurity reflection at 20 =- 23.68 can be assigned to NbZOS. The observed

(Philips APD) and calculated d-spacings and intensities are presented in Table 38.

 

 

 

 

 

 

 

3200
2400 -
>4
1’: E
m _, L
C 1600 a
9 .§
.._
C -1 o"
43‘
z
8004 5
0 r r

10 80

45
Two Theio

Figure 48 X-ray powder diffraction pattern for Li ,6Nb2N80

rablt
Limb

128

Table 38 Miller indices and observed and calculated d-spacings and intensities for
LimszNso

 

 

h 1t 1 do de 1; I.
0 1 2 , 4.8063 4.8005 100 100
0 0 6 4.2544 4.2473 30 24
1 0 4 4.0251 4.0202 70 72
3.7313 11
1 1 0 2.9902 2.9920 38 31
0 0 9 2.8316 4
1 1 —3 } 2°82“) { 2.8221 22 12
0 1 8 2.7147 2.7138 19 23
2 0 2 2.5377 2.5392 19 18
1 1 21:6 2.4451 2.4460 33 11/16
0 2 4 2.3992 2.4002 16 17
1 0 10 2.2877 2.2869 10 14
0 0 12 2.1237 2.1237 6 5
2 0 8 2.0097 2.0101 6 7
2 1 -2 1.9360 10
1 2 2 } 19345 { 1.9360 15 6
2 1 4 1.8722 6
1 2 -4 1 13709 { 1.8722 15 10
0 2 10 1.8165 1.8169 5 5
1 1 :12 1.7313 1.7318 28 5/23
3 0 0 1.7256 1.7274 31 22
0 1 14 1.7175 1.7174 13 5
1 2 8 1.6685 4
2 1 -8 } “5676 { 1.6685 7 5
0 3 6 1.6002 4
3 0 6 } 15990 { 1.6002 10 4
1 2 -10 1.5530 4
2 1 10 } 15520 { 1.5530 5 3
1 0 16 1.5232 1.5225 4 4
2 2 0 1.4945 1.4960 4 4
2 0 14 1.4892 1.4895 3 4
3 0 9 1.4747 <1
0 3 9 } 1.4722 { 1.4747 3 <1
2 2 3 1.4733 <1
3 1 2 1.4283 4
1 3 -2 } ”27° 1 1.4283 5 2
2 2 :6 1.4098 1.4110 5 2/3
3 1 4 1.4021 2
1 3 4 } ”008 { 1.4021 5 4
0 2 16 1.3569 1.3569 3 4
0 3 12 1.3401 3
3 0 12 } 13393 { 1.3401 6 3
2 1 -14 1.3334 3
1 2 14 } 13332 { 1.3334 4 3
3 1 8 1.3101 3
1 3 -8 } 13°93 { 1.3101 3 2
0 4 2 1.2879 1.2889 2 2
1 1 21:18 1.2795 1.2797 3 2/2

 

Figure

and 30*
Curie t
NbN0 8.

a trace

[Qm

129
A plot of the magnetic susceptibility of LinszsO against T(K) is presented in

Figure 49. The data show an almost temperature-independent paramagnetism between 10
and 300 K (average 7.948 x 10“ emu/mol). The data at 10 and 15 K evidence a small
Curie tail. The sharp drop in susceptibilities below 10 K is presumed due to traces of
NbN”, a superconductor with Tc = 8.90 K (124). The small peak at 50 K is due to

a traces of oxygen absorbed in the sample during specimen loading procedures.

 

1O TIVI—[T'YUFTIUIITTI'UIIIIIIIII‘ITTT

l

XxIOE+04
O3
'U‘l"

 

.b
rrhlrarlrrrlrrrlrrrlLr

 

-2 1|Illlllllllll'JllllllllllLlllJIll

O 50 100 150 200 250 300 350
T(K)

Figure 49 Plot of magnetic susceptibility (emu/mol) of Li 16szN 80 against

temperature (K)

MZVN
(29), :
isostn
quatel
is iso:
ofldz
reacti
lattice
likely

grow

SrZN
at 7:
Synu
at 8(
11nde
then

ratio

of e}

130

VI. Li3Sr2NbN4

Several group-V transition metal nitrides have been reported recently: Ca3W3Q9),
M2VN, (M = Ba, Sr)(_l;5_), Li7NbN4L31), Li,TaN,L3_8_), TaThN,(1_2§), BaszN,
(99, and MzTaN3 (M = Ba, 8mg). Among these compounds, BazTaN3 and SrzTaN3 are
isostructural while Ba2VN3 and Sr2VN3 not. Recently we reported structures of two new
quaternary nitrides Li3Ba2MN4 (M = Nb, Ta) and also found that SerbN3 (see Chapter 4)
is isostructural with BangN3QQ). These observations suggest that the structure analogue
of Li,Ba,MN,, (M = Nb, Ta) should be preparable. X-ray powder diffraction data of the
reaction product of UN 4» SerbN3 could be indexed on orthorhombic symmetry , with
lattice parameters that give a cell volume reasonable for Li3Sr2NbN4. Thus Li,Sr,NbN,
likely exists but has a structure different from that of Li3Ba2NbN4. Single crystals must be

grown in order to solve the structure of the new quaternary nitride.

1. Experimental

Synthesis: The polycrystalline Li3Sr2NbN, was synthesized by reaction of MN and
SerbN3 in molar ratio of Li3N: SerbN3 = 1:1 with about 10% excess of Li3N in a Nb tube
at 750°C for 30 hours under an Ar atmosphere. The unreported nitride SerbN3 was
synthesized from Ser and NbN. Ser was prepared by heating Sr metal in flowing N,(g)
at 800°C for 36 hours. NbN was prepared by heating NbCl5 (Johnson Matthey, grade 1)
under flowing NH,(g) (Na purified) to 700°C in 3 hours, heating it at 700°C for 8 hours,
then quenching. SerbN3 was prepared by heating a mixture of Ser and NbN in the molar
ratio of Serb = 2:1 at 900°C for one day under flowing NH,(g), followed by quenching.

Single crystals of Li,Sr2NbN, were grown from Li, Sr, and Nb metals. A mixture

of elemental Li, Sr (99%, Cerac, Inc.), and Nb powder (99%, E. H. Sargent & Co., 80

131

mesh) in a 3:1.5:1 molar ratio was confined in a Nb boat which was then placed in a quartz
reaction tube. The mixture was first heated under flowing Ar (AGA Gas, Inc.) to 800°C
at a rate of 110°C/h, held at this temperature under flowing Ar for about 40 minutes, and
then NH,(g) was slowly diffused into the Ar-filled reaction system. The sample was heated
under the flowing Ar(g) + NH,(g) for 29 hours. The volume ratio of Ar(g) and NH,(g)
varied between approximately 1:1 and 3:2. The sample was next kept at 800°C under
flowing Ar for 5 hours and finally cooled to 200°C under flowing Ar at a rate of 10°C/h.
To ensure that the NH3 gas was dry, some Sr metal was placed before and after the sample.
Light-yellow transparent single crystals of Li3Sr2NbN4 were isolated from crushed product
which was both air and moisture sensitive. The Ar gas was purified by molecular sieves
(4-8 mesh, Aldrich) and De-Ox catalyst (Johnson Matthey). Ammonia was dried over
sodium metal and stored in a stainless-steel vessel. All sample manipulations were handled
in an Ar-filled glove box whose typical moisture and oxygen contents are <0.5 ppm and
<1 ppm, respectively.

X-ray diffraction: X-ray powder diffraction data were obtained by using both a
114.6 mm Guinier camera (quartz monochromatized CuKa,) and Philips APD
diffractometer system (graphite monochromatized CuKa radiation) with a Philips XRG-
3000 powder X-ray generator. The CuKtlt,z radiation component was stripped with the APD
software and the data corrected for the 0-compensating slit as described previously(9_6). To
achieve a better printing plot for the diffraction pattern, a Rietveld refinement program,
DBWS-9411(5)) together with program DMPLOTQZ), was used to plot the powder
diffraction pattern based on the collected intensity data. The programs VISSERL6_9),
TREORSQQ), and DICVOL92(7_1) were used to index the Guinier X-ray powder pattern

of Li3Sr2NbN4. NBS certified Si (a = 5.43082(3) A) served as internal standard The

132
program LAZY PULVERIXQZ) was used for intensity calculations. X-ray single crystal

diffraction was carried out on a Rigaku AFC6S 4 circle diffractometer with graphite
monochromated Mo Kot radiation (21:0.71069 A).

Structure determination: An irregular crystal of approximate dimensions 0.15 x 0.20
x 0.30 mm3 was selected and sealed in a 0.2 mm glass capillary in a Nz-filled glove bag.
Lattice parameters were obtained by least-squares refinement of the angle settings of 20
carefully-centered reflections in the range 21.61 < 20 < 29.80°. The choice of space groups
was reduced to Pnnm (#58) and Pnn2 (#34) by systematic absences (0M, 1; + _l_ at 211; 1101,,
h + l at 211), and the structure was solved based on space group Pnnm (#58).

Data were collected at 26i1°C by the 03-20 scan technique to 20 S 60°. A total of
1952 reflections were collected, 970 were unique (Rh, = 0.130). The intensities of three
representative reflections which were measured after every 150 reflections declined by
-3.70%. A linear correction factor was applied to account for this phenomena. The linear
absorption coefficient for Mo K01 radiation, 206.0 cm", was used to make an empirical
absorption correction which resulted in transmission factors that ranged from 0.66 to 1.33.
The data were corrected for Lorentz and polarization effects and for secondary extinction
(coefficient = 02:01 x 10°).

The structure was solved by direct methods with the program SHELXS86(1_05_). All
atoms were refined anisotropically with the refinement based upon F. The final cycle of
full-matrix least-squares refinementw, which was based on 408 observed reflections (I
> 3.000(1)) and 55 variable parameters, converged with unweighted and weighted agreement
factors of:

R = 2| lFal-lFal IIXIFal = 0033

WR = [(2w(|F°|- IF,,|)2/2;wFo2)]"2 = 0.036

 

133

The maximum and minimum peaks in the final difference Fourier map, were 1.81
and -l.49 e‘lA’, respectively.

Neutral atom scattering factors were taken from Cromer and Waber(M). All
calculations were performed with the TEXSANlOtj) crystallographic software package.
Data collection and atomic position parameters are listed in Tables 39 and 40, respectively.
Thermal parameters (Uij) are presented in Table 41.

Magnetic susceptibility: Data were measured with a Quantum Design SQUID
magnetometer at temperatures between 4 and 300 K. Since magnetic susceptibilities are
slightly field dependent measurements were also performed at various magnetic fields
between 200 and 6000 G and the susceptibilities at fields of 200, 400, and 600 G were

extrapolated to zero reciprocal field to eliminate ferromagnetic impurity contributions.

2. Results and Discussion

Li:,Sr,NbN4 is a light-yellow air sensitive compound. It decomposes in air to release
ammonia gas within minutes. The polycrystalline form of Li3Sr2NbN, was synthesized by
heating a mixture of Li3N and Sr,,NbN3 in a sealed Nb tube under an Ar atmosphere at
750°C. Use of too great an excess of Li3N and a higher reaction temperature may result
in the formation of Linszso unless the starting materials are absolutely free from oxygen
contamination. The Guinier X-ray powder pattern for the reaction product of Li,N and
SerbN3 can be indexed based on an orthorhombic cell with a cell volume of 548.0 A3,
suggestive that the new phase is not an isostructural niuide of Li3Ba2NbN4. However, upon
comparing the lattice cell volumes for Li3Ba2NbN4, BaszN3, and SerbN3 which are 620.3,
954.3, and 842.4 A3, respectively, we found that the ratio of VMN/VSM, ~ IAdhfldwvnflu,

m. So based on the synthetic stoichiometry and the cell volume relationship, we believe

 

134
that the new phase is likely Li38r2NbN4. Since a structure model could not be found, single
crystal growth becomes necessary for structural analysis.

For crystal growth, the volume ratio of Ar(g) to NH,(g) was thought to be critical
and was controlled between approximately 1:1 and 3:2 in order to achieve a mild reaction.
The advantage of growing niuide crystals under flowing NH,(g) is that the reaction can be
carried out at lower temperature. The use of NH,(g) will probably allow many reactions
to proceed that are otherwise difficult under flowing N,(g). Both Li and Sr metals react
easily with NH,(g) to form amides. The low melting point of LiNHz, 380°CL1fl),
allows the reaction system to be maintained in the liquid state. The reaction temperature,
however, must not be too high since LiNH2 becomes too volatile at elevated temperatures.

The crystal structure of Li,Sr2NbN,, shown in Figure 50, is different from that of
Li3Ba9MN4 M = Nb, Ta). Selected bond distances and angles are given in Table 42. As
shown in Figure 50, the Nb and Li(2) atoms are tetrahedrally coordinated by N atoms.
Each NbN4 tetrahedron connects three Li(2)N, tetrahedra, one by edge sharing and two by
comer sharing to form a two dimensional layer in the bc plane. A mirror plane which
crosses over atoms Nb, Li(2), N(1), and N(3) can be seen from the sum of the four bond
angles, N(1)-Nb-N(3) (115.4°), N(1)-Li(2)-N(3) (915°), Nb-N(l)-Li(2) (734°), and Nb-

N(3)-Li(2) (79.7°), which total 360°.

Table 39 Summary of Crystal and Diffraction Data for Li3Sr2NbN4

 

 

Chemical formula Li3Sr2NbN,
Formula weight 345.00
Space group Pnnm(#58)
a (A) 8.713(6)
b (A) 9.007(4)
c (A) 7.006(5)
v (A3) 550(1)
Z 4
Dtalc (g/cm’) 4.167
T(°C) 26 t 1
Crystal color, Habit light yellow, irregular
Crystal dimensions (mm) 0.150 x 0.200 x 0.300
20 max (deg) 60.0
Scan type 00-20
X-ray radiation (71) MoKa (A=O.71069 A)
Monochromator Graphite
Octants collected hkl;-ha
Absorption coeff 11 (cm") 205.98
Measured reflections 1952
Observed reflections' 408
Unique reflections 970
F000 616
No. of variables 55
Max (min) peak in final diff. map 1.81 (-149) e°/A3
R2”, wR2c 0.033, 0.036
' I > 3.000(1)

b R = 2" lFol'IFcl l/ZIFOI
° wR = [(zw( |F°|- II:c [)2/2wF02)]m

136
Table 40 Atomic Positions and B(eq) for LiaSerbN4

 

Atom a 1 _z_ B(equ’)
Nb 0.2145(2) 0.1368(1) 0 045(5)
Sr(l) 0.2577(2) -0.2458(2) 0 102(6)
Sr(2) 0.5 0 0.2602(2) 0.67(5)
N(1) 0.049(2) 0.284(1) 0 1.0(5)
N(2) 0.198(1) 0.0205(8) 0.234(1) 1.1(4)
N(3) 0.427(2) 0.220(1) 0 0.7(5)
Li(l) 0.017(2) -0.137(2) -0.264(2) 1.0(7)
Li(2) 0.294(4) 0.425(2) 0 1(1)

 

The equivalent isotropic temperature factor is defined as (112):

3 3
2.: ”u“! “r 3'53]
1-1 1-1

. .8353
B(eq) 3

 

Table 41 Anisotropic thermal parameters, Uij, for Li,Sr2NbN,,

 

Atom U11 U22 U33 U12 U13 U23

Nb 0.0058(6) 0.0068(6) 0.0044(5) 0.0008(6) 0
Sr( 1) 0.01 17(8) 0.01 12(7) 0.0159(8) -0.0022(5) 0
Sr(2) 0.0093(8) 0.0094(5) 0.0067(6) 0.0015(6) 0
N(1) 0.004(7) 0.019(6) 0.015(7) 0.004(5) 0
N(2) 0.016(5) 0.017(4) 0.009(4) 0.005(4) 0.000(5) 0.004(4)

COCO

N(3) 0.017(8) 0.011(6) 0.000(6) -0.001(5) 0 0
Li(l) 0.01(1) 0.013(7) 0.012(8) 0.00(1) -0.003(8) -0.00(1)
Li(2) 002(2) 001(1) 001(1) -0.00(1) 0 0

 

The anisotropic temperature factor coefficients Uij are defined as:
exp(-21t2(g"Unh2 + b'zUzzkz + 9’2U3312 + Zg’b'Unhk + ZQ'Q'Unhl + ZQ'Q‘Uzakl»

 

137

Table 42 Selected Bond Distances (A) and Angles (deg) in Li,8r,NbN,

 

Bond distances

Nil-N(1)
Nb-N(2)
Nb-N(3)
Li(1)-N(1)

Li(1)-N(2) {

Li(1)-N(3)
Li(2)-N(1)
Li(2)—N(2)
Li(2)-N(3)
Sr(1)-N( 1)

Sr(1)-N(2) {

Sr(l)-N(3)
Sr(2)-N(1)
Sr(2)-N(2)
Sr(2)-N(3)
Nb-Sr(1)
Nb-Sr(2)
Nb-Li(1)
Nb-Li(2)

Sr(1)-Li(1) {

Sr(1)-Li(2)
Sr(2)-Li(1)
Sr(2)-Li(2)
Li(1)-Li( 1)
Li(1)-Li(2)

1.96(1)
1.953(8)
200(1)
235(2)

215(2)
213(2)

215(2)
248(3)
205(1)
218(3)
270(1)

2.954(8)
2.836(8)

275(1)
2.608(9)
265(1)
2.767(9)
3.466(3)
3.321(2)
2.74(2)
268(2)

2.97(2)
299(2)

298(2)
3.28(2)
3.13(3)
248(3)
240(3)

2x

2x

2x
2x

2x
2x

2x

2x

2x
2x

2x
2x

Bond angles

N(1)-Nb-N(2)
N(1)-Nb-N(3)
N (2)-Nb-N(2)
N(2)-Nb-N(3)

N(1)-Li(1)-N(2)

N(1)-Li(1)-N(3)
N(2)-Li(1)-N(2)

N (2)-Li(1)-N (3)

N(1)—Li(2)-N(2)
N(1)-Li(2)-N(3)
N(2)-Li(2)-N(2)
N(2)—Li(2)-N(3)

N(1)-Sr(1)-N(2)

N(2)-Sr(1)-N(2)

N(2)-Sr(1)-N(3)
N(1)-Sr(2)-N(1)
N(1)-Sr(2)-N(2)

N(1)-Sr(2)—N(3)
N(2)-Sr(2)-N(2)
N(2)-Sr(2)-N(3)

N(3)-Sr(2)-N(3)
Nb-N(1)-Li(2)
Nb-N(3)-Li(2)

{

{

107.9(3)
1 15.4(6)
l 14.5(5)
105.7(3)

89.2(8)
1 18.6(9)

108.8(7)
108.1(7)

124.2(9)
107.8(9)

104.3(9)

91 .5(9)
130(1)

109.6(9)

85.8(3)
92.4(3)

67.6(3)
172.7(2)

105.21(6)

81 .9(2)

96.2(3)
85.8(3)

99.8(4)

99.0(3)
86. 1(3)

85.8(2)
157.2(4)

172.2(4)

71 .2(3)
103.5(3)

97.6(3)
73.4(7)
79.7(9)

2x

2x

2x

2x

2x
2x

2x
2x

2x
2x

2x
2x

2x
2x

2x
2x

 

 

138
The average bond distances for Nb-N and Li(2)-N in Li,Sr2NbN4 are 1.97 and

2.24 A, respectively, close to those in Li3Ba2NbN4 which has an Nb-N distance of 1.95 A
and an average Li-N distance of 2.18 A. The Nb-N bond distance reported for Li7NbN,,

where the Nb atoms are also tetrahedrally coordinated by N atoms, is also 1.95 AM).

 

 

 

 

 

e 69 ea 9 ea 0 O b
Nb Sr(l) Sr(2) N(1) N(2) N(3) Li(l) Li(2) c

Figure 50 The structure of Li3Sr2NbN4. Li(l) atoms link the layers formed by NbN4

and Li(2)N, tetrahedra

clarit
Li(l)
struc1

that i

COOTC

each 0

139

In the layer structure shown in Figure 51 the Li(l) and Sr atoms are omitted for
clarity. The layers formed by the Nb and Li(2)-nitrogen tetrahedra are linked together by
Li(l) atoms, also tetrahedrally coordinated by N atoms, to form the three dimensional
structure (Figure 50). The average Li-N distance of 2.22 A in LigserbN, is larger than
that in LiaBaszN4 because one of the two independent Li atoms in the latter case is 3-fold

coordinated by N atoms. The threefold coordinated Li-N distance in Li3N is 2.106 A(110).

 

 

 

 

 

 

 

e o o a 0
Nb N(1) N(2) N(3) Li(2)

Figure 51 The Li3Sr2NbN4 layer structure. NbN4 and Li(2)N4 tetrahedra connect

each other form layers in the bc plane

(Figure 5
with Sr-l

based or

140

The two Sr atoms occupy the channels formed by N atoms along the 9 direction
(Figure 50). Their coordination environments are shown in Figures 52 and 53, respectively,
with Sr-N distances that range from 2.608 to 2.954 A. The resultant octahedra are distorted

based on the bond angles listed in Table 42.

? N(1)

N(2)

Sr(l) 41%
&, N(2)

N(2) ‘ N(2)

 

 

 

(9 N(3)

Figure 52 Coordination environment of Sr(l)

Q N(2)

N(1)

ea;

 

 

QS N(2)

Figure 53 Coordination environment of Sr(2)

LhBa
indept
a basi

by ed;

weak
Theo
the m:
exhibi

10'3 e:-

141

Li3Sr2NbN4 shares some common structural features with previously reported nitrides
Li3Ba2MN4 M = Nb, Ta). In the structure of Li3BaQNbN,, Nb atoms and one of two
independent Li atoms are also tetrahedrally coordinated by N atoms. Both compounds have
a basic structure unit in which one NbN4 tetrahedron connects with one LiN, tetrahedron
by edge sharing. Each NbN4 tetrahedron in Li3Ba2NbN4, however, only connects two, not
three, Li(2)N,, tetrahedra to form a one dimensional chain and not a layer as in Li38r2NbN4.

The X-ray powder pattern for Li:,Sr2NbN4 is shown in Figure 54. There are two
weak impurity reflections at about 20 = 29.911 and 34.608° which can be assigned to SrO.
The observed and calculated d-spacings and intensities are shown in Table 43. A plot of
the magnetic susceptibility of Li3Sr2NbN4 against T(K) is presented in Figure 55. The data
exhibit a temperature independent paramagnetism between 4 and 300 K ( average 5.605 x

10" emu/mol).

142

eZnZ~emnE .80 83 causeway a 55 5830 e332— 003336 05 026030 «an Bani

 

0.05

cannula

0.00 0.0. 0.0M 0.0a

L
‘

 

q 11 1

 

0.0

 

-
--
”--
w
v

 

 

..00fi

.2 com

..00n

 

 

 

[ado] Kareuaaur

 

 

Table 43 Miller indices and observed and calculated interplanar distances and intensities

143

 

 

for Li3Sr2NbN4

h k 1 d, de 10 1, h k 1 do de 1, 1c
1 1 0 6.2729 6.2624 19 51 4 2 2 1.7111 27
0 1 1 5.5362 5.5300 15 33 1 5 1 1.7100{1.7107 33 {21
2 0 0 4.3564 4.3565 10 20 4 3 1 1.7098 3
1 2 0 4.0075 4.0007 22 39 2 3 3 1.6970 1.6976 5 8
0 0 2 3.5050 3.5030 8 14 1 1 4 1.6874 1.6868 2 3
2 l 1 3.4241 3.4221 14 26 1 2 4 1.6040 1.6045 3 5
2 2 0 3.1314 3.1312 31 51 4 4 0 1.5648 1.5656 4 4
1 1 2 3.0568 3.0572 18 32 3 3 3 1.5557 1.5563 6 12
2 2 1 2.8585 2.8587 3 5 5 1 2 15356{1.5373 6 {9
1 3 0 2.8392 2.8385 16 16 3 5 0 ' 1.5308 2
0 2 2 2.7650 00 2 2 4 1.5279 1.5286 7 13
3 1 0 2.7637{2.7642100 {‘40 5 3 0 1.5059 1.5071 4 6
0 3 1 2.7596 15 4 2 3 1.4990 1.5017 3 4
2 0 2 2.7290 2.7299 65 95 3 5 1 1.4944 1.4956 11 13
1 2 2 2.6355 20 1 3 4 1.4883 1.4906 3 4
1 3 1 2'6318{2.6308 5° {56 3 1 4 14784 1.4795 6 9
2 1 2 2.6080 2.6126 3 4 1 6 0 ‘ {1.4794 {2
3 1 1 2.5614 2.5713 8 8 5 3 1 1.4717 1.4734 3 4
3 2 0 2.4409 2.4408 7 7 2 6 0 14187 1.4193 3 3
2 3 1 2.3319 2.3313 9 13 3 4 3 ' {1.4154 {1
0 1 3 2.2606 2.2606 3 5 1 5 3 1.4065 1.4076 7 11
0 4 0 2.2516 2.2517 5 6 3 5 2 1.4009 1.4027 3 6
1 3 2 2.2055 2.2054 8 13 0 4 4 13807 1.3825 6 2
4 0 0 2.1782 2.1783 19 28 6 2 0 ° {1.3821 {7
3 1 2 2.1675 2.1700 3 6 4 0 4 1.3650 11
3 2 2 2.0026 5 1 6 2 1'3635{1.3628 7 {5
3 3 1 2.0005{2.0005 25 {29 6 2 1 1.3545 1.3560 2 3
2 4 0 2.0003 4 5 4 1 1.3522 3
0 4 2 1.8928 1.8942 4 5 4 1 4 1'3503{1.3496 3 {2
4 2 1 1.8866 1.8883 5 9 3 3 4 1.3418 1
4 0 2 1.8496 1.8498 2 1 6 0 2 13402113415 5 {8
0 3 3 1.8414 1.8433 2 3 2 1 5 1.3182 1.3195 1 3
3 0 3 1.8178 1.8200 2 1 2 4 4 1.3166 1.3177 2 2
4 1 2 1.8102 1.8120 3 4 3 5 3 1.2792 1.2803 5 8
1 3 3 1.8019 1.8034 11 17 5 3 3 1.2649 1.2663 2 2
3 3 2 1.7923 1.7932 19 26 1 3 5 12552{1.2565 3 5
3 1 3 1.7825 1.7839 1 2 5 5 0 ° 1.2525 {2
1 5 0 1.7626 1.7641 4 6 3 6 2 12414 1.2463 2 2
0 0 4 1.7511 1.7515 12 23 1 5 4 ° {1.2429 {3
3 4 1 1.7232 1.7248 4 5

 

144

 

8 UT'U'V'VV'U‘U'lrj'VlTUWU'T‘U'

03

AAAAAAAAAAAA

.h

I
N O N
omtitlrrfilrr

 

Molar susceptibility x IOE+03

llLlLLLlllJlll

lllllllllllllllllllllllll

50 100 150 200 250 300
T(K)

F

 

Figure 55 Plot of magnetic susceptibility (emu/mol) of Li3Sr2NbN4 against temperature (K).

 

VII

Li3
Li3
Sr;
un

SC:

CE

[:1"

St

145
VII. I..i._,Sr2TaN4

1. Experimental

Synthesis: Polycrystalline LiasrzTaN4 was synthesized in a Ta tube by reaction of
L13N and SrzTaN3(_1_7) in the molar ratio of Li,N:Sr2NbN3 = 1:1 with about a 7% excess of
Li,N. The mixture was heated at 750°C for 30 hours under an Ar atmosphere. The nitride
SrzTaN3 was synthesized by heating 8er and Ta3N5 in 3:1 molar ratio at 970°C for 2 days
under flowing N,. Ser and NbN were prepared as described previously (see Li3Sr2NbN4
section).

Single crystals of Li3Sr2TaN4 were grown in exactly the same way as were those for
Li3Sr2NbN4, except that Ta metal was used instead of Nb.

X-ray diffraction: X-ray powder diffraction data were obtained by using a 114.6
mm Guinier camera (quartz monochromatized CuKorl). NBS certified Si (3 = 5.43082(3)
A) served as internal standard. The program LAZY PULVERIXLZQ) was used for intensity
calculations. X-ray single crystal diffraction was carried out on a Rigaku AFCGS 4 circle
diffractometer with graphite monochromated Mo Ka radiation (A=0.71069 A).

Structure determination: An irregular crystal of approximate dimensions 0.14 x 0.10
x 0.14 mm3 was selected and sealed in a 0.15 mm glass capillary in a Nz-filled glove bag.
Lattice parameters were obtained by least-squares refinement of the angle settings of 10
carefully-centered reflections in the range 7.38 < 20 < 18.09°. The choice of space groups
was reduced to Pnnm (#58) and Pnn2 (#34) by systematic absences (0M, 5 + 1 at 23; mg,
g +1: 21.1). and the structure was solved based on space group Pnnm (#58).

Data were collected at 25:1:1°C using the (1)-20 scan technique to 20 S 60°. Omega
scans of several intense reflections, made prior to data collection, had an average width at

half-height of 043° with a take-off angle of 60°. Scans of (1.15 + 0.30 tan 0)° were made

at a S
I'CSCal

reflec

atom
refin.
the t
1310111
was I

With

CaICu

Data

Then

146

at a speed of 2.0°lmin (in omega). The weak reflecrions were rescanned (maximum of 3
rescans) and the counts were accumulated to assure good counting statistics. A total of 965
reflections were collected. The intensities of three representative reflections which were
measured after every 150 reflections declined by -0.50%. A linear correction factor was
applied to account for this phenomena. The linear absorption coefficient for Mo K01
radiation, 383.6 cm", was used to make an empirical absorption correction which resulted
in transmission factors that ranged from 0.86 to 1.16. The data were corrected for Lorentz
and polarization effects and for secondary extinction (coefficient = 0.51:0.08 x 10").

The structure was solved by direct methods with the program SHELX886(1M05). All
atoms were refined anisotropically with the refinement based upon F. Anisotropic
refinement of N(3) and two Li atoms results in a message of "non-positive definition" for
the three atoms. Thus in the ORTEP structural plots, the N(3) and two Li atoms were
plotted isotropically. The final cycle of full-matrix least-squares refinementMQ), which
was based on 617 observed reflections (I > 3.006(1)) and 55 variable parameters, converged
with unweighted and weighted agreement factors of:

R = 23| IF.l-|F.| IIZIF.| = 0.035
wR = [(ZW(|F.|- IF.|)2/}-7wF..2)l"2 = 0.045

The maximum and minimum peaks in the final difference Fourier map, 3.84 and -2.16 e’
/A’, respectively, are close to either Ta or Sr atoms.

Neutral atom scattering factors were taken from Cromer and Waber(1_07_). All
calculations were performed with the TEXSANLOS) crystallographic software package.
Data collection and atomic position parameters are listed in Tables 44 and 45, respectively.

Thermal parameters (0,) are presented in Table 46.

whic
U331
to 0
L138
tetra
mp0

1.96

sam,

147

2. Results and Discussion

Li,Sr2TaN4, a light-yellow air sensitive compound, decomposes in air to release
ammonia gas within minutes. The crystal structure of Li;,Sr2TaN4 is shown in Figure 56.
Figure 57 shows an extended plot of Figure 56. Li,Sr2TaN, crystallizes isotypically to
Li,Sr2NbN4. Selected bond distances and angles are given in Table 47. As shown in Figure
56, the Ta and Li(2) atoms are tetrahedrally coordinated by N atoms. Each TaN,
tetrahedron connects three Li(2)N4 tetrahedra, one by edge-sharing and two by comer-
sharing to form a two dimensional layer in be plane.

The average Ta-N bond distance in Li3Sr2TaN4 is 1.96 A, close to that in Li313a2TaN4
which has an average Ta-N distance of 1.95 A. The tetrahedrally coordinated Li(2) in
Li,Sr,TaN, has Li(2)-N distances of 2.05, 2.19, and 2.48 A (average 2.24 A), comparable
to the Li(1)-N distances which range between 2.21 and 2.43 A (average 2.32 A) in
Li3BaQTaN4 where the Li atom is also tetrahedrally coordinated by N atoms. The UN
tetrahedra in Li3Sr2TaN‘ are apparently more distorted. Note that the Ta-N bond distance
reported for Li,T‘aN4 where Ta atoms are also tetrahedrally coordinated by N atoms is also
1.964 AQ8) .

The coordination environments for the Sr(l) and Sr(2) atoms are essentially the

same as those in Li;,Sr2NbN4 (see Figures 52 and 53).

Tabl

Table 44 Summary of Crystal and Diffraction Data for Li:,Sr2TaN4

 

 

Chemical formula Li:,Sr2TaN4
Formula weight 433.04
Space group Pnnm(#58)
a (A) 8.700(6)
b (A) 9.004(4)
c (A) 7.000(3)
v (A3) 548.3(5)
Z 4
Dub (g/cm’) 5.245
T(°C) 25 i 1
Crystal color, Habit light yellow, irregular
Crystal dimensions (mm) 0.140 x 0.100 x 0.140
20 max (deg) 60.0
Scan type (1)-20
X-ray radiation (1) Mom (A=0.71069 A)
Monochromator Graphite
Octants collected hkl
Absorption coeff 11 (cm") 383.64
Measured reflections 965
Observed reflections‘ 617
Unique reflections 861
F000 744
No. of variables 55
Max (min) peak in final diff. map 3.84 (-2.16) e'/A‘-'
R", wRc 0.035, 0.045
‘ I > 3.000(1)

" R = 2| lF.l-IF.| I/ZIF.|
° wR = [(2w( |F, |- |l=c |)’/2‘w1=,2)]”2

 

149
Table 45 Atomic Positions and B(eq) for Li._,Sr2TaN4

 

Atom 5 x z B(eqXA’)
Ta 0.214420) 0.137110) 0 015(2)
Sr(l) 0.2582(2) 0.2457(2) 0 076(6)
Sr(2) 0.5 0 0.2603(2) 038(5)
N(1) 0.048(2) 0.284(2) 0 0.6(5)
N(2) 0.197(1) 0.019(1) 0.234(1) 0.5(3)
N(3) 0.425(2) 0.219(2) 0 1.0(6)
Li(l) 0.018(2) 0.137(2) -0.262(3) 0.5(7)
Li(2) 0.292(5) 0.426(4) 0 1(1)

 

The equivalent isotropic temperature factor is defined as (112):

81:2 3 3
B(eq) = —5—[ 2 : Uua, a, a, 4,]
1-1 j-l

Table 46 Anisotropic thermal parameters, Uij, for Li._,Sr2TaN4

 

 

Atom U11 U22 U33 U12 U13 U23

Ta 0.0022(3) 0.0021(3) 0.0013(3) 0.0002(3) 0
Sr(l) 0.0088(7) 0.0066(8) 0.0134(8) -0.0026(5) 0
Sr(2) 0.0047(6) 0.0059(6) 0.0037(7) 0.0006(5) 0
N(1) 0.001(6) 0.015(7) 0.008(7) -0.005(5) 0
N(2) 0.003(4) 0.013(4) 0.004(4) -0.003(3) -0.000(4) 0.000(4)

COCO

N(3) 0.005(7) 0.002(6) 0.03(1) 0.005(5) 0 0
Li(l) 0.02(1) -0.011(7) 0.01(1) 0.002(7) -0.004(8) -0.003(8)
Li(2) 005(2) 000(1) -0.00(1) 001(2) 0 0

 

The anisotropic temperature factor coefficients Uij are defined as:
exp(-21t2(§‘2Uuh2 + b‘zUnk’ + g’zUnl2 + ZQ'Q‘Unhk + Za'g'Unhl + 2b‘g'U23kl))

 

Table 4

Bonc
Ta-b
Li(lj
Li(l‘

Li(l
Li(2
Li(2
Li(2

Sr(l
Sr(l

Sr(T

Sr(i
Sr(j

Sr(i
Ta-

Ta-

Ta.

Ta.
Ta.

Sr(

Sr(
Li(
Li(
Li(

150
Table 47 Selected Bond Distances (A) and Angles (deg) in Li3Sr2TaN4

 

Bond distances Bond angles

Ta-N 1.96(1) 2x N(1)-Ta-N(2) 108.1(4)
Li(l)-N(1) 233(2) 2x N(1)-Ta-N(3) 115.7(6)
. 2.l6(2) N(2)-T -N(2) 113.3(6)
“(”Nm 1 211(2) N a
(2)-Ta-N(3) 105.9(3)
Li(l)-N(3) 217(2) 2x . 897(9)
Li(2)-N(1) 248(4) N(1)‘L‘(1)‘N(2) 119.0(10)
Li(2)-N(2) 205(2) N(1)-Li(l)-N(3) 108.8(8)
Li(2)-N(3) 219(4) N(2)-Li(1)-N(2) 107.7(7)
2.69(1) . 124.0(10)
5’11”“) 1 3.894(7) 2x N(2)'L‘(1)'N(3) 107.0(10)
294(1) 2x N(l)-Li(2)-N(2) 104.0(10)
Sr(l)-N(2)
1 2°85“) 2" N(1)-Li(2)-N(3) 91.0(10)
Sr(l)-N(3) { 38%;) 2x N(2)-Li(2)-N(2) 131.0(10)
Sr(2)-N(1) 2.600) 2x N(2)-Li(2)-N(3) 109.0(10)
Sr(2)-N(2) 2.65(l) 2x N(1)-Sr(1)-N(2) 33.38;
22:21:. 2*
a- a . N2-S 1-N2 172.8(4)
3.468(2) ( ) r1) () 105.28(6)
Ta-Sr(1) { 4.227(3) 817(4)
3.663(1) 2x N(2)-Sr(l)-N(3) 32.081
”Sr(2) { 3.319(2) 2x -
4.120(2) 2x N(1)-Sr(2)-N(1) 99.8(4)
. 273(2) 2x 99.0(4)
T344“) 1 352(2) av. 4x N(1)'S‘(Z)'N(2) 86.2(4)
Ta-Li(2) 268(3) N(1)_Sr(2)_N(3) 1 39381
S r(1)—S r(2) { 3.555(2) 2x '
3.620(2) 2x N(2)-Sr(2)-N(2) 171.9(5)
3.645(3) 70.8(4)
5’12)‘S’(2) 1 3.356(3) N(2)’S’(Z)'N(3) 103.6(4)
Li(1)-Li(1) 249(3) N(3)-Sr(2)-N(3) 97.5(4)
Li(l)-Li(2) 241(4) Ta-N(1)-Li(2) 73.4(9)
Li(2)-Li(2) 3.86(9) Ta-N(3)-Li(2) 80.0(10)

2x

2x

2x

2x

2x
2x

2x
2x

2x
2x

2x
2x

2x
2x

2x
2x

 

 

and

151

 

 

 

 

 

 

 

 

 

/
8
$— 8
Q ‘9 Li(2)
m 6? a
m w 8 Q Sr(l)
N(2) Ta
N(1)
7 7
§ N(3) l ' c
Sr(2) Li(l)

Figure 56 The structure of Li,Sr2TaN4. Li(l) atoms link the layers formed by TaN4

and Li(2)N4 tetrahedra.

152

 

Figure 57 Extended view of the Li,Sr2TaN4 structure

I. Extc
1. Sub
Attem;

powde

I‘Q

Su

'WaSC
at 85‘

0f tht

+Sr

950C

CHAPTER 6 SOME UNSUCCESSFUL TRANSITION METAL TERNARY NITRIDE
SYNTHESIS EXPERIMENTS

-- REFERENCE INFORMATION FOR FUTURE WORK

1. Extended synthetic experiments based on the nitrides Li3Ba,_M_N4(l_vi_ = Nb, Ta)
1. Substitution of Er for Ba
Attempts to synthesize Li,Eu,_MN,(_M_ = Nb, Ta) at ~ 860°C were unsuccessful based on the

powder X-ray diffraction analysis.

2. Substitution of Na or Cu(l) for Li

Reaction conditions were similar to those for Li3Ba2MN,(_l\_/l = Nb, Ta). No crystal
was obtained in either cases. Cu metal does not react with the rest of the reactants either
at 850°C or 960°C under flowing N2. When Na metal was used, the powder X-ray pattern
of the black product was complicated and did not suggest the formation of NagBazMN4 M

= Nb, Ta).

3. Simultaneous replacement of Nb” or Ta" and one Li“ by 80‘4 and Zn”, respectively
Since Zn is very volatile at the reaction conditions, a mixture of 4 Li + Zn + 2 Ba
+ Sn + 2NaN3 + I2 was sealed in Nb tube under Ar, heated to 950°C at 77°C/h, kept at

950°C for 18 h, and then cooled to 500°C in 9°C/h. No crystal was found.

4. Crystal growth for Li-Ba-M-N system, M = M0, W

Li, Ba, M0 or W metals were mixed in 2:1:1 molar ratios and heated to 850°C

153

 

series
and ‘.
for L
heate

COD ta

III.

in 8.

Prep

816C

Slmfl'

154
under flowing Ar, held at 850°C for 27 h, and finally cooled to 150°C at 10.6°C/h. Only

red crystals of Ba3M0N4 and yellow crystals of Ba3WN4 were found. The structures of

Ba.,MoN4 and Ba3WN. have been reported(1_5).

II. Efforts on the synthesis of rare-earth metal-containing ternary nitrides

Rare earth metals have a common oxidation state of +3. We tried to synthesize the
series of compounds with formula of MMoN3, where M = Y, La, Ce, Sm, Eu, Gd, Er, Tm,
and Yb, which potentially have the perovskite-type structure. The rare earth metal (except
for La) was mixed with Mo powder and confined in a Ta or Nb boat. The samples were
heated under flowing N2 with Li or Li+Ba as flux. No crystal was obtained For the La-
containing system, LaN, Mo,N, NaN3, I2 , with NaCl-KCl as flux were mixed in the molar
ratio of 2:1:2:1:2 and sealed in a Nb tube under Ar. Here NaN3 serves as nitriding agent,
I2 reacts with Na to form NaI which can become part of the flux, NaCl-KCl is flux. The
mixture was heated at 830°C for 30 hours. The X—ray powder diffraction did not show any
new phase. When Tm metal was used, yellowish orange colored crystals were produced.

But unfortunately, they are Li,MoN,, whose crystal su'ucture has been reportedLl_2_).

III. Experiments with Alkali Metal Amides MNHz, M = Na, K) as the Nitrogen Source
in Sealed Nb Tubes

The structure of Na3MN3 (M = M0, W)(_2_2,2_3) have been reported. Na3MoN3 was
prepared by heating Mo metal with an excess of NaNH2 (molar ratio 1:10) in autoclaves
at 600°C. Na3WN3(2._3_) was prepared similarly, but at 900°C with molar ratio of WzNaNH2
= 1:6. We tried to synthesize K3MN3 (M = M0, W) in arc welded Nb tubes by using

similar reactants. KNH2 was prepared by using the procedure for preparing

 

 

NaNH2<
WITH
several

conditi

Was
blac
But

diff

nur

€13

155
NaNH,(128). CAUTION : KNH2 IS EXTREMELY EXPLOSIVE UPON CONTACT
WITH VERY SMALL AMOUNT OF WATER! Mo powder and KNH2 were mixed in

several different molar ratios (1:3, 1:6, 1:10) and are welded in Nb tubes. The reaction

conditions are listed below.

Molar ratio (Mo : KNHZ) Reaction condition
1 : 3 600°C, 6 days, -19°/h to 150°
1 : 6 1) 600°C, 3 days, -19°/h to 150°

2) 600°C, 6 days, -19°/h to 150°

3) 750°C, 6 days, -12.5°/h to 150°
1 : 10 1) 600°C, 8 days, -14.6°/h to 250°

2) 550°, 10 days, -14.6°/h to 200°

3) 550°, 10 days, then quenched.

All of the products were embedded in K metal, which was washed away with liquid
ammonia. In the most of the cases, the products were just a black powder; no single crystal
was obtained. When the molar ratio of Mo : KNH2 = 1 : 10 was used, some regular-shaped
black crystals were obtained under the all three different reaction conditions listed above.
But none of the crystals diffracted for reasons that are still unclear. X-ray powder
diffraction was not carried out because of an insufficient quantity of crystals. The X-ray
powder diffraction analysis on the product of the sample with 1:3 molar ratio gave
numerous weak reflections, suggestive that one or more new phases had been formed.

Similar reactions were carried out by using Moocontaining reactants other than

elemental Mo. MoCls, MoCl3, M003, and M002 were all tried, but the results were

promi

the pr

CW8!

the ]

VI.

F0111
tube

DUI

156
promising. Only black powders resulted In the case of M002, Y-MOZN was identified in

the product.

For the W system, the following three reactions were tried.

Molar ratio (W : KNHz) Reaction condition
1 : 6 1) 600°C, 3 days, -19°/h to 150°
2) 600°C, 6 days, -19°/h to 150°

3) 750°C, 6 days, -12.5°/h to 150°

When the sample was heated at 600°C for 6 days, some small (< 0.01 mm) red
crystals were found in the product. X-ray powder diffraction showed these crystals to be

the K,4W6N,6NI-I(2_4) rather than our expected K3WN3.

VI. Sodium Azide (NaN3) as the Nitrogen Source in Sealed Nb Tubes

Most experiments focused on crystal growth of MszN3 where M = Sr, Ba
Following is the list of some of the experiments uied. All sample were are welded in Nb
tubes and heated under flowing Ar or N2. CAUTION : CARE MUST BE TAKEN

DURING THE ARC WELDING SINCE NaN3 IS POTENTIALLY EXPLOSIVE.
1) 2 Ba + Nb + 3 NaN3 + 3 NaCl-KCI
830°C, 1 day, -6.6°/h to 500° and then -20°/h to 200°C.

NaCl-KCI served as flux.

2) 2 Ba + Nb + 3 NaN, + 3 NaCl-KC1+ 1.512

3)

4)

5)

6)

7)

8)

157
830°C, 1 day, -6.6°/h to 500° and then -20°/h to 200°C

I2 can react with Na to form NaI which serves as flux.

28r-1-Nb+6Li-1-3NaN,-1~1.512

950°C, 1 day, -7.3°/h to 600° and then -20°/h to 300°C

ZBa+Nb+2NaN3-1-0.8I2

950°C, 1 day, -10°/h to 450° and then -20°/h to 250°C

4Ba-1-Nb+2NaN3+0.8I2

950°C, 1 day, -10°/h to 450° and then -20°/h to 250°C

2Ba+28r+Nb+2NaN3+0812
i) 950°C, 1 day, -10°/h to 450° and then -20°/h to 250°C

ii) 1000°C, 20 h, -10°/h to 500° and then -20°/h to 300°C

ZBa+28r+Nb+2NaN3+I2

1000°C, 20 h, -10°/h to 500° and then -20°/h to 300°C

2Ba+Nb+3NaN3+2YCl3
830°C, 1 day, -6.6°/h to 500° and then -20°/h to 200°C

YCl3 served as flux (mp. 721°C).

Except for #3 above, these reactions usually produced yellowish or yellowing

brown
in p0“
from t

indica

158
brown powders, sometimes together with Na metal. BaQNbN3 and SerbN3 do forms but

in powder form only. For #3 above, some light yellow transparent crystals were isolated
from the crushed product. But cell constants obtained by X-ray single crystal diffraction

indicated the product to be the reported compound, Li7NbN4Cfl).

 

pa

SC?

CC

81

S(

If

bi

p:

p:

01'

CHAPTER 7 CONCLUSION

1. A method for synthesizing transition metal nitrides by heating nanoscale metal
particles under flowing N2(g) or NH,(g) was explored. Nanometer-size metal particles are
scientifically and technologically important because of their high reactivity and high surface
area. Very few transition metal nitrides can be prepared at relatively low temperatures by
reacting a chloride or oxide with ammonia gas. Conventional high temperature preparative
procedures for synthesis of transition metal nitrides from the elements require severe
conditions (e.g., T 2 1200°C for NbN and TaN) and produce specimens with very low
surface areas, a detriment for their potential use as catalysts or for their use as solid state
synthesis precursors.

Homogeneous reduction of metal chloride(s) by an alkalide in dimethyl ether
solution produces highly reactive nanoscale metal particles. This work shows that these
nanoscale metal particles are very good precursors for the synthesis of binary or ternary
transition metal nitrides such as Fe,Mo3N where bOth metals are transition metal elements.
Several of these types of pure nitrides were successfully synthesized and synthesis
temperatures in some cases were significantly lower than those required by conventional
methods. This synthesis method may prove useful for the preparation of transition metal
binary niuide catalysts where a large surface area is required. The purity of the metal
particle products needs to be further improved. The organic impurities in the nanoscale
particles interfere the synthesis reaction when more electropositive metals such as the alkali
or alkaline earth metals are used as reactants because these metals are more sensitive to the

organic impurities than are the transition metals.

159

gasn

binar
A hi
imp<

CXCfi

Ta :
and
nitr

atO'.

8-f

qu;
10
isc
gn
Un
re?

re.

160

2. For synthesis of ternary and quaternary nitrides by the conventional liquid-solid-
gas method, N2(g) and NH,(g) are still the most common nitriding reagents. Reactants can
be a mixture of two (or three) different metals or binary metal nitrides or metal(s) and
binary metal nitride(s). The reaction temperature is normally between 800 and 1000°C.
A higher temperature is required when reaction takes place in a N2(g) atmosphere. Another
important factor in nitride synthesis is the need for very pure N2 or NH3 gas. With the
exception of the first transition series, the transition metals in the nitrides reported-to-date
are always in their highest common oxidation states. The coordination around the Nb and
Ta atoms is teu'ahedral both in the ternary and quaternary nitrides presented in this work
and tetrahedral coordination is also most common for Nb and Ta atoms in other reported
nitride studies. Varieties of coordination environments are observed for the N atom. N
atom coordination by metal atoms can be 4-fold (distorted tetrahedron in BangN3), 6-fold
(distorted octahedron in BaQNbN3), 7-fold (capped distorted octahedron in Li3Sr2NbN4), and
8-fold (irregular polyhedron in LizBaszN4 and distorted cubic in LimszNaO).

It is still a challenge to grow single crystals for transition metal ternary and
quaternary nitrides. The most common method is to heat the reactants between 800 and
1000°C under flowing N2(g) in a Li metal or Li3N flux. The single crystals of the two
isostructural quaternary nitrides, Li3Ba7_MN, M = Nb, Ta), presented in this work, were
grown in this way. The successful growth of single crystals for Li.‘,Sr,.M,N4 M = Nb, Ta)
under flowing NH,(g) in this work provides a new way for niuide crystal growth at
relatively low temperature. Here alkali and alkaline earth metals can be used both as
reactants and flux because they form a mixture of alkali and alkaline earth metal amides
with low melting points. An appropriately controlled partial pressure of NH3(g) needed to

maintain mild reaction conditions is probably a very important factor for successful crystal

grov

growth.

161

10.

11.

12.

13.
14.

15.

l 62
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