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GRAZING INCIDENCE FLUORESCENCE EXAFS AND NEAR EDGE SPECTROSCOPY FOR
CORRODED GLASS SURFACES
N. Barrett, G. Antonini, G. Greaves, F. Thornley, A. Manara
To cite this version:
N. Barrett, G. Antonini, G. Greaves, F. Thornley, A. Manara. GRAZING INCIDENCE FLUORES- CENCE EXAFS AND NEAR EDGE SPECTROSCOPY FOR CORRODED GLASS SURFACES.
Journal de Physique Colloques, 1986, 47 (C8), pp.C8-879-C8-882. �10.1051/jphyscol:19868170�. �jpa-
00226075�
G R A Z I N G I N C I D E N C E F L U O R E S C E N C E EXAFS AND N E A R EDGE S P E C T R O S C O P Y F O R C O R R O D E D G L A S S S U R F A C E S
N.T. BARRETT, G.M. ANTONINI', G.N. GREAVES"', F.R. THORNLEY*"' and A. MANARA
Physics Division, Joint Research Centre Euratom, I-21020 Ispra, (VA), Italy
'GNSM and Physics Department. University of Modena, Via Campi 213/A, I-41100 Modena, Italy
"'SERC Daresbury Laboratory, GB-Warrington WA4 4AD, Great-~ritain
"'~epartment of Applied Physics, University of Strathclyde, GB-Glasgow G1 IXL, Great-Britain
A b s t r a c t
EXAFS a t g r a z i n g i n c i d e n c e w i t h f l u o r e s c e n c e d e t e c t i o n has been used t o i n v e s t i g a t e t h e l o c a l atomic s t r u c t u r e around uranium a t t h e s u r f a c e of b o r o s i l i c a t e g l a s s e s . S t r u c t u r a l changes have been observed a s a r e s u l t of l e a c h i n g i n w a t e r a t 1 0 0 ~ ~ . These i n d i c a t e t h a t uranium m i g r a t e s go t h e s u r f a c e t o form i s l a n d s t r u c t u r e s i n a s u r f a c e l a y e r which, s u b s e q u e n t l y , p e e l s o f f t o r e v e a l f r e s h s u r f a c e f o r f u r t h e r c o r r o s i o n .
G l a s s s u r f a c e c o r r o s i o n induced by w a t e r i s a s u b j e c t of i n c r e a s i n g s c i e n t i f i c and t e c h n o l o g i c a l i n t e r e s t , p a r t i c u l a r l y i n view o f concern about an a c c i d e n t a l r e l e a s e of r a d i o a c t i v e p r o d u c t s from v i t r i f i e d n u c l e a r waste / I / . Fluorescence EXAFS a t g r a z i n g i n c i d e n c e is p r o v i n g t o be a p a r t i c u l a r l y u s e f u l t o o l f o r t h e s u r f a c e i n - v e s t i g a t i o n o f changes i n t h e atomic neighbourhood o f a r a d i o a c t i v e waste m e t a l , e s p e c i a l l y a s t h e wet s t a t e can be maintained d u r i n g measurements.
A
d e s c r i p t i o n and some t h e o r e t i c a l background o f t h i s t e c h n i q u e can be found i n t h e l i t e r a t u r e /2,3/.
The samples (U-glasses) were b o r o s i l i c a t e g l a s s e s (53.7 SiOZ, 16.7 B2O3, 21 Na20, 5.6 ~ 1 ~ 0 3 , i n weight p e r c e n t a g e ) , w i t h 3%
U02. Asr e f e r e n c e samples,
Um e t a l , an uranyl compound and U02 were used. The g l a s s was p r e p a r e d a t S t a z i o n e Sperimentale d e l Vetro, Murano, I t a l y , by m e l t i n g o f t h e mixture i n a i r w i t h i n a platinum cru- c i b l e . T h i s procedure e n s u r e s an o x i d i z i n g environment.
The uranium c o n c e n t r a t i o n i n t h e g l a s s was scanned by an e l e c t r o n microprobe and no evidence was d e t e c t e d f o r any inhomogeneities. G l a s s s l a b s were o p t i c a l l y p o l i s h e d t o b e t t e r t h a n h/10 b u t , a f t e r l e a c h i n g , a porous m i c r o s t r u c t u r e was o b s e r v a b l e i n t h e s u r f a c e , probably t h e r e s u l t o f t h e formation of a h y d r a t e d g e l l a y e r . Leaching was performed dynamically under a flow o f b o i l i n g w a t e r a t 1 0 0 ~ ~ i n a Soxholet appa- r a t u s . X-ray measurements were made immediately a f t e r l e a c h i n g , t h u s t h e samples were s t i l l i n t h e "wet" s t a t e .
Ani n c r e a s e of t h e
p Hvalue of t h e l e a c h a n t was measured a f t e r l e a c h i n g , i n d i c a t i n g t h e d i s s o l u t i o n o f a l k a l i n e p r o d u c t s from t h e g l a s s s u r f a c e , X-ray measurements were made on t h e w i g g l e r EXAFS s t a t i o n 9.2 of The Synchrotron R a d i a t i o n Source a t t h e Daresbury Laboratory. T y p i c a l f l u o r e s c e n c e i n t e n s i t i e s a t g r a z i n g i n c i d e n c e a t t h e 1,111 edge o f uranium a r e r e p o r t e d i n F i g . l . Measurements were made a t an angle i n c i d e n c e of h a l f t h e c r i t i c a l a n g l e f o r t o t a l e x t e r n a l r e f l e c t i o n a t t h e s u r f a c e o f a b o r o s i l i c a t e g l a s s . With t h i s geometry, t h e
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868170
C8-880 JOURNAL DE PHYSIQUE
p e n e t r a t i o n depth probed by t h e i n c i d e n t beam i s expected t o be about 50 i n t h e unleached g l a s s .
A smay be seen more c l e a r l y from t h e i n s e r t , t h e r e i s an i n c r e a s e i n t h e f l u o r e s c e n c e i n t e n s i t y w i t h l e a c h i n g t i m e , p o i n t i n g t o an i n c r e a s e i n t h e uranium c o n c e n t r a t i o n a t t h e s u r f a c e . D e t a i l e d t o t a l r e f l e c t i o n measurements g i v e some evidence f o r t h e g e n e r a t i o n o f a s u r f a c e l a y e r whose c r i t i c a l a n g l e f o r t o t a l e x t e r n a l r e f l e c t i o n i s l e s s t h a n t h a t of t h e b u l k g l a s s , i . e . t h e l a y e r h a s a lower e l e c t r o n d e n s i t y .
1 2 ~ 1 ~ h leach
%eAc=.=z=;--.=2z=~=:
6 * h leach
0.09-
20' leach
1
17100 17130 17160 17190 17220 17250 17280 Incident energy (eV)
Fig. 1. Near edge f l u o r e s c e n - c e a t g r a z i n g i n c i d e n c e a t t h e ULIII edge. I n t h e i n s e r t ,
A 1i s t h e s t e p h e i g h t and
I i st h e i n t e n s i t y of pre-edge s c a t t e r i n g .
From t h e r e s u l t s shown i n Fig. 1, t h e r e i s c l e a r evidence f o r s a t u r a t i o n i n t h e i n c r e a s e o f U f l u o r e s c e n c e i n t e n s i t y a f t e r 100 ' . Accordingly, s h o r t l e a c h i n g t i m e s o f 1 5 ' , 30' and 9 0 ' were chosen f o r
EXAFS measurements. Fluorescence EXAFS s p e c t r awere recorded f o r l e a c h e d and unleached samples a t o n e h a l f o f t h e c r i t i c a l a n g l e f o r t o t a l e x t e r n a l r e f l e c t i o n . Reference compounds were measured from powders u s i n g t h e t r a n s m i s s i o n geometry. Least s q u a r e s f i t s t o k 3 weighted d a t a were o b t a i n e d u s i n g t h e curved wave program EXCURVE a v a i l a b l e a t Daresbury Laboratory
/4/.I n o r d e r t o a c c u r a t e l y determine phase s h i f t s and b a c k - s c a t t e r i n g amplitudes f o r
U-0 and U-Up a i r s s p e c t r a , t h e r e f e r e n c e compounds mentioned e a r l i e r were a c c u r a t e l y f i t t e d t o t h e i r c r y s t a l l o g r a p h i c r a d i a l d i s t r i b u t i o n f u n c t i o n s . Fig.
2shows t h e comparison between experiment and t h e o r y f o r uranyl-fluoride-fluorammonium.
A si n o t h e r u r a n y l compounds, t h e n e a r e s t neighbour o c t a h e d r a l ,oxygen s h e l l i s s p l i t i n t o two s u b s h e l l s
( 2 : 4 ) . The s t r u c t u r a l parameters a r e summarized i n t h e i n s e r t . No
U-Uc o r r e l a t i o n s a r e d e t e c t a b l e f o r t h i s molecular s t r u c t u r e . U02, on t h e o t h e r hand, h a s t h e f l u o r i t e s t r u c t u r e w i t h 8 n e a r e s t neighbour oxygens and 12 n e a r e s t neighbour ura- n i u m ~ . Both s h e l l s a r e r e a d i l y r e s o l v e d i n EXAFS a s we have found f o r o t h e r f l u o r i t e o x i d e s /5/. The
U-0d i s t a n c e and Debye-Waller f a c t o r s were 2.41 8 and 0.003 (5) g2.
For
U-Uc o r r e l a t i o n s t h e corresponding v a l u e s were 3.89 g and 0.005
( 0 )8'. The more
1 .o
0
Fig. 2. k 3 weighted experimental EXAFS
(SO-l i d l i n e ) and f i t (dashed l i n e ) f o r r e f e r e n -
-1 .oSHELL COORDINATION DISTANCE DWF
c e u r a n y l compound (uranyl-fluoxide-fluor-
NUMBER
(a)
( k )1 2 185 00035
2 4 2 34 0 0045
ammonium), u s i n g modified phase s h i f t s from EXCURVE. Data were f i l t e r e d t o i n c l u d e t h e
f i r s t two oxygen s h e l l s .
c o r r e c t t o 1% o r b e t t e r , c o r r e c t c o o r d i n a t i o n numbers and r e a l i s t i c Debye-Waller f a c t o r s . These parameters were used t o f i t t h e FLEXAFS s p e c t r a o b t a i n e d from t h e g l a s s s u r f a c e s . I n o r d e r t o avoid p o s s i b l e a r t i f a c t s from K i e s s i g f r i n g e s t h a t might be a s s o c i a t e d with t h e g e n e r a t i o n of a s u r f a c e g e l l a y e r , t h e k-range was l i m i t e d t o 3 - 14 8-l.
Analysis o f a l l t h e g l a s s s p e c t r a r e v e a l e d r a d i a l d i s t r i b u t i o n f u n c t i o n s w i t h t h r e e major f e a t u r e s : one due t o
U - 0c o r r e l a t i o n s and t h e o t h e r two t o U-U c o r r e l a t i o n s . These can be seen i n F i g . 3 where r e s u l t s of two d i f f e r e n t l e a c h i n g p e r i o d s a r e shown. I n p a r t i c u l a r , t h e f i r s t f e a t u r e
(U-0d i s t a n c e s ) c o n s i s t s o f two main c o n t r i - b u t i o n s r e m i n i s c e n t of an u r a n y l r a d i c a l . The p a i r d i s t r i b u t i o n f u n c t i o n f o r t h e f i t shown i n Fig.
2i s a l s o i n c l u d e d i n F i g . 3 ( d o t t e d c u r v e s ) , where t h e resemblan- c e can be c l e a r l y seen. Indeed, t h e chemical s h i f t f o r t h e ULIII edge f o r t h e g l a s s s p e c t r a i s c l o s e t o t h a t f o r t h e u r a n y l r e f e r e n c e a s o t h e r a u t h o r s have found
/6/.I t
is obvious from Fig. 3, however, t h a t t h e r a d i a l d i s t r i b u t i o n f u n c t i o n s f o r t h e g l a s s s u r f a c e a r e more complicated t h a n e i t h e r o f t h e oxide r e f e r e n c e compounds. I n o r d e r t o a n a l y s e this, we used t h e procedure of s u b s h e l l a n a l y s i s , c o n s t r u c t i n g t h e oxygen c o o r d i n a t i o n s p h e r e from s e v e r a l s u b s h e l l s w i t h the same i n t e g r a t e d coordi- n a t i o n number. T h i s i s p a r t i c u l a r l y a p p r o p r i a t e f o r s t u d y i n g t h e d i s t o r t e d metal environments i n many g l a s s y and c r y s t a l l i n e o x i d e s
/7/.From Fig. 3 i t can be seen t h a t compared t o t h e u r a n y l r e f e r e n c e , t h e second s u b s h e l l around
2.32 is broadened i n t h e g l a s s s p e c t r a and i s a l s o skewed - t a i l i n g o f f t o l o n g e r d i s t a n c e s . Where
U-Uc o r r e l a t i o n s a r e n o t s e e n i n t h e uranyl r e f e r e n c e compound, t h e y a r e seen i n t h e g l a s s s p e c t r a and, indeed, can be modelled t o a p l a n a r s q u a r e l a t t i c e /8/ a s pro- posed e a r l i e r by Knapp e t a l .
/6/.Fig. 3.
k 3weighted EXAFS experimental ( s o l i d l i n e ) and f i t t e d (dashed l i n e ) s p e c t r a
from leached U-glasses and t h e i r corresponding a i r - d i s t r i b u t i o n f u n c t i o n s . Expe-
r i m e n t a l d a t a a r e F o u r i e r f i l t e r e d from 1 t o 5 !. Leaching times i n minutes a r e
marked i n t h e f i g u r e . Dotted c u r v e s on t h e r i g h t correspond t o t h e p a i r d i s t r i b u -
t i o n f u n c t i o n f o r t h e u r a n y l r e f e r e n c e compound from Fig.
2.C8-882 JOURNAL DE PHYSIQUE
The two atomic d i s t r i b u t i o n s shown i n Fig. 3 demonstrate t h a t t h e l o c a l environment of uranium changes a s c o r r o s i o n advances.
W e
have found t h e two U-U f e a t u r e s become more prominent with l e a c h i n g . This can b e seen i n Fig. 3 and a l s o i n Table I where t h e normalized weights of U-U ( 1 ) and U-U ( 2 ) a r e l i s t e d f o r d i f f e r e n t l e a c h i n g times. This d a t a s u g g e s t s t h a t i n t h e i n i t i a l build-up of uranium a t t h e s u r f a c e(Fig. I ) , t h e u r a n y l - l i k e complexes agglomerate i n t o l a r g e r i s l a n d s . A broadening of t h e U-0 f e a t u r e
i s
a l s o observed, i n d i c a t i n g t h e oxygen environment may becoming more d i s o r d e r e d w i t h p r o g r e s s i v e c o r r o s i o n . This would be expected, f o r i n s t a n c e , i f t h e s u r f a c e g r a d u a l l y became more hydrated. A f t e r 9 0 ' of l e a c h i n g , however, a uranium environment r e m i n i s c e n t o f t h e unleached p o l i s h e d s u r f a c ei s
recovered(Table I ) . This behaviour
i s
c o n s i s t e n t with a p e e l i n g o f f of a s u r f a c e l a y e r , r e - v e a l i n g f r e s h m a t e r i a l f o r an aqueous a t t a c k .TABLE I
-
Weights o f t h e U-U ( 1 ) and'U-U( 2 )
f e a t u r e s normalized t o t h e U-0 fea- t u r e i n t h e r a d i a l d i s t r i b u t i o n f u n c t i o n s f o r t h e s u r f a c e s t r u c t u r e s a s a f u n c t i o n of l e a c h i n g timeLeaching time U-U ( 1 ) U-U ( 2 )
The p r e s e n t g r a z i n g a n g l e s p e c t r o s c o p i c r e s u l t s a t t h e ULIIT edge i n d i c a t e s t h a t t h e p r o c e s s o f b o r o s i l i c a t e g l a s s l e a c h i n g i n w a t e r i n v o l v e s t h e m i g r a t i o n of.uranium t o t h e s u r f a c e with t h e complexing of u r a n y l - l i k e c e n t r e s , accompanied by t h e genera- t i o n o f a s u r f a c e l a y e r . This
i s ,
s u b s e q u e n t l y , shed with a t o t a l time cycLe o f between 30'
and 90' .
Re f e r e n c e s
P.B. Adams, J. Non C r y s t . S o l i d s ,
67
(1984) 193.S.M. Heald, K. K e l l e r and E. S t e r n , phys. L e t t . , (1984) 155.
F.R. Thornley, N.T. B a r r e t t , G.N. Greaves and G.M. Antonini, J. Phys. C. ( i n p r e s s )
.
4.
G.P. Diakun, G.N. Greaves, S.S. Hasnain and P.D. Quinn, Daresbury Laboratory, t e c h n i c a l r e p o r t DL/SCI/TM~~E ( 1984).5. P.D. B a t t l e , C.R.A. Catlow,
A.V.
Chadwick, G.N. Greaves and L.M. Moroney ( t h i s c o n f e r e n c e ) .6.
G.S. Knapp, B/W/ Veal,D.J.
Lam, A.P. P a u l i k a s and H.K.Pan,
M a t e r i a l s L e t t . ,2
(1984) 253.7.
-
G.N. Greaves, N. s i n s t e a d and C.M.B. Henderson, i n "EXAFS and Near Edge S t r u c - t u r e III", ed. K.O. Hodgson, B. Hedman and J.E. Penner-Hahn, S p r i n g e r , B e r l i n(1984) p.297.
8. G.N. Greaves ( t h i s c o n f e r e n c e ) .
