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SODIUM ENVIRONMENTS IN GLASS
G. Greaves
To cite this version:
G. Greaves. SODIUM ENVIRONMENTS IN GLASS. Journal de Physique Colloques, 1981, 42 (C4),
pp.C4-225-C4-228. �10.1051/jphyscol:1981447�. �jpa-00220904�
SODIUM ENVIRONMENTS IN GLASS
G.N. Greaves
Science and Engineering Research CounciZ, Daresbury Laboratory, Daresbury, Warrington WA4 4AD, England
Abstract.- Sodium environments have been measured d i r e c t l y i n s e v e r a l oxide g l a s s e s u s i n g EXAFS a t t h e sodium K edge. The e x i s t e n c e of l o c a l S t r u c t u r e around sodium i n g l a s s c o n t r a d i c t s t h e c o n v e n t i o n a l Zachariasen model.
A n a l y s i s o f t h e EXAFS i n d i c a t e s t h e r e a r e s i g n i f i c a n t d i f f e r e n c e s r e l a t i n g t o t h e g l a s s modifier: g l a s s former chemistry - t h e d e t a i l s o f which demon- s t r a t e s i m i l a r i t i e s w i t h c r y s t a l l i n e s i l i c a t e s and b o r a t e s .
I n t h e t o t a l d i f f r a c t i o n spectrum of a g l a s s t h e c o r r e l a t i o n s a s s o c i a t e d w i t h t h e modifying component (Nap0 i n t h i s c a s e ) a r e masked by t h o s e a s s o c i a t e d w i t h t h e more s t r o n g l y bound g l a s s forming network (e.g. S i 0 2 o r BpO3). The EXAFS t e c h n i q u e however e n a b l e s t h e s e c o r r e l a t i o n s t o b e s e p a r a t e d o u t . J u s t t h i s approach h a s been used f o r t h e f i r s t t i m e w i t h t h e f o l l o w i n g t h r e e g l a s s e s : sodium d i s i l i c a t e , s o d a - l i m e - s i l i c a g l a s s and sodium d i b o r a t e . A t t h e Na K edge (11.5 8 ) d i s t i n c t EXAFS s p e c t r a have been measured demonstrating t h a t t h e sodium environments i n t h e s e widely d i f f e r e n t compositions a r e w e l l d e f i n e d f o r each g l a s s . The f i n e s t r u c t u r e , however, i s weaker t h a n t h e t e t r a h e d r a l S i environment o b t a i n e d from a n e a r l i e r s t u d y (1) of t h e S i K Edge (6.7
g ) .
I t i s now c l e a r u s i n g EXAFS why Na-0 c o r r e l a t i o n s a r e b a r e l y i d e n t i f i a b l e u s i n g x-ray o r neutron d i f f r a c t i o n t e c h n i q u e s12)
-
Normalized sodium EXAFS s p e c t r a f o r t h e t h r e e g l a s s e s a r e d i s p l a y e d i n f i g . 1 ( s o l i d c u r v e s ) . These were o b t a i n e d from t r a n s m i s s i o n measurements o f sub-micron t h i c k specimens f a b r i c a t e d by blowing t h i n f i l m s from a molten p e l l e t . Synchrotron r a d i a t i o n from t h e ACO s t o r a g e r i n g was employed and monochromatized using t h e LURE two c r y s t a l s o f t x-ray spectrometer. Experimental d e t a i l s a r e g i v e n elsewhere ( 3 ) . F o u r i e r t r a n s f o r m s of t h e normalized s p e c t r a a r e p r e s e n t e d i n f i g . 2 . These were c o r r e c t e d u s i n g c a l c u l a t e d phase s h i f t s f o r sodium and oxygen. The broad f e a t u r e s of t h e atomic d i s t r i b u t i o n s were t h e n used a s f i r s t a t t e m p t s f o r c u r v e - f i t t i n g t h e wave v e c t o r s p e c t r a . The f i n a l f i t s ( d o t t e d c u r v e s i n f i g . 1 ) were o b t a i n e d u s i n g Lee and P e n d r y ' s s p h e r i c a l wave formalism ( 4 ) . D e t a i l s o f t h e d e r i v e d c l u s t e r s a r e l i s t e d i n t h e accompanying t a b l e .
The Debye-Waller f a c t o r s (0;) f o r a l l t h e g l a s s e s l i s t e d i n t h e t a b l e a r e many t i m e s g r e a t e r t h a n t h o s e f o r J s i l i c o n (0.001
g2
( 3 ) ) . S p e c i f i c a l l y t h e Debye- Waller f a c t o r measured i n EXAFS i s t h e c o r r e l a t e d bond l e n g t h v a r i a n c e . I t r e f l e c t s t h e s t r e n g t h of t h e bond and t h e o v e r a l l r i g i d i t y o f t h e l a t t i c e . The l a r g e d i f f e r e n c e s between t h e sodium and s i l i c o n Debye-Waller f a c t o r s demonstrates why Na-0 c o r r e l a t i o n s a r e l o s t i n t h e p r e s e n c e of Si-0 o r B-0 c o r r e l a t i o n s i n t h e t o t a l d i f f r a c t i o n spectrum. ~t a l s o e x p l a i n s why t h e sodium r e l a t e d phonon band i n t h e IR a b s o r p t i o n spectrum of sodium d i s i l i c a t e g l a s s (5) i s s o much b r o a d e r t h a n t h e bands r e l a t i n g t o SiOp. I n f a c t t h e r e s p e c t i v e s t r e t c h i n g bands a r e s e p a r a t e d i n frequency by approximately an o r d e r o f magnitude. T h i s d i f f e r e n c e combined w i t h t h e l i g h t e r weight of t h e atom p a i r i m p l i e s Debye-Waller f a c t o r s r a t h e r l a r g e r t h a nArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981447
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t h e measured v a l u e s given i n t h e t a b l e . E v i d e n t l y i n g l a s s e s t h e r i g i d g l a s s form- i n g network o f S i 0 2 o r B2O3 i s i m p o r t a n t i n c o r r e l a t i n g t h e motions of t h o s e oxygens c o o r d i n a t e d with t h e sodium c a t - i o n s .
The Zachariasen model f o r g l a s s (6) w h i l s t emphasising t h e back-bone f u n c t i o n o f t h e c o v a l e n t network s u b o r d i n a t e s t h e s t r u c t u r a l r o l e of t h e i o n i c modifying component. C a t i o n s l i k e sodium o r c a l - c i m a r e p i c t u r e d a s p r o v i d i n g a g e n e r a l f i l l e r f o r h o l e s and v o i d s p r e s e n t i n t h e network a s a r e s u l t o f t o p o l o g i c a l d i s - o r d e r . I t i s c l e a r from c r y s t a l l i n e chemistry, however, t h a t , e v e n though t h e c o o r d i n a t i o n o f a l k a l i o r a l k a l i n e e a r t h c a t i o n s i s never t h e same from compound t o compound, f o r a given c r y s t a l it i s u s u a l l y f a i r l y well-defined.
A very s i m i l a r p i c t u r e h a s emerged from t h e p r e s e n t s t u d y of t h e sodium EXAFS o f s i l i c a t e and b o r a t e g l a s s e s . The atomic d i s t r i b u t i o n s given i n f i g . 2 confirm t h e average Na-0 bond l e n g t h t o b e 2.3
8.
The c l u s t e r c a l c u l a t i o n s ( s e e t a b l e ) however show t h e a c t u a l oxygen c o o r d i n a t i o n depends on t h e remaining composition. ( I n c i d e n t a l l y t h e coordina- t i o n numbers, N . given i n t h e t a b l e a r eI'
e s s e n t i a l l y normalized w i t h r e s p e c t t o t h e t e t r a h e d r a l c o o r d i n a t i o n o f S i i n t h e s i l i c a t e g l a s s e s and a s such a r e good t o
-
20% ( s e e r e f ( 3 ) ) . So t h e r e a r e approximately 5 oxygens around each sod- ium i n sodium d i s i l i c a t e , 6 i n sodium d i b o r a t e b u t o n l y 2 i n t h e soda-lime- s i l i c a g l a s s . D i f f e r e n c e s i n l o c a l s t r u c t u r e can a l s o be s e e n beyond t h e oxygen l i g a n d ( f i g . 2).
I n sodium d i b o r a t e t h e r e i s a second s h e l l of approximately 6 sodiums, t h e borons a r e n o t s e e n because t h e b a c k - s c a t t e r i n g i s s o much weaker compared t o oxygen o r sodium. For sodium d i s i l i c a t e a second s h e l l i s l e s s obvious. Sodium and s i l i - con s h e l l s a r e n o t pronounced probably because t h e s i m i l a r i t y of t h e r e s p e c t i v e Fig. 1 s c a t t e r i n g f a c t o r s makes t h e d i s t i n c t i o nof s e p a r a t e s h e l l s ambiguous. I n t h e Normalized sodium EXAFS s p e c t r a f o r s o d a - l i m e - s i l i c a g l a s s , however, t h e low t h e t h r e e g l a s s e s sodium d i s i l i c a t e , oxygen c o o r d i n a t i o n number s u g g e s t s t h e s o d a - l i m e - s i l i c a and sodium d i b o r a t e . second s h e l l , which i s d i s t i n c t i n t h e For d e t a i l s s e e t e x t . F o u r i e r t r a n s f o r m ( f i g . 2 1 , must a l s o be
made up of oxygens. The c l u s t e r c a l c u l a - t i o n s ( f i g . 1 and t h e t a b l e ) s u p p o r t t h i s i n t e r p r e t a t i o n . P a r a l l e l compositionally r e l a t e d s t r u c t u r a l d i f f e r e n c e s a r e t o be
for the three glasses shown in fig.1 See text for details.
Curve f i t t i n g p a r a m e t e r s f o r t h e t h r e e g l a s s e s Sample
N. atom t y p e and number 7
r . s h e l l r a d i u s 3
0 . Debye-Waller f a c t o r
TABLE
N1 rl 0 1
11
112N2 1 2 0 2
11
22Na2 s i 2 05
1
5 0's 2.3(0) 0.005(6)N3 '3 0 3
X
112Na2CaSi2012
Na2 B2 0 5
I I
I
2 0 ' s 2 . 4 ( 3 ) 0.01 ( 4 )
6 0 ' s 2 . 2 ( 5 ) 0.02 ( 8 )
1 0 2.8(6) 0 . 0 2 ( 8 )
6 N a ' s 3 . 8 ( 1 ) 0 . 0 2 ( 8 )
3 0's 3.3(31 0.02(6)
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found i n t h e sodium environments o f c r y s t a l l i n e s i l i c a t e s and b o r a t e s . I n sodium d i b o r a t e t h e uxygen c o o r d i n a t i o n i s between 6 and 7, i n sodium d i s i l i c a t e it i s 5 w h i l s t i n t e r n a r y a l k a l i - a l k a l i n e e a r t h s i l i c a t e s it i s g e n e r a l l y a s low a s 2 o r 3
( 7 ) . The s i m i l a r i t y w i t h t h e l o c a l s t r u c t u r e i n t h e g l a s s e s i s s t r i k i n g . I n a d d i t i o n t o t h e Na EXAFS r e p o r t e d h e r e f o r s i l i c a t e and b o r a t e g l a s s e s , d i s t i n c t EXAFS a t Rb K edge (0.82
8)
i n s e v e r a l germanate g l a s s e s h a s a l s o been observed. D e t a i l s o f t h e s e measurements w i l l be g i v e n i n a f u t u r e p u b l i c a t i o n .References
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5,
(1977) 87; MISAWA M., PRICE D.L. and SUZUKI K., J. Non-Cryst. S o l i d s ,37,
(1980) 85.3. GREAVES G.N., FONTAINE A., LAGARDE P., RAOUX D. and GURM?+N S.J. ( t o b e p u b l i s h e d i n N a t u r e ) .
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G,
(1975) 27955 . ELLIS E., GASmLL P.H. and JOHNSON D.W., on-Crystalline S o l i d s (Trans.
Tech. P u b l i c a t i o n s , 1977) P. 312.
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54,
(1932) 3841.7. WYCKOFF R.N. G., C r y s t a l S t r u c t u r e s (New York. Wiley, 1964)
