Fig. 1. Total and partial structure factors, S X,N (Q) a n d S i j ( Q ) , r e s p e c t i v e l y , f r o m t h e R M C m o d e l (blue lines) for vitreous B 2 O 3 , in comparison with the experimental total structure factors (red lines). 1.5 1.0 0.5 0.0 2.0 1.5 1.0 0.5 0.0 6 4 2 0 -2 -4 -6 S N ( Q ) S X ( Q ) S ij ( Q ) 35 30 25 20 15 10 5 0 Q (Å -1 ) H i g h - e n e r g y X - r a y ( E 3 0 k e V ) d i f f r a c t i o n ( H E X R D ) u s i n g a s y n c h r o t r o n r a d i a t i o n s o u r c e a f f o r d s s e v e r a l a d v a n t a g e s i n s t u d i e s o f t h e s t r u c t u r e s o f e n c a p s u l a t e d l i q u i d s a n d g l a s s e s i n t r a n s m i s s i o n g e o m e t r y , i n c l u d i n g : ( i ) h i g h e r r e s o l u t i o n i n r e a l s p a c e d u e t o a w i d e r a n g e o f m o m e n t u m t r a n s f e r ( Q ) , ( i i ) s m a l l e r c o r r e c t i o n HIGH-ENERGY X-RAY STUDY OF THE STRUCTURE OF VITREOUS B 2 O 3 s t r u c t u r e f a c t o r s S ( Q ) , s i m u l t a n e o u s l y . O n t h e basis of the RMC model, we discuss the validity of the boroxol ring model for vitreous B 2 O 3 . The experimental X-ray-weighted and neutron- w e i g h t e d s t r u c t u r e f a c t o r s , S X ( Q ) a n d S N ( Q ) , a r e s h o w n a s r e d l i n e s i n F i g . 1 . S X ( Q ) e x h i b i t s significant oscillations up to the maximum Q value terms (especially for absorption), ( iii ) the u s e o f e x t r e m e s a m p l e e n v i r o n m e n t s ( h i g h - t e m p e r a t u r e , h i g h - p r e s s u r e ) , a n d ( i v ) t h e a b i l i t y o f d i r e c t c o m p a r i s o n b e t w e e n X - r a y a n d n e u t r o n d i f f r a c t i o n d a t a . T h e H E X R D t e c h n i q u e , i n c o n j u n c t i o n w i t h m o d e l c a l c u l a t i o n s a n d n e u t r o n d i f f r a c t i o n e x p e r i m e n t s , c o m p r i s e s o n e o f t h e b e s t m e t h o d s available for investigating the structure of liquids and glasses. This study was undertaken in order to i n v e s t i g a t e t h e s h o r t - a n d i n t e r m e d i a t e - r a n g e s t r u c t u r e o f v i t r e o u s B 2 O 3 . H i g h - e n e r g y ( 4 0 . 9 k e V [ 1 ] a n d 6 1 . 7 k e V ) syn chr otr on X-r ay dif fra cti on exp eri men ts w e r e p e r f o r m e d t o o b t a i n a n a c c u r a t e structure factor, S ( Q ), for vitreous B 2 O 3 up to high Q (~ 35 Å -1 ), with small systematic c o r r e c t i o n s . T h e X - r a y d i f f r a c t i o n me as ur em en ts w er e ca rr ie d ou t at BL 04 B2 and BL14B1 bending magnet beamlines. T h e r e v e r s e M o n t e C a r l o ( R M C ) modelling technique [2] was then applied t o b o t h t h e H E X R D a n d t h e p u b l i s h e d n e u t r o n d i f f r a c t i o n d a t a [ 3 ] . R M C simulations were carried out on a system c o n t a i n i n g 4 0 0 0 a t o m s u s i n g t h e X - r a y - w e i g h t e d a n d n e u t r o n - w e i g h t e d [ 3 ] t o t a l 4 3 2 1 0 4 3 2 1 0 B-O-B 120 60 0 180 120 60 0 O-B-O O-O-O B-B-B θ θ (degree) Probability ( a.u. ) Fig. 2. Bond angle distributions for vitreous B 2 O 3 . o f 3 5 Å - 1 , a l t h o u g h t h e o s c i l l a t i o n s i n S N ( Q ) are at a considerably higher Q (> 4 0 Å - 1 ) . T h e r e i s a l a r g e d i s c r e p a n c y a r o u n d 3 - 4 Å - 1 , b e c a u s e o f t h e d i f f e r e n c e i n t h e i n t r i n s i c c o h e r e n t scattering cross sections. The results of the RMC simulation are plotted as blue lines in Fig. 1 for comparison. Excellent a g r e e m e n t i s o b t a i n e d f o r b o t h t h e s t r u c t u r e f a c t o r s , a p a r t f r o m m i n o r d e v i a t i o n i n S X ( Q ) . T h e R M C m o d e l s h o u l d b e a b l e t o p r o v i d e r e l a t i v e l y detailed structural information, because of the con tra st bet wee n the S N ( Q ) and S X ( Q ) . T h e p a r t i a l s t r u c t u r e f a c t o r s , S i j ( Q ) , a r e a l s o s h o w n i n F i g . 1 . T h e f i r s t s h a r p d i f f r a c t i o n p e a k ( F S D P ) o b s e r v e d a t Q ~ 1 . 6 Å - 1 i m p l i e s t h e p r e s e n c e o f i n t e r m e d i a t e - r a n g e o r d e r d u e t o t h e c a g e s f o r m e d b y t h e t o p o l o g i c a l c o n n e c t i o n o f B O 3 u n i t s i n t h e n e t w o r k [ 4 ] . T h i s i n t e r p r e t a t i o n i s s u p p o r t e d b y t h e R M C r e s u l t s , a s t h e F S D P i s f o u n d t o b e a p o s i t i v e f e a t u r e i n a l l t h r e e p a r t i a l structure factors at Q ~ 1.6 Å -1 in Fig. 1 . The bond angle distributions for B-B-B, O-O-O, B-O-B, and O-B-O are given in Fig. 2 . The O-B-O and O-O-O distributions show a maximum close to th e ex pe ct ed va lu es fo r a re gu la r BO 3 tr ia ng le of 120˚ and 60˚, respectively. The B-O-B angles are als o nar row ly dis tri but ed aro und a max imu m nea r 1 2 0 ˚ . T h e p r e s e n c e o f a l a r g e n u m b e r o f p l a n a r boroxol rings ( B 3 O 6 ) is indicated by the small sharp p e a k a t 6 0 ˚ i n t h e B - B - B b o n d a n g l e d i s t r i b u t i o n . Recent neutron scattering studies of vitreous B 2 O 3 indicate a model in which the majority of the boron atoms are in boroxol groups [3] . Figure 3 represents a 5 Å thick section of the R M C c o n f i g u r a t i o n , i n w h i c h t h e p r e s e n c e o f t h e b o r o x o l r i n g s i s c l e a r l y v i s i b l e . T o e s t i m a t e t h e fract ion of the borox ol rings in vitre ous B 2 O 3 from the RMC configuration, the probability of finding a boron atom, which forms the B-B-B angle of 60˚ in t h e f i r s t c o o r d i n a t i o n s h e l l f o r B - B c o r r e l a t i o n , i s ca lc ul at ed in Fi g. 3 . Th e va lu e of th e pr ob ab il it y w a s f o u n d t o b e a b o u t 0 . 2 , i m p l y i n g t h a t t h e fraction of boroxol rings in vitreous B 2 O 3 is not so large. In order to obtain a more detailed understanding of th e fr ac ti on of bo ro xo l ri ng s in th e st ru ct ur e of v i t r e o u s B 2 O 3 , s t r u c t u r a l m o d e l s o f p r e s e r v e - compacted B 2 O 3 glasses are being generated from R M C f i t s t o h i g h - e n e r g y X - r a y a n d n e u t r o n diffraction data. Fig. 3. A 5 Å thick slice of part of the largest RMC configuration for the borate (B-O) networks. B: O: 20 Å Kentaro Suzuya a and Shinji Kohara b (a) SPring-8 / JAERI (b) SPring-8 / JASRI E-mail: suzuya @ spring8.or.jp References [ 1 ] K . S u z u y a , S . K o h a r a , Y . Y o n e d a a n d N . Umesaki, Phys. Chem. Glasses 41 (2000) 282. [2] R.L. McGreevy and L. Pusztai, Molec. Simul. 1 (1988) 359. [ 3 ] A . C . H a n n o n e t a l . , J . N o n - C r y s t . S o l i d s 1 7 7 (1994) 299. [ 4 ] A . C . W r i g h t e t a l . , J . N o n - C r y s t . S o l i d s 1 2 9 (1991) 213.
home
- JP
- EN