Actinide Chemistry

6

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on June 9, 2016 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0071.ch006

Fluorite-Related Oxide Phases of the Rare Earth and Actinide Elements LEROY

EYRING

Arizona State University, Tempe, Ariz. Cerium, praseodymium, and terbium oxides display homologous series of ordered phases of narrow composition range, disordered phases of wide composition range, and the phenomenon of chemical hysteresis among phases which are structurally related to the fluorite-type dioxides. Hence they must play an essential role in the satisfactory development of a comprehensive theory of the solid state. All the actinide elements form fluorite-related oxides, and the trend from ThO to CmO is toward behavior similar to that of the lanthanides already mentioned. The relationships among all these fluorite-related oxides must be recognized and clarified to provide the broad base on which a satisfactory theory can be built. x

x

' H p h e i n c e n t i v e for s c h e d u l i n g a s y m p o s i u m o n the c h e m i s t r y of

the

l a n t h a n i d e s a n d a c t i n i d e s is d e r i v e d i n p a r t f r o m the v a l u e of e x a m i n i n g the s i m i l a r i t i e s a n d i n t e r r e l a t i o n s h i p s w h i c h exist b e t w e e n analogous sequences of elements.

these

T h e c h e m i c a l analogies w h i c h

first

suggested the i n g e n i o u s a c t i n i d e hypothesis m a n y years ago are n o w w e l l d e v e l o p e d a n d g e n e r a l l y r e c o g n i z e d . It is n o w possible to go b e y o n d this to e x a m i n e subtle s i m i l a r i t i e s a n d v a r i a t i o n s . I n d e e d , i n the t w o c h e m i c a l l y r e l a t e d / - s h e l l groups n e a r l y one t h i r d of the k n o w n elements p r o v i d e e x t e n d e d series of elements a n d c o m p o u n d s w h o s e properties v a r y c o n t i n u a l l y i n e a c h sequence b u t w i t h o v e r l a p to p r o v i d e one of the most v a l u a b l e testing g r o u n d s i n a l l of c h e m i s t r y for a n y t h e o r y w h i c h may

be advanced.

T h e i s o m o r p h i s m e x i s t i n g a m o n g the oxides of the

a c t i n i d e a n d l a n t h a n i d e elements i n the c o m p o s i t i o n r a n g e R O ^ , 1.5

axis of the u n i t c e l l ) ,

Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

6.

EYRING

Oxide

71

Phases

planes c o n t a i n i n g o n l y m e t a l or o x y g e n sequence Ac aCb

cBa

atoms a p p e a r s t a c k e d i n the

h A, w h e r e c a p i t a l letters represent m e t a l planes

a n d l o w e r case letters those of o x y g e n

(23).

T h e C - t y p e rare e a r t h s t r u c t u r e ( space g r o u p Ia3 ), w h i c h is the other e n d - m e m b e r of the

fluorite-related

series of phases, has the same s t a c k i n g

sequence, b u t o n e - f o u r t h of the oxygens are m i s s i n g f r o m e a c h

oxygen


p l a n e i n a n o r d e r e d w a y . T h e result of this o r d e r i n g is that i n the C - t y p e s t r u c t u r e a l l the o x y g e n vacancies m a y be c o n s i d e r e d as l a y i n g i n strings a l o n g the f o u r < 1 1 1 >

d i r e c t i o n s of the fluorite c e l l .

T h e s e strings are

n o n - i n t e r s e c t i n g , a n d their closest a p p r o a c h removes oxygens f r o m the face d i a g o n a l s of the i n t e r v e n i n g [ R O ] cubes g i v i n g six c o o r d i n a t i o n of s

one of t w o types to a l l the m e t a l atoms. T h e i n t e r m e d i a t e o r d e r e d phases o b s e r v e d i n the rare e a r t h oxides h a v e structures w h i c h are o b v i o u s l y r e l a t e d to those of the e n d - m e m b e r s d e s c r i b e d a b o v e i f one compares

the x - r a y d i f f r a c t i o n patterns

T h e structure of the t phase, P r O i , has b e e n d e t e r m i n e d . 7

(46).

It m a y b e

2

r e p r e s e n t e d i n terms s i m i l a r to those u s e d a b o v e b y v i s u a l i z i n g it as c o n s i s t i n g e n t i r e l y of the strings of six c o o r d i n a t e d m e t a l atoms r u n n i n g i n one

direction only.

T h e c r e a t i o n of the s t r i n g

generates

" s h e a t h s " of seven c o o r d i n a t e d m e t a l atoms s u r r o u n d i n g i t f o r m i n g a r o d . W h e n these rods are a l i g n e d p a r a l l e l to one another y i e l d i n g the P r O i T

2

s t r u c t u r e (23, 31, 32, 46) o n l y six a n d seven c o o r d i n a t i o n exists. It is s u g gested (32)

t h a t the strings are the s t r u c t u r a l e n t i t y r e l a t i n g a l l the i n t e r

m e d i a t e oxides a n d the e n d - m e m b e r s . S i n c e 1 / n t h of the cations are i n the strings a n d e a c h c a t i o n has t w o of its o r i g i n a l eight oxygens m i s s i n g , a composition R 0 n 2

=

R, 0 „_ ?

2

T h e other phases w i t h η > r u n n i n g i n o n l y one < 1 1 1 >

2

is o b s e r v e d .

7 are b e l i e v e d to consist also of strings

d i r e c t i o n , b u t for e a c h of these there m u s t

b e i n c r e a s i n g regions of [ R O ] groups as the c o m p o s i t i o n P r 0 s

2

(η =

oo )

is a p p r o a c h e d . T h e d i s o r d e r e d σ phase is b e l i e v e d to consist of a C - t y p e oxide, w i t h some of the o x y g e n positions a l o n g the strings

filled,

i n t e r r u p t i n g the

strings b u t m a i n t a i n i n g the c u b i c s y m m e t r y . T h e a phase, o n the other h a n d , m u s t consist of a w i t h segments

of

strings b u n d l e d together

o x y g e n is lost f r o m the P r 0

2

matrix as

structure. T h e b u n d l e s are b e l i e v e d to b e

at r a n d o m a l o n g the f o u r < 1 1 1 > cubic symmetry.

fluorite-type

i n i n c r e a s i n g amounts

d i r e c t i o n s of the fluorite c e l l p r e s e r v i n g

W h e r e the b u n d l e s so o r i e n t e d a p p r o a c h e a c h other,

regions of C - t y p e are created.

T h i s is suggested b y the w e a k s u p e r

structure reflections i n x - r a y d i f f r a c t i o n f r o m the a phase of the C e 0 - Y 0 2

m i x e d oxides (12).

2

3

T h e complexes of rods are not fixed i n the α p h a s e

b u t are free to translate. T h e α-σ m i s c i b i l i t y g a p represents the r e g i o n i n w h i c h the l a b i l e a transforms to the r i g i d σ.


72

LANTHANIDE/ACTINIDE

P r o p o s e d p a r t i a l (T,x)

phase d i a g r a m s for the C e O

x

CHEMISTRY

(13) and T b O

x

( 32 ) are s h o w n i n F i g u r e s 2 a n d 3. T h e existence of some of the h o m o l o gous series are seen i n e a c h , b u t the p r i n c i p l e feature is the w i d e r a n g e of σ a n d a phases w i t h a m i s c i b i l i t y gap. —r—

12

1 f

hk


10 1 X Ρ 6

\\

Ί

-

-

a

\

8

ϊ

\

*—

4 -L L

i

1.5

1.6

-r|-σ

U

ι

1.7 1.8 1.9 χ i n Ce O

i

2.0

x

Figure 2.

Figure 3.

Projection of the phase diagram

Projection

CeO O

of the TbO -0

x

x

2

2

phase diagram

A l l three p o l y m o r p h s of the rare e a r t h sesquioxides are s h o w n b y either C e 0 (A-type), P r 0 ( A a n d C - t y p e ) or T b 0 (A, B, a n d C - t y p e ) . M u c h w o r k is n o w u n d e r w a y o n the existence a n d r e l a t i o n s h i p 2

3

2

3

2

3


6.

EYRING

Oxide

73

Phases

of these three types, a n d there is a great d e a l of d i s a g r e e m e n t as to w h e t h e r t h e y d o i n fact represent p o l y m o r p h s i n the t h e r m o d y n a m i c sense.

B r a u e r (15,

16)

r e v i e w s progress i n this area of r e s e a r c h to the

e n d of 1965, b u t c o n s i d e r a b l e w o r k has b e e n r e p o r t e d since t h e n w i t h l i t t l e agreement.

T h e areas of h i g h t e m p e r a t u r e t r a n s f o r m a t i o n are d i s -

cussed b y F o ë x (24, 2 5 ) , w h o has o b s e r v e d phase transitions u p to the


m e l t i n g p o i n t of the oxides; b y B o g a n o v a n d R u d e n k o (14) kova and Boganov

(27)

and Glush-

w h o address themselves to r e v e r s i b i l i t y of the

transitions a n d the differences i n c o m p o s i t i o n ; b y H o e k s t r a ( 2 9 ) ,

who

reports extensive w o r k o n the pressure d e p e n d e n c e of the transitions a n d reversibility among them.

Fluorite Related Phases of the Actinide Metal Oxides A l l the a c t i n i d e elements w h o s e oxides h a v e b e e n s t u d i e d c a n b e m a d e as

fluorite-type

d i o x i d e s . F o r most of t h e m this is t h e i r most stable

f o r m i n a i r at r o o m t e m p e r a t u r e .

The

fluorite-related

phases of

each

a c t i n i d e e l e m e n t k n o w n w i l l b e discussed i n d i v i d u a l l y b e f o r e a c o m p a r i son is m a d e w i t h the r a r e e a r t h oxides Th0

Thorium. b u t i t forms

2

(45).

is one of the most t h e r m a l l y stable oxides k n o w n ,

a s l i g h t l y oxygen-deficient,

ThOi.998 ( 3 )

dioxide w i t h a =

2

Th0

2

is a

2

is k n o w n , b u t no e v i d e n c e exists

phases w h i c h almost c e r t a i n l y c o u l d b e m a d e .

a ?

fluorite-type

5.999A.

Protoactinium. F l u o r i t e - t y p e P a 0 for P a 0 .

congruently-vaporizing solid

at t e m p e r a t u r e s of a b o u t 2 5 0 0 ° C .

Nonfluorite-

t y p e h i g h e r oxides ( i.e., P a O , ) h a v e b e e n s t u d i e d to some extent. 2

r

U r a n i u m . O f a l l oxide systems i n v e s t i g a t e d , those of u r a n i u m m u s t b e the most t h o r o u g h l y s t u d i e d , r e v e a l i n g i t to b e one of the most c o m p l e x b i n a r y systems k n o w n . W e s h a l l c o n c e r n ourselves h e r e o n l y w i t h the fluorite-related

phases U 0

2

± . A phase d i a g r a m of the r e g i o n of interest x

is s h o w n i n F i g u r e 4, w h i c h is a c o m p o s i t e of the U 0 b y R o b e r t s (43)

2 + x

region proposed

a n d the UO. r e g i o n o b s e r v e d b y M a r t i n a n d E d w a r d s x

(39). T h e extra o x y g e n i n

fluorite-related

t r o n d i f f r a c t i o n studies b y W i l l i s (50)

U0

2 + x

has b e e n l o c a t e d i n n e u -

to b e a c c o m m o d a t e d i n the

fluorite

l a t t i c e b y the g e n e r a t i o n of c o m p l e x groups d e s c r i b e d as 2 : 2 : 2 c o n f i g u r a tions c o n s i s t i n g of t w o i n t e r s t i t i a l oxygens d i s p l a c e d a b o u t 1 A . a l o n g the

d i r e c t i o n f r o m the holes i n the center of the fluorite u n i t c e l l ,

a n d t w o o x y g e n interstitials d i s p l a c e d a b o u t 1 A . a l o n g the < 1 1 1 > t i o n f r o m the t w o n o r m a l sites v a c a t e d .

direc-

T h e positions of the u r a n i u m

atoms are unaffected b y this r e a r r a n g e m e n t . The U 0 4

9

phase results w h e n one of these 2 : 2 : 2 complexes i n e v e r y

t w o u n i t cells of the p a r e n t U 0

2

is l i n k e d together w i t h its n e i g h b o r s i n

a n o r d e r e d w a y . T h e c o m p l e t e s t r u c t u r e is not k n o w n . A t h i g h t e m p e r a -


74

LANTHANIDE/ACTINIDE

tures the U 0 4

phase m a y exist w i t h c o m p l e x concentrations

2 + x

than i n U 0 .

A c c o r d i n g to A n d e r s o n ( 6 )

9

CHEMISTRY

greater

the t r a n s f o r m a t i o n b e t w e e n

U4O9 a n d U 0 o . 2 5 is a c c o m p a n i e d b y o n l y a s m a l l e n t r o p y increase, 2 +


i n d i c a t i n g some b u t n o t great d i s o r d e r i n g i n the t r a n s i t i o n .

uo

1.60

1.70

1.80

2

1.90 2.00 χ in UO

2.10

2.20

2.30

x

Figure 4.

Projection

of the U0

2

M a r t i n and E d w a r d s (39) U0 -U 2

U0

2

2

2

T h e l a t t i c e p a r a m e t e r of the U 0

2+W

phase diagram

x

phase as s k e t c h e d i n F i g u r e 4. have observed a U 0

5.4714A., a s i g n i f i c a n t l y l a r g e r u n i t c e l l t h a n for U 0

O / U ratio from a = of \J0

2

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

a n d U ; h o w e v e r , A c k e r m a n n et al. (2)

of a =

-0

x

h a v e s t u d i e d the phase d i a g r a m for the

r e g i o n , w h i c h shows a U 0 .

Usually hyperstoichiometric U 0

±

4

9 ν

2

phase

2

o-

phase d e c r e a s e d w i t h i n c r e a s i n g

2 + a

5.4705A. for U 0 . V a l u e s of the l a t t i c e p a r a m e t e r s

a n d \] 0 .

end-members U 0

2 0

2

are o n s m o o t h curves l y i n g a b o v e a l i n e j o i n i n g the

and U 0 4

4

(35).

9

5.4453A. ) A c t u a l l y , the U 0

9

( T h e p s e u d o - c e l l of the l a t t e r is a

=

s t r u c t u r e is c u b i c w i t h w e a k s u p e r s t r u c t u r e

lines i n d i c a t i n g that the true u n i t c e l l has a n e d g e of 4a =

21.8A. a n d

the b o d y - c e n t e r e d s p a c e - g r o u p I43e£ ( 5 0 ) . Neptunium.

The

fluorite-type

Np0

2

is the stable o x i d e f o r m e d i n

a i r w h e n n e p t u n i u m oxysalts are d e c o m p o s e d , b u t almost no studies h a v e b e e n c a r r i e d out i n the oxygen-defect

r e g i o n . A c k e r m a n n et al. ( I )

s t u d y i n g the v a p o r i z a t i o n process of N p O ^ . o b s e r v e d A - t y p e N p 0 2

3

in in

q u e n c h e d samples w h i c h h a d b e e n 7 0 % v a p o r i z e d . It is l i k e l y t h a t the t w o phases w e r e f o r m e d f r o m a n o n s t o i c h i o m e t r i c N p 0 . 2

x

phase b y d i s

p r o p o r t i o n a t i o n as the s a m p l e w a s c o o l e d .


6.

EYRING

Oxide

75

Phases

R o b e r t s a n d W a l t e r (44) Np O 3

s

s t u d i e d the i r r e v e r s i b l e d e c o m p o s i t i o n

b u t o b s e r v e d n o phases i n t e r m e d i a t e to N p 0 3

stoichiometric N p 0

2

on

and N p 0 . Hyper-

8

2

has not so f a r b e e n r e p o r t e d .

P l u t o n i u m . G a r d n e r et al. (26)

have made a careful h i g h tempera

ture x - r a y d i f f r a c t i o n s t u d y of the p l u t o n i u m - o x y g e n system i n the r a n g e f r o m r o o m t e m p e r a t u r e to 9 0 0 ° C . o b s e r v i n g d i f f r a c t i o n f r o m o x i d e s a m


ples c o n t a i n e d i n s i l i c a c a p i l l a r i e s . T h e y r e v i e w b r i e f l y p r e v i o u s apropos

work

of phase transformations (i.e., t h e r m a l a n d e l e c t r i c a l m e a s u r e

m e n t s ) a n d construct a phase d i a g r a m as s h o w n i n F i g u r e 5. T h e phases t h e y o b s e r v e d w e r e P u O i ,io ( b . c . c , a =

11.05); P u O i . i

r

(assumed b.c.c, a =

10.99); P u O i .

c o m p o s i t i o n l i m i t of P r 0

(f.c.c, a =

9 8

(f.c.c, a =

2

hex., a — 3.8417 =b 0.0003A., c =

5.396); β P u 0 2

5.9530 ±

6

5.40) w h i c h is the l o w e r (PuOi. io ±

3

0.005

5

0 . 0 0 5 A . ) ; a n d at least one

c u b i c phase of w i d e l y v a r i a b l e c o m p o s i t i o n P u O , 1.61 < tT

χ < 2.0.

Be

t w e e n 300° a n d 6 0 0 ° C . i n this c o m p o s i t i o n r a n g e there is a m i s c i b i l i t y g a p w h i c h c o n t i n u o u s l y n a r r o w s u n t i l i t is s u p p o s e d l y 1

1

1

i

1

σ

700

I

ι

closed. -

α

500 Ρ 300 . l c + σ — ι 100

1

C+F

1.5

1.6

1.7

1.8

x in PuO Figure

5.

1.9

2.0

x

Projection of the phase diagram

Pu0^0

2

C e r t a i n features of the s t u d y w e r e e m p h a s i z e d s u c h as the fact t h a t a l t h o u g h the P u O i . i phase w a s a s s u m e d b . c . c , the p o w d e r patterns w e r e 6

not g o o d e n o u g h to s h o w the s u p e r s t r u c t u r e lines. A l s o the e u t e c t o i d i n the p r o p o s e d d i a g r a m of C h i k a l l a et al. (22) x - r a y studies.

w a s not o b s e r v e d i n the

N o satisfactory e x p l a n a t i o n of e l e c t r i c a l r e s i s t i v i t y a n d

t h e r m a l e x p a n s i o n measurements w h i c h l e d to this e a r l i e r c o n s t r u c t i o n has e m e r g e d .

G a r d n e r et al. (26)

also l o o k e d for b u t d i d n o t see a n y

i n d i c a t i o n of o r d e r e d i n t e r m e d i a t e phases at l o w temperatures s u c h as w e r e d e s c r i b e d a b o v e for the r a r e e a r t h oxides nor d i d t h e y observe a f a i l u r e of the m i s c i b i l i t y g a p to close as w o u l d h a v e o c c u r r e d i f the e n d members, P u O i i a n d P u O i 6

9 8

, h a d different symmetries.

They


could

76

LANTHANIDE /ACTINIDE

CHEMISTRY

not, h o w e v e r , h a v e o b s e r v e d a n a r r o w t w o - p h a s e r e g i o n since h i g h reso l u t i o n a n d s h a r p d i a g r a m s w o u l d h a v e b e e n r e q u i r e d . O n e c a n observe f r o m t h e i r p u b l i s h e d d a t a t h a t a shift occurs i n slopes of the a vs. χ lines f o r oxides b e t w e e n P u O i . 9 o a n d P u O i i , w h i c h m a y suggest a d i s c o n t i 7

6

7

n u i t y i n this r e g i o n p e r m i t t i n g the c o n s t r u c t i o n of the n a r r o w m i s c i b i l i t y g a p r e q u i r e d i f the P u O i . e i a n d P u O i .

9 8

h a v e different symmetries. A b r i e f


d i s c u s s i o n of this p o i n t is g i v e n b e l o w . I f this g a p exists, the system bears e v e n greater r e s e m b l a n c e to the c e r i u m o x i d e system a n d for m a n y m i x e d oxides (12)—especially

i f one

assumes t h a t o r d e r i n g u p o n c o o l i n g is b l o c k e d i n some w a y . T h i s is n o t likely, however,

since the h i g h t e m p e r a t u r e d i s o r d e r e d phase

easily b e q u e n c h e d i n other systems.

B r e t t (18)

cannot

f o u n d t h a t samples

q u e n c h e d f r o m 900 ° C . i n l i q u i d n i t r o g e n r e s u l t e d i n a - P u 0 2

and P u 0 .

3

2

T e n s i m e t r i c studies w o u l d h e l p to settle t h e q u e s t i o n of phase r e l a t i o n ships i f one

c o u l d achieve e q u i l i b r i u m r a p i d l y enough

at

reasonable

temperatures a n d the e x t r e m e l y l o w pressures r e q u i r e d . M a r k i n et al. (38)

h a v e d e t e r m i n e d the e.m.f. of h i g h t e m p e r a t u r e

g a l v a n i c cells i n v o l v i n g the p l u t o n i u m o x i d e - o x y g e n system.

T h e plots

of p a r t i a l m o l a l free e n e r g y of o x y g e n vs. t e m p e r a t u r e s h o w a p r o f o u n d c h a n g e i n the c o m p o s i t i o n i n t e r v a l 1.691 a n d 1.812.

I n m a n y respects

the b e h a v i o r of PuO# is q u i t e s i m i l a r to C e O ^ . Americium.

Some years ago A s p r e y a n d C u n n i n g h a m ( 7 )

the t h e r m a l d e c o m p o s i t i o n of P r 0 volume.

2

and A m 0

2

studied

i n a calibrated reaction

T h i s w o r k p r e s a g e d the t e n s i m e t r i c w o n d e r of the

PrO -0 i P

s y s t e m a l r e a d y discussed a n d i n d i c a t e d the r e l a t i v e s i m p l i c i t y of AmOar0

2

system i n the accessible t e m p e r a t u r e range.

to lose o x y g e n s m o o t h l y to a c o m p o s i t i o n of A m O i .

8 5

Am0

2

2

the

appeared

at 1 4 0 0 ° C . w h e r e

it h a d a n e q u i l i b r i u m o x y g e n pressure of 13 m m . H g i n contrast to the m u c h easier b u t i n t e r r u p t e d loss for P r O ^ r e s u l t i n g f r o m the s e v e r a l stable i n t e r m e d i a t e phases. A s p r e y ' s curves d o s h o w some m i n o r breaks i n t h e l o g ρ vs. l/T plots w i t h a c h a n g e of slope at A m O i .

8 7 7

i n one of the r u n s .

It is n o t at a l l c e r t a i n that these are significant since t h e y w e r e n o t cor r o b o r a t e d i n the w o r k d i s c u s s e d b e l o w . A recent s t u d y b y C h i k a l l a a n d E y r i n g (21) of the A m O * system, i n c l u d i n g t e n s i m e t r i c a n d x - r a y d i f f r a c t i o n measurements, has b e e n c o m p l e t e d . T h e results agree w i t h A s p r e y ' s measurements w i t h i n t h e e x p e c t e d a c c u r a c y of his w o r k . A r e v e r s i b l e single phase A m 0 . r e g i o n for 0 < χ < 0.2 is o b s e r v e d at 1 1 7 2 ° C . a n d 0 < x < 0.007 at 866° C , w h i c h w e r e t h e l i m i t s of c o m p o s i t i o n a v a i l a b l e to the i s o t h e r m a l t e n s i m e t r i c t e c h n i q u e . A s t r i k i n g feature of the isotherms is the d i s t i n c t c h a n g e i n slope w h i c h occurs at a c o m p o s i t i o n of A m O i . at 8 6 6 ° C . T h e b r e a k occurs at d e c r e a s i n g c o m p o s i t i o n s r e a c h i n g A m O i . at 1 1 7 2 ° C . T h i s feature is r e m i n i s c e n t of a b r e a k i n the c u r v e of l a t t i c e p a r a m e t e r vs. c o m p o s i t i o n 2

i P

9 9

9 7


6.

EYRING

Oxide

77

Phases

of the t e r n a r y o x i d e C e ^ Y i . ^ O ^ at M O i

9 5

w h i c h was considered b y B e v a n

to be c a u s e d b y a t r a n s i t i o n f r o m r a n d o m to c o m p l e x e d defects.

et al. (12)

T h e c h a n g e is not s h a r p b u t occurs r e v e r s i b l y over a c o n s i d e r a b l e pressure r a n g e i n a l l cases. T h i s b r e a k n a t u r a l l y shows u p i n the d e r i v e d t h e r m o d y n a m i c q u a n t i t i e s , AS a n d AH.

C h i k a l l a also points out t h a t the i m p u r i t y

c o n c e n t r a t i o n w o u l d u n d o u b t e d l y affect the b e h a v i o r i n the r e g i o n


2.0 > χ >

AmO , x

1.99.

X - r a y d i f f r a c t i o n studies b y C h i k a l l a (21)

on cooled and quenched

samples s h o w t h a t the single phase A m O , 1.8

< χ < 2.0, stable at h i g h

r

t e m p e r a t u r e s , d i s p r o p o r t i o n a t e s too r a p i d l y to b e q u e n c h e d ; a l l the r o o m temperature diagrams show two p h a s e s — A m 0 . o o and A m O i . g .

I n the

2

r a n g e A m O i > , Ο < χ < 0.20, a c o n t i n u o u s l y c h a n g i n g C - t y p e phase is r

+a

o b s e r v e d as s h o w n b y the c o n t i n u a l shift i n the l a t t i c e p a r a m e t e r as a f u n c t i o n of c o m p o s i t i o n .

B o t h c o o l e d a n d q u e n c h e d samples g i v e the

same results; e v e n one q u e n c h e d f r o m 4 6 0 ° C . seems to b e i n the s i n g l e phase r e g i o n . I n terms of the r a r e e a r t h oxides a m i s c i b i l i t y g a p b e t w e e n A m 0 2

A m O i . s exists, w i t h some c o m p l i c a t i o n s i n the A m O i . n to A m O i

region.

8

B r o a d σ a n d a regions exist at h i g h t e m p e r a t u r e a n d m a y be s e p a r a t e d b y a miscibility gap. C u r i u m . A c e n t r a l p r o b l e m i n the c u r i u m - o x y g e n system is the l a c k of p r e c i s e k n o w l e d g e of the c o m p o s i t i o n s for the phases w h i c h h a v e b e e n observed.

G e n e r a l l y , it has b e e n a s s u m e d t h a t there is a n a n a l o g y b e

t w e e n this system a n d those d i s c u s s e d above.

The

fluorite-type

phase

w i t h the smallest l a t t i c e p a r a m e t e r is c o n s i d e r e d to b e C m 0 , a n d the 2

most f u l l y r e d u c e d C - t y p e o x i d e is a s s u m e d to h a v e the Cm0

1 5

composition

.

W a l l m a n n ( 4 9 ) has p r e p a r e d C - t y p e C m 0 2

3

(a =

11.01 ±

0.01)

by

igniting c u r i u m nitrate on a p l a t i n u m plate i n air (this yields a black intermediate oxide), followed by reduction w i t h purified hydrogen t e m p e r a t u r e s f r o m 600° to 8 5 0 ° C .

at

T h e C - t y p e s e s q u i o x i d e transforms

s p o n t a n e o u s l y at r o o m t e m p e r a t u r e i n a f e w days to the h e x a g o n a l A - t y p e s e s q u i o x i d e (a — 3.80 ± 0.02A., c — 6.00 ±

0.03A.), presumably a result

of r a d i a t i o n effects. U s i n g a n a u t o m a t i c r e c o r d i n g t h e r m a l b a l a n c e P o s e y et al. (41,

42)

h a v e m a d e i s o b a r i c a n d i s o t h e r m a l studies i n d i c a t i n g the existence regions

of

stability

having

approximate

compositions

C m O i . 8 2 , as w e l l as phases of v a r i a b l e c o m p o s i t i o n C m O i

5 + A

CmOi

7 i

of

and

and C m 0 . .

T h e breaks are not u s u a l l y s h a r p ; hence, the stable phases s h o w

2

x

an

a p p r e c i a b l e range of c o m p o s i t i o n e s p e c i a l l y for the C m O i . g a to C m O i . r e g i o n w h i c h m a y i n v o l v e s e v e r a l different phases.


7 8

78

LANTHANIDE/ACTINIDE

CHEMISTRY

A c o m p l e t e i s o b a r at 159 m m . of H g shows the n o r m a l breaks at C m O i . 8 a n d C m O u i n the r e d u c t i o n h a l f of the c y c l e b u t e x h i b i t s a n extreme hysteresis l o o p i n the o x i d a t i o n p a r t of the c y c l e w h i c h d i d not close u n t i l a c o m p o s i t i o n near C m 0

was r e a c h e d .

2

T h i s b e h a v i o r , to a

lesser degree, is e x h i b i t e d b y the rare e a r t h o x i d e systems discussed a b o v e . I s o t h e r m a l measurements

seem to c o n f i r m the m o r e expressive

iso


b a r i c results a n d even i n d i c a t e a p o s s i b l e c o m p l e x i t y i n the C m O i . s o - i . s a region. Smith

has o b s e r v e d a C m O ,

(47)

lattice parameter

(a

fluorite-type

r

5.38 to 5 . 5 2 A . ) .

=

entire c o m p o s i t i o n r a n g e b e t w e e n C m 0

2

phase of

variable

T h i s p r o b a b l y represents

the

a n d C m O i — t h e d i a g r a m s not 5

b e i n g g o o d e n o u g h to see s u p e r s t r u c t u r e lines c h a r a c t e r i s t i c of a C - t y p e , σ phase or a n y of the other i n t e r m e d i a t e phases. s e r v e d a — 2 ( 5 . 5 0 ) for C - t y p e

( W a l l m a n n (49)

ob

Cm 0 ). 2

3

P e r h a p s w i t h s u c h intense r a d i o a c t i v i t y c o m p l e t e o r d e r i n g is i m p o s sible, a n d a l t h o u g h the t e n s i m e t r i c measurements

definitely show

the

g r e a t l y i n c r e a s e d s t a b i l i t y at c e r t a i n concentrations the o r d e r is not w e l l e n o u g h e s t a b l i s h e d to s h o w u p i n the x - r a y d i a g r a m s . o b s e r v e d i n the m i x e d C e

0 2

Tb

0 8

0.

r

T h i s b e h a v i o r is

system w h e r e the presence of

Ce

prevents the c o m p l e t e o r d e r i n g necessary to g i v e r e s o l u t i o n i n the h i g h t e m p e r a t u r e x - r a y d i f f r a c t i o n patterns (20),

b u t the t e n s i m e t r i c m e a s u r e

ments i n d i c a t e u n m i s t a k a b l y the f o r m a t i o n of the t phase Ternary Actinide-Lanthanide

Oxide

(33).

A n interesting

Phases.

exists b e t w e e n the o r d e r e d o x i d e phases of the l a n t h a n i d e elements

link on

the one h a n d a n d the a c t i n i d e oxides o n the other. B a r t r a m describes the p r e p a r a t i o n a n d structures of U0

3

' 3R 0 2

3

t e r n a r y oxides h a v i n g the

composition

or U R O i , w h e r e R represents a rare e a r t h a t o m 6

2

T h e s e structures are i s o m o r p h o u s w i t h the R O 7

cussed for the b i n a r y C e , P r , a n d T b oxides.

i 2

(10).

phases p r e v i o u s l y d i s

I n the t e r n a r y oxides the

u r a n i u m atoms fill a l l the m e t a l positions a l o n g the strings a n d the oxygens are s h i f t e d i n t o w a r d the v a c a n t sites a l o n g the s t r i n g . T h e R atoms are seven c o o r d i n a t e d

i n the sheaths s u r r o u n d i n g the strings as i n d i c a t e d

above.

Observed Trends The

fluorite-related

o x i d e phases w h i c h are k n o w n i n the l a n t h a n i d e

a n d a c t i n i d e series are d i s p l a y e d i n T a b l e I I for closer c o m p a r i s o n .

The

m o s t o b v i o u s feature is that the o x i d e systems of C e , P r , a n d T b r e v e a l greater c o m p l e x i t y t h a n a n y of the a c t i n i d e elements so far s t u d i e d . d i o x i d e s of the a c t i n i d e elements from T h 0 thanides.

2

to C m 0

2

are m o r e easily r e d u c e d as one

The goes

s h o w i n g a n a p p r o a c h to the b e h a v i o r of the l a n

M o r e c o m p l e t e measurements

on C m O

x

a n d BkO

x

may well

s h o w m a r k e d s i m i l a r i t y to the rare earths.


6.

EYRING

Oxide

79

Phases

A l l 12 elements l i s t e d i n T a b l e I I f o r m d i o x i d e s of the same structure. M o s t o f t h e m f o r m sesquioxides h a v i n g t h e Α - ,Β-, or C - t y p e r a r e e a r t h structure a n d demonstrate s i m i l a r p o l y m o r p h i s m . T h e analogous i n t e r m e d i a t e oxides are also o b v i o u s l y possible, a n d the R„0 n-2

homologous

2

series c o u l d exist. T h e q u e s t i o n as to w h e t h e r s u c h phases are i n fact o b s e r v e d depends e n t i r e l y o n the f o r m of t h e free e n e r g y surface. T h e


factors d e t e r m i n i n g t h e shape a n d r e l a t i v e positions of t h e free m i n i m a of e a c h phase are n o t u n d e r s t o o d . observed

energy

E v e n metastable phases are

to b e f o r m e d u n d e r circumstances w h e r e o r d e r i n g is m u c h

s l o w e r t h a n t r a n s f o r m a t i o n to another d i s o r d e r e d phase (i.e., σ

ηι

—>

a

m

inPrCy. F i g u r e 6 shows t h e l a t t i c e p a r a m e t e r s vs. c o m p o s i t i o n of t h e phases d i s c u s s e d above.

F o r t h e C - t y p e sesquioxides or σ phases t h e p s e u d o \

5.600

1

I

i

I

\

_Ce

_

-

\ ο

° Th "

Ν. Ο

5.500

Pu "Am 1 \ N . • Cm°

° Pa -

-

·\

\

-

°u >v ° Np Ce ' \ / \ Pu " \ Pr > Am " °Cm

•

5.400 -

-

.Tb

°Bk -

ο

5.300

ο

°Tb 5.200

J-

ι

I

1.5

1.6

1.7

_,J

1.8

I

1.9

i-

2.0

x i η ROv Figure 6. Lattice of some hnthanide

parameters of fuorite-related oxides and actinide elements as a function of composition


LANTHANIDE /ACTINIDE

80

fluorite

CHEMISTRY

c e l l d i m e n s i o n is u s e d for c o m p a r i s o n . T h e l a t t i c e parameters of

a l l the i n t e r m e d i a t e oxide phases l i e a b o v e a straight l i n e j o i n i n g values f o r the d i o x i d e a n d C - t y p e sesquioxide. Existence Diagram for R 0 - 2 Phases of Some Lanthanide and Actinide Elements

Table II.

n

2 u


Phase

A

Type η

4

x(ROJ

Β

C

σ

4

4

1.5 < χ

1.50

X X X

Ce Pr Tb U/Y Th Pa U Np Pu Am Cm Bk

Actinide Chemistry

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