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J lnorg. Nucl. Chem., 1961, Vol. 18, pp. 154 to 165. Pergamon Press Ltd. Printed in Poland

T H E U R A N I U M - O X Y G E N S Y S T E M : U3O s-UO 3

H. R.

HOEKSTRA and S SIEGEL

Argonne National Laboratory, Argonne, lllinois

(Received 18 April 1960)

Abstract--Preparative methods for fiv e crystalline mod ifications and an am orphous form of

uranium trioxide are described. Some properties of the UO3 phases are discussed, including the

density, crystal structure, heat of solution, thermal stability and infra-red spectrum. Data on the

UO2.9 phase are also given.

U RA NIU M t r i o x i d e i s o n e o f t h e f o u r w e ll e s ta b l i s h e d o x i d e s o f t h e e l e m e n t . M e t h o d s

f o r i t s p r e p a r a t i o n a n d d e s c r i p t i o n s o f i ts p r o p e r t i e s a p p e a r i n s o m e o f th e e a r li e st

p u b l i c a ti o n s o n u r a n i u m c h e m i s t ry . H o w e v e r , n o c r y s t al li n e f o r m o f th e t r io x i d e

h a s b e e n r e p o r t e d u n t i l r e c e n t ly . A t p r e s e n t a t l e a s t f iv e c r y s t al l in e m o d i f i c a t i o n s

o f U O 3 h a v e b e e n p r e p a r e d a n d o n e t y p e w h i c h i s a m o r p h o u s t o X - r a y s ( cr ys ta ll it es

l es s t h a n 1 00 ,A i n d i a m e t e r ) . T h r e e o f t h e c r y s t a l li n e p h a s e s w e r e d e s c r i b e d b y

SHEFT et. aL ), a f o u r t h b y

W A I T 2 ) ,

a n d a f i f th b y K A T Z a n d

G R U E N 3 ) .

T h e l a s t

o f th e s e p h a s e s h a d b e e n p r e p a r e d e a r l ie r b y G AT ES et ak ) b y t h e a c t i o n o f n it r ic

a c id v a p o u r s o n U 3 0 s, b u t t h e p r e p a r a t i o n w a s n o t d e s c ri b e d i n a j o u r n a l a rt ic le .

I n a r e c e n t p u b l i c a t i o n , (5) w e h a v e r e v i e w e d b r i e fl y t e n m e t h o d s f o r t h e p r e p a r a t i o n

o f U O 3, a n d c o n f i r m e d t h e e x is te n c e o f f i ve c r y st a ll in e a n d a n a m o r p h o u s f o r m o f

t h e t r i o x i d e . T h e p r e p a r a t i v e m e t h o d s i n v e s t i g a t e d a r e s u m m a r i z e d i n T a b l e 1 .

C r y s t a l l o g r a p h i c d a t a o n t h e s e p h a s e s a r e i n c o m p l e t e ; f u l l s t r u c t u r a l d e t e r m i n a -

t i o n s h a v e b e e n r e p o r t e d f o r o n l y t w o o f t h e f i v e t y p e s (0~ a n d ~ ).

Z A C H A R I A S E N 6 )

f o u n d ~ - U O 3 t o b e h e x a g o n a l w i t h a = 3 .9 71 A , c = 4 . 1 68 A . T h e s i m i l a r it y

b e t w e e n t h e 0 t- U O 3 a n d U 3 0 s st r u c t u r e s s u g g e s t s t h e e x i s t e n c e o f a c o n t i n u o u s s o l id

s o l u ti o n b e t w e e n t h es e c o m p o u n d s , b u t t h is h a s n o t b e e n c o n f i r m e d e x p e r im e n t a l ly .

A m o r p h o u s U O a is k n o w n t o d e c o m p o s e t o U 3 0 s i n t w o d i st in c t s te p s r a t h e r t h a n

v i a s o li d s o l u t i o n , tT~ T h e i n t e r m e d i a t e p h a s e , U O 2 . 9 , i s r e p o r t e d t o b e s t a b l e b e t w e e n

5 2 0 C a n d 6 1 0 C a n d t o h a v e a s t r u c t u r e q u i t e s i m i l a r t o U 3 0 8.

P FR IO a) h a s s u g g e s t e d a n o r t h o r h o m b i c c e ll f o r ~ , -U O 3 w i t h a : 1 3.0 1 A ,

b - - 1 0.7 2 A a n d c = 7 . 50 A , b u t d i d n o t a t t e m p t t o l o c a t e t h e a t o m s w i t h i n t h e

* Based on w ork' performed und er the auspices of the U .S. Atomic Energy Comm ission.

(J~ 1. SHEFT, S. FRIED and N. DAVIDSON,J'. Amer. Chem. Soc. 72, 2172 (1950).

I~ E. WALT, /.

Inorg. Nucl. Chem.

1, 309 (1955).

(a) j . j . KAT Z and D. M. GRUEN, 3 . Amer. Chem. Soc. 71, 2106 (1949).

(~) J. GATES, R .B . PiTT and I. O . Ar~DREWS, Manhattan Project Rep ort C D- 49 5 1945) .

15) H. R. HOEKSTRA nd S.

SIEGEL,

Second Proceedings of the International Conference on the

Peaceful Uses of Atomic Energy, Geneva, 15/P/1548 (1958).

~) W. H. ZACHARIASEN, ~Ct i Cv)/sI. l, 265 1948).

~ A. BOULL~and M.

DOMIN[-BERGF:S,

C. R. Acad. Sci., Paris 227 , 1365 (1948); 228, 72 (1949).

t*) p. PERIO,Bull. Soc. Chim. Ft . 776 (1953).

[ l ~ ]

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T h e u r a n i m n - o x y g e n s y s t e m U3Os--UO3 155

cell. Recently CONNOLLY 9) deduced a monoclinic lattice with the dimensions a =

1 3 ~ 8 A , b = 1 5 . 4 8 A , c = 6 . 9 1 A a n d [~ = 8 9 . 6 3 f o r t h e s a m e p h a s e .

A c o n s i d e r a t i o n o f t h e p r e p a r a t i ve c o n d i t i o n s a n d o f t h e r e l a t iv e s t a b i li t y o f

t h e p h a s e s i n q u e s t i o n h a s l e d t o t h e a s s u m p t i o n ) t h a t t h e s e q u e n c e

UO3(A)

\ .

~-UO3 > [~-UO3 > :~-UO3

/

U 3 0 8

is followed during reaction of amorphous UO 3 or U30 s with oxygen under pressure.

T A ItLg I . - - P i t I ~ tA T I O N O F U R AN IU M T RIO X ID E

T y p e

A m o r p h o u s

U O 3 ( A ) *

a - U 0 3

~-u03

T - U O 3

& U O 3

t - U 0 3

C o n d i t i o n s

I g n it io n o f U O 4 . 2 H 2 0 , U O 3 . 2 H 2 0 , U O 2 C 2 0 4 3 H 2 0 , o r

(NH4)4UO2(CO3h in air to 400C.

U O 3 ( A ) h e a t e d i n 4 0 a r m 0 2 a t 4 7 0 - 5 0 0 C .

U 3 O s h e a t e d i n 4 0 a r m 0 2 a t 5 0 0 - 5 5 & C .

I g n it io n o f ( N H 4 ) 2 U ~ i n a i r t o 5 0 & C .

I g n i t i o n o f U O 2 ( N O 3 ) 2 6 H ~ O i n a i r a t 4 0 0 - 6 0 0 C .

H e a t a - , [ 3 -, B -, - a n d a m o r p h o u s U O 3 i n 4 0 a t m 0 2 a t

650 C .

I g ni ti o n o f ~ U O 3 H 2 0 i n ai r t o 3 7 5 C ( > 2 4 h r ).

H e a t U 3 0 s i n N O 2 a t 2 5 0= 3 75 C.

* A m o r p h o us u o 3 ~ u b e ~ b 7 U O 3 ( A ) in th is p ~ e r .

t T he p t m e d e ~ m e o m . r e - - r o e m n : a ,e s a d ~ . u o 3 m p h , ~ i , n . ri d m o f SH ]E FT ,

et al.

B~UO is

WA IT's cubic plume, and s.UO 3 is KA TZ and O RU EN 's plural

T A s ~ 2 . - - I q to m m T m s o r U O 3 A ND U O 2 .9

D e n s i t y

(g/cm )

U

A t o m s

T y p e C o l o u r S t r u c t u r e

m e a s . X - r a y c e l l

U O 3 ( A )

~ - U O 3

[~-UO3

y - U O 3

& U O 3

- U O 3

U O 2 . 9

o m n g e

~ n

o r a n g e , r e d

y e l l o w

d a r k r e d

r e d

o l i v e g r e e n

6 80

7 . 04 8 . 34

8 25 - -

7 75 7 ' 2 7

8 16

6-99 6 .67

8 54 -

7-72 8 .41

hex ag, a = 3 .971 A ,

c = 4 .17 A

o r t h o r h o m b i c ?

c = 14 . 3 ) [

( o r t h o r h o m b i c a : 1 3 ~ 1 ,

b = 15 . 48 , c = 7 . 5 0A )

o r

(m on oc l in ic a ---- 13 08 ,

b = 15 .48 , c = 6 .91 A )

[I = 89.63

c ub ic a = 4 . 16

?

o r t h o r h o m b i c a = 6 - 91 ,

b - - 3 . 92 , c - - 4 . 16 A

1

16)

24)

1

~s~ D . E . CONNOLLY

Acla Crys t .

12 , 949 ( 1959 ) .

~10~ j . j . K A T Z a n d

E . R A l n m ow r r cs T he C ~ m t s t r y o f U r a ~ m

P l u t o n i u m P r o j ec t R e c o r d N N E S ,

D i v . V I I I , V o l 5, p . 2 7 6 . M c G r a w - H i l l , N e w Y o r k ( 1 95 1 ).

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156 H.R. HO~KS'n~Aand S. SW.O~L

The struc ture of ~-UOs was reported by WArl 2) as cubic with a = 4 15 )~.

The lattice is the ReO 3 type, which gives ~-UO 3 the distinction o f being the only

known oxide of uranium (VI) in which all uranium-oxygen bonds are of equivalent

length.

Several modifications of U30 s have also been reported, although only one is

co mmon ly encountered (0~-U3Os:a = 6.71 A, b ---- 11.97 A, c ---- 4.15 A). Its

structure was determined by ZXCH^aIASeN t~) and by GRONVOLD12) using X- ra y

diffraction. Recently ANDRESEN 13) suggested an alternative ar rangeme nt of oxygen

atoms on the basis of neutron diffraction data. HOEKS~RX e t a i . ~4) have reported

a second orthorho mbic structure [3-U30s, with a = 7.05 A, b = 11.72 A, c =

8 29 A. High temperatu re X-ray studies also revealed the conversion of a- U30 s

to a t rigonal form (a ---- 6 815, c -- 4.136 A) at 400C, em although this material

may have been slightly oxygen-deficient. Recent ly WILSON

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The uranium-oxygen system U3O s-UO3 157

The pu r i ty of ou r UO3 preparat ions was determined by uranium and by oxygen analyses. The

uranium content was w i th in 0.2 per cent o f the theoret ica l 83.22 per cent for each of the ctystaUine

phases, bu t the amorphous oxide was a lways 0.3-0-5 per cent low in uranium . The maximum

discrepaney occurred on samples prepared from ura ny l oxaiate o r acetate. Carbon analyses on

these preparations indicated a sm all residue of the element, and infra -red investigation revealed

the presence of carbonate.

Ox ygen -uranium ratios w ere determined by tw o methods, 1) d i rect determination o f oxygen

wi th bromine tr i f luor ide, 2) and 2) weight cha nje on ig ni t ion o f the sample at 750-800C in a i r

for several hours. I t has been ou r exper ience ha t any uranium oxide wi l l he converted to UO2.670.Ol

under these condit ions.

Tr iox ide preparat ions from amorphous UO3 and U3O s under oxygen pressure we re carried

ou t i n an lncon e l r eac to r s imi l a r t o t ha t desc r ibed by Smu~r et a l. 0) T a n k o x y g en w a s u s e d w i t h o u t

fu r the r pu r i f i ca t ion ; a i r was r emoved f rom the r eac to r by r epea ted f lush ing wi th oxygen to 20

a r m . X - r a y d i ff r ac t io n d a t a o n t h e o x i d e s w e r e o b t a in e d w i t h C u - K ~ r a d i a t i o n u s in g a c a n ~ a o f

9 0 m m d i a m e t e r t o o b t a i n d i f fr a c ti o n p a t t e r n s o n f ilm . A G E r e c o r d in g s p e c tr o m e t e r w a s e m p l o y e d

on ce r t a in p r epara t ions when inc r eased r eso lu t ion was des i r ed . S ing le c rys t a l W ei ssenberg pho to -

graphs were taken of the [3 .phase.

He a t o f so lu t ion da t a on UO3 phases were measured in a ca lo r imete r o f t he type desc r ibed

b y K A T Z m

et al. 21)

He at ca pac i t ie s o f t he ca lo r imete r and o f t he so lu t ions were de t e rmined by e l ec -

t r ic a l h e a ti n g ; a c c u r a c y o f t h e m e a s u re m e n t s w a s t e st e d b y m e a s u r i n g th e h e a t o f s o l u t io n o f s o d i u m

c a r b o n a t e .(22) Tw o m easuremen t s ag reed to l e ss t han 1 pe r cen t and the average was wi th in 0 .1 kca l

o f t he l i t e r a tu r e va lue .

I t had been hop ed to use 1M HCIO4 fo r t he ca lo r imet r i c de t e rmina t ions , bu t a m ore concen-

t r a t ed ac id was r eq u i r ed to e f fec t so lu t ion o f t he l e ss so lub le phases w i th in a r e asonab le time ; a s

a r esu l t, 5M HNO 3 was used fo r mo s t o f the ca lo r imet r i c measuremen t s . S ince the so lu t ion da t a

a r e be ing used to m ake a com par i son be tween the t r i ox ide phases , t he ac id concen t r a t ion i s no t

c r i ti ca l . Fo r t hese measurem en t s 1 g samples o f t he t r iox ide were d i sso lved in 300 ml o f ac id .

In f r a - r ed da t a were ob ta ined wi th a Beckman IR4 r eco rd ing spec t ropho tomete r on po tass ium

bromide d i sks con ta in ing one pe r cen t o f t he t r i ox ide .

D I S C U S SI O N A N D R E S U L T S

P r e p a r a t io n o f u r a n i u m t r i o x i d e

T h e r e la t i v e i n s t a b i l i t y o f U O 3 n e c e s s it a te s t h e u s e o f m o d e r a t e t e m p e r a t u r e s

i n i ts p r e p a r a t i o n . A s w i l l b e s h o w n i n a l a t e r s e c t i o n , m a x i m u m p e r m i s s i b l e t e m p e r a -

t u re s v a r y f r o m 4 0 0 to 7 0 0 C . T h i s l i m i t a t i o n l e a d s t o d i f f i c u lt i e s i n p r e p a r i n g p u r e

p h a s e s , e s p e c ia l l y i n t h e c a s e o f t h e a m o r p h o u s o x i d e a n d ~ ,-U O 3. I t h a s b e e n s h o w n

t h a t r e s id u a l t ra c e s o f n i t r o g e n ( f r o m n i t r a te a n d a m m o n i u m i on s ) , a n d c a r b o n

( f r o m c a r b o n a t e a n d o r g a n i c r e s i du e s ) a r e e x t r e m e l y d i ff ic u l t to r e m o v e w i t h o u t

d e c o m p o s i t i o n o f t h e t r io x i d e . F o r t h i s r e a s o n i t i s p r e f e r a b le , w h e n e v e r p o s s i b l e ,

t o u s e U 3 0 s a s a s t a r t i n g m a t e r i a l .

T h e r e t e n t i o n o f r e s id u a l c a r b o n a s c a r b o n a t e i n a m o r p h o u s o x i d e p r e p a r e d f r o m

u r a n y l o x a l a t e a n d a c e t a t e h a s b e e n i n d i c a t e d . T h e r m a l s t a b i l i t y e x p e r i m e n t s o n

U O 2 C O 3 r e v ea l e d t h a t t h e c a r b o n a t e m u s t b e h e a t e d a b o v e 5 0 0 C i n a i r b e fo r e

a p p r e c i a b le d e c o m p o s i t i o n o c c u rs , a n d c o m p l e t e d e c o m p o s i t i o n c a n n o t b e e ff ec t e d

cs0~ H . R . H O E K S T R A a n d J . J . K A T Z ,

A r ia l yt . C h e m .

2 5 , 1 6 0 8 1 9 5 3) .

c..l~ . I . K A T Z l N , D . M . S I M O N a n d J . R . F r d m A l ~ O , J . A m e r . C h e m . ~ c . 7 4 , I f 9 1 0 9 5 2 ) .

c2,~ F . R . B I C f l O W S K Y a n d F . D . R O S S I N I ,

7 ~ r m o c ~ r ~ $ t r y o f C b e m l c a l S u b s t a n c e s

p . 1 4 4 . R e i n -

h o l d , N e w Y o r k 0 9 3 6 ) .

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1 5 8 H . R . HOEKSTR a n d S . SIEGEL

b e l o w 60 0 C . C o n s e q u e n t l y , p u r e a m o r p h o u s o x i d e c a n b e s t b e p r e p a r e d f r o m

u r a n y l p e r o x i d e . P r e c ip i t at e s o b t a i n e d f r o m u r a n y l s o l u ti o n s s h o u l d b e w a s h e d t o

m i n im i z e a n i o n c o n t a m i n a t i o n . T h e p e r o x i d e ca n b e f o r m e d f re e o f c o n t a m i n a t i o n

b y t h e a c t i o n o f H 2 0 2 o n U O 2 o r U a O s , b u t e v e n h e r e , c a re m u s t b e t a k e n t o m i n i -

m i z e e x p o s u r e t o a i r, s i nc e C O 2 is t a k e n u p b y t h e p e r o x i d e t o f o r m u r a n y l c a r b o n a t e .

P u r i t y o f ~ - U O 3 is l ar g e ly d e t e r m i n e d b y t h e c o m p o s i t i o n o f t h e s t a r ti n g a m o r -

p h o u s o x i d e , a l t h o u g h s o m e p u r i f ic a t i o n d o e s o c c u r d u r i n g c r y s ta l li z a ti o n . C o m p l e -

t i o n o f th e c r y s t a l l i z a t i o n p r o c e s s is d if f ic u l t t o a s c e r t a i n , b u t a t 4 8 5 C a p p r o x i m a t e l y

f o u r d a y s a r e r e q u i re d . T h e t i m e c a n b e s h o r t e n e d c o n s i d e r a b l y b y h e a t i n g a t 5 0 0 C ,

b u t a b o v e t h i s t e m p e r a t u r e 4 0 a t m o x y g e n a re i n s u ff ic ie n t t o p r e v e n t s o m e o x y g e n

loss, i .e . t o U O 2 . 9 8 .

O x i d a t i o n o f U 3 0 s is f o u n d t o f o l lo w s e v e ra l r o u t e s, a n d t h e c o n d i t i o n s r e q u i r e d

t o o b t a i n e a c h p r o d u c t h a v e n o t b e e n c l e a r ly d e f i n e d . I n g e n e r a l , ~ 3-U O3 i s o b t a i n e d

a t 5 0 0 - 5 5 0 C w i t h 3 0 - 4 0 a t m 0 2 w h i l e y - U O 3 is f o r m e d s l o w l y a t t h is t e m p e r a t u r e

i n 6 - 1 0 a t m 0 2. A t 6 5 0 C a n d 4 0 a t m 0 2, a ll m o d i f i c a t io n s o f U O 3 w i ll b e c o n v e r t e d

t o t h e y - p h a s e . I n s e v e r a l e x p e r i m e n t s a t 5 0 0 C a n d 4 0 a t m 0 2 t h e o x i d a t i o n f o l l o w e d

a t h i r d r o u t e t o f o r m t h e U O 2 . 9 p h a s e . A t t e m p t s t o c o m p l e t e t h e o x i d a t i o n t o ~ - U O 3

w e r e u n su c c es s fu l . W e h a v e b e e n u n a b l e t o c o n f i r m t h e f o r m a t i o n o f ~ - U O s a t a n y

t im e d u r i n g t h e o x i d a t i o n o f U 3 0 s u n d e r a n o x y g e n p r e s s u re . I t i s p o s s ib l e , h o w e v e r ,

t h a t t h i s r e a c t i o n c o u l d b e a c h i e v e d a t h i g h e r p r e s s u r e s t h a n w e w e r e a b l e t o m a i n t a i n

i n o u r s y s t e m .

T h e f o l l o w i n g o x i d a t i o n r o u t e s h a v e t h u s b e e n o b s e r v e d :

U 3 0 x

~ - U O ~

b~3 - 6,yOo

500

~Ooo ~,o

~ U 0 2 . 9

y - U O 3

I d e n t i ty o f t h e p r o d u c t p h a s e m a y b e g o v e r n e d b y it s n u c l e a t io n e a r ly in t h e o x i d a -

t i o n p r o c e s s .

O u r i n v e st ig a t io n o f th e u r a n y l n i t r at e d e c o m p o s i t i o n r e a c t i o n h a s b e e n l im i t e d

b e c a u s e t h i s r e a c t i o n h a s b e e n s t u d ie d i n t e n s iv e l y e ls e w h e r e , 23,24) W e d i d c o n f i r m

t h e o b s e r v a t i o n t h a t d e h y d r a t i o n o f U O 2 ( N O a ) 2 . 6 H 2 0 b e f o r e d e n i t r a ti o n r e su l ts

i n a n a m o r p h o u s p r o d u c t r a t h e r t h a n y - U O a .(ts)

W A n c2) h a s r e p o r t e d t h a t 8 - U O 3 is o x y g e n - d e f i c i e n t w h e r l p r e p a r e d b y h e a t i n g

rq -U O 3 H 2 0 a t 4 1 5 C . W e w e r e a b l e t o p r e p a r e s t o i c h i o m e t r i c 8 - U O 3 b y h e a t i n g

t h e m o n o h y d r a t e l o n g e r ( > 2 4 h r ) a t s o m e w h a t l o w e r t e m p e r a t u r e s ( 3 7 5 - 4 0 0 C ) ,

b u t t h e o x i d e d o e s n o t e x h i b i t a n a p p r e c i a b l e t e m p e r a t u r e s t a b i li t y r a n g e ( s e e b e l o w ) .

2s) D .A . VAUGttAN, J. R . BRIDGE and C. M . SCH WA RTZ, BM I Report 1205 (1957).

(~4) B. A .J . LmTERand R.J . RICHARDSON, AE RE, C/R 1874 (1954).

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The uranium oxygen system U3Os UO3 159

KATZ and GRUEN 3) gave 250-350C as the temperature limits for the preparation

of ~-UO 3. The reaction rate was observed to increase rapidly to 350C, where it

levelled off. We find that the phase can be prepared at 375C, that oxidation is

very slow at 400C, and negligible at 425C. The upper temperature limit is apparently

governed by the stability of -UO 3 (see below) rather than by dissociation of

the NO 2.

Properties o f UO3 and UO2.9

Some of the properties of the six UO 3 phases and UO2 9 are summarized in

Table 2. Although the ~-form has been reported to be orange or red, our preparations

were always tan. Colour of the respective phases is dependent to some extent on the

method o f preparation, since the [~-phase prepared from ammonium uranate is a

dark orange, while the U30 s oxidation product is a cherry red. The colour change

may be due to variations in particle size.

The measured densities (vacuum pycnometric) represent the average of at least

two preparations of each of the phases. Densities of UO2. 9 and ~-UO3 are consider-

ably below the theoretical X-ray density. If the assumed structures are correct, the

low observed densities can probably he ascribed to poor crystallinity and the exist-

ence of voids within the crystals. The [~-UO3 density should be close to the theoretical

value, since measurements were made on a highly crystalline material. Our y-UO3

density is somewhat higher than the value reported by PEtlO (7 24 g/cma),

and suggest that possibly more than sixteen molecules are present in his

postulated unit cell. Cubic ~-UO 3 has an open structure with the lowest theoretical

density of any of the phases. It is not surprising to find that the experimental density

of the poorly crystalline oxide somewhat exceeds the theoretical density. The presence

of any of the remaining phases would lead to a measured density in excess of the

X-ray value. The density of .-UO3 corresponds almost exactly to that expected for

the addition of oxygen into the U3Os structure without expansion of the uranium

lattice. It is apparent from the X-ray diffraction pattern, however, that rearrangement

of the uranium atoms is involved.

The unit cell given for UO2.9 contains two uranium atoms and aproximately

5.8 oxygen atoms. This cell probably represents a pseudo-cell, but the present X-ray

results do not warrant consideration of a larger cell with an integral number of

oxygen atoms. One possibility for such a cell would have the formula

UI2035

i.e.

U02.92.

A comparison of the a b ratios of the relevant oxides (referring ~-UO 3 to ortho-

hexagonal axes) suggests that UO2.9 is not part of a continuous solid region between

~-UO 3 and ~-U30 s. Rather it appears to be more closely related to [3-U30 , thus

the

a b

ratios for ~-UO 3 ---- 0.577, for UO2.9 ---- 0.567, for [3-U30s = 0.554, and for

~-U30 s = 0 594.

Table 3 gives data on the diffraction patterns of ~3-UO3, ~-UO3 and UO2.9. Our

data on ~'-UO3 are in complete agreement with those published by DAwsoN 9) and

CONNOt.LY7). The structures of the ~- and ~-phases have been reported, and need

not be repeated here.

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160 H . R . HOEKSTRA an d S. StEOEL

Z AC HA RIA SEN 25) h a s d i s c u s s e d t h e b o n d i n g i n u r a n y l a n d r e l a t e d c o m p o u n d s . H e

p o i n t s o u t t h a t c o m p l e t e s t r u c t u re d e t e r m i n a t i o n s a r e a v a i l a b l e o n o n l y a r e s t r i c t e d

n u m b e r o f c o m p o u n d s i n t h is c a t e g o r y . In e a c h i n s t a n c e th e u r a n y l g r o u p s a r e

s y m m e t r i c a l a n d c o l l in e a r . I n a d d i t i o n t o t h e p r i m a r y u r a n y l b o n d s , e a c h u r a n i u m

a t o m f o r m s f o u r t o s ix s e c o n d a r y b o n d s t o o x y g en o r h a l i d e a t o m s . T h e b o n d

l e n g t h s w i t h i n t h e u r a n y l g r o u p a s w e l l a s in t h e s e c o n d a r y l : o n d s v a r y f r o m o n e

s t r u c t m ' e t o a n o t h e r . Z AC HA RIA SE N w a s a l s o a b l e t o s h o w t h a t t h e o b s e r v e d U - - O

d i s ta n c e s , f o r b o t h p r i m a r y a n d s e c o n d a r y b o n d s p l o t t e d a g a i n s t t h e b o n d s t r e n g t h

f a l l o n a s i n g le s m o o t h c u rv e . T h i s c u r v e w a s r e v i s e d t o s o m e e x t e n t i n a l a t e r

p u b l i c a t i o n , t2 o

TABLE 3.--DIFFRACTION DATA ON ~ UO3, UO3 AND UO2. 9

~ - U 0 3

sin20 1

- U 0 3

sin20

U02 .9

sin20

1 hk l

0-01144

0-02288

0.02578

0.03773

0.04252

0.04633

0.05022

0.05137

0.06185

0-06303

0.06439

O.O9426

0.09755

0.1477

0.1552

0.1572

0-1620

0.1665

0.1820

0.1853

0 2012

0.2051

0.2125

0.2358

0.2470

0-2523

0 2581

v w

0.0283

v v w

0-0347

v w

0.0363

w w

0.0439

v w

0 0495

s 0.0508

w 0 0549

wm 0.0747

v w 0 0 7 6 4

w m

0 0786

w m

0.0874

v w

0.0928

v w

0.1052

v v w

0 1395

w 0.1468

v w

0-1571

v w

0.1591

v w

0 1641

v v w

0 1740

w 0 1762

v w

0 1817

v w

0 2054

v v w 0.2175

v v w

0 2208

v v w

0.2255

v v w 0.2530

v v w

0'2591

v v w

0 0344

s 0.0500

vw 0 0515

v v w 0 0 8 4 0

w m

0 0858

w m

0.1374

m 0.1507

w 0 1557

v w

0.1849

v w

0.1879

v v w

0-1977

v w

0 . 2 0 4 7

v w

0.2331

v w 0 2392

v w 0 2 8 6 8

v v w

0 2916

v v w

0'3083

v w

0 3 3 5 4

v v w

0.3428

v w

0 3595

v v w

0 3743

v w

0'3823

v v w 0 7 8 2

v v w

0 3964

v v w

0.4590

v v w

0.5131

v v w 0 5 4 6 4

0.5982

s 001

w m 2 0 0

s 110

w 201

s 111

m 002

m 102, 310

w m

020

m 311

s 202, 112

w 400

w m 220

w m 401

m 221

w m 312

w 022

w + 003

w 4 0 2

w + 222, 501

m 203, 11 3, 130

v v w

412

v w

511

v w

421

v w

213

w m

313, 330

w 223

w 413, 430

wm 204

A v a i l a b l e s t r u c t u ra l d a t a o n u r a n i u m ( V I ) c o m p o u n d s i n d ic a t e s t h a t t h ey m a y

b e g r o u p e d i n t o t h e t h r e e g e n e r a l t y p e s i l l u s t r a te d b e l o w :

tls~ W . H . Z A C ~ A m A S E N , A e t a C v y s t 7 , 7 9 5 ( 1 9 5 4 ).

c s i ~ W . H . Z A C H ~ A S E N a n d H . A . P L E T T I N G E R , A C t Q C r y M | 2 ~ 5 2 6 ( 1 9 5 9 ).

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The uranium oxygen system U3Ot~U03

161

T y p e (~ ) is c h a r a c t e r iz e d b y d e f i n i te u r a n y l ( U O ~ ~ ) g r o u p s i n w h i c h t h e u r a n y l

o x y g e n s a r e b o n d e d t o a s i ng le u r a n i u m a t o m , T h e d o u b l e - b o n d U - - O d i st a n c e

i s a p p r o x i m a t e l y 1 . 7 A . T h e n o n u r a n y l s e c o n d a r y b o n d s a r e m u c h w e a k e r w i t h

c o r r e s p o n d i n g l y l o n g e r b o n d d i s t a n c e s, w h i c h m a y a p p r o a c h 2 .5 A i n l e n g t h . E x -

a m i d e s o f t h i s t y p e i n c l u d e N u U O 2 ( O A c ) 3 a n d U O .z F2.

I n T y p e (b ) t h e i n d iv i d u a l u r a n y l g r o u p s a r e re p l a c ed b y c o n t in u o u s - U - O - U - O -

c h a i n s i n w h i c h t h e o x y g e n a t o m s a r e b o n d e d t o t w o , r a t h e r t h a n o n e , u r a n i u m

a t o m . T h i s a r r a n g e m e n t l e a d s to a p r i m a r y b o n d s t r e n g th o f u n it y , a b o n d d i st a n c e

o f a p p r o x i m a t e l y 2 .0 7 A , a n d a r e p e a t g r o u p ( U O ) r a t h e r t h a n ( U O 2 ) . H e r e a g a i n

w e a k s e c o n d a r y b o n d s ( f o u r , f i v e o r s ix ) a re e n c o u n t e r e d . T h e i r r e la t iv e s t r e n g t h

i s g r e a t e r t h a n i n t y p e ( a ) a n d t h e b o n d d i s t a n c e c o r r e s p o n d i n g l y s h o r t e r . W h e n

f o u r s e c o n d a r y b o n d s a r e p r e se n t , th e i r s tr e n g t h b e c o m e s e q u a l t o t h e p r i m a r y b o n d s

a n d t h e t w o t y p e s a re i n d is ti n g u is h a b le . S u c h a n a r r a n g e m e n t i s s h o w n b y c u b i c

B - U O 3 . T w o - t h i r d s o f t h e u r a n i u m a t o m s o f U s O s ( a s d e du __ ~ d b y A m w ,m ~ 14)

a re c o - o r d in a t e d t o o x y g e n t h r o u g h f iv e s e c o n d a r y b o n d s , a l t h o u g h t h e b o n d l e n gt h s

a r e a d j u s t e d s o t h a t t h e o x i d a t i o n s ta t e o f a ll u r a n i u m a t o m s a r e e q u i v a le n t . H e x a g -

o n a l x -U O 3 is a n e x a m p l e o f u r a n i u m c o . o r d i u a t i o n t h r o u g h s ix se c o n d a r y b o n d s .

0 0 0

0 0 0

I

o l b ) c )

A n i n t e r m e d i a t e t y p e o f b o n d i n g b e t w e e n ( a ) a n d ( b ) i s a l s o o b s e r v e d . T h i s

a r r a n g e m e n t , t y p e (c ), h a s b e e n i d e n t i fi e d i n t h e u r a n a t e s . H e r e t h e s t r e n g t h o f t h e

p r i m a r y b o n d s a s w e ll a s t h e s e c o n d a r y b o n d s c a l c u la t e d f r o m t h e b o n d l e n g t h s a r e

i n te r m e d i a t e be t w e e n (a ) a n d ( b). B a r i u m a n d m a g n e s i u m o r t h o u r a n a t e h a v e f o u r

s e c o n d a r y b o n d s , w h i le c a l c i u m a n d s t r o n t i u m u ~ n _ A t es h a v e s ix .

W h e t h e r t h e t h r e e t y p e s o f b o n d i n g a r e d ix t in c t e n ti ti e s o r m e r e ly p a r t s o f a

c o n t i n u o u s c h a n g e i n b o n d t y p e f r o m t h e t w o e x t re m e s ((a ) a n d (b )) r e m a i n s t o b e

e s t a b li s h e d a s a d d i t i o n a l s t r u c t u r e d e t e r m i n a t i o n s a r e f o r t h c o m i n g . F u r t h e r w o r k w i l l

a l s o d e t e r m i n e w h e t h e r th e a p p a r e n t c o r r e l a t i o n o f u r a n y l sa l ts w i t h t y p e ( a ), u r a n i u m

o x i d e s w i t h ( b) , a n d u r a n a t e s w i t h (c ), w i ll b e m a i n t a i n e d , K 3 U O 2 F s , w h i c h e x h ib i t s

t y p e ( c ) b o n d i n g , i s , a t p r e s e n t , t h e o n l y e x c e p t i o n t o t h i s r u le .

A c o m p a r i s o n o f U O 3 X - r a y d i f f ra c t i o n p a t t e r n s s h o w s t h a t t h e ~ - a n d B -p ha se s

h a v e r e la t iv e l y s i m p l e p a t t e r n s , w h i l e t h e r e m a i n i n g p h a s e s a r e m o r e c o m p l e x . T h e

~ , B a n d c p h a s e s h a v e a s t r o n g d i f f r a c t i o n l in e a t a n a n g l e ( s in 2 0 = 0 . 0 34 7 ) c o r re s -

p o n d i n g t o a d v a l u e o f 4 '1 4 A , w h i l e t h e [5 a n d y p h a s e s d o n o t . ( T a b l e 3 ) . T h i s d

s p a c in g i n ~ - a n d B -U C ) c o r r e s p o n d s to t h e U - U s e p a r a ti o n i n th e c o n t i n u o u s

- - U - - O - - U - - O c h a in s , a n d m a y h a v e a si m i la r s ig u if ic a n c e i n g - U O 3. A b s e n c e

o f t h i s s t r o n g l i n e i n t h e ~ a n d T - U O 3 p h a s e s m i g h t b e c o n s t r u e d a s s u g g e s t i n g

a n a l t e r n a t i v e a r r a n g e m e n t o f U - O b o n d i n g , b u t m a y a l s o a r i s e f r o m i n t e r f e r e n c e

e f fe c ts . A t p r e s e n t t h e r e i s n o d e f i n i t e e v i d e n c e f o r t h e p r e s e n c e o r a b s e n c e o f t h e

U - - O c h a i n s in t h e ~ , y a n d f o r m s o f U O 3 .

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162 H . R . HOEKSTRA an d S . S IEtZEL

A surpr i s ing feature o f the U3Os-[i -UO3 system is tha t one can con ver t s ingle

crys t a ls o f e i t her o f t hese phases t o t he o the r w i thou t en t i r e ly l os ing t he s ing le c rys t a l

charac t e r o f t he so l i d . Th i s phe nom eno n occu r s i n sp i te o f the f ac t t ha t a m ajo r

rear rangem ent i n t he c rys t a l s t ruc tu re t akes p l ace dur ing the ox ida t ion o r r educ t ion

process . Un for tuna t e ly , ~ -UO 3 crys t a ls , p repa red f rom twinned U 30 s c rys t a ls , w ere

a l so twinned abou t one ax i s . We were ab l e t o measure one ax i s ( t he twinn ing ax i s )

a s 1 4 .3 k X . A h ex ag o n a l p s eu d o - ax is co u l d b e o b t a i n ed f r o m t h e t w i n p a t t e rn

cor respon d ing to t he a -ax i s o f g -U O 3 a t 3 99 A. Ox ida t ion o f U 30 s s ing le c rys t a l s

to z -UO3 l eads t o b re aku p o f t he so l id i n to a microcrys t a i li ne p ow der ; t he sa m e

o b s e r v a t io n i s t ru e f o r t h e ~ - U O 3 - - - > y - U O 3 co n v e r s io n .

eat o f so lu t ion

H ea t s o f s o l u ti o n w er e o b t a i n ed o n t h e s ix U O 3 p h as es i n o r d e r t o p r o v i d e a

com par i son o f t he i r t herm ody nam ic s t ab i li ty . The r esu l ts a re sum m ar i zed in Tab le 4.

TABLE 4.--H EA T OF SOLUTION OF U Oa

H e a t o f s o l u t i o n ( k c a l /m o l e ) H e a t o f f o r m a t i o n

P h a s e ( k c a l / m o l e )

I n 1 M H C I O 4 I n 5 M H N O 3

U O 3 ( A ) - 2 2 . 5 0 . 2 - - 2 1 . 0 4 - 0 .3 - -2 9 1

g - U O 3 - - - - 1 8 . 5 4 - 0 .1 - - 2 9 4

~ - U O 3 - 1 9 . 2 0 . 2 - - 1 8 . 1 q - 0 . 3 - - 2 9 4

, '-UO3 -- - -1 7.3 4- 0.1 ,~ -- 29 5

~ - U O 3 - - - - 18 . 3 -4- 0 . 2 ] - - 2 94

~ - U O 3 - - - - 18 7 - 4 - 0 . 1 [ - - 29 3

BREW I/P. (27) es t imated a h ea t o f fo rm at ion o f - -291 -4 -3 kca l /mole fo r a ,-UO3,

- 2 9 2 k ca l /mo l e f o r ~ - U O 3 an d - - 2 9 3 k ca l / mo l e f o r y - U O 3 . Th e e st ima t e s w e r e

based to a large extent On BXLTZan d M O L L E R ' S 2 s) t en si me t ri c d a t a o n t h e U 3 O s - U O 3

sys t em, wi th t he assumpt ion tha t t he t r i ox ide i n t h i s work was t he hexagonal phase .

S ince BILTZ and MOLLER'S sam ple was p repa red f rom urany l perox ide , t he phase

mu s t h av e b een amo r p h o u s r a t h e r t h an c r y st a ll in e . O u r d a t a w o u l d l e ad t o a v a l u e

o f 2 9 5 k ca l / mo l e f o r t h e - f -p h as e .

Thermal s tab i l i ty

Th e t h e r ma l s / ab il it y an d d eco m p o s i t i o n p a t t e r n o f each o f t h e six U O 3 p h as es

i n a i r h a s b een s t u d i ed . S i n ce t h e U O 3 - > U 3 0 s convers ion i s i r revers ib le at a tmos-

pher i c p ressu re , one canno t ob t a in equ i l i b r ium pressu res i n t h i s compos i t i on r ange .

We have , however~ s tud i ed t hermal s t ab i l i t i es under cond i t i ons which g ive a r eason-

ab l e ap p r o ac h t o eq u i l ib r iu m. O x i d e s amp l e s w e i g h in g ap p r o x i ma t e l y l g w e r e

hea t ed i n a i r fo r a t l eas t 24 h r i n approx imate ly 15C in t e rva l s be tween 300 and

7 50 C . F u r n ace tem p er a t u r e s w e re co n t r o l led t o I C . C h an g es i n co m p o s i t i o n

c2~ L. BaEWER, Chem. Rev . 52 , 2 (1953) .

c2s~ W . BILTZ an d H. MOLLER, Z. Anorg. Chem. 163 , 257 ( 1927 ) .

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The u r an ium oxy s en s y s tem U3 O s U O3 163

were fo l l o wed b y o b s e rv in g co lo u r an d we ig h t ch an g e , an d b y p e r i o d i c X- ray d i f -

f rac t ion iden t i f i ca t ion o f the phases . R esu l ts o f th i s s tudy are p lo t ted in F ig . I .

I t w i ll b e o b s e rv ed t h a t o n ly am o rp h o u s U O 3 d eco m p o s es b y w ay o f t h e i n t e r-

m ed ia t e p h as e , UO2 . 9. So m e ~ -UO 3 s am p le s s h o w a m in o r i n f l ex io n a t t h is co m p o s i -

t io n . S in ce 0 c-UO i s p rep a red f ro m th e am o rp h o u s fo rm , t h e i n f lex io n m ay r e -

p re s en t r es i d ua l am o rp h o u s o x id e i n t h e s am p le . L a t t i c e p a ram e te r s m e as u red

3 . 1

3 . 0

2 . 9

o

[**

( [

n .

o 2 . 8

. . ' . . . U O ( A )

- _ - _ - = . . . . . , ,

I

. . .

_ _

4 0 0 4 5 0 ~ 0 0 5 eO

I I I I I I

" T - -

Is ru03s

t t

t ;

i

I

I

I

I

|

i

i

|

t

D

:

' U O 3

, ,

oo .oo oo

I ' L e ~ I l l~ R A T U R E * N ' C

F I G . | .

d u r in g d eco m p o s i t i o n o f a -U O 3 r ev ea l t h a t t h e p ro d u c t a t co m p o s i t io n UO = . 9 is

n o t i d en t ica l w i th th e p h as e o b t a in ed d u r in g d eco m p o s i t io n o f t h e am o rp h o u s

ox ide . The a -U O 3 deco m pos i t ion appea rs to invo lve ex tens ive so lid so lu t ion .

T r a n s it io n o c c u rs g r a d u a l l y f r o m h e x a g o n a l ~ - U O 3 t o t h e o r t h o r h o m b i c U 3 0 s

s t ruc tu re .

T h e UO2 .9 p h as e wh ich is fo rm e d b y p a rt i a l d eco m p o s i t i o n o f t h e am o rp h o u s

o x id e is a d i s ti n c t co m p o u n d i n t h e u ran iu m -o x y g en s y s tem , s in ce i ts O /U ra t i o

remains v i r tua l ly cons tan t over a 100C tempera tu re in te rva l . I t s poss ib le re la t ion-

ship to the 0~- an d ~-U3Os s tructur es ha s be en discussed.

X- ray d i f f r ac t io n st u d ie s an d v i s u a l o b s e rv a t io n o f t h e o x ides i n t h e U3 O s -Y-UO3

s y s tem g ive n o ev id en ce fo r s o l id s o lu t io n b e tween t h e e n d -m e m b e rs . Sam p le s w i th

th e em p i r ica l co m p o s i t i o n UO2,7 ~ an d U O2 . ~ co n t a in ed d e t ec t ab l e am o u n t s o f b o th

phases .

T h e 8 an d fo rm s co n v e r t d i rec t ly t o U 3 0 s i f t h e h ea t i n g r a t e i s f a i r ly r ap id

s ev era l h o u r s ) . H o wev e r , t h e s l o w h ea t i n g r a t e em p lo y ed i n o u r s t u d y p e rm i t t ed

re-ox ida t ion o f the par t i a l ly reduc ed ox ides to y -UO ~. P oor c rys ta l l in i ty o f these

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164

H R HOEKSTRA an d S SIEGEL

samples l ed to som e decrease i n s t ab i l i ty o f the r esu l t an t T-UO 3; decom pos i t i on

was in it ia t ed a t 25 -50C be low the t em pera tu re l imi t observe d fo r wel l c rys t a l-

l i z ed y - U O3.

Infra red spectra

Prev ious r epor t s on the i n fra - red spec t ra o f hexavalen t u ran ium have dea l t

pr imar i ly wi th uranyl sal t s , and were reviewed by RAmNOW]TCH29~. A bso rpt io n of

the UO22+ g roup has genera l ly been fou nd a t 860 and 920 cm -1 . T he re l a ti ve ly we ak

abso rp t ion a t 860 cm -1 ha s been ass igned to t he sym me t r ic s t r e t ch ing (V l ) and the

s t ro n g e r 9 2 0 cm - ~ ab s o r p t i o n t o t h e a s y m me t r i c s tr e tch i n g ( v 3 ) . A p p ea r an ce o f

the fo rb idd en sym m et r ic v ib ra t ion in t he i n f ra - red is ascr ibed to c rys t a l f i e ld

e f fec ts . N o a t tem p t w as m ad e , h o w ev e r , to co r r e l a te o b s e r v ed s p ec t ra w i t h U - O

di s t ances i n t he u rany l g roup , o r t o ca l cu l a t e fo rce cons t an t s .

1200 I1 00 I000 4JO0 800 ?00 1200 I1 00 I0~0

CM I CM I

Flo. 2.

Rec ent ly , JONF_S '3~) d iscussed the system at ics in v ibra t ional spe ct ra of u ra ny

co mp l ex es an d a t t emp t ed t o ca l cu l a te th e U - O b o n d d i st an ce f r o m t h e i n fr a - red

s p ec t ra o f s ev e ra l u l an y l- co n t a in i n g co m p o u n d s . H e co n c l u d ed t h a t t h e m ax i mu m

er ro r i nvo lved in t r ea t ing the u rany l g roup as a fr ee i on fo r ca l cu l a t ion o f t he U -O

f o r ce co n s t an t w as 3 p e r cen t .

JONES rep or te d a n asym m etr ic v ibra t ion (v3) and co m bina t ion f requenc ies (v l~-v3)

fo r f i ve u rany l complexes . F orce cons t an t s w ele ca l cu l a t ed f rom the observed spec t ra .

{ ~ E . R A B m o w r r c H , A N L - S I 2 2 0 9 5 3 ) .

is, ; L . H . J o i n s , S p e e t r oc h i m . A c t a I 0 , 3 9 5 ( 1 9 3 8 ) .

t s u L . H . J o ~ m ,

S p e e t r o c h i m . A c t a

1 1 , 4 0 9 ( 1 9 59 ) .

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The uranium-oxysen system U3Os-UO3

165

A p p l i ca t i o n o f

BAD GER S 32)

r u le p e r mi t t ed an e s t ima t i o n o f t h e U - O d i st an ce .

JONES con clude d tha t t he v man d v3 f r equencies shou ld serve as a sens it ive m easure

o f R u o i n u ran y l co mp l ex es s in ce a 1 6 p e r cen t ch an g e i n t h e f o r ce co n s t an t co r r e -

s p o n d s t o o n l y a 0 .0 3 A ch an g e i n t h e U - O b o n d d i s t an ce .

As we have po in t ed ou t , i so l a t ed u rany l g rou ps d o no t ex is t in ~ -UO3, ~ -UO3,

U O 2 .9 o r U 3 0 s . W h e t h e r t h ey d o ex is t i n an y o f t h e r ema i n i n g U O 3 p h as es r ema i n s

t o b e d e te r mi n ed . T h e i n f r a -r ed s p ec t r a o f t h e s ix U O 3 p h as es , U O 2 .9 an d U 3 0 8

f r o m 1 00 0 t o 6 5 0 cm - I a r e g i v en i n F i g . 2. I t w o u l d ap p ea r f r o m t h es e t h a t u t an y l

g rou ps a re n o t p resen t i n any o f t he ox ide phases i nves t iga t ed . As a r esu l t, d i r ec t

co mp ar i s o n w i t h J O N ES w o r k o n u r an y l s a l t s an d u r an a t e s i s n o t p o s s i b l e .

W e a r e co n t i n u in g o u r w o r k o n th e i n fr a -r ed s p ec t r a o f t h e s e c o m p o u n d s an d

h o p e t o b e ab l e t o r ep o r t f u r t h e r o n t h em i n t h e n ea r f u t u r e .

A c k n o w l e d t e m e n t s W e a r e indebted to R.A. GARBER who was responsible for construction of

the calorimetric apparatus and the heat o f solution measurements. We are also indebted to the

analytical group headed by g . BANe for analyses on the compounds used in these studies, and to

L.H. Fuctts for the UO2CO3 preparation.

at) R. M. BADGER, /.

Chem. Phys.

3, 710 1935).