XPS - American Chemical Society

10 x-Ray Photoemission Spectroscopy (XPS) Study of Uranium, Neptunium, and Plutonium Oxides in Silicate-Based Glasses

Downloaded by NANYANG TECHNOLOGICAL UNIV on June 3, 2016 | http://pubs.acs.org Publication Date: May 19, 1983 | doi: 10.1021/bk-1983-0216.ch010

D. J. LAM, B. W. VEAL, and A. P. PAULIKAS Argonne National Laboratory, Materials Science and Technology Division, Argonne, IL 60439

Using XPS as the principal investigative tool, we are in the process of examining the bonding properties of selected metal oxides added to silicate glass. In this paper, we present results of XPS studies of uranium, neptunium, and plutonium in binary and multi -component silicate-based glasses. Models are proposed to account for the very diverse bonding properties of 6+ and 4+ actinide ions in the glasses. S i l i c a t e g l a s s e s a r e b e i n g c o n s i d e r e d as p o s s i b l e m e d i a f o r long term storage o f r a d i o a c t i v e wastes. A c t i n i d e elements are among t h o s e a c t i v e s p e c i e s w h i c h must be i s o l a t e d f r o m t h e b i o s p h e r e f o r many c e n t u r i e s . Thus i t i s i m p o r t a n t t o u n d e r s t a n d bonding p r o p e r t i e s o f a c t i n i d e s i n the g l a s s e s i n order t o f o r m u l a t e t h e most e f f e c t i v e s t o r a g e medium and t o d i s c o v e r p o t e n t i a l f a i l u r e modes. As p a r t o f a more e x t e n s i v e s t u d y o f m e t a l o x i d e s d i s s o l v e d i n s i m p l e g l a s s e s (1-5), we r e p o r t r e s u l t s f o r U, Np, and P u o x i d e s d i s s o l v e d i n s o d i u m s i l i c a t e g l a s s . F o r t h i s w o r k , we r e l y p r i m a r i l y o n XPS d a t a . V a l e n c e s t a t e s o f t h e i n c o r p o r a t e d i o n s can be q u i t e r e l i a b l y d e t e r m i n e d t h r o u g h a c a r e f u l e x a m i n a t i o n o f b i n d i n g e n e r g y s h i f t s , 5f e l e c t r o n i n t e n s i t i e s , a n d the s a t e l l i t e s t r u c t u r e a s s o c i a t e d w i t h core l e v e l e x c i t a t i o n ( I ) · For s u i t a b l y c o n c e n t r a t e d samples, the a c t i n y l s t r u c t u r e c a n be r e c o g n i z e d i n t h e s p l i t t i n g o f t h e An ( a c t i n i d e ) 6P3/2 l e v e l (2) · ( D a t a f o r h e x a v a l e n t Np o x i d e s a r e s i m i l a r t o r e s u l t s r e p o r t e d f o r U ^ compoounds ( 6 ) ) . These ( a c t i n y l ) r e s u l t s can a l s o be t e s t e d w i t h EXAFS s p e c t r a where t h e An-0 s e p a r a t i o n can be d i r e c t l y measured (7)· The s p e c t r a l a n a l y s e s , s o l u b i l i t y d a t a , and o b s e r v e d p r o p e r t y changes a s s o c i a t e d w i t h a d d i t i o n o f a c t i n i d e e l e m e n t s t o g l a s s e s have e n a b l e d u s t o s u g g e s t b o n d i n g m o d e l s o f An 4+ and 6+ i o n s i n s i l i c a t e glasses· +

0097-6156/83/0216-0145$06.00/0 © 1983 American Chemical Society

Carnall and Choppin; Plutonium Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Experimental The s a m p l e s were p r e p a r e d by m e l t i n g a p p r o p r i a t e amounts o f c o n s t i t u e n t o x i d e s w i t h powdered s o d i u m t r i s i l i c a t e g l a s s . To minimize the compositional v a r i a t i o n of t h e samples, a l a r g e b a t c h o f s o d i u m - t r i s i l i c a t e g l a s s was p r e p a r e d . T h i s g l a s s was t h e n u s e d t o p r e p a r e t h e Np and Pu g l a s s e s as w e l l as t h e m u l t i c o m p o n e n t g l a s s s a m p l e s . The g l a s s e s were m e l t e d i n a n a t m o s p h e r e o f f l o w i n g oxygen o r i n a r e d u c i n g a t m o s p h e r e o f c o n t r o l l e d CO/C0 r a t i o , a t 1250°C i n p l a t i n u m c r u c i b l e s f o r s e v e r a l h o u r s . The samples were c r u s h e d , t h o r o u g h l y m i x e d and r e m e l t e d s e v e r a l t i m e s t o i n s u r e h o m o g e n e i t y . X - r a y powder d i f f r a c t i o n p a t t e r n s were t a k e n f o r s e v e r a l r a n d o m l y s e l e c t e d samples t o check f o r t h e presence o f c r y s t a l l i n e p h a s e s . A f t e r t h e f i n a l m e l t i n g , t h e samples were v i s u a l l y examined f o r g r o s s nonuniformity of color d i s t r i b u t i o n . I f f o u n d , t h e sample was c r u s h e d and r e m e l t e d . Most o f t h e samples s t u d i e d were f u r n a c e c o o l e d f r o m 1250°C t o room t e m p e r a t u r e . To c h e c k t h e e f f e c t o f c o o l i n g r a t e on t h e XPS s p e c t r a , s e v e r a l s a m p l e s were a i r quenched and r e a n n e a l e d a t 400°C. No a p p a r e n t d i f f e r e n c e s were f o u n d between t h e f u r n a c e c o o l e d and r e a n n e a l e d specimens· Two b a r s h a p e d samples m e a s u r i n g a p p r o x i m a t e l y 3x3x15 mm were c u t f r o m t h e i n g o t s w i t h a diamond w a f e r i n g saw u s i n g m e t h a n o l a s coolant. The b a r s were s c o r e d and were s u b s e q u e n t l y f r a c t u r e d i n t h e u l t r a - h i g h ( 6 x l 0 ~ ^ T o r r ) vacuum chamber o f t h e s p e c t r o m e t e r t o expose c l e a n s u r f a c e s o f t h e b u l k g l a s s c o m p o s i t i o n f o r e x a m i n a t i o n w i t h XPS. The s p e c t r o m e t e r was a P h y s i c a l E l e c t r o n i c s M o d e l 548 m o d i f i e d f o r emplacement i n a g l o v e b o x so t h a t a c t i n i d e s a m p l e s c o u l d be e x a m i n e d . S p e c t r a were t a k e n u s i n g A1K r a d i a t i o n (1486.6 e V ) . The o v e r a l l e n e r g y r e s o l u t i o n o f t h e s p e c t r o m e t e r was ~1.2 eV u s i n g an a n a l y z e r p a s s e n e r g y o f 25 e V . The s p e c t r o m e t e r c o n t r o l was i n t e r f a c e d t o a N i c o l e t 1180 m i n i c o m p u t e r p r o v i d i n g a u t o m a t i c d a t a a c q u i s i t i o n and a n a l y s i s c a p a b i l i t y .


2

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Results

We have p r e v i o u s l y r e p o r t e d t h a t d i s s o l u t i o n o f U 0 o r UO3 i n sodium s i l i c a t e g l a s s e s i n an atmosphere o f oxygen o r a i r produces a g l a s s i n which uranium appears o n l y i n the hexa v a l e n t s t a t e (_2). In order t o o b t a i n d i s s o l v e d t e t r a v a l e n t uranium i o n s , i t i s necessary t o melt the g l a s s i n very r e ducing c o n d i t i o n s . I n c o n t r a s t , t h e m e l t i n g o f N p 0 and P u 0 i n t o s i l i c a t e g l a s s i n f l o w i n g o x y g e n w i l l n o t o x i d i z e t h e Np o r Pu t o a h i g h e r v a l e n c e s t a t e . The s o l u b i l i t y o f N p 0 a n d P u 0 i n s o d i u m t r i s i l i c a t e g l a s s a t 1250°C i s t h r e e a n d two mole p e r c e n t , r e s p e c t i v e l y . When a t t e m p t s were made t o d i s s o l v e h i g h e r c o n c e n t r a t i o n s o f Np o r Pu o x i d e , n o n g l a s s y r e s i d u e s were o b t a i n e d . F o r U 0 d i s s o l v e d i n t o t h e same g l a s s 2

2

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at 1250°C, t h e s o l u b i l i t y i s f i v e ( f o r U4+) and t w e n t y ( f o r U6+) mole p e r c e n t when m e l t e d i n a t m o s p h e r e s o f CO/CO2 = 7 and i n a i r , r e s p e c t i v e l y . The l a r g e d i f f e r e n c e i n s o l u b i l i t y between h e x a v a l e n t and t e t r a v a l e n t u r a n i u m i n t h e s e g l a s s e s w i l l be d i s c u s s e d b e l o w . The v a l e n c e s t a t e s o f Pu and Np i o n s i n complex b o r o s i l i c a t e g l a s s e s ( d e s i g n a t e d 76101 and 7668, g l a s s e s p r e p a r e d by B a t t e l l e P a c i f i c N o r t h w e s t L a b o r a t o r i e s f o r w a s t e f o r m s i m u l a t i o n ) were p r e v i o u s l y i n v e s t i g a t e d w i t h XPS (J_)· N o m i n a l c o m p o s i t i o n s o f t h e s e g l a s s e s a r e g i v e n i n réf. 1. B o t h t r i v a l e n t and t e t r a v e l e n t s t a t e s o f Pu were r e a d i l y o b s e r v e d i n t h e g l a s s e s b u t Np was p r e s e n t e s s e n t i a l l y i n a s i n g l e v a l e n c e s t a t e (+4), i n both g l a s s e s . S i m i l a r r e s u l t s a r e seen i n F i g . 1, w h i c h shows an XPS s p e c t r u m t h a t i n c l u d e s a c t i n i d e 4 f c o r e l e v e l s f o r t h e 76101 g l a s s i n t o w h i c h one mole p e r c e n t e a c h o f UOo, N p 0 , and PuOo has been a d d e d . Two v a l e n c e s t a t e s f o r Pu (+4 and +3) and U (+6 and +4) a r e d i s c e r n a b l e w h i l e Np a p p e a r s o n l y i n t h e 4+ s t a t e . The T i 2p d o u b l e t a p p e a r s on t h e e x t r e m e l e f t s i d e o f F i g . 1. T i 0 was added t o t h e g l a s s b e c a u s e i t i s p r e s e n t i n n u c l e a r w a s t e and a l s o b e c a u s e i t i s u s e f u l a s an i n t e r n a l r e f e r e n c e f o r b i n d i n g e n e r g y d e t e r m i n a t i o n s (J_)· The s o d i u m t r i s i l i c a t e glasses with various additive oxides, l i s t e d i n T a b l e I , were s t u d i e d by XPS. A l t h o u g h m u l t i p l e v a l e n c e s t a t e s were f r e q u e n t l y o b s e r v e d w i t h X P S , c a u t i o n s h o u l d be e x e r c i s e d i n a s s o c i a t i n g t h e s e valence states w i t h the unperturbed bulk g l a s s . Particularly f o r P u ^ , a s i g n i f i c a n t amount o f r e d u c t i o n ( t o P u ) o c c u r r e d d u r i n g t h e XPS d a t a a c c u m u l a t i o n t i m e . This i s p a r t i c u l a r l y t r o u b l e s o m e f o r d i l u t e samples where l o n g d a t a a c q u i s i t i o n t i m e s a r e needed t o o b t a i n an a d e q u a t e s i g n a l - t o - n o i s e r a t i o . The e f f e c t i s i l l u s t r a t e d i n F i g . 2 w h i c h shows p r o g r e s s i v e changes o f Pu 4 f s p e c t r a f o r g l a s s I ( s e e T a b l e I ) as a f u n c t i o n of data a c q u i s i t i o n time. S p e c t r u m ( a ) i s a 10 m i n u t e scan a c q u i r e d immediately a f t e r f r a c t u r e . Each s u c c e s s i v e s p e c t r u m , r u n f o r an a d d i t i o n a l t e n m i n u t e s , was a c q u i r e d i m m e d i a t e l y a f t e r t h e p r e v i o u s s p e c t r u m . The s p e c t r a were t h e n smoothed, u s i n g a 5 p o i n t S a v i t s k y - G o l a y r o u t i n e s u p p l i e d by N i c o l e t . A f t e r extended X-ray exposure ( s e v e r a l h o u r s ) , t h e r e l a t i v e c o n c e n t r a t i o n s o f 4+ and 3+ i o n s became a p p r o x i m a t e l y equal. S i n c e t h e p h o t o e l e c t r o n e s c a p e d e p t h i s ~20-30 Â, i t a p p e a r s t h a t Pu i o n s i n t h e t o p ~15 A l a y e r were r e d u c e d . A l o w e n e r g y e l e c t r o n beam was u s e d d u r i n g measurement t o m a i n t a i n c h a r g e n e u t r a l i z a t i o n a t t h e sample s u r f a c e . To t e s t f o r p o s s i b l e r e d u c t i o n r e s u l t i n g f r o m t h e n e u t r a l i z a t i o n beam, t- _ sample was e x p o s e d o v e r n i g h t t o t h e beam. No r e d u c t i o n .om t h i s s o u r c e was d e t e c t a b l e . ( T h i s s o u r c e a l s o p r o d u c e s more sample h e a t i n g t h a n t h e X - r a y beam s o we r u l e o u t t h e r m a l d e c o m p o s i t i o n . ) Thus r e d u c t i o n o f t e t r a v a l e n t Pu and h e x a 2

2

+

3 +

1

r

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BINDING ENERGY leV)

F i g u r e 1. XPS 4 f c o r e l e v e l s p e c t r a f o r g l a s s 76101 c o n t a i n i n g d i s s o l v e d UO3, N p 0 and P u 0 - M u l t i p l e v a l e n c e s t a t e s a p p e a r f o r U and P u . I n t e n s i t i e s o f t h e l o w v a l e n c e s t a t e s a r e enhanced by i n - s i t u r e d u c t i o n d u r i n g t h e XPS measurement. 2

2

T a b l e I . C o m p o s i t i o n o f V a r i o u s Samples G l a s s Samples

C o m p o s i t i o n (mole

A

0.25 Na O«0.75

Β

( 0 . 2 5 Na O-0.75

C

( 0 2 . 5 Na O*0.75 S i O )

2

2

2

Si0

SiO ) .964( 2

0

2

0 e 9 8 5

(0.25Na O-0.75SiO )

Ε

(0.25Na O-0.75SiO )

2

2

2

2

F

(0.25Na O-0.75SiO )

G

(0.25Na O*0.75SiO )

H

2

2

(CaO)

fraction

2

D

2

Studied

0 e 9 1

0 e 9

0 e 9 4

2

T i 0

2>0.036

(PuO ) 2

4(TiO ) 2

44(TiO ) 2

9(TiO ) 2

0 # 9 0 9

0 # 0 1 5

0 e 0

2

6

0 # 0 3 6

0 e 0 3 6

(TiO )

3 (UO ) 3

(NpO ) 2

(PuO )

0 e 0 2

0

0 #

0i5

(PuO )

0 e 0 1 5

2

0 e 0 3 6

0 e 0 5 0

2

0 # 0 4 0

(0.25Na O*0.75SiO ) 2

2

0 # 8 8 9

(TiO ) 2

0 # 0 3 6

(PuO )

0 e

0l5

(PuO )

0 e 0 1 5

2

Zn

( °)0.060 I

(0.25Na O*0.75SiO ) 2

(Β Ο ) 2

3

2

0 e 8 0 9

(TiO ) 2

0 e 0 3 6

0 # 1 4 0


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F i g u r e 2.

4f core l e v e l s p e c t r a f o r g l a s s I (Table I) demonstra t i n g t h e s t r o n g t e n d e n c y of t h e P u ^ i o n s t o r e d u c e i n t h e XPS s p e c t r o m e t e r . S p e c t r a ( a ) t h r o u g h ( e ) a r e s h o r t (10 min) s e q u e n t i a l r u n s s h o w i n g a c c u m u l a t i v e r e d u c t i o n of P u to P u . +

4 +

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v a l e n t U was c a u s e d by t h e X - r a y f l u x u n d e r UHV c o n d i t i o n s . P r o b a b l y , o x y g e n d e s o r p t i o n was s t i m u l a t e d by p h o t o n s o r s e c o n d a r y electrons· I t i s n o t known w h e t h e r t h o s e o x y g e n atoms i n t h e v i c i n i t y o f a c t i n i d e i o n s a r e p r e f e r e n t i a l l y removed o r i f 0 atoms come f r o m t h e s u r f a c e r a n d o m l y . (We n o t e , however, t h a t s i m i l a r r e d u c t i o n b e h a v i o r o f t e n occurs f r o m s i m p l e u r a n a t e s a l t s where t h e i o n s a l w a y s have a r e l a t i v e l y c l o s e a s s o c i a t i o n w i t h a l l o x y g e n atoms.) The e a s e o f r e d u c t i o n o f t h e Pu 4+ i o n s a p p a r e n t l y c a n be a f f e c t e d by i n c l u s i o n i n t h e g l a s s o f a d d i t i o n a l a d d i t i v e s . Whereas t h e a d d i t i o n o f T i 0 2 a p p a r e n t l y does n o t a f f e c t i n - s i t u r e d u c t i o n , t h e a d d i t i o n o f CaO, ZnO, o r B2O3 a p p r e c i a b l y a c c e l e r a t e d r e d u c t i o n o f t h e Pu 4+ i o n s . T h i s e f f e c t i s demon s t r a t e d w i t h 4 f c o r e l e v e l s p e c t r a o f g l a s s e s F, G, H, and I shown i n F i g . 3. E a c h sample was s u b j e c t e d t o t h e same X - r a y e x p o s u r e , i . e . , t h e same number o f s c a n s a t t h e same f l u x . ( N o t e t h a t t h e c o n c e n t r a t i o n s o f a d d i t i v e o x i d e s d i f f e r . No a t t e m p t h a s b e e n made t o s c a l e t h i s e f f e c t w i t h a d d i t i v e c o n centration) · This curious reduction effect i s not e a s i l y u n d e r s t o o d b u t emphasizes t h e complex n a t u r e o f t h e g l a s s e s i n c l u d i n g the p o s s i b l e cooperative involvement of the m u l t i p l e c o m p o n e n t s . S i m i l a r l y c o m p l e x phenomena m i g h t i n f l u e n c e l e a c h i n g b e h a v i o r i n t h e complex, multicomponent g l a s s e s o f i n t e r e s t f o r r a d i o a c t i v e waste s t o r a g e . Discussion A consideration of the s t r u c t u r e of s i l i c a t e glasses provides a u s e f u l s t a r t i n g p o i n t f o r examining bonding proper t i e s o f e l e m e n t s i n c o r p o r a t e d i n t o t h e glasses· I t i s commonly b e l i e v e d t h a t added m e t a l i o n s a r e r a n d o m l y s i t u a t e d i n l a r g e i n t e r s t i t i a l s p a c e s o r h o l e s o f t h e amorphous S1O2 n e t w o r k . V a r i a t i o n o f t h e bond a n g l e s between S1O4 t e t r a h e d r a g i v e r i s e t o t h e h o l e s o f i r r e g u l a r shape a n d s i z e . T h i s model o f g l a s s s t r u c t u r e , d e v e l o p e d by Z a c h a r i a s e n ( 8 ) and Warren (_9), w o u l d i m p l y t h a t s o l u b i l i t i e s o f i n c o r p o r a t e d e l e m e n t s s h o u l d be d e t e r m i n e d , i n l a r g e p a r t , by t h e i o n i c s i z e o f t h e added e l e ment. W i t h t h i s v i e w , i t i s s u r p r i s i n g t h a t t h e d i f f e r e n c e i n t h e s o l u b i l i t y o f h e x a v a l e n t ( i o n i c r a d i u s , y = 0.8 A) a n d t e t r a v a l e n t ( y = 0.97 Â ) u r a n i u m i s so l a r g e , i . e . , 20 v e r s u s 5 mole p e r c e n t . In contrast to experimental observation,i t w o u l d a l s o be e x p e c t e d t h a t t h e s o l u b i l i t y o f t e t r a v a l e n t Np i o n s ( Y = 0.95 Â ) and P u i o n s ( y = 0.93 A) s h o u l d be l a r g e r o r a t l e a s t e q u a l t o t h a t o f t h e t e t r a v a l e n t U i n t h e same g l a s s . Previous studies of hexavalent U i n s i l i c a t e glasses have i n d i c a t e d t h a t U a p p e a r s i n t h e u r a n y l s t r u c t u r e Ç2, _7, 10)· I f b o t h t h e U i o n s and t h e UO^" i o n s a r e s i t u a t e d i n the i n t e r s t i t i a l holes of t h e g l a s s network, then the s o l u b i l i t y o f υοΐ* s h o u l d be s m a l l e r , i n s t e a d o f much l a r g e r t h a n t h e U+ i o n . c

c

C

c

+ 4

4


10.

LAM ET AL.


4

I

151


+

:

'-λ

F Ti 2p's Pu 4f's

I 470

L

460

I

I

450

I

I

440

I

I

430

I 420

BINDING ENERGY (eV)

F i g u r e 3.

4f core l e v e l s p e c t r a f o r a s e r i e s of g l a s s e s c o n t a i n i n g Ca, & i , o r Β o x i d e a d d i t i v e s ( g l a s s sample F, G, H, and I o f T a b l e I ) . The r a t e o f r e d u c t i o n o f Pu " " t o Pu d u r i n g XPS measurement i s a p p a r e n t l y i n f l u e n c e d by t h e c h o i c e o f a d d i t i v e ( s e e t e x t ) . 4


1


152

PLUTONIUM CHEMISTRY

F o r o u r p u r p o s e s , we u s e a c r y s t a l c h e m i s t r y a p p r o a c h and consider c r y s t a l s t r u c t u r e s of s i l i c a t e s w i t h chemical composi t i o n s approximating the g l a s s e s o f i n t e r e s t . Structural f e a t u r e s o f t h e s e c r y s t a l f o r m s s h o u l d be p r e s e r v e d i n v a r y i n g degrees i n t h e g l a s s e s . F o r example, t h e SiO^ t e t r a h e d r o n i s a s t r u c t u r a l e l e m e n t common t o a l l o f t h e s i l i c a t e g l a s s e s · Of c o m p a r a b l e i m p o r t a n c e , b u t more d i f f i c u l t t o d e m o n s t r a t e , i s the p o s s i b i l i t y t h a t extended range chains o r s h e e t s , r e a d i l y o b s e r v a b l e i n t h e c r y s t a l l i n e f o r m s , may a l s o a p p e a r i n t h e g l a s s e s and p l a y a n i m p o r t a n t r o l e i n d e t e r m i n i n g t h e p h y s i c a l p r o p e r t i e s o f a p a r t i c u l a r glass· Consider the a - N a 0 . 2 S i 0 c r y s t a l l i n e form (11) . T h i s s i l i c a t e c o n s i s t s of i d e n t i f i a b l e sheets that a r e assembled f r o m S i 0 4 t e t r a h e d r a . T h r e e " b r i d g i n g " o x y g e n bonds j o i n t h e t e t r a h e d r a w i t h bond s t r e n g t h s c o m p a r a b l e t o t h o s e f o u n d i n p u r e Si02« A s i n g l e " n o n b r i d g i n g " o x y g e n ( j o i n i n g an a l k a l i atom) on e a c h t e t r a h e d r o n p r o v i d e s a t e r m i n a t i o n l a y e r o f a l k a l i atoms on t h e s h e e t s u r f a c e . T h u s , t h e s h e e t s , w h i c h c o n t a i n s t r o n g i n t e r n a l bonds, are weakly j o i n e d t h r o u g h i o n i c a l l y bonded, m o n o v a l e n t a l k a l i a t o m s . However, i f m e t a l atoms o f h i g h e r v a l e n c e were added t o t h e compound and a d j a c e n t s h e e t s were bonded ( c r o s s l i n k e d ) t h r o u g h t h e s e p o l y v a l e n t a d d i t i v e s , one m i g h t e x p e c t t o f i n d p r o f o u n d changes i n t h e p h y s i c a l p r o p e r t i e s o f the g l a s s . An example o f t h i s c o n t r a s t i n b o n d i n g c o n d i t i o n s f o r c r y s t a l s s i l i c a t e s i s p r o v i d e d by t h e compound L i 2 0 . 2 S i 0 2 ( 1 2 ) w h i c h d i s p l a y s v e r y weak i n t e r s h e e t b o n d i n g and t h e m i n e r a l p e t a l i t e ( 1 3 ) , w h i c h h a s t h e c o m p o s i t i o n L i 0 . A l 0 3 . 8 S i 0 · To convert the d i s i l i c a t e t o p e t a l i t e r e q u i r e s the replacement of t h r e e L i atoms w i t h one A l , a p r o c e s s t h a t i s a c c o m p l i s h e d e n t i r e l y w i t h i n the intersheet region without s i g n i f i c a n t l y p e r t u r b i n g t h e s h e e t s t r u c t u r e o f t h e p a r e n t d i s i l i c a t e . The A l t h u s c r o s s l i n k s t h e s i l i c a s h e e t s w i t h a s t r o n g bond and profoundly a l t e r s thep h y s i c a l p r o p e r t i e s . I t i s o u r v i e w t h a t s i m i l a r phenomena o c c u r i n g l a s s y s i l i cates. Probably, chains or sheets of s i l i c a w i t h strong i n t e r n a l bonds f o r m i n t h e m e l t a t an e a r l y s t a g e o f c o o l i n g . A d d i t i v e e l e m e n t s t h a t f o r m s t r o n g c r o s s l i n k s between t h e s e s h e e t s w i l l t h e n be e x p e c t e d t o s i g n i f i c a n t l y a f f e c t t h e g l a s s rigidity. Such h a r d e n i n g ( a n a l a g o u s t o t h e c r o s s l i n k i n g o f c h a i n s i n p o l y m e r s ) i s commonly o b s e r v e d i n g l a s s e s . One m a n i f e s t a t i o n of the cross l i n k i n g i s increased v i s c o s i t y , often a p p a r e n t when p o l y v a l e n t m e t a l s a r e added t o s i m p l e s i l i c a t e glasses (14). U t i l i z i n g t h i s d e s c r i p t i o n o f s i l i c a t e g l a s s f o r m a t i o n , we s u g g e s t models t o e x p l a i n t h e i n c o r p o r a t i o n o f 4+ and 6+ a c t i n i d e i o n s i n sodium d i s i l i c a t e g l a s s . D u r i n g cooldown, SiO^ t e t r a h e d r a l i n k together t o form a s h e e t - l i k e s t r u c t u r e 2

2

2

2

2



10.

LAM ET AL.

153


w i t h , on a v e r a g e , one n o n b r i d g i n g o x y g e n p e r t e t r a h e d r o n . The s o d i u m i o n s , b e i n g c o m p l e t e l y i o n i z e d ( 5 ) and r e l a t i v e l y l a r g e , a r e s i t u a t e d i n t h e i n t e r s t i t i a l s p a c e s between t h e s h e e t s , weakly bonding those sheets together. Presumably, the t e t r a valent a c t i n i d e ions are also located i n the " i n t e r s h e e t " r e g i o n s b u t t h e y bond w i t h g r e a t e r s t r e n g t h t h a n a l k a l i s t o oxygens o f t h e t e t r a h e d r a , thus c r o s s l i n k i n g t h e d i s t o r t e d s i l i c a l a y e r s . T h i s g l a s s y s t r u c t u r e w i l l be f r o z e n i n as t h e sample i s c o o l e d . T h i s " s t r o n g c r o s s l i n k i n g " model i s c o n s i s tent with the ( q u a l i t a t i v e ) observation that, with a d d i t i o n of a c t i n i d e 4+ i o n s , t h e g l a s s v i s c o s i t y i n c r e a s e s . I t w o u l d a p p e a r t h a t a r a t h e r d i f f e r e n t p i c t u r e i s needed t o e x p l a i n t h e b o n d i n g o f U 6+ i n s i l i c a t e g l a s s e s . Strong e v i d e n c e i s now a v a i l a b l e t o s u p p o r t t h e a s s e r t i o n t h a t U 6+ appears i n a u r a n y l c o n f i g u r a t i o n (2,7,10). In the glasses, t h e UO^" i o n has a l i n e a r s t r u c t u r e w i t h U-0 s e p a r a t i o n o f a b o u t 1.8 Â ( 7 ) . A l k a l i u r a n a t e s c o n t a i n i n g h e x a v a l e n t u r a n i u m i n t h e u r a n y l s t r u c t u r e f o r m e a s i l y and i n c o n s i d e r a b l e v a r i e t y . T h e y o f t e n d i s p l a y s h e e t s t r u c t u r e s (sometimes d i s t o r t e d ) w i t h s h e e t s s e p a r a t e d by a l k a l i o r a l k a l i n e e a r t h atoms· These same f e a t u r e s , s h e e t s s e p a r a t e d by a l k a l i s , a r e c h a r a c t e r i s t i c of c r y s t a l l i n e a l k a l i d i s i l i c a t e s . I t thus a p p e a r s l i k e l y t h a t , d u r i n g c o o l d o w n o f U 6+ g l a s s e s , u r a n y l s t r u c t u r e s f o r m and bond i n t o t h e i n t e r s t i c e r e g i o n s between s i l i c a sheets· T h i s d i s s o l u t i o n mode p e r m i t s a l a r g e U 6+ solubility. Since a l k a l i s play the r o l e of d e l i n e a t i n g s i l i c a o r u r a n a t e s h e e t s , s t r u c t u r a l r i g i d i t y s h o u l d be weak. The r e l a t i v e l y l o w v i s c o s i t y o f U 6+ s i l i c a t e g l a s s e s ( a g a i n a q u a l i t a t i v e observation) supports t h i s p i c t u r e . Uranates are n o t s t a b l e a t 1250°C where t h e s a m p l e s were m e l t e d so we do n o t e x p e c t m a c r o s c o p i c phase s e p a r a t e d r e g i o n s o f U 6+ u r a n a t e . ( F u r t h e r m o r e , X - r a y d i f f r a c t i o n d a t a do n o t show t h e p r e s e n c e o f c r y s t a l l i n e s e c o n d phase r e g i o n s . ) Rather, i t appears that a t h i n u r a n a t e l a y e r i s f o r m e d between s i l i c a s h e e t s and h e l d i n p l a c e by l a y e r s o f a l k a l i atoms· + 4

+ 6

The d i f f e r e n t b o n d i n g c h a r a c t e r i s t i c s o f U and U in a l k a l i s i l i c a t e g l a s s e s may have i m p o r t a n t i m p l i c a t i o n s t o t h e l e a c h i n g p r o p e r t i e s of h i g h l e v e l waste g l a s s e s . I n t h e UO3 g l a s s e s , the u r a n y l s t r u c t u r e s probably occur i n a s s o c i a t i o n w i t h a l k a l i s ( s i m i l a r t o a l k a l i u r a n a t e s ) , an a s o c i a t i o n t h a t d r a m a t i c a l l y i n c r e a s e s b o n d i n g o f t h e sodium i o n s i n t h e g l a s s . T h i s c o n c l u s i o n i s s u p p o r t e d by q u a l i t a t i v e l e a c h i n g s t u d i e s o f s o d i u m d i s i l i c a t e g l a s s c o n t a i n i n g UO3 ( 1 5 ) . I t was o b s e r v e d t h a t t h e g l a s s m a i n t a i n e d i t s s t r u c t u r a l i n t e g r i t y and no s i g n i f i c a n t change i n t h e l e a c h a n t ph was d e t e c t e d a f t e r ( s t a t i c ) l e a c h i n g i n d i s t i l l e d w a t e r f o r 180 days a t room temperature· I n c o n t r a s t , d i s i l i c a t e g l a s s e s c o n t a i n i n g U 4+ showed much l e s s l e a c h r e s i s t a n c e . The 4+ g l a s s was o n l y s l i g h t l y more l e a c h r e s i s t a n t than pure sodium d i s i l i c a t e g l a s s w h i c h i s w a t e r s o l u b l e . F o r 4+ g l a s s e s , t h e ph o f t h e l e a c h s o l u t i o n



154

PLUTONIUM CHEMISTRY

rose to 10.7 under the same leaching conditions used for the U 6+ glass · The U 4+ ions , which are soluble in low concentra tion, apparently cross link silica sheets to produce signifi cant physical property changes. However, this cross linking process would not appear to significantly affect the bonding of alkali ions in the glass, so that the ability of a simple silicate glass to withstand leach attack is not appreciably increased by adding U 4+ ions. More quantative leaching data is needed for well charac terized sodium silicate glasses containing U + ^ and U"^ ions. It is clear, however, that the general practice of using airmelted uranium glass (U^+) to simulate leaching behavior of transuranic actinides in waste glasses should be discarded. Since transuranics in silicates do not, in general, appear in the 6+ valence state, this is not a reliable simulation. Also U 6+ ions appear to have bonding properties (associated with a strong tendency to form the uranyl structure) in glasses that are not typical of transuranic ions· Acknowledgment Work supported by the U. S. Department of Energy. Literature Cited 1.

Karim, D. P.; Lam, D. J. ; Diamond, H.; Friedman, A. M.; Coles, D. G.; Bazan, F.; McVay, G. L., Proc. of Materials Res. Soc. Symp. 1982, p. 67. 2. Lam, D. J.; Veal, B. W.; Chen, H; Knapp, G. S. "Scien tific Basis for Nuclear Waste Management;" McCarthy, G. J., Ed.; Plenum Publ. Co., New York, 1979, Vol. I, p. 97. 3. Veal, B. W.; Lam, D. J., Proc. Int. Conf. on Physics of SiO2 and its Interfaces 1978, p. 299. 4. Lam, D. J.; Paulikas, A. P.; Veal, B. W. J. Non-Cryst. Solids 1980, 42, 41. 5. Veal, B. W.; Lam, D. J.; Paulikas, A. P.; Ching, W. Y. J. Non-Cryst. Solids 1982, 49, 309. 6. Banks, R., Carnall, W. T., Veal, B. W., Lam, D. J., and Paulikas, A. P., to be published. 7. Detailed x-ray absorption experimental results will be published elsewhere. 8. Zachariasen, W. H. J. Am. Chem. Soc. 1932, 54, 3841. 9. Warren, Β. Ε. J. Am. Ceram. Soc. 1934, 19, 249; J. Am. Ceram. Soc. 1941, 24, 256; J. Appl. Phys. 1942, 13, 602. 10. Karraker, D. G. J. Am. Ceram. Soc. 1982, 62, 53. 11. Pant, A. K., Cruickshank, D. W. J., Acta Cryst., 1968, B24, 13. 12. Liebau, F., Acta Cryst., 1961, 14, 389. 13. Liebau, F., Acta Cryst., 1961, 14, 309 14. Morey, G. W., "The Properties of Glass"; Reinhold Publ. Co., New York, 1954. 15. Veal, B. W., Lam, D. J., Paulikas, A. P., and Karim, D. P., Nucl. Tech. 51, 136-142 (1980). RECEIVED January 21, 1983


XPS - American Chemical Society

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