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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
<
x < 2.0, gave e a r l y s u p p o r t to the a c t i n i d e h y p o t h e s i s , b u t n o w a m o r e c a r e f u l s c r u t i n y of the details is possible a n d s h o u l d l e a d to a greater a p p r e c i a t i o n of the q u a l i t y of the v a r i a t i o n i n c h e m i c a l b e h a v i o r a l o n g 67
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
68
LANTHANIDE/ACTINIDE
CHEMISTRY
the t w o series. W e are not d i s c o u r a g e d b y the fact t h a t oxides i n this c o m p o s i t i o n r a n g e b e h a v e n e i t h e r l i k e l a n t h a n u m n o r a c t i n i u m oxides. P e r h a p s of greater significance is that this d o u b l e sequence of c o m p l e x oxides w i l l p l a y a c e n t r a l r o l e i n e l u c i d a t i n g the n a t u r e of n o n s t o i c h i o m e t r y i n c h e m i c a l systems.
T h e p h e n o m e n a of o r d e r e d i n t e r m e d i a t e
phases of n a r r o w c o m p o s i t i o n
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phases
of
(homologous series), nonstoichiometric
widely variable composition
resulting from
order-disorder
transformations, a n d c h e m i c a l hysteresis are e x h i b i t e d a m o n g t h e m w i t h u n u s u a l v a r i a t i o n . T h e entire q u e s t i o n of the n a t u r e of n o n s t o i c h i o m e t r i c b e h a v i o r is r a p i d l y a p p r o a c h i n g a n e w l e v e l of u n d e r s t a n d i n g , a n d c o n t i n u e d c a r e f u l d e v e l o p m e n t of k n o w l e d g e of these oxides is necessary i n e v o l v i n g the t o t a l p i c t u r e . T h e state of k n o w l e d g e of these o x i d e systems is not yet r e a d y for a f u l l r e v i e w . R a t h e r , this d i s c u s s i o n sketches the present state of k n o w l e d g e w h i c h is f a i r l y c o m p l e t e for some o x i d e systems, s k i m p y for others, a n d v i r t u a l l y nonexistent for a f e w , w i t h the h o p e that i t s h a l l b e c o n t i n u e d u n t i l this m o t h e r l o d e s h a l l h a v e b e e n e m p t i e d of h e r treasure. Studies a n d discussions of the past several years h a v e l e d to the r e a l i z a t i o n t h a t b i n a r y i n o r g a n i c c o m p o u n d s of elements e x h i b i t i n g a l t e r v a l e n t c h a r a c t e r , s u c h as the t r a n s i t i o n m e t a l oxides, t e n d to s h o w a n u n m i x i n g of p r e v i o u s l y o b s e r v e d ranges of c o m p o s i t i o n i n t o a sequence of o r d e r e d phases b e l o n g i n g to a n h o m o l o g o u s series w h o s e structures are r e l a t e d b y some s i m p l e s t r u c t u r a l p r i n c i p l e . T h e earliest i n d i c a t i o n s of this w e r e seen b y Magnéli (36, 37) o n the m o l y b d e n u m - o x y g e n
system
a n d h a v e b e e n e n l a r g e d a n d e x t e n d e d b y h i m a n d his colleagues to a w i d e r a n g e of t r a n s i t i o n m e t a l oxides. A g e n e r a l r e v i e w of the extent of s u c h studies a n d the s t r u c t u r a l p r i n c i p l e s i n v o l v e d has b e e n p r e s e n t e d b y W a d s l e y (48),
w h o has greatly e x t e n d e d o u r k n o w l e d g e of o r d e r e d i n t e r -
m e d i a t e phases.
N o n s t o i c h i o m e t r y has b e e n discussed i n d e t a i l over the
years b y A n d e r s o n (5, 6), w h o has r e c e n t l y s y n t h e s i z e d the extant ideas. T h e g e n e r a l p a t t e r n of b e h a v i o r of m e t a l oxides c a p a b l e of v a r i a t i o n i n c o m p o s i t i o n is to f o r m a h o m o l o g o u s series of phases h a v i n g some s i m p l e r e l a t i n g s t r u c t u r a l p r i n c i p l e , a n d d i s o r d e r e d phases of w i d e c o m p o s i t i o n r a n g e i n w h i c h the d i s o r d e r is not at the l e v e l of p o i n t defects b u t r a t h e r of d e f e c t complexes or d o m a i n s of the o r d e r of five to 20 u n i t cells i n l i n e a r d i m e n s i o n — i . e . , there is short- b u t not l o n g - r a n g e order.
Coherence
m a k e s s u c h phases t h e r m o d y n a m i c a l l y stable. T h e o c c u r r e n c e of h y s t e r e sis, the m a g n i t u d e of the t h e r m o c h e m i c a l p r o p e r t i e s , as w e l l as c o n s i d erations of the e n e r g y r e q u i r e m e n t for l a r g e concentrations defects, suggest that s u c h d o m a i n structures exist (11,
of
point
28).
It m u s t b e k e p t i n m i n d that each n o n s t o i c h i o m e t r i c system is u n i q u e , a n d t h a t e a c h o r d e r e d phase, e a c h p o l y m o r p h , a n d e a c h d i s o r d e r e d phase has its o w n range of s t a b i l i t y .
W h e n e v e r tangents to the free
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
energy
6.
EYRING
Oxide
curves (G,x)
69
Phases
for the v a r i o u s stable phases c a n be c o n s t r u c t e d , t w o - p h a s e
regions are i n d i c a t e d . S u c h t w o - p h a s e regions are u s u a l l y r e a l i z e d o n l y for the lowest surfaces i n a n y r e g i o n of Τ a n d x. T h i s means t h a t n o t w o o x i d e systems are e x p e c t e d to b e i d e n t i c a l b u t w i l l v a r y as the s t a b i l i t y ranges of a l l possible species dictate. It is c o n v e n i e n t to discuss the most t h o r o u g h l y s t u d i e d o x i d e systems a n d e s t a b l i s h a n o m e n c l a t u r e for the
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phases w h i c h c o u l d b e e x p e c t e d
i n e a c h of the o x i d e systems of
the
l a n t h a n i d e a n d the a c t i n i d e elements.
Fluorite Related Phases in the Rare Earth Oxides C o m p r e h e n s i v e studies of the P r O - 0 x
2
system h a v e c u l m i n a t e d i n a
d e t a i l e d p a p e r b y H y d e et al. ( 3 0 ) , w h o r a t i o n a l i z e p r e v i o u s w o r k a n d d e s c r i b e i s o b a r i c studies w h o s e results w e r e a p o w e r f u l t o o l i n e x p o s i n g the i n t r i c a c i e s of that system.
The T b O - 0 x
s t u d i e d b y H y d e a n d E y r i n g (32) results of p r e v i o u s TbO -Q x
(13).
2
2
system has l i k e w i s e b e e n
u s i n g the i s o b a r i c t e c h n i q u e a n d the
studies r e v i e w e d
a n d discussed.
system is c o m p a r e d w i t h that of P r O - 0 x
References
I n a d d i t i o n , the (30)
2
and C e O - 0 x
details a n d for a d i s c u s s i o n of p r e v i o u s w o r k .
B r a u e r (15,
has r e
16)
c e n t l y r e v i e w e d c u r r e n t studies o n a l l the rare e a r t h oxides.
0
2
13, 30, a n d 32 s h o u l d b e c o n s u l t e d for e x p e r i m e n t a l
10
1.50
20
30
Weight change, mg 40 50 60 70
1.60
1.70
1.80
Projection
of the PrO ~O x
z
90
1.90
Composition, χ in PrO Figure 1.
80
100
2.00
x
phase diagram
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
70
LANTHANIDE/ACTINIDE A phase d i a g r a m of the system P r O , 1.5 < a ;
χ <
CHEMISTRY
2 . 0 , is s h o w n i n
F i g u r e 1. A p p a r e n t i n this d i a g r a m are a n h o m o l o g o u s series ( R » 0 „ _ , 2
η =
2
4 , 7 , 9 , 1 0 , 1 1 , 1 2 ) of phases of n a r r o w c o m p o s i t i o n range w h i c h d e
c o m p o s e to g i v e d i s o r d e r e d n o n s t o i c h i o m e t r i c phases at h i g h e r t e m p e r a ture. P h a s e designations a d o p t e d for this system, s h o w n i n T a b l e I, s h a l l
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b e u s e d h e r e to i n d i c a t e phases b e l i e v e d to b e i s o m o r p h o u s or analogous. Table I.
Members of the R C>2n - 2 Series and Disordered Phases of the Rare E a r t h Oxides
ue of η 4
n
Stoichiometric formula C e 0 (hex.) P r 0 (hex.) T b O (hex.) T b 0 (moncl.) P r 0 (b.c.c.) T b 0 (b.c.c.) CeOi ± δ PrO ± δ TbO ± δ Ce 0 Pr 0 Tb 0 Ce 0 Pr»O Ce 0 Pr O 2
3
2
Phase symbol
χ in RO
w
1.500
A
1.500 1.500
Β C
1.5-1.7
σ
3
2
s
2
3
2
3
2
3
6
i e
l e
7
7
1 2
7
9
1.714
ι
1 2
7
1 2
9
1 6
1.778
ζ
1.800
e
1.818
δ
1.833
β
1.7-2.0
a
2.000
F
l e
10
5
9
B
e
11 Pr O TbuO Pr O Tb O (?) CeOi j + δ PrO + δ TbOx.7 + δ ( ? ) Ce0 Pr0 TbQ n
2 0
6
n
2 0
12
e
n
x
00
7
2
2
2
Praseodymium
dioxide
c r y s t a l l i z e s i n the
fluorite-type
structure
( s p a c e g r o u p F m 3 r a ) w i t h f o u r p r a s e o d y m i u m atoms a n d e i g h t o x y g e n atoms p e r u n i t c e l l . T h i s s t r u c t u r e m a y b e v i s u a l i z e d e a s i l y as a n infinite a r r a y of c o o r d i n a t i o n cubes ( e a c h c o n s i s t i n g of a P r a t o m at the center w i t h e i g h t Ο atoms at the c o r n e r s )
s t a c k e d so that a l l c u b e edges are
shared. If one v i e w s this c o n f i g u r a t i o n at r i g h t angles to the b o d y d i a g o n a l of one of the cubes ( p e r p e n d i c u l a r to the < 1 1 1 > 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
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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.
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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
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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
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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
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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 -
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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 +
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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 .
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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
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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
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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
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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
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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
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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.
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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
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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.
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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
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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
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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
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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
<
1.7
e
2
3
+
1.7
9
10
11
12
1.80
1.82
1.83
Χ
Χ χ
X X χ
Χ
Χ χ χ χ
χ
|0
? ?
χ χ χ
?
χ
?
=
? χ
change
χ
00
2.00
Χ Χ ρ
Χ χ χ χ χ χ χ χ χ χ χ χ
?
χ
X X X
2
<
w i t h the c h a n g e i n the heats of a t o m i z a t i o n of the L
respective oxides p e r e q u i v a l e n t 0
2
( F i g u r e 7 ) illustrates w e l l the o b
s e r v e d trends i n s t a b i l i t y . A difference of 10 k c a l . p e r e q u i v a l e n t of o x y g e n i n the energies of a t o m i z a t i o n corresponds to o x y g e n d i s s o c i a t i o n pressure differences of a b o u t 100 orders of m a g n i t u d e . T h e o x y g e n d i s s o c i a t i o n pressure of a p a r t i c u l a r oxide c o m p o s i t i o n shows a definite t r e n d w i t h i n e a c h series i n c r e a s i n g w i t h a t o m i c n u m b e r . T h e order, i n the r e g i o n of o v e r l a p of the t w o series, is i n d i c a t e d b y the o x y g e n d i s s o c i a t i o n pressure, at some t e m p e r a t u r e , of R 0
1 7
i
4
w h i c h increases i n the o r d e r P u , C e , A m ,
Pr, T b , and C m . C h e m i s t s , a c c u s t o m e d to the vagaries of the t w o series of elements, r e a l i z e that facts f o r b i d a n y s i m p l e p r o g r e s s i o n i n properties. T h i s is true for the l a n t h a n i d e elements because of the p e r t u r b a t i o n of
properties
a c c o m p a n y i n g the s t a b i l i z i n g effect o n the electrons of a n e m p t y , h a l f filled
a n d f u l l 4/ shell. It is a c c e n t u a t e d i n the case of the actinides w h e r e
one has 6d, 7s, a n d 5/ levels so close together i n e n e r g y that the e a r l y m e m b e r s of the series b e h a v e as t h o u g h they h a d no / electrons.
However,
as one progresses d o w n the a c t i n i d e series the / electrons d e f i n i t e l y be c o m e less a v a i l a b l e for b o n d i n g as t h e y do i n the 4f elements. A c k e r m a n n a n d T h o r n ( 4 ) e m p h a s i z e the i m p o r t a n c e , i n i n t e r p r e t i n g h i g h t e m p e r a t u r e v a p o r i z a t i o n results, of k n o w i n g the c o m p o s i t i o n a n d
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
6.
EYRING
Oxide
81
Phases
n a t u r e of the s o l i d phase. E v e n T h 0 t i o n at 2 5 0 0 ° C .
(4).
2
lost o x y g e n i n c o n g r u e n t v a p o r i z a
A l l other a c t i n i d e d i o x i d e s s t u d i e d h a v e a h i g h e r
o x y g e n d i s s o c i a t i o n pressure t h a n T h 0
2
at the same t e m p e r a t u r e a n d
w o u l d b e e x p e c t e d to f o r m a n a phase.
T h i s was o b s e r v e d to a great
degree i n U 0 . , s t r o n g l y suggested i n N p 0 . , , a n d c l e a r l y d e m o n s t r a t e d 2
x
2
r
in P u 0 _ , A m 0 . , and C m 0 _ . In addition, C m 0 . 2
x
2
x
2
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i n t e r m e d i a t e phases at C m O i , +3
and + 4
x
2
and C m O i .
7 1
8
valences w i t h a 5f
s h o w e d stable
x
T h e fact that C m shows
c o n f i g u r a t i o n for C m
3 +
emphasizes the
greater a v a i l a b i l i t y of the 5f electrons for b o n d i n g t h a n exists i n the r a r e earths w h e r e G d
3 +
w i t h 4f electrons forms o n l y the sesquioxide. 1
Ι
ΐ
ι
ι
Pu
45
-
40
V
Ce
-
35
\
30
Am° 25
°o
\
20
15
-
10
-
\ \
OPr
Tb 1 4
12
ι
I
16
18
Δ (Δ H ) A T O M
ι
20
1
22
IZATION
Figure 7. Free energy change for the reaction R0 (c) = V2R O (c) + V4 0 (g) correlated with the differences in atomization energy per equiv alent of oxygen for R0 (c) and R 0 (c). All values are given in kcal. per equivalent 2
2
s
2
2
2
3
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
82
LANTHANIDE /ACTINIDE
T h e structural principles enumerated
a b o v e for
the
CHEMISTRY
intermediate
phases of the r a r e earths, i n c l u d i n g the a phase, p r e s u p p o s e a s t r u c t u r e w h i c h w o u l d be better c h a r a c t e r i z e d b y o x y g e n vacancies t h a n b y m e t a l interstitials. I t is t a c i t l y a s s u m e d t h a t the a phases of t h e a c t i n i d e oxides w o u l d be analogous.
However, Atlas and Schlehman (8)
have studied
t h e pressure d e p e n d e n c e of e l e c t r i c a l c o n d u c t i v i t y o n c o m p o s i t i o n
and
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o x y g e n pressure a n d c o n c l u d e f r o m m a s s - a c t i o n analysis that the defects i n P u 0 - ^ i n the t e m p e r a t u r e r e g i o n 1 1 0 0 ° - 1 6 0 0 ° C . are p r e d o m i n a n t l y 2
i n t e r s t i t i a l p l u t o n i u m ions, u s u a l l y P u . 4 +
I n this t r e a t m e n t the a r b i t r a r y
a s s u m p t i o n is m a d e t h a t the defects d o n o t interact. T h e d a t a c o v e r t h e c o m p o s i t i o n r a n g e P u 0 . o - i 8 , w h e r e one w o u l d expect extensive defect 2
i n t e r a c t i o n s . C h i k a l l a ( 2 1 ) has c o n s i d e r e d the v a r i o u s attempts to determ i n e the t y p e of defects i n the o x y g e n deficient fluorite phases, P u 0 _ 2
and A m 0
CeOy-œ,
2
. H e
concludes
that
although
the
Xy
mass-action
t r e a t m e n t agrees i n a l l cases w i t h a m e t a l i n t e r s t i t i a l m o d e l a n o x y g e n v a c a n c y m o d e l m i g h t e x p l a i n the results as w e l l , e s p e c i a l l y i f t h e defects are p a i r e d or c o m p l e x e d . defect concentration
O n the other h a n d there m a y b e no a p p r e c i a b l e
at a l l i n a w e l l a n n e a l e d a phase
with
domain
structure. A s a m a t t e r of p r i n c i p l e a n y of the systems s h o w i n g b o t h the s e s q u i oxide
( e v e n the C - t y p e )
a n d the
miscibility gap somewhere. t h e a phase.
fluorite-type
dioxide must exhibit a
F o r this reason the σ is d i s t i n g u i s h e d f r o m
I n a l l cases w h e r e t h o r o u g h studies h a v e b e e n p o s s i b l e , a
g a p has i n d e e d b e e n f o u n d . S o l i d s o l u b i l i t y i n r a r e e a r t h o x i d e systems has b e e n s t u d i e d e x t e n s i v e l y . T h e r e is n o d o u b t of c o m p l e t e s o l u b i l i t y , often c a l l e d i s o m o r p h o u s r e p l a c e m e n t b e t w e e n o x i d e phases of the same s y m m e t r y as i n m i x t u r e s of P r 0
and C e 0
2
2
or i n the sesquioxides of one t y p e .
V e g a r d ' s l a w appears to h o l d p r e c i s e l y
I n these cases
(40).
B e c a u s e of the close r e l a t i o n s h i p b e t w e e n the C - t y p e a n d the
fluorite-
t y p e structures m u c h a t t e n t i o n has b e e n g i v e n to the r a n g e of s o l u t i o n between
phases
h a v i n g these structures.
i n d i c a t e d that i n solutions i n v o l v i n g C e 0
Brauer and Gradinger 2
or T h 0
r a r e earths, p h a s e s e p a r a t i o n is m o s t c o m m o n ,
2
(17)
w i t h the t r i v a l e n t
b u t there w e r e
w h i c h a p p e a r e d to f o r m c o m p l e t e s o l i d s o l u t i o n (i.e., C e 0
2
cases
with S m 0 , 2
3
G d 0 , or D y 0 ) over the entire r a n g e of c o m p o s i t i o n . 2
3
2
3
T h i s is apropos
of the b a s i c q u e s t i o n of w h e t h e r or not one m a y go
continuously between
different structures h a v i n g different
I n p r i n c i p l e this s h o u l d n o t b e p o s s i b l e (19, 34);
symmetries.
h o w e v e r , c e r t a i n cases
h a v e b e e n r e p o r t e d , a n d the one m e n t i o n e d a b o v e is u s e d b y Z e r n i k e (51)
against the b a s i c p h i l o s o p h i c a r g u m e n t s . B e v a n (12)
a n d his colleagues h a v e s t u d i e d p r e c i s e l y those systems
r e p o r t e d to b e s i n g l e phase b y B r a u e r a n d G r a d i n g e r a n d h a v e
Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
shown
6.
EYRING
Oxide
83
Phases
t h e m to b e at least t w o phase. A m i s c i b i l i t y gap at about 6 0 %
sesquioxide
is o b s e r v e d i n e a c h case. T h i s removes one of the p r i n c i p l e a r r o w s f r o m the b o w of those w h o w o u l d argue against the r e q u i r e m e n t s of
basic
t h e o r y a n d raises the q u e s t i o n as to w h e t h e r , i n fact, there are
any
exceptions. T h i s is a q u e s t i o n w h i c h c a n o n l y b e settled b y p r i n c i p l e since more a n d more careful structural determinations m a y be made a n d c o u l d
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r e v e a l n a r r o w t w o - p h a s e regions. C h i k a l l a (21)
has c o n s i d e r e d the p o s s i b i l i t y t h a t r a d i a t i o n d a m a g e
is a m a j o r cause of f a i l u r e to o r d e r a m o n g the i n t e r m e d i a t e a c t i n i d e oxides. T h i s w i l l c e r t a i n l y b e a factor against o r d e r i n g , b u t the d e c i d e d l y greater t e n d e n c y of the short l i v e d C m O to s h o w o r d e r i n g indicates t h a t x
it is o n l y one of m a n y factors. X - r a y d i f f r a c t i o n patterns m a y f a i l to r e v e a l the presence of o r d e r e d i n t e r m e d i a t e phases i n C m O b e c a u s e of the h i g h x
radiation.
H o w e v e r , t e n s i m e t r i c measurements m a y r e v e a l order
x - r a y measurements f a i l , as w a s true for the C e T b i . O y
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 measurements
y
x
system
where (33).
u s i n g h i g h intensity
tubes m i g h t be r e q u i r e d to see the true s t r u c t u r a l changes at t e m p e r a t u r e a n d at e q u i l i b r i u m for h i g h l y r a d i o a c t i v e m a t e r i a l s .
Acknowledgment T h i s p a p e r is d e d i c a t e d to James C . W a l l m a n n , a c t i n i d e e x p e r i m e n talist e x t r a o r d i n a r y w h o s e u n t i m e l y d e a t h r e m o v e d a d e v o t e d
scientist
f r o m his l a b o r a t o r y i n the p e a k of his p r o d u c t i v i t y . T h e loss to science is great a n d to his f a m i l y a n d friends is w i t h o u t relief. T h i s w o r k was p e r f o r m e d u n d e r the auspices of the A t o m i c E n e r g y C o m m i s s i o n w h o s e resources
a n d far s i g h t e d p o l i c i e s h a v e m a d e this
p a p e r a n d most of the w o r k d i s c u s s e d h e r e i n possible.
R. T . Sanderson
a n d J . O . S a w y e r h a v e h e l p f u l l y discussed c e r t a i n aspects of this w o r k .
Literature Cited
(1) Ackermann, R. J., Faircloth, R. L., Rauh, E. G., Thorn, R. J., Inorg. Nucl. Chem. 28, 111 (1966). (2) Ackermann, R. J., Gilles, P. W., Thorn, R. J., J. Chem. Phys. 25, 1089 (1956). (3) Ackermann, R. J., Rauh, E. G., Thorn, R. J., Cannon, M.C.,J.Phys. Chem. 67, 762 (1963). (4) Ackermann, R. J., Thorn, R. J., Proc. Symp. Thermodynamics, IAEA Vienna 1965,I,243 (1965). (5) Anderson, J. S., ADVAN. CHEM. SER. 39, 1 (1963). (6) Anderson, J. S., Proc. Chem. Soc. p. 166 (1964). (7) Asprey, L. B., Cunningham, B. B., U. S. At. Energy Comm. Rept. UCRL329 (1949). (8) Atlas, L. M., Schlehman, G. J., Proc. Intern. Conf. on Plutonium, 3rd, London (1965). Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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84
LANTHANIDE/ACTINIDE CHEMISTRY
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(49) Wallmann, J. C.,J.Inorg. Nucl. Chem. 26, 2053 (1964). (50) Willis, Β. T. M., I. A. E. A. Symp., Vienna, 1964. (51) Zernike, J., "Chemical Phase Theory," p. 169, Ν. V. Uitgevers-Maat schappij AE. E. Kluwer Antwerp. RECEIVED October 17, 1966.
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