fl DERSETY AE‘ED S?§-Z?JC?URE STUDEES OF
LANTHANUM-PRASEQQYMEQM QXEC‘ES

Thesis for the Degree of Ph. D.
M!CHEGAN STATE COLLEGE
W‘ENiam Roby? Read

1954

:rHESIS

7‘

A

  
   

L [B R x! R \x"
Nlichigjxn ‘imzc
University

DENSITY AHD STRUCTURE STUDIES
OF LAfiTHANUM~PRASBODYHIUM OXIDES

8!
WILLIAM Roevar REED

A THESIS

Submitted to the School of Graduate Studiee of Michigan
State College of Agriculture and Applied Science
in partial fulfillnent of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1954

ctCXNOVLKDGfiENT

The author wiehee to eXpreee hie sincere appre-
ciation for the able and inspiring direction by Dr. L. L.
Quill throughout the research program end during the
preperetion of the theeie. The guidance and underetanding
of the other nembere at the faculty ie eleo appreciated.

TABLE OF GOIITENTS

Lint of Figure: . . . . . . . . . . . . . .
Introduction
1. Structure and Preparation of Lanthanon
Oxidee . . . . . . . . . . . . . . . .
II. Lonthanuu Oxide. Exyerinentel . .'. .
Sumner, . . . . . . . . . . . . . . .
III. Praeeodyliul*Oxidee, Rietorioel . . .
Praeeodymiun Oxides. EXperinentel . .
Bummer! . . . . . . . . . . . . . . .
IV. nixed Oxides, Hietcrioel . . . . . . .
nixed Oxidee, EXperieentel . . . . . .
Summary . . . . . . . . . . . . . . .
Conciueione . . . . . . . . . . . . . . . .
Literature cited . . . . . . . . . . . . . .

App.nd 1x 0 O O O O C O O O C O O O O O O O O C

Page Ho.

7
. 3
. 7
. 14
. 18
. 20
. 27
. 28
. 52
. 56
. 58
. 60
. 65

I.

II.

III.

IV.

LIST OF FIGURES

Variation of the Cubic Structure with
Gompoeition . . . . . . . . . . . . .

Variation of the Hexagonal Structure with

conp°.1t1°n e e e e e e e e e e e e e e 0

Variation of 'Exceee' Oxygen/Hole Pr203

Vita COIpOIltlon e e e e e e e e e e e e e

Variation in Deneity with Oompoeition

Page Ho.

. 4O

. 41
. 48
. 53

IETHODUCTIOH

the queetion of .0113 solution formation vereue
compound formation between higher oxidee of electropoeitive
elements and oxidee of the elemente showing only the posi-
tive one. two, or three oxidation etate hue been debated for
many yeare. The two poeeibilitiee have been of intereet in
rare earth oxide etudiee. Kixed oxide eyetene may be of two
types: (1) synthetic nixturee prepared by combining the pure
oxidee or (2) naturally occurring nixturee. Heny euch eye-
teme have been shown to follow the additivity rules. However,
some eyetene ouch en the air-ignited mixed oxide eyeteme of
lanthanum and praeeodyniun or neodymium and praeeodymiun do
not completely follow the ndditivity rules. Since the
lanthanum and prceeodymiun oxide eyetem doee occur naturally
and hae proven to be difficult to analyze because it doee
not follow the additivity rules. it ie of interest to know
more coupletely the behavior of thie eyeten an the compo-
sition in varied.

The virtual ineolubility of ceriua dioxide, when
pure, in mineral ecide hue been explained by aeeuming
compound formation. Prnndtl(49) stated that Prboll end

Frog are solid eolutione of Przoa and Préo Zintl end

50
Oroetto(7l) and Corleon(l4) etudied mixtures of lanthanu-
oxide and ceriue dioxide and the variations of the proper-

tiee of the eyeten were believed to be evidence of solid

solution formation. Zintl and horavietz(72) prepared
fiaECeOa. RaEPrOS, and NaPrOQ. From X-ray etudiee they found
that each of the oxidee has a eodiuo chloride etructure.
Hoffman(29.30) prepared BaCeOs. Brceoa, and Barroa. From
x-ray studiee he found that BaGeO3 hae the perovekite struc-
ture and that Burroa and aroeo3 are cubic. The air-ignited
lanthanum oxide-praeeodyuiun oxide eyeten hae been etudied
by 8aluteky(de) and Prandtl and Huttner(50) and they found
that at high concentratione of praeeodyoiun oxide, oxygena-
tion of the praaeodymiun beoouee eaeier than for the pure
oxide whereas in prueeodyniun-poor aixturee oxygenation of
the praeeodyniuu becouoe very difficult.

The preeent etudy has as its object the study of
the air-ignited lanthanum oxide-praeeodyniun oxide system
and the accompanying variations of crystal etructure, den-
aitiee, and amounts of oxygenation with change: of composi-
tion to learn whether or not the queetion of eolid eolution

vereue compound formation for the eyeten nay be resolved.

SEGTICfl I

atructure and groperatiog of lanthanon Oxides

Each of the rare earths forms a basic oxide in
which the natal is trivalent. These oxides are by defini-
tion the original rare earths. Except for the oxides of
cerium. praseodyaiun, and terbiun these sesquioxides are
stable toward oxidation at higher temperatures or under
vigorous oxidizing conditions. A review of sons of the,
properties of the eesquioxides will be followed by a brief
discussion of those rare earth oxides in which these elements
exhibit a higher oxidation state.

all of the rare earth eesquioxides except those of
cerium, praseodynium, and terbiun can be formed by direct
combination of the free metals with oxygen and by ignition
of the hydroxides, nitrates. carbonates. oxalates, sulfates,
or other salts formed with anions of volatile or easily
decomposed oxy-acids. Praeeodynium and terbiuo sesouioxides
may be prepared by thereduction of the higher oxides in a
stream of hydrogen at about 900° 0. or by ignition of the
higher oxides in vacuum at above 800° 6. however. the
reduction of ceric oxide to the seaquioxide in a stream of
hydrogen is difficult and is not rapid until the temperature
is about 2000° C. and the hydrogen pressure is around 150
atmospheres. Ceric oxide is not reduced upon ignition in

vacuum 0

The rare earth sssquioxidee are readily soluble in
most acids. Those of lanthanus, praseodyaiuo. and neodymium
readily hydrate and carbonate in air and torn normal salts
with cost of the weaker acids except hydrocyanic and hydro-
sulfuric acids which are too weak. this type of behavior of
the sesquioxides contrasts vith that of ceric oxide which is
difficultly soluble even in strong acids.

In asking structural studies Goldschnidt. Ulrich.
and Barth(25) found that the lanthanide sesquioxides crystal-
lies in three structural types: a, hexagonal: B, pseudo-
trigonal: and O. body-centered cubic. The A or hexagonal
for. is common fro. lanthanu- through sanariun and is
obtained by high temperature ignition of appropriate con-
pounds. Goldschnidt originally reported that only the
sesquioxides of the elements saaariu- through luteciua
assumed the 0 or body—centered cubic structure. However.
Lohberg(38), Isndclli(34), and Bonner(7) have prepared cubic
sesouioxides of the remaining rare earths. The C structure
is forced at lower temperatures than the A form. The less
common 3 or pseudotrigonal structure has been reported for
the sesquioxidee of praeeodysiua(?), neodymius, saoariun.
gadolinius, and dyeprosiua(?) and is forced at intermediate
temperatures.

Since this study deals with lanthanum and praseo-
dyniua oxides. the a. 0. and fluorite structures will be

reviewed. Velle(65) in his revise of the structures of the

rare earth.oxides states that the hexagonal structure con-
tains one lonthanus oxide soleculs per unit cell. The natal
ion has a coordination nuaber of seven with the odd oxide
ion being located above one face of an octahedron which is
distorted by separating the oxide ions at the corners of
that face. The four nearer oxygen neighbors to the lanthanum
ion are at 2.42 A.snd the renaining three are at 2.69 A.
Values for the distances between the praseodyniua and oxide
ions are not given but the structures of the two sesquioxides
are isotypic and the radii of the two ions are 1.04 A. for
lanthanum oxide and 1.00 A. for praeeodyniua sesquioxide.
The 0 or body-centered cubic structure. having 16
aoleculee of sesquioxide per unit cell. is closely related
to the fluorite structure froa which it may be derived by
removing oneoquarter of the anions and rearranging the ions
slightly so that all the natal ions are six coordinated..
Unlike the other lanthanons. cerius, praseodyniua,
and terbiua. upon ignition of the free setals or salts of
volatile or easily decomposed oxybacids in air or oxygen
fora oxides in which the oxidation number of the metals is
higher than three. Ceriua sesquioxide left in the open air
slowly transforms to ceriuu dioxide. Upon air ignition,
praseodyniun and terbiun or their apprOpriate compounds form

Frso and Tb407. respectively. Of these three higher

11
oxides ceriun dioxide has the fluorite structure whereas

., I .
Pr'ao11 and Tb407 fora defect fluorite structures.

There are two possible arrangements for a ”defect“
fluorite structure: namely. a lattice in which all the cation
positions are filled and sons of the anion positions are
vacant or a lattice in which all the cation and anion poai~
tions are filled and the excess cations are located intsr~
stitially. thullough(42) and hund and Peeta(33) think that
the first suggestion is the more probable one for the etruc~
ture. however, Vickery(64) using data by Hartin(40) and
Foes and Loriers(23) believes the second possibility to be
more probable. ho comprehensive structural studies have
been reported for Tb407. Victory also states that the cubic
lines characteristic of the 0 structure of the sesquioxides
have been observed in the X-ray powder diagrams of ”e°11 and
Tb407. This latter idea implies that lines characteristic
of both body-centered and face~centered cubic structures
would have to be present on the ease powder diagram. However,
in this investigation only cubic lines corresponding to the

face-centered cubic fluorite structure were found.

SECTION II

Lgnthenun Oxide

 

Experimental

since lanthanum oxide is a part of the system
studied in this investigation. it was thought advisable to
recheck the values of the density and of the lattice con-
etants of the hexagonal structure. Except for the earliest
values there is good agreement among the reported density
values. It was also felt that the reported lattice constant
values were not too precise. Therefore. it was decided to
redetereine the density and lattice constants for the oxide
having the hexagonal structure.

Lanthanun from four different sources was used in
this investigation. Two of the samples were from the lan-
thanua-rich ends of two different extensively fractionated
double magnesium nitrate recrystallisation series from this
laboratory. The lenthanue eaterial was recovered from the
fractions by double precipitation as the oxalate.’ The
absorption spectrum of an almost saturated solution of the
chloride was checked visually through a layer five centio
esters thick for the presence of other lanthenidee exhibiting
absorption in the visible range. Sons 'spectroscopically'
pure material was obtained from Dr. L. L. Quill. The fourth
sample of lanthanun aaterial was obtained tron the Lindsay

Uremicel Company as 99.95 per cent pure and was checked for

the presence of other lanthanons in the same manner as the
first two source materials. All the oxide samples after
ignition were pure white.

Three oxide samples were prepared from each source
material. By ignition. one sample was prepared free the
nitrate. one from the carbonate. and one from the oxalate.
Each sample was air ignited for at least ten days in an
electric muffle furnace at 900° C. to insure complete cal-
cination and annealling of the oxide product so very sharp
X-ray lines would be obtained. Each sample upon removal
from the furnace was placed in a desiccator to cool. For
X—ray analysis Lindenann capillaries were filled with the
oxides as soon as possible after cooling.

Powder diagrams were taken on a Rorelco X-ray
diffraction machine using a Hull-DebyeoSchsrrer type camera
of 57.3 millimeter radius. Capper radiation filtered through
nickel foil was used. Films were censured with a steel
metric scale having a sliding vernier reading to $0.008
centimeters. The linear readings were converts! to 20
values by the method recommended by Straunanis(61). For
samples having the hexagonal structures the lattice con-
stants were chlculated from the equation.

sinEO ' F1(h9+hssk?) + 72(1)2 ’

and the rclationehips,

a I A and O 3 A
3 \/il ‘4 F2.

 

Since one till obtained was unreadable, only
eleven tilts were lessured. The 80 values for each plane
producing a reflection were averaged. The 20 values,
relative intensities, einzo values. and planes producing
reflections are given in reble I. There was good agreeeent

among all the 29 values for all files.

 

 

 

 

 

 

 

 

new _1_
X-rg Date £93: Lanthanu Oxide
Relative
Line Plane Integsity so sues
1 10‘1'0 V 26.23 0.05168
2 03%? w 229.25 .06375
3 1 vs 30.12 .06701
4 idle e 30.63 .11490
0 11‘50 s 4:. .15400
6 10135 e 02 28 .19410
7 2690 new 51.94 .20563
e 1122 s 55.250 .21710
9 2021 w 06.10 .2211:
10 0004 vvw 60. :52 .25394
11 2622 vw 62.41 .20343
12 101'4 vw 67.04 .30490
1:5 202:3 w 72 22 .34732
14 2130 m 73.53 .seeee
1a 2131 e 75.46 .37447
10 11154 w 79 . 20 .4063;
17 2132 v7: 81.00 .42178
18 101's 1: e: .09 .44670
19 3030 vw 35.47 .46060
2 213: 1: 90.04 .eooas
21 3039 w 92 . 73 . 52131
ein20 - 0.05130(h2+hk+k2) + 0.0115770)2
e 8 W ' 3.930 1- 0.001A. ,
a \/ 0.00130
¢=___1_._§_4_1_g___= 6.13910.0C-2 A.
4 Jo.o1e77

 

 

 

10

Lattice constants of pure lanthanus eesquioxide
were calculated free these data as follows. For 0n (as!

1.5410 1..
siege 3 0.00130(h‘5‘+hx+12) + 0.0157741)?

 

 

and
. a 1.0418 and o 1' 1.0418
3 J0.03130 d V0.01”?

free which the lattice constants are a". 3.930 t 0.001 A.
and c 3 6.139 t 0.002 A. The uses square error was calcu-
lated according to the method of least squares. These values
were used also to calculate the x-rsy density as will be
shown later.

table II suemariees the literature data and that

or this study for the lattice constants of lanthanus oxide.

 

 

 

 

 

 

 

 

 

 

 

 

 

TABLE 1;
kettles Constants‘ggbLanthanus Oxide
‘17 Hexagonal
W iszm no.
a c
3.93. 3.12 Goldschsidt‘gg 5;. 23
3.93 6.12 Zachariasen 68
3.943 6.101 Pauling . 4s
3.927 0.114 Zintl and Croett 71 -
~s.922 6.120 Orcstto 13
3.930 6.189 This work
.
W Ms: 53’0"“.
a
11.4 Lohber; 08
11.40 Bonner 7
11.33 landelli . 34

 

 

 

 

 

11

The agreesent anon; the values for the‘g axis is
good except for the values reported by Pauling and by Groattc.
agreement of the values for the length or the 3 axis is not
as good. For this work the value found for the.g axis is
somewhat higher than the reported values with the exception
of the one given by Pauling.

Density seasureeents of lsnthanus oxide were nads
at 26.00 0.02° 0. One weight pycnoseter has a thersoseter
and a smell capillary side are and the other has a ground
glass stopper with a capillary hole. Both pycnoseters were
calibrated with redistillsd water. the density of water
being taken as 0.99705 g./Il. at 03.00° 0. Th. 331.“. “Cid
was the .1001. one-third of freshly distilled c.p. isoserie
xylene. At three different tines newly distilled xylene
was prepared. the three different samples used had densities
of 0.85900. 0.83869. and 0.86828 3./sl.. respectively.

For a typical density detersination, a freshly
ignited sample of lanthanus oxide weighing about three grass
was weighed into a clean dry pycnoeeter. xylene was intro-

duced over the solid. and then the unit was placed in a
vacuua system to renove any occluded air bubbles. The xylene
was allowed to boil for at least a half hour at roo- tes-
psrature. the unit was resoved from the vacuus systes.
tilled completely with xylene and then placed in a thereo-
stated bath a: 20.00 0.02° o. The tine necessary for
establishnent of thermal equilibrius was determined by

12

placing the pycnoneter having the thermometer in the thermo-
stated bath and finding how long it took for the temperature
in the pycnometer and bath to become equal. This time
(fifteen minutes) was quadrupled to guarantee thermal soni-
libriun. Accordingly, after one hour the pycnometer was
removed from the bath. capped. cooled in a stream of cold
water, wiped dry. and placed in the balance case for
weighing. Each sample was weighed four times with the
pycnometer being treated in the same nanner described above
between each weighing. Densities were calculated by the

following formula,
v .d

 

do . a solv.
dsolv.vpyc. ’ vunit ’ 'e
da 2 density of the solid
d301,. 8 density of the solvent
w. a weight of the solid sample
“unit 3 weight of the solid plus solvent
contained in the pycnoneter
prc. = volume of the pycnoneter

The pycnonetric densities for three samples were
found to be 6.68. 6.46. and 6.63 g./cc., the average being
6.56 g./oo.

The X-ray density for lanthanum oxide was calcu-
lated from the previously nentioned lattice constants

according to the equation,
fi.¥.,

da 3
K a’c sin 60°

 

325.84
0.6036‘10.24(8.930‘10;E)26.139410‘§°0.86605

 

13

where, M.W molecular weight of the solid
' (1942 atomic weight!)

N. dvogadrc'e number

a and c 3 the lattice constants of the

hexagonal structure

the density calculated according to the above method is
6.585 g./cc.
The density values given for hexagonal lanthanum
oxide are tabulated in Table III.
we;
Eycnometrlc Densities

Hexagonal Lanthanun Oxide

 

 

Density Worker Literature
6.48 Hilson and Patterson 46
6.41 Brenner ll
6.51 frandtl 49
6.67 Zachariasen - 68
6.55 Zintl and Croatto 71
6.52 Croatto 16
6.56 vThis wort

 

 

 

 

 

the pycnonetric density determined for hexagonal
lanthanum oxide in this study agrees very well with litera-
ture values except for those by Brenner and Eileen and
Patterson. All densities agree within one per cent. The
x~ray density. 6.685 g./cc. is slightly higher than the
pycnocetric density. This is expected since microscopic
cracks and crystal imperfections will be formed that cause

voids which are not filled with the immersing liquid.

14

SUHMARY

The lattice constants of hexagonal Lagos have been
refleternined. The length of the‘g axis found for the unit
cell egress well with the literature values but the'g
value of the exis found in this work is larger than all but
one value found in the literature. The cubic lettice
dimension was not redeternined but literature values are
given.

the redeterninetion of the density of hexagonal
Lego3 shows good correspondence with the literature values

and the density calculated from X-rey date.

lb

SBOTIOH III
Preseodyniun Oxides

Historical

The three common oxides of preseodyniun ere
Przoa, Frog, and Prt°11- The sesquioxide and dioxide must
be prepared under special conditions. The seequioxide is
prepared by reduction from e higher oxide in e stream of
hydroaen or by ignition in a vacuum. I! the reduction is
performed et higher temperatures. the hexagonal structure
results; if at temperetures below 700° 0., the cubic etruc-
ture terns.

Foex(22), measuriné the change in length of a
small cylinder of the sesquioxide with change in temperature,
found the oxide undergoes an irreversible transition from
the cubic to the hexagonal structure et about 900° 0. with
e decrease in volume. Since the molecular volumes of the
cubic and hexagonal structures of the eeequioxide. es calcu~
lated tree the lettice constants reported by Eyring. Lohr,
and Cunninghen(le). are 52.0? and 46.69, respectively, a
contraction of the oxide during the transition from the
cubic to the hexagonal structure would be expected. The
only way to convert the hexagonal back to the cubic form is
to ignite the oxide to Prso11 and again reduce to the seequi-
oxide below 'IOOo 0. The B etructure(£b) is obtainable only

upon the ignition of preeeodynium sulfate in a stream of

hydrOgen at 900° C.

18

Preseodyniue sesouioxide readily dissolves in
nest ecids to form bright green solutions end is converted
completely to FrGOn upon sir ignition.

The most easily prepared and most common presen-

dyniun oxide is Pr 0 This oxide has n face-centered

6 .
cubic 'dsfeot' fluorite structure and is obtained upon sir
ignition of the sppropriete salts. The exect structure of
the oxide is still unknown. As ststed earlier in this
triting. RcQullough(42) end Hund end Peetz(33) believed the
defects to be enion vscnnciss whereas Victery(63) believed
that the fluorite lattice was completely filled end the
excess cstions were scconoodeted interstitially. In general
its chemical nature is sieiler to that of the dioxide.

The positive four oxidation etete hes been observed
'only on the oxides. However. Keketsuke end Gheng(46)
reported that they prepared preeeodyniun(IV) end neodymiun(lV)
ions by air oxidation and anode oxidation of smooniecsl
solutions of the rare earth ions in e solution of B-quino-
linol-o-sulfonie acid. Rensey. Douglse, and Xost(54)
repeated their experiments and felt that fiskntsuke and Chang
only found new complexes of the two rsre earth ions in the
positive three state.

harnh(39) suggested Pro es enother possible oxide
of preeeodyniuc. However, it was obteined by hydrogen reduc-
tion of a solid solution of preseodyniuo oxides in thorium

dioxide and no other evidence has. been given for its existence.

1?

Preeeodyniue dioxide, the other etoichionetric
compound of preseodyniun, hes the face-centered cubic
fluorite etructure. Page} end Brinton(47) prepared 99.2
per cent pure preeeodyniun dioxide by ignition of Przo3 in
pure oxygen et about 25 etnospheree preesure and 355° 0. for
ebout five hours. ucCullough(42.43) and Syring. Lohr. and
Cunninghal(18) using the eene principle but pressures of
above fifty eteoepheree produced Pro? for X-rsy analysis.
The dioxide in dark brown to black.

Several etudies have been Inde ot the prnseodymiun-
oxygen system. Psgel and Brinton(47) etudied the effects of
temperature, ignition tins, oxygen pressure. and rate of
cooling on the amount or oxygenation of preeeodyniun oxide.
They found that the snount of oxygenation of air-ignited
preseodyniun oxide varies with the conditions of ignition
and cooling end that the enount or oxygenation depende on
the oxygen pressure end tenpereture of ignition. Hertin(40)
made structural studies and dieeocietion pressures neeeure-
nents on e series of nonetoichionetric oxides of prneeodyniun
as well as conductivity measurements on Preoll. He reported
that between Pr01.50 end Pral.5E5 the eystee is heXagonel:
between Pr01.55 end 191-01.?5 the hexagonal and cubic struc-
tures coexist and above Pr01.75 the eysten is cubic. He
stated that below 780° 0. the air-ignited oxide is an
electron conductor whereas above that tenpereture it is a

positive hole conductor.

18

Ferguson(19) and Ferguson, Guth, on: Eyring(£0)
who investigated the isothermal dissociation preeeuree of
the prneeodyniun-oxygen systen at different temperatures
and oxygen pressures. reported that there appears to be two
compositions at which the oxide is particularly stable.
namely: rr01.715 and Pr01.eo. neprey(5) node X-rey studies
of the etructure of some nonetoichionetric oxides and found

the following:

 

 

 

 

 

 

: Lattice
Oxide Structure Constant
F'Ol.66 body-centered cubic 5.532 A.
rr01.718 face-centered cubic 5.499 A.
Fro: 833 face-centered cubic 6.467 A.
Proz face-centered cubic 5.392 A.

 

The information from Ferguson and neprey indicates
that. under their respective experimental conditions,
changes of echo physical properties in the praeeodymiun-
oxygen eyetec are not linear with changes in the~nmount of
oxygenation. ‘

Foex(21) from his obeervntione that the resin-
tivity of Freol1 in such lower than that of the seequioxide
believes that PrGO11 is e salt like compound or at leaet e
phenomenon similar to that observed in F9304 and T1305.

Rnbidenu and Glockler(53) prepared oxides of

praseodyniun intermediate between Yrboll end Frog by

19

oxidising lover oxides at room temperature and stmoepheric
pressure with ozone.

Gruen, Ioehler, and Katz(£6) using atomic oxygen,
prepared rr02.02 which has the fluorite structure with a
lattice constant of 6.380 A.

Foex and Loriers(23) studied the thermal decon-
position of the preseodymiun-oxygen systee with increasing
temperature by gravinetric and diletonetric methods. Samples
were prepared by ignition of nitrates to 650° 0. and cooling
in air at 0.5° 0. per hour. The samples were slowly heated
in air from 18° to 800° C. and variations of ease and size
studied with temperature increase. Dilatonetric data showed
nexina in density at Frog and Praoll. Gravinetric data
showed plateaus corresponding to Proz at 265° C. and
resell at 350° c.

Prseeodzcium Oxides

Experinental

Praeeodymiuc materials used in this investigation
for deternining the lattice constants of preseodymium oxides
and the density of Pre°11 were obtained from Dr. L. L. Quill
and labeled 'spectroeconically' pure. The Prboll was
prepared by igniting freshly precipitated oxalates in an
electric muffle furnace at 900° 0. for two or aore days to
insure complete calcination and to anneal the aaterial so
sharper I-ray lines could be obtained. Preo3 was prepared
by reducing Pr6011 in a stream of hydrogen at 900° 0. in
a tube furnace. After reduction the sample was allowed to
cool to about 75° 0. in hydrogen before renewing it from the
system.

Since Pr'60n is a nonetoichionetric oxide, it was
necessary to determine the amount of oxygenation of the sire
ignited eaterial. This amount of oxygen over that required
for Ergo:5 is called 'excess' oxygen. It was determined by
the method reconaended by daluteky{58). freshly ignited
oxide eacples weighing 0.0000 to 0.1260 grams were weighed
into 250 nl. iodine flasks. To the aanple in each flask
are added first 10 al. of 0.115'potaesiun iodide solution
and then 25 ml. of o‘fi sulfuric acid. The flash is immedi-
ately stoppered and gently swirled to diseolve the oxide.

The solution tine varies depending on the nature of the

h:-
H

oxide and the particle size. The liberated iodine is
titrated with 0.03 g sodium thiosulfate using 3.4 ml. of
1 per cent starch solution as the indicator.

The calculation for the per cent 'excess' oxygen
is

Per cent 'Fxcess' Oxygen 8 EV . 0 - 100
2000 an“.

n reyresents the normality of the sodium thiosulfate solu-

 

tion, V its volume, the nilliequivalent weight of

 

2000
oxygen, and s.fl. the sample weight. The amount of “excess“

oxygen in air-ignited preseodyuiun oxide was determined to
be 3.1e weight per cent. The celculeted emount is 3.13
weight per cent.

The methods of taking and measuring the X-rey
films and of calculating the lattice constants of Przoa
were the same as for Lagos. The 20 values. relative
intensities, singo values, and planes producing reflections
ere given in Table IV.

The lattice constants of hexagonal PrEOa were
calculated from the equation sinto 8 0.05335(h8+hk+k2) e
O.01647(l)2 and the relationships.

, . 1.5413 and o x 1.5419 .
a casein”: ‘ :608T345

The lattice constants calculated are a t 3.854 t 0.001 and
c a 6.007fl1 0.002 A. Literature values for the cubic and
hexagonal structure determined in this study are given in

Table v. There is good agreement between the literature

TABLB‘31
Kora! Data for Hexagonal Praeeodzgiun Besguioxide

 

 

 

 

 

Relative
Line r153; Intensity 2e Binge
1 1010 w 26.34 0.05307
2 0002 w 29.79 .oeeoe
s 1 1 vs 30.70 .07007
4 10 2 s 40.ee .11957
5 11 0 e 47.22 .15041
5 10 5 e 03.38 .20175
7 20 ever 65.03 .21343
a 1122 e 50.78 .22507
9 2021 e 57.54 .25010
10 0004 vvvu 61.€0 .26288
11 2032 vvw 68.80 .27926
12 1014 vvvv 63.50 .51075
15 2025 vv 73.91 .36142
14 2130 vvvv 75.51 .37520
15 2131 v 77.27 .58032
15 1124 vv 81.17 . 2524
17 2132 vvv 8“.02 .45924
18 1015 vv 85.88 .46408
19 30 77w 87.08 .47976
20 2133 w 92.49 .52172
21 3032 vs 95.19 .5452:

 

 

 

 

 

.1529 - 0.05505(52.hx.12) + 0.01047u)2

a ' __1;21l§___. - 5.054 2 0.001 a.
5 J0.05356‘ -

e : 1-6410 . 0.007 2 0.002 a.
e J0.01547

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

IAILE‘I
Lattice constants 2; Praeeodzgiua aegggiexides
Synmetry Lettiee constants Yorkers Literature
Hexagcna Cubic ‘
a e a

Cubic 11.116 Banner 7
Cubic 11.136 Iandelli 34
Cubic 11.14 McCullough 42
cubic 11.136 Gruen. Koehler,

and Sat: 26
Cubic 11.14 Eyring. Lchr.

and Culninghal 10
3.:150fl81 3.85 6.00 Goldsoh-idt.

et a1. 20
Hexagonal 3.80 6 .00 235321-111.» so
Hexagonal 3.851 6.996 fouling 48
Heragonal 3.85 6.00 Rarheaat and

loriere 6
Hexagonal 3.859 6.008 Eyring, Lohr.

and cunninghal 18
Hexagonal 3.854 6.007 This work
values of the lattice constants of the hexagonal structure
and those of this study. the lattice constant for the cubic
structure was not determined but the literature values are

included so they nay be used in a later discussion.

hateriais for determining the lattice constant of

Praoll were the ease as used for X-ray deter-instions of the

hexagonal seequioxide.

Freshly precipitated proseodyniun

oxalate was ignited for two days in an electric muffle

furnace at 900° C. to insure couplets calcination and to

anneal the sample for obtaining sharper lines on the powder

diagraee.

The freshly ignited material was removed from

the lufrle furnace. allowed to cool for about a minute and

placed in a desiccator to cool. As soon as possible a

24

Lindeaann capillary tube was filled for X-ray analysis. Two
files were prepared and aeasured in the some manner as
described for Lagos. a lattice constant was calculated fro-

the equation.

2
2 3 2 shg+hgel I

sin20

for each reflection line. The lattice constants were averaged
arithmetically to obtain an average lattice constant for the
film. The relative intenaities. 29 and sinzo values. and
planes producing reflections are given in Table VI. Values
fro. the first two lines were not averaged because they
varied too greatly from the rest of the values. The lattice
constants deter-ined were a ‘ 5.460 1 0.003 a. and a 0 6.464
t 0.004 A., the average being 5.462 1 0.004 A.

TABLE :2
X-ray Data for 9:60

 

 

 

 

 

 

 

 

Line Plane Relative Film 472 File 526

Intensity 22 sings 22 e 2
1 111 e 28.40 0.06018 99.41 0.06022
2 800 I 32.86 .08000 82.91 .08024
3 220 e 47.07 .1694! 47.12 .1097?
4 511 e 60.86 .2193? 55.8? .21946
5 222 vw 53.5? .9392? 58.5? .2392?
6 400 vw 68.66 .31806 68.83 .31943
7 351 V 75.94 .37853 76.93 .37944
8 420 w 98.20 .39775 78.28 .39944
10 533 V 94.16 .5362? 94.34 .53784

511
2 2
‘ " L3" ”1”“! 5.4e4ic.ooa 546030.002;
ainee

 

 

 

 

 

25

Literature values for the lattice constants of
Proz oxides having a cubic structure uni x greater than
1.00 are given in Table VII. the value determined in this
work for the lattice constant of F’boll agrees very well
with those reported in the literature. A lattice constant
of 2:02 was not determined in this investigation but litera-
ture values are given for purposes of later conpsriscn.

Letting Constants 1; Higher Oxides‘gg Egaeeodzgiua

 

 

 

 

 

for-ula Lattice tortsr Literature
constant

Pro]. .66 5 e 5312 £833", 5
PIT-21.718 5.400 Alp", 5
Ir 0 10.98 Soldechcidt e1. 86
:vrgofi 5.4.6? Asprey ‘23 "" a
””6011 5.469 HcJullough 42
Fro? 5.39 Scherrer and Palncioe 60
Frog 5.392 Asprey 8
frog 5.394 XcSullough 42
1:02 5.40 Barbeaat ana Loriers 6
231-02. 5.395 Eyring. iohr, and

Cunninghaa 1.
15302.02 8.380 Gruen. Koohler. and

Into £6

 

 

 

 

 

 

As the amount of oxygen in the cubic prosecdyaiuo
oxide structure inureaeen, the lattice contracts. $hs
change of the lattice constant with change in aaount of
oxygen in the oxide is discontinuous.

The pycnouetric ienoity or ¥r6011 was deter-iced
in the ears nnnner as for L810: and was found to be 6.88
3./cc. The K-ray density is calculated to be 6.83 3./cc.,

do . 2/3 P’GOII . 2/3 1021.52 3 6,33 gym,
- W
5 s (5.462oio 3)3 0.6026-102‘

 

Densities reported by other workers are given in
Table VIII. It is noted that the density found in this
study is greater than previously reported values but agrees
closely with the X-rsy density value.
was

Regorted Densities 25
Airblgnited Prseeodzmiuu Oxide

 

 

Value Worker Literature
6.704 Brauner 11
6.71 Prandtl 49
6.61 Prandtl and Huttner 60
6.83 This work

 

 

 

 

 

2?

SUHfiARY

The lsttice constants of hexnboncl Prgos have
been redetermined and round to be in good egreement with
the literature values. The redeterminstions of the lattice
constant and density of Prboll show good correspondence of
the lsttice constant with the literature values but thet
the density value is higher.

The amount or excess oxygen calculated for Pres!1
is 3.13 per cent by weight. The amount or excess oxygen
determined for two samples of air-ignited preseodyniuu was

3.14 per cent by veight.

28

SECTION IV
hired Oxides
Historicsl

Since the system being studied involves a dioxide
having the fluorite structure end rare earth sesouioxides.
this type of system will be reviewed.

A number of vorhere(e, 9, 14. 15. El. 32. 39,

41. 42, 43, 44, 49, 50. so, 71) have reported observations
on systems of dioxidee having the fluorite structure with
lsnthsnon sesquioxidss. Hood and Durrvschter(31) working
with the lsnthsnuntlll) - thoriun(lV) oxide system and
Zintl and Grostto(7l) and Carlson(l4) with the lanthanun(lll)
- ceriun(lV) oxide system reported that the fluorite struc-
ture remained as the sesquioxide was sdded to the dioxide
to fern solid solutions. They further stated that anion
vacancies were created to sccocmodate the deficiency of
oxide ions rather than the fluorite structure rensining
completely filled and the excess cations being situated
interstitielly. X-ray investigations by these workers also
showed that the change in lsttice constant of the fluorite
structure is linesr with change in concentration of Lego3
and that the fluorite structure became saturated at about
fifty mole per cent sesquioxide. This concentration of
sesquioxide in the fluorite structure corresponds very

closely to that found by HartlnféO).

McGullough(42,43) stated that for binary systems
of lanthanum. praeeodyniuc. or neodymius sesquioxides with
cerium, proseodyuiue, or thorium dioxides the fluorite
structure becomes saturated at some concentration depending
on the oxide pair and undergoes no further change except to
decrease in amount as the concentration of sssouioxide is
further increased. Brauer and Gradinger(8) found that the
yttrium(lll) — ceriumfIV) oxide system tends to undergo a
continuous transition from the face-centered cubic fluorite
structure of the dioxide to the bodybcentersd cubic 0
structure of the yttrium oxide. NcCullough and Britton(44)
studying the sane system as well as the yttriun(lll) -
preseodynium(IV) oxide system came to the same conclusion.

Investigations by Salutsky(68) and Prandtl and
Huttner(50) of the air-ignited lanthanum oxide-prance-
dymiuu oxide system showed that as the amount of lanthanu-
cxide increased. the 'excess' oxygen first increases over
that predicted for all the praseodyoiue existing as Frfioll
to a peak and then rapidly decreases as the amount of Lang};5
increases further.

harsh(39) states that at least two factors influ-
ence the amount of oxygenation of praseodyniun in mixtures
of one mole of P’203 to two moles of some other rare earth
shoving only the positive three oxidation state; namely,
the relative stabilities of the hexagonal and cubic struc-

tures and the ionic radius of the metal ion of the

30

sesquioxide used for dilution. As the ionic radius of the
lanthanon ion in the diluting sesquioxide decreases and as
the hexagonal structure becomes more stable, the amount of
oxygenation of the praseodyniua in the mixture decreases.
Accordingly. as the atomic number of the lanthanon ion of
the diluting sesquioxide increases. the amount of oxygenap
tion of preseodymiun in mixtures will reach a peak at some
intermediate lenthanon and decrease with further increase
in atomic number.

Brauer and Haag(9) and Carlson(l4) determining
the rate of solubility of the Lagos - Geog system in
mineral acids found that at high concentrations of Geog
the rate of solubility was very slow but began to increase
when the concentration had decreased to about 70-80 mole
per cent.

Although binary oxide systems of Lagos, Rdgos.
Sages, ¥b203. or 8c£03 with U303 do not involve a pure
dioxide having the fluorite structure. it is felt that
information concerning these systems reported by Hund and
Feets(32) is pertinent. They report solid solutions form
which have the fluorite structure in the composition range
of about 25-70 sole per cent of the sesquioxide. The
fluorite structure forms with anion vacancies until all
the fluorite structure is completely filled and any excess
oxygen is distributed statistically among the octahedral

vacancies.

When two compounds are nixed, the question of
whether there is solid solution formation or compound
formation arises. Prandtl(49) reported that Pre°11 and
Pro2 are types of praseodynates. Though the praseodynium -
oxygen system has been studied by several methods. the
question of solid solution formation versus compound

formation is still unresolved.

Mixed Oxides

Experimental

Praeeodyniun-poor samples for this study were
obtained by separation using honogeneous precipitation of
the carbonate of material from the lanthanum-rich end of
a double magnesium-rare earth nitrate recrystallization
series. '

For this procedure a method developed by
Salutsky(5e) was used. Freshly ignited oxide is dissolved
in s sinisun amount of 25 per cent trichloroacetic acid,
the solution diluted to give a concentration of about ten
to twelve grams of rare earth oxide per liter of solution.
The solution is heated to 90° 0. to decompose the trichloro-
acetate ion to chloroform and carbonic acid. The rare
earth ions were precipitated as carbonates. In order to
calculate the time required to precipitate 80 per cent of
the rare earth ions from solution, rate constants for the
decomposition of the trichloroacetate ion were obtained
from the work by Verhoek(62). For 90° C. the time required
was 86 minutes.

The pyramidal method of fractional precipitation
given in Illustration 1 was used. To designate fraction
steps the precipitate from the first precipitation was
numbered one and the supernatant liculd was numbered two.

Decomposition of the trichloroacetate ion in fraction two

ILLU STRAT IO N I
Pzresidal Fractional Precipitation Method

Orig.
1////// Sample \\\\\V
0:. ° Precipitate——A
_____;>

Supernate

was allowed to continue to give precipitate fraction four
and supernatant liquid fraction five. Fraction five was

not separated further. Fraction one was redissolved in a
minimum of trichloroacetio acid and decomposed as previously
described to give precipitate fraction three and super—

natant liquid fraction four. Supernatant liquid fraction

four and preedpitete traction four were nixed to give free-
tion four. Two individual eeteriele were fractieneted in
the uenner deecribed above to give frectiene having the
conpoeition lieted in Teble IX.

WE

anggeitign‘gg Frgcsione fieteined‘;g Prectignetign
‘21 Hoeogenggue grecigitetion‘gg the Oerbcnetge

 

 

 

 

 

81:11! L aeriee H
T Semple Coupoeiticn sample Oonpoeition
Deeignetien Height per cent Deeignetion weight per cent
P9203 FPEOS
Original ;
“at.r‘.1 9.0 i “-6 1.3
In-” 3.5 “-8 10‘
L‘ll Vol “’1‘ 3e].
LolO 18.6 “-18 4.0

 

 

 

 

Free theee dete it ie ebeerved thet (e) procee-
dyniue concentratee in the precipitate traction end (b) the
rete or concentration ie teirly repid in prneeedyeiul-poor
eanplee.

One frectionetien ueing diuethyl exelete ee
euggeeted b1 Seluteky(be) wee porter-ed. The preeeedyuiue
concentratione in the precipitete end in the eupernetent
liquid were 13.0 end 6.0 weight per cent of Przoa. reopen-
tivelr.

Belplee of verioue coupoeiticne were node by
mixing cone of the preeecdyniuI-poor Interiele, prepared

ae deecribed previouely. with material labeled 98 per cent

35

pure preseodyniun oxide obtained from the Lindsey Chemical
Company. rThe spectrum snelysis report of this preeeodynium-
rich materiel showed less than 0.5 per cent of neodymium
oxide end only e trace of gadolinium oxide present in the
sample. is e further check the absorption et 794.5 up of

a chloride solution of e concentration or about 20 3.]100
ml. of oxide mixture was checked with s Becknen DU spec-
trophotoloneter calibrated with e etenderd solution of pure
neodymium chloride having e concentration of 0.650 g./lOO
ml. of nagoa which showed approximately the some amount of
absorption as the sample. The amount of Nd203 in the
sample. uncorrected for absorption by preseodyniun,-corre-
spended very well with that obtained from emission spectra
measurements.

For the various studies fresh oxide samples were
prepared by precipitating the rare earth ions from slightly
acid nitrate solutions as the oxelstee end igniting at
900° 0. in an electric muffle furnace. For X-rey studies,
samples were ignited for s minimum or two days in order to
insure complete celcinetion end to anneal the crystalline
material so sharp Xorey lines would result. For other than
x-rsy determinations samples were ignited for a minimum of
twelve hours.

Heterisl labeled “spectroBGOpicnlly pure“
praseodymiun oxide obtained from Dr. L. L. Quill was used

to calibrate e Beckmen DU spectrophotolometer for the

determination of the amount of preseodymium present in the
oxide mixtures used. The oxide was ignited in a stream of
hydrOgen at 900° 0. for several hours to insure complete
reduction to the sesquioxide. The commercial hydrogen was
dried by passing it through anhydrous magnesium perchlorate.
The reduced oxide was cooled in hydrogen to prevent
reoxidation. After cooling, the oxide was removed from
the furnace. Samples for the preparation of different
standard solutions were ouickly weighed to minimize absorp-
tion of water and carbon dioxide. These samples were
dissolved with hydrochloric acid. evaporated to dryness,
and redissolved in water and a minimum amount of 3.§ H61.
The samples were then transferred quantitatively to 25 ml.
volumetric flasks and diluted to the mark.

For the measurement of variation of the absorp-
tion of the preseodymium(lll) ion with concentration a
Beckman DU spectrOphotolometer was set to transmit the
spectral band at 444.5 m}; where an intense absorption band
characteristic of praeeodymiun occurs. A slit width of
0.025 mm. was used. The Optical densities of these samples
of different concentrations were determined. These
measurements also indicate that absorption or the prosec-
dymium(III) ion in a chloride medium for the concentrations
used follows Beer's law. calibration data are given in

Table I.

(A
'0

TABLE 25
Calibration g! the Recknsn SpectrOphotolometer
for Determination 23 Preseodymium(lII)

Ln 5 chloride Medium

 

g. PrZOa/ES m1. 0.0918 0.1343 0.150? 0.2026
103 transmission 0.224 0.330 0.366 0.488

 

 

 

g. Pr203/25 ml. 0.2270 0.2859 0.2957 0.3122
log transmission 0.642 0.694 0.702 0.741

 

 

 

 

 

 

 

 

 

slit width 3 0.025 on. C 444.6 nu.

 

For a series of mixtures X-rsy diagrams were made
and densities were determined as described for pure L8203,
Prgos. and P'boll' These data are compiled in Table X1. as
well as the data for total weight per cent of praseodymlum
expressed as Prgoa. and weight per cent 'excees' oxygen.

Since the weight per cent and sole per cent
composition are directly related and since some preperties
of oxide sixtures can be compared more easily on the basis
of mole per cent. the weight per cent composition was
converted to mole per cent. the mole per cent composition
of the oxide mixtures was calculated on the basis that the
mixture was composed of Lagos, Freos. end Frog. The weight
per cent of total praseodymium in the sample was determined
spectrophotometrioslly as described before and eXpressed as
F’EOG' The weight per cent 'excess' oxygen in the air-

ignited sample was determined also. The weight per cent

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

soo.o coo.» o.o 0.00” mouse
eso.e use.» 0.00 «.me sameness a“
00050.0 0a
oo~.o and.» v.0s s.vm oncogene an
cocoons 0”
000.0 00.0 0H.» 0.0 0.00 00
00.0 00.0 n.e 0.00 am
,000.0 00.0 00.0 0.0 0.00 00
000.0 00.0 00.0 0.0m 0.00 an
000.0 00.0 0.0g 0.00 0H
00.0 0.00 0.00 on
000.0 00.0 0.00 0.00 0H
000.0 0e.u e.00 0.00 0a
000. v0.0 00." “.00 0.00 vn
000.0 000.0 000.0 00.n 0.00 n.0v 0a
000.0 000.0 ~00.» 00.0 00.0 0.10 0.00 «a
000.0 000.0 000.0 00.0 0.00 0.H0 an
000.0 mac.» 0.00 0.00 00
000.0 0H0.0 00.0 0.00 0.0a a
v0~.0 000.0 0.00 0.0” 0
~H~.0 000.0 00.0 0.00 0.0g 0
HHH.0 000.0 ”.00 0.0” 0
000.0 000.0 0.00 0.0 0
0HH.0 000.0 00.0 0.00 «.0 v
000.0 «00.0 0.00 0.0 0
00H.0 vwa.0 0.e0 v.0 m
00H.0 000.0 00.0 0.00n 0.0 H
.d! o L? d . 000.080 00004 000.8
00000 ueeouewem huaeoea .eeeoxm. «coo hem coco Lem sadism
endow-coo scanned peso hem one: unaues
nevunm.0ex«0 venuewnnaua new even
mmwuomea

59

“excess“ oxygen was converted to weight per cent PrOe by
the following equation,

Wt. per cent Pr02 ' (Vt. per cent “excess” oxygen) PPOZ
0/2
a (Wt. per cent “excess oxygen) 355.84 .
8

The weight per cent Przos in the sample was calculated from
the equation,
wt. per cent Preoa S Wt. per cent 'excess' oxygen + Wt. per
cent total Pr as Pr203 - Ht. per cent Pr02.
The weight per cent La203 was calculated from the equation,
Wt.per cent Lagos I loo-wt.per cent Prgoa—Wt.per cent Frog.
The lattice constants of the cubic and hexagonal
structures observed in the various mixed oxides versus the
mole per cent of Lagos are plotted in Figures I and II.
respectively. Several interesting observations can be made.
Curve B represents the changes in the lattice constants of
pure face-centered cubic Preoll and the body-centered cubic
lanthanum oxide in a mixture, if the changes were additive.
Curve A represents the change in the lattice constant of the
3 axis of the hexagonal structure of Pr203 and Lagos in a
mixture, if the changes were additive. Curve 0 is the
change in the 3 axis of the hexagonal structure for Pr203
and Lagos in a mixture, if the changes were additive. With
an increasing amount of Lagos the lattice constant of the
cubic structure increases. Also, only the cubic structure

is present until the L3203 concentration reaches about 48

4O

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20:69:30 It; manhozmhm 44209.me my; “.0 20....<_¢<>
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42

mole per cent. Fro- about 48 to about 72 mole per cent
Lagos the cubic and hexagonal forms coexist. It is observed.
too. that the lattice constant for the cubic system does
not increase linearly. For sxample,for the initial
increases in the lattice constants. the curve has a less
steep slaps than does the additivity Curve 3 for the solid
solutions of cubic Preoll and cubic Lagos. From about 28
to 44 mole per cent Lagos the rate of increase of the _
lattice constant becomes slightly greater. Above 44 mole
per cent La203 the rate of increase of the lattice con-
stant of the cubic structure decreases to less than that or
Curve B. The curve of the observed cubic lattice constant
crosses Curve 8 at about 67 mole per cent Lego; and the
lattice constant of aixei lanthanum an: praseodymiuu oxide
is less than that predicted from Curve 8. The lattice
constants of the cubic structures for compositions of less
than about 56 mole per cent Legoa are slightly greater

than those predicted :.5. Curve B.

In contrast to the change of the cubic lattice
constant found in this study of the air-ignited lanthanum-
praseodymium oxide system, 0arlson(l4) and Zintl and
Croatto(71) investigating the Lagos-Cece system found that
the cubic lattice constants of the single phase system
chambed linearly with changes or composition. To learn if
the observed and calculated values agree for the lattice

constants of sir-ignited oxide mixtures having compositions

corresponding to pure Preoll and to samples 15. 17, 18.
19, 20, and El. lattice constants were calculated according
to the additivity rule from the lattice constants of Frog
and cubic rrzos reported by Eyring, Lohr, and Cunninghan(18)
and the lattice constant reported by landelli(34) for cubic
L820: by the following equation,

Lem I aole per cent La203 2 LOL‘ e aole per cent

Prgoa x LOP, e mole per cent PPOZ x L°rr02

Where L0a I the predicted lattice constant of the mixture.
LcLa 3 the lattice constant of cubic Lagos
LCrr 8 the lattice constant of cubic Prgos

and L0Pr02= the lattice constant of Frog.

The calculated lattice constants are given in Table XII
and are plotted in Figure 1. Trends for the changes of
lattice constants with changes of composition are the same
for both the eXperimental and the theoretical lattice con-
stante. The theoretical values are much lower than the
experimental values but this night be caused by the
presence or the larger lanthanum(lll) and praseodyaius(III)
ions.

when the hexagonal and cubic structures begin to
coexist at from 40 to 48 mole per cent lanthanum oxide. it
is also noted that the lattice constant of the cubic struc-
ture does not change as rapidly. This is indicated by a

definite change in slope of the curve above 48 mole per cent

 

44

has: 2.2.;
Calculated agg'Ohserved Lattice Constants
91 Single M M M Solutions 91
Airvlgnited lanthanum and {raseodynium Oxides

 

 

 

 

 

 

 

 

Sample Mole per Lattice Constants
cent Lagos Calculated Observed
22 0.0 5.429 5.464
20 8.6 5.446 5.488
19 15.0 5.452 5.498
18 16.5 5.458 5.506
16 26.0 5.485 5.526
15 57.7 5.522 5.558
14 39.1 5.529 5.562

 

 

lanthanum oxide. Above approximately 72 mole per cent
Lagos the cubic structure disappears.

Also. as the La20a concentration increases in
the composition range when the two structures coexist, the
rate of change of the‘g axis is slow. Free the curve one
observes that the rate of change of the‘g axis compared to
that of the'g axis is more rapid with increase of Lagos.
The irregularity of the rates or change or the‘a and'g
axes of the hexagonal structure is probably caused by a
distortion of the lattice due to the presence of the
smaller praseodysium(IV) ions in solid solution within the
hexagonal structure.

Above approximately 57 mole per cent lanthanum
oxide the‘g axis of the hexagonal structure is shorter than
predicted from Curve 6. Below this concentration the 2

axis is longer than predicted.

45

Since it is of interest to determine what occurs
when a homogeneous mixture of Lagos and Preoa is prepared.
two freshly air-ignited oxide mixtures of lanthanum and
praseodymiuu oxides were reduced in hydrogen to give
samples having 50.8 and 75.4 mole per cent Laeoa, respec-
tively. The X-ray powder diagram of the sample containing
50.8 male per cent Lagos had lines which corresponded to
the lanthanon oxide A-type structure. The lattice constants
predicted according to the additivity rule using the
lattice constants of lanthanum and preseodymiua sesquioxide
as determined in this study were a 3 5.895 A. and
c i 6.074 A. The observed lattice constants were
a 3 3.852 A. and c ' 6.075 A.

The x-ray powder diagram of the other sample
(75.4 mole per cent Lagoa) had lines corresponding to the
lanthanon oxide a-type structure and several other uniden-
tified lines. Part of the second sample was graycwhite
instead of pale green. Upon treatment of the sample with
dilute RSI a gas was evolved that smelled like rotten
5558 and turned lead acetate-saturated paper black which
indicates that the gas was HES. A rubber stapper was used
in part of the system and got very hot. Rubber steppers
have a large amount of sulfur in than which can distill
from the stepper to the oxide sample. Black particles were
observed after dissolution of the gray material. Two sets

of diffraction lines were observed in the X-ray powder

46

diagram of this sample. One set could not be explained
except that they were not caused by hexagonal crystal
lattice. The other set corresponded to the lanthanon
oxide A-type structure. Its composition was 75.4 mole
per cent Lagos. The predicted lattice constants are

a 8 5.911 A.. and c I 6.106 A. The observed lattice con-
stants are a i 5.911 A. and c 8 6.105A. The relative
intensities, 20 values, planes producing reflections,
sin28 values. and the factors Fl and 72 for each film are
given in Table XIII.

To better understand the trends in the lattice
constant changes. the curves of Figures I, II. and III can
be compared. In Figure III the number of atomic weights of
’excess' oxygen per mole of Prgoa, with all the praseodymiun
expressed as the sesquioxide regardless of the degree of
oxygenation. is plotted against the mole per cent La203.
The line drawn parallel to the abcissa at 0.667 corresponds
to the value that should be observed if all the presse-
dyaium in the mixtures exists as Pr6011. As the Lagos
concentration is increased in the airbignited mixture,
starting at pure praseodyeiun oxide, the ratio goes above
that corresponding to the assumption that all the praeeo-
dymium in the mixtures exists as rreoll. This means that
the presence of a small amount of 1.3203 in the mixture
favors an increase in the oxygenation of the praseodymiuu.

As the amount of Lagos is further increased the degree of

TABLE XIII

47

X-gez Data {gr Solid Solutions‘2£_Lanthenun and
{reseodzgign Seeguioxides Obtained‘gl Redugtiog

2;,Air-Ignited Mixes Ozides vith Bldrogen

 

 

n.1gegy. Sample 100 Sample 15
Line Plane Intensity 26 flin‘Q 20 61626
1 10f0 v 20.66 0.06166 26.46 0.06267
6 1o 1 vs 60.14 .06766 60.27 .06616
4 10 2 e 69.77 .11666 40.03 .11716
6 1120 e 46.66 .16616 46.74 .16764
6 1016 e 62.66 .19604 2.60 .19770
7 2020 see 64.17 .20761 64.46 .2093?
6 11F2 e 66.62 .21610 66.16 .22167
9 2021 v 66.62 .22276 66.71 .22666
10 0004 vvv 61.12 .25681
11 6022 vv 62.66 .27060 66.07 .27666
12 1014 vvw 67.26 .60676 67.66 .61011
13 2026 v 92.67 .66106 76.04 .66416
14 213d 6 76.66 .67716 76.29 .66160
16 1121 v 72.76 .4110: 60.26 .41666
16 2132 vv 81.40 .42626 62.00 .43641
17 ldTS v 66.46 .46176 64.60 .46666
16 6030 vv 66.92 .46442 66.41 .46669
19 2136 w 00.76 .60667 91.22 .61066
20 6032 vv 96.64 .62§13 96.67 .66462

 

 

 

 

 

 

Sample 10 Siege

e 8 1.5418

0.06176
0 - 1.6416

4 0.01594

' 3.912 A.

f_’e 6.106 A.

. 0.06176(h?+hk+k?) + 0.61664(1)2

 

 

56mplo 16 61696 - 0.06260(62¢hx¢x9) + 0.01610(1)2

e C 1.8418

___' . 8689?. A.
3 40.05230 1 .

0 . .5418 a 5.075 A.

4 V0.01610

 

 

48

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49

oxygenation reaches a peak of about 0.86 at approximately
28 mole per cent 16203 and then drape sharply to about
0.04 at 70 mole per cent Lagos then proceeds at a very slow
rate of change to zero at 100 mole per cent Lszoa. The
resultant curve compares well with the one prepared by
Salutsky(58).

It has been demonstrated by Fund and Durrwachter
(31). Zintl and Croatto(71), and Carlson(14) that as a
rare earth sesquioxide is added to a dioxide having the
fluorite structure. a solid solution results by creating
an anion-deficient structure and that as the amount of the
sesquioxide is increased, the lattice constant of the cubic
structure is increased. In sir-ignited mixtures of lan-
thanum and praeeodyniun oxides the same phenomenon occurs
except that the eesquioxide is derived from two sources.
partly from the Fri-,03 present due to the fact that all the
praeeodyniun present is not oxidized to the positive four
state and partly from the 16203. As the preseodyniue oxide
is diluted by the addition of La205 the lattice of the
cubic structure is expanded and more anion deficiencies
result; both of these effects favor increased oxygenation
of praeeodyniun. As more preeeodymiun is oxygenated. more
praseodyaiun(IV) ions having a smaller ionic radius are
produced. The increase of the number of the smaller
praseodynium(IV) ions in the mixture relative to the amount

in Preoll will cause the lattice constant for the cubic

50

structure to increase less rapidly than would be eXpected
if all the praseodymiun in the mixture existed as F’BOII'
This change is observed in the first part of the plot of
the lattice constant of the cubic structure against the
mole per cent L620; as shown in Figure I.

Prandtl and Huttner(50) investigating the air-
ignited system ceriun and praseodyniun oxide and Hccullough
and Britton(44) investigating the air-ignited system
yttrium and praseodyeiun oxide reported that when the con-
centration of the praseodyeiuu was low, it was sore
difficult to oxygenate it. Host systems containing cerium
oxide upon air ignition yield cerium dioxide which has the
fluorite structure. Yttrium oxide has the rare earth
oxide 0 structure which is closely related to the fluorite
structure. It follows from the above inforuation that high
dilution favors a decreased oxygenation of the praseo-
dyuiuu though the fluorite structure remains in all eixtures.
This can be the reason for the decrease in the amount of
oxygenation after about 28 mole per cent L620a observed in
this work.

Above about 28 mole per cent Lagos the rate of
change of the lattice constant of the cubic structure
increases more rapidly than expected if all the preseo-
dyuiuu exists as P’6011- Owing to the decreased amount of
oxygenation of the prassodyniun the quantity of preseoe

dyniua(IlI) ions, relative to the amount in 7’60110

51

increase and cause the increased rate of change of the
lattice constant of the cubic structure. As the com.
position changes and the cubic structure only is present,
the change of the calculated and observed lattice con-
stants parallels the change of the amount of 'excess'
oxygen in the mixtures.

As the concentration of the Lagos increases
and the amount of the oxygenation of the praseodyaiua
decreases. the concentration of lanthanua(lll) and praseo-
dyaiun(III) ions attains such a concentration that the
formation of the hexagonal structure is favored. The
presence of praseodyaiua(1V) ions in the hexagonal oxide
lattice is not favored because they are too small. In
this concentration range the cubic and hexagonal structures
coexist. As the concentration of the L8203 in the systen
increases. the L'aos enters both structures. Since there
are two structures involved as the La203 concentration
increases, the rate change of the concentration of the
Lagos in either structure will be lower than if only one
structure were present. Because of this fact the lattice
constants of the two structures change more slowly as the
Lagos concentration increases than if only one structure
were present. As the cubic structure disappears, the
amount of oxygenation of the praseodyaiun becomes very low

and its rate of change with change of Lagos concentration

82

is very slow indicating that the hexagonal structure does
not accommodate readily the presence of preseodyuiua(IV)
ions.

since there is a relationship between the struc-
ture of a mixture and its density. it is necessary to know
what changes of density occur when the composition is
varied. Inspection of Figure IV reveals that in preseoe
dyaiua-rich oxides the density of the mixture decreases as
the Lagos concentration increases and that a minimum is
reached at around 45 mole per cent Lagos. then increases
to a naxicua at about 20 mole per cent 1.3203 and finally
decreases again toward the density corresponding to pure
Lagps.

For purposes of coaparison of the variation of
density and structure as the composition of the mixtures
are varied, the densities of the various structures of the
pure oxides are given in Table XIV.

W321

Qensities‘gg the Pure Oxides

 

 

Oxide Structure ‘ X-ray density Observed density
Lego3 Cubic 6.9a g./os.3
Lagos Hexagonal 6.686 6.66 g./cm.3
Page3 Cubic 6.34
Preoa Hexagonal 7.03

, P’fioll cubic c.83 6.83

 

 

 

 

 

 

The X-rsy density of cubic L320a was calculated
from the lattice constant reported by Iandelli(34). The

53

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54

x-ray density of cubic Pr203 was calculated free the lattice
constant of the cubic structure reported by Eyring, Lohr,
and Cunninghan(18). The rest of the X-ray densities were
calculated from lattice constants determined in this
investigation. The observed densities were determined in
this work. By couparing the densities of the cubic and
hexagonal ferns. it can be seen that the hexagonal fora is
more dense. Also the hexagonal and cubic forms of PP203
are more dense than corresponding forms of Lagos. P’6011
is more dense than the cubic forms of the sesquioxides.
X-ray diagrams show that only two structures occur through-
out the entire composition range, both of which correspond
to the structures of the pure components but with the
lines displaced. when only the cubic structure is present
the decrease in density of the oxide nixtures with the
increasing amount of La203 can be accounted for by the
expansion of the lattice structure in the mixture. than
the hexagonal structure begins to appear. the density
increases with the increasing amount of Lagos because the
proportion of the denser hexagonal structure increases.
when the cubic structure disappears. the density reaches a
maximum and then decreases as the La203 is less dense than
hexagonal Pr203.

From the foregoing discussion it follows that the
variations of the densities and structures of the oxide

mixtures with variation of composition parallel one another.

55

hsither quantitative studies of the relative
solubilities of the oxide mixtures in water nor their rates
or solubility in dilute mineral acids were investigated.
Qualitatively. however. the rates of solubility were
observed during “excess“ oxygen determinations. It was
noted that above 68.: mole per cent La203 the samples
dissolved quite rapidly whereas below that concentration
the rate of solubility decreased rapidly as the concen-

tration of Lagos decreased.

56

SUM-1 A3!

The change of densities, structures present and
their lattice dimensions, and the amount of “excess' oxygen
versus composition of air-ignited lanthanum and praseodycium
oxide mixtures have been investigated. I

For pure praeeodysium oxide and for this oxide
with increasing amounts or Lagos the face-centered cubic
structure characteristic of Pr6011 is the only structure
present to about 48 mole per cent Lagos. Then, the A or
hexagonal structure characteristic of pure Lagos coexists
with the race-centered cubic structure from around 48 to
approximately 72 mole per cent Lagos. Above about 72 sole
per cent La2oa the hexagonal form is the only structure
present. There are breaks in the curve of the change of
the cubic lattice constant with changing composition at
around 27 and 47 mole per cent Lagos. there is a break
in the curve of the change of the length or theIQ axis
of the hexagonal torn versus changing composition at
about 80 sole per cent La203.

The curve of the l'exceae" oxygen per mole of
Ergo: in the sample. expressing all preseodyniun in the
sample as Frgoa regardless of oxidation state. versus
changing composition has been redeterained. In quali-
tative agreement with work by 3s1uteky(58) it attains

a maximum at approximately 28 mole per cent Ls2oa and

57

then decreases sharply with increasing concentration
of Lezoa until about 70 mole per cent Lagoa.‘then the
change is very gradual toward 100 mole per cent Lagos.

Starting at pure Freoll the densityof the air-
ignited oxide mixtures decreases with increasing amounts
of [egos until about 42 mole per cent then increases to
a maximum at about 80 mole per cent Lagoa and finally
decreases toward the density of pure Lagos.

In view or the above changes it is highly prob-
able that solid solution formation is the only thing that
occurs when mixtures of air-ignited lanthanum and praseo-
dynium oxides are prepared. no new structures or struc-
tural changes were observed which usually occur with
compound formation.

It has also been shown that lanthanum and praaeo-
dymium eesquioxidee form solid solutions in which the

hexagonal lattice dimensions follow the additivity rule.

58

GOECLUSIONS

l. The lattice constants of hexagonal Laioa
have been redeternined and found to be a - 3.930 and
c I 6.139 A. The pycnonetric and the x-ray density values
were found to be 6.86 and 6.585 g./cc.. respectively. All
values were in reasonable agreement with previously deter-
mined values.

2. The lattice constant value for P'ooll was
found to be 6.462 A. The pycnonetric and X-ray density
values for Pre°11 were redetsrained and found to be 6.83
and 6.83 g./cc.. respectively. The lattice constants
for hexagonal Frgoa were found to be a 8 3.864 and
c ' 6.00? A. All values are in reasonable agreement with
previously determined values.

3. The variation of the density, crystal
structure, crystal lattice dimensions, and amount of
“excess" oxygen has been determined for the air-ignited
lanthanum-prsecodyniun oxide system. It has been shown
that the system undergoes solid solution formation through-
out with the cubic and hexagonal structures coexisting
in the concentration range from about 47.5 to about 69
mole per cent Lagos. It has been shown that hexagonal
Lagca and Pr20a for: solid solutions that follow the

additivity rule for the lattice dimensions.

4. It has been shown that the changes in the
lattice constants of the air-ignited lenthanum~prase0~
dyniun oxide system does not follow the additivity rule
which fact is in qualitative agreement with the observation
that the amount of oxygenation of preseodymiun also flees

not follow the additivity rule in this system.

4.

6.

8.

9.

10.

11.
12.

13.
14.

60

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APPENDIX

66

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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8.3910 10 85-910 11
711-;lv 000 02?
L100 Plan.
1 10'1’0 26.33 26.40
2 0002 20.34 29.47
a 1011 30.14 20.27
4 1012 39.77 40.03
6 11‘20 46.69 46.74
0 101's 62.06 02.00
7 2020 54.17 04.40
a 1122 65.02 66.16
9 2021 56.32 66.71
10 0004 61.12
11 2022 62.69 66.07
12 1014 67.26 67.60
13 2023 72.67 73.04
14 2130
10 213.1 76.60 76.29
16 1124 70.76 00.20
17 2132 61 .40 02 . 00
1a 1010 64.46 04.90
19 6030 85.92 06.41
20 2130 90.73 01.22
‘ 21 fig 96.34L 90.27
71 0.06176 0.05230
7 0.01004 0.01610
‘Eittico a. 3.912’ 3.655
Constant 0 6.106 6.076
H010 par
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9072210 11 Sample 12
I; I 609 ggg 6g? 491 Qfifi
L1ne rlnne »
1 111 27.60 27.57 57.62 27.82 27.79
2 200 31.90 31.88 31.00 32.18
3 220 46.39 46.66 46.62 46.06
4 511 64.58 54.33
a £22 —-—-— ———- 57.29
6 400 -——* —-——— -——* 67.06
7 331 -—-—- --— -—-—- 73.98 74.23
B 420 —-- --- -—-—- 76.15 76.13
9 422 -———- -———— -———— 84.95 84.76
10 333 -———- -‘——— '———‘
011
Lattice
Constant e 6.578 5.613 5.61? 5.062 6.09.
Averafia
Lattice
Constant e 5.603 0.090
3Ilp10 13 Sample 14
F11. 501 512 813 492 Ill
Line Plene
1 111 27.72 27.82 27.?7 21.93 27.01
2 800 52.13 32.12 32.13 52.29 32.22
3 220 46.01 06.99 45.97 46.25 06.18
4 311 . .04 54.50 54.44 54.80 04.67
5 222 67.14 67.46 67.83
6 400 67.0? 67.44 67.30
7 331 73.98 74.41 74.31
8 420 76.09 76.28 76.15 76.61 76.47
9 422 84.91 85.44 85.39
10 335 . --— --- 91.96 92.10
611
Lattice
“n.‘flfi‘ . 5-6“ 50680 gJefi‘ 5.060 5.564
erece
Lcttlce
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Sample 15 "amplo 16
711- 475 3.10 519 504 517
Line Piene ,
1 111 27.94 27.92 27.94 28.01? 28.01
2 200 52.55 52.55 52.50 32.42 55.42
5 220 46.25 46.25 46.87 46.45 46.45
4 511 54.75 54.86 54.79 54.97 55.04
5 222 57.51 57.47 57.59 57.88 57.74
5 400 57.45 57.33 £7.56 68.00 67.70
7 551 74.40 74.54 74.57 75.01 74.71
5 480 76.55 75.64 76.52 77.52 77.06
9 488 85.57 85.51 85.54 86.54 85.97
10 55: 92.09 92.07 92.00 95.00 92.88
5
Lattlce
Constant a 5.555 5.559 5.551 5.519 5.554
Iverage
Lattace
Constant I 5.558 5.525
Sample 18 Sample 19
711- 474 505 505 505 548 547
Line Plane .
1 111 28.12 28.15 28.27 - 26.16 28.16 28.20
2 200 52.55 52.67 52.65 - 52.57 52.52 52.66
5 220 45.55 45.75 46.77 45.75 46.71 46.75
4 511 55.52 55.57 55.59 55.47 55.45 55.47
5 222 57.97 58.12 58.04 58.15 58.09 58.18
6 400 68.05 68.19 68.07 68.50 88.2 68.26
7 551 75.10 75.26 75.?4 75.52 75.£8 75.58
8 420 77.56 77.56 77.44 77.52 77.84 77.58
9 422 85.17 85.58 86.57 86.70 86.71 86.75
10 533 86.42 95.50 95.28 93.56 93.48 93.52
511
Lnt5noe
80555385 I 5.508 5.505 5.507 5.497 5.500 5.495
vere‘e
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Samplo £0
’11. 585 544 545
Lsno Plano
111 28.20 28.19 28.19
2 2'00 32‘.“ 38.69 5? 70
.‘5 220 46.88 46.80 46.87
4 511 55.50 55.57 55.58
5 2 2 58.20 58.25 5
5 400 58. 68.59 58.40
7 551 75.59 75.40 75.45
8 420 77.59 77.80 77.81
9 422 86.91 86.95 86.97
10 355 95.82 93.67 95.58
511
Lntt1oo
Constant 5' 5.487 5.489 5.489
average
Lattice
Constant 5 5.480

 

 

 

 

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2.0040 0.342 7.
1.8995 0.315 9.
0.4909 0.159 15
1.2005 0.300 13.7 2.2810 0.721
1.2040 0.400 15.? 1.1980 0.445
0.4450 0.210 20.0 0.4800. 0.200

 

0.5523 0.879 24.6 0.0487 0.382
0.5427 0.404 31.7 0.6017 0.500
0.2005 0.256 39.1 0.8034 0.250

0.2913 0.586 40.1 0.2790 0.322
0. .300 40.0 0.2751 0.
0.2540 0.355 50.7 0.2805 0.375
0.2585 0.372 50.5 0.2517 0.875
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