HAL Id: jpa-00220896
https://hal.archives-ouvertes.fr/jpa-00220896
Submitted on 1 Jan 1981
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
INTRINSICALLY BROKEN CHALCOGEN CHEMICAL ORDER IN STOICHIOMETRIC
GLASSES
W. Bresser, P. Boolchand, P. Suranyi, J. de Neufville
To cite this version:
W. Bresser, P. Boolchand, P. Suranyi, J. de Neufville. INTRINSICALLY BROKEN CHALCOGEN
CHEMICAL ORDER IN STOICHIOMETRIC GLASSES. Journal de Physique Colloques, 1981, 42
(C4), pp.C4-193-C4-196. �10.1051/jphyscol:1981439�. �jpa-00220896�
JOURNAL DE PHYSIQUE
CoZZoque C4, suppZ6ment au nolO, Tome 42, octobre 1981 page C4-193
INTRINSICALLY BROKEN CHALCOGEN CHEMICAL ORDER IN STOICHIOMETRIC GLASSES
W . J . B r e s s e r , P. Boolchand, P. S u r a n y i and J . P . d e ~ e u f v i l l e *
Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, U.S.A.
"
~ n e r ~ ~ Conversion Devices, Troy, Michigan 48048, U. S . A.A b s t r a c t . - The c h a l c o g e n s i t e i n b u l k g l a s s e s of GeSe2 and GeS2 h a s been probed v i a n u c l e a r q u a d r u p o l e i n t e r a c t i o n s u s i n g 1 2 g m ~ e i m p u r i t y atoms a s p a r e n t i n 1291 Miissbauer e m i s s i o n s p e c t r o s c o p y . Two d i s t i n c t t y p e s of c h a l - cogen s i t e s a r e observed and a r e shown t o p r o v i d e d i r e c t e v i d e n c e f o r i n t r i n - s i c a l l y broken chemical o r d e r i n g i n t h e s e network g l a s s e s . The c o m p o s i t i o n a l dependence of t h e s i t e p a r a m e t e r s i n pseudo-binary a l l o y g l a s s e s i n d i c a t e s t h e p r e s e n c e of c h a r a c t e r i s t i c c l u s t e r s r a t h e r t h a n a c o n t i n u o u s random network.
I n t r o d u c t i o n . - The n a t u r e of t h e e q u i l i b r i u m network d e s c r i b i n g t h e s t r u c t u r e of t h e good g l a s s f o r m e r s GeX2, where X = S and Se, h a s evoked p a r t i c u l a r r e c e n t i n - terest. Most w o r k e r s have d e s c r i b e d t h e s e c h a l c o g e n i d e g l a s s e s t o c o n s i s t of a c h e m i c a l l y o r d e r e d c o n t i n u o u s random network of Ge(XLJ4 u n i t s , a s was o r i g i n a l l y proposed by Z a c h a r i e s e n . However s u c h s i m p l i s t i c s t r u c t u r a l models have been q u e s t i o n e d on t h e b a s i s t h a t i n t e r a c t i o n s between c h a l c o g e n l o n e p a i r e l e c t r o n s s h o u l d l e a d t o a m u l t i p l i c i t y of chemical bonding c o n f i g u r a t i o n s i n t h e s e
m a t e r i a l s (1). R e c e n t l y i t h a s been s u g g e s t e d t h a t t h e s e g l a s s e s may a c t u a l l y con- s i s t of p a r t i a l l y polymerized c a t i o n - r i c h (Ge-rich) and a n i o n ( c h a l c o g e n - r i c h ) c l u s t e r s ( 2 ) . The p a r t i a l l y polymerized c l u s t e r (PPC) model (3) s p e c i f i c a l l y r e - q u i r e s t h a t some like-atom bonds (homopolar) e x i s t s i n t h e network, i . e . t h e Ge and X c h e m i c a l o r d e r of t h e g l a s s network b e i n t r i n s i c a l l y broken. I n t h i s p a p e r we p r e s e n t d i r e c t e v i d e n c e f o r broken c h a l c o g e n c h e m i c a l o r d e r i n t h e s e s t o i c h i o m e t r i c b u l k g l a s s e s and show t h a t t h e s e r e s u l t s are. i n a c c o r d w i t h t h e PPC model.
E x p e r i m e n t a l method.- We have probed t h e c h a l c o g e n s i t e i n b o t h g-GeSe2 and g-GeS2 u s i n g N u c l e a r Quadrupole I n t e r a c t i o n s (NQI). An i m p o r t a n t consequence of l o n e - p a i r p e l e c t r o n s i n p n i c t i d e s and c h a l c o g e n s i s t h a t t h e EFG i s s t r o n g l y c o r r e l - a t e d t o t h e l o c a l c h e m i c a l c o o r d i n a t i o n and s e r v e s a s a v a l u a b l e probe of s h o r t r a n g e o r d e r . (< % a t . %) t h e s e l a s s e s w i t h t h e i s o e l e c t r o n i c S i n c e S and Se a r e n o t amenable t o N I measur m e n t s , we have doped
'3
5 ~ e and e29?Ce i s o t o p e s and measured N Q I a t lzgTe and 1 2 9 ~ u s i n g Massbauer s p e c t r o s c o p y (4).G l a s s e s o f GeSez-, Te, i n t h e r a n g e 1.20
2
x2
0.005 and of GeSz-, Tex x = 0.005 were p r e p a r e d s t a r t i n g from 99.9999% p u r e e l e m e n t s and s e a l i n g t h e s e i n e v a c u a t e d (< 10-6 T o r r ) q u a r t z ampules (3mm d i a . ) , and s l o w l y h e a t i n g t o 9 5 0 ' ~ f o r a n hour. The m e l t s ( c a . 400 mg) were e q u i l i b r a t e d f o r 24 h o u r s a t 9 0 0 ~ ~ b e f x e quenching i n c o l d w a t e r . The g l a s s e s p o s s e s s e d a w e l l d e f i n e d T which was found t o i n c r e a s e w i t h x i n e x c e l l e n t agreement t o t h e work of S a r r a c f e t a 1 ( 3 ) . These Tg r e s u l t s s u g g e s t t h e a b s e n c e of p h a s e s e p a r a t i o n i n t h i s p s e u d o b i n a r y .Mijssbauer s p e c t r a were t a k e n (4) a t 4.2K u s i n g a c o n s t a n t a c c e l e r a t i o n d r i v e . 125sb d i f f u s e d i n Cu was used a s a s o u r c e of monoenergetic 35.5 keV gamma r a y s i n t h e 1 2 5 ~ 8 e x p e r i m e n t s . Anhydrous Na 1291 was used a s a n a b s o r b e r t o a n a l y s e t h e e m i s s i o n s p e c t r a of t h e 1 2 9 m ~ e l a b e l l e d g l a s s e s .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981439
JOURNAL DE PHYSIQUE
Ex e r i m e n t a l r e s u l t s . - 1 2 5 ~ e s p e c t r a of t h e g l a s s e s show p a r t i a l l y r e s o l v e d doub- l e f s , which we a t t r i b u t e t o a q u a d r u p o l e s p l i t t i n g ( A ) i n t h e 3/2+ l e v e l . By f i t t i n g t h e s p e c t r a t o two e q u a l l y i n t e n s e l i n e s , we have e x t r a c t e d a mean < A >, t h e c o m p o s i t i o n a l v a r i a t i o n of which shows a s t r i k i n g l y l a r g e i n c r e a s e from 7.6 mm/s i n GeTe2 t o 10.8 m m / s i n GeSe2. I n I291 spectroscopy, p r e s e n c e of a u n i q u e N Q I l e a d s t o a 1 2 l i n e p a t t e r n which was a n a l y s e d k e e p i n g Q */Q f i x e d a t 1.238, t o y i e l d t h e q u a d r u p o l e c o u p l i n g ( e 2 q ~ ) ,
n,
and t h e isomer s h i f t ( 6 ) . Examples i n - c l u d e t r i g o n a l Te ( t - T e ) , amorphous Se (a-Se) and p l a s t i c S (p-S)(see F i g . 1 and T a b l e 1 ) . I n t h e s e e x p e r i m e n t s , one measures t h e N Q I a t 1291 formed from B decay of 1 2 g r n ~ e p a r e n t , and o b s e r v e s i n v a r i a b l y a s t a t i c i n t e r a c t i o n . Because of t h e narrow l i n e w i d t h (2Tn = 0.58 mm/s), one c a n r e s o l v e a t l e a s t two c h e m i c a l l y in- e q u i v a l e n t 1291 s i t e s i n t h e s p e c t r a of t h e p s e u d o b i n a r y g l a s s e s . T h i s i s shown i n F i g . 1 , where a q u a l i t a t i v e improvement i n t h e f i t t o t h e spectrum of g-GeSe2 r e s u l t s i n going from a one s i t e t o a two s i t e f i t . F u r t h e r , a d e c i d e d l y s u p e r i o r f i t t o t h e s p e c t r a of t h e g l a s s e s r e s u l t s o n l y i f a -ve s i g n i s chosen f o r e Z q ~ a t b o t h t h e sites. F i g s . 2 summarises t h e NQI r e s u l t s . The s i t e h a v i n g a lowerl e 2 q ~ l , which e x h i b i t s a weak dependence on x , w i l l h e n c e f o r t h b e c a l l e d s i t e A.
The o t h e r s i t e , h a v i n g t h e l a r g e r l e 2 q ~ l , which e x h i b i t s a s h a r p i n c r e a s e f o r x ( 0 . 4 , w i l l h e n c e f o r t h b e l a b e l l e d a s s i t e B.
I
I I * I-10 -5 0 5 10
VELOCITY ( mm/s )
F i g . 1 ( L e f t ) : 1 2 9 1 Massbauer s p e c t r a of 1 2 9 m ~ e s o u r c e s i n i n d i c a t e d h o s t s . F i g . 2 ( t o p ) : 1291 N Q I p a r a m e t e r s p l o t t e d a s a f u n c t i o n o f x i n g-GeSe2-,Tex.
Discussion.- Both 1 2 5 ~ e and 1 2 9 ~ s p e c t r a of t h e g l a s s e s a r e dominated by n u c l e a r quadrupole e f f e c t s and a r e s u g g e s t i v e of chemically i n e q u i v a l e n t Te s i t e s g i v i n g r i s e t o i n e q u i v a l e n t I s i t e s . The s t r i k i n g l y l a r g e i n c r e a s e (- 30%) i n < A > w i t h Se c o n t e n t , which i s n o n - l i n e a r , i s a l r e a d y an important r e s u l t . T h i s r e s u l t cannot be understood on t h e b a s i s of a chemically ordered network of c o r n e r s h a r i n g Ge(XQ4 u n i t s . Such a model of t h e g l a s s p e r m i t s only one type of a Te s i t e , which is 2 f o l d c o o r d i n a t e d t o Ge, and which remains i n d e endent of x. Because of t h e broad n a t u r a l l i n e w i d t h of t h e l 2 5 ~ e resonance (21nP= 5.2 mmls), one i s unable t o c l e a r l y r e s o l v e t h e asymmetric Te environments ( c h a r a c t e r i z e d by l a r g e A) which a p p a r e n t l y dominate a s x + 0. These s i t e s on t h e o t h e r hand a r e c l e a r l y r e s o l v e d i n t h e I291 experiments.
The -ve s i g n of eZqq s e e n a t t h e A and B s i t e s i m p l i e s t h a t t h e s e s i t e s r e p r e s e n t I291 t h a t is a bonded t o a Se o r a Ge o r a Te n e a r neighbor i n a nomi- n a l l y one f o l d c o o r d i n a t i o n (4) and f u r t h e r t h a t t h e 1 2 9 m ~ e p a r e n t i s 2-fold coor- d i n a t e d . T h i s i s m a n i f e s t a t i o n of a l o c a l bond rearrangement (4) t h a t o c c u r s on t h e Te + I chemical t r a n s f o r m a t i o n . One of t h e Te a bonds b r e a k s and forms a l o n e p a i r w h i l e t h e o t h e r IT bond becomes an I o bond c a u s i n g t h e d i r e c t i o n of eq t o f l i p by 90° and t o change i t s s i g n . From t h e weak x dependence of e2qq and constancy of b o t h 0 and 6 a t s i t e A, we immediately recognize t h i s s i t e t o b e an I-Ge a bond r e s u l t i n g from a two f o l d c o o r d i n a t e d Te s i t e : Ge-Te-Ge. A s expected t h i s s i t e a l s o appears i n g-GeS2 a s r e v e a l e d by t h e s i m i l a r i t y of s i t e A N Q I r e s u l t s (Table 1) between g-GeS2 and g-GeSe2. The
n
of z 0.5 i s s u g g e s t i v e t h a t t h e r e i s s i g n i f i - c a n t IT bonding of I t o i t s second neighbor Ge. And we b e l i e v e t h a t t h e s m a l l in- c r e a s e (-. 12%) i n e2qq w i t h x f o r s i t e A, r e s u l t s from a reduced I-Ge a bond l e n g t h < r > i n a GeSe2 g l a s s , c a u s i n g eq a r-3 t o b e somewhat l a r g e r t h a n i t s v a l u e i n a GeTe2 g l a s s .Host 2
e QVzz (MHz)
t-Te -397(2)
a-Se -1314(10)
P-s
-1453(6)GeSe2 A -860(12)
B -1360(9)
GeS2 A -936(15)
B -1432(10)
I d e n t i f i c a t i o n of s i t e B, a t l e a s t f o r x + 0, i s made uniquely p o s s i b l e by c o n t r a s i n g N Q I r e s u l t s of t h i s s i t e w i t h t h o s e i n a-Se (Table 1 ) . One i s s t r u c k by t h e c l o s e s i m i l a r i t y of e 2 q ~ , 0 and 6 f o r t h e s e s i t e s , and on t h i s b a s i s , we i d e n t i f y s i t e B w i t h an I-Se a bond. It i s remarkable in- deed, t h a t a n a l y s i s of t h e spectrum of g-GeS2 i n terms Table 1 l Z 9 1 N Q I parameters i n i n d i c a t e d h o s t s . of 2 s i t e s y i e l d s f o r s i t e B
N Q I parameters t h a t a r e i n e x c e l l e n t agreement t o t h o s e known f o r p-S (Table I ) , t h u s s t r o n g l y s u g g e s t i n g t h a t t h i s s i t e l i k e w i s e r e p r e s e n t s an I-S a bond. I d e n t i f i c a t i o n of t h e s e s i t e s i s f u r t h e r supported by t h e 6 ' s which a r e lower f o r s i t e A i n r e l a t i o n t o s i t e B
(Table 1 ) . We s u g g e s t t h i s i s due t o a h i g h e r i o n i c i t y of t h e I-Ge bond compared t o I-Se bond, causing e x c e s s 5p charge t o be l o c a l i z e d on I, which lowers t h e n e t 5 s charge d e n s i t y (isomer s h i f t ) a t t h e 1291 n u c l e u s by s h i e l d i n g a p a r t of i t .
The high i n t e n s i t y of B s i t e s ( I B > > I A ) i n b o t h g-GeSe2 and g-GeS2 demonstrates t h a t t h e r e occurs a l a r g e f r a c t i o n of I-S o r I-Se bonds i n r e l a t i o n t o I-Ge bonds i n t h e s e g l a s s e s . Since Te a s a d i l u t e impurity i s expected t o r e p l a c e S o r Se i~'.
t h e s e g l a s s e s , t h e d a t a imply t h e presence of Se-Se o r S-S homopolar bonds i n t r i n - s i c t o t h e s t r u c t u r e of t h e s e g l a s s e s . This i s t o s a y t h a t t h e chemical o r d e r of t h e g l a s s network i s i n t r i n s i c a l l y broken about t h e chalcogen s i t e s . Within t h e c o n t e x t of t h e PPC model, t h e p r o p o s a l of d i m e r i t e d chalcogen c o n f i g u r a t i o n s on t h e edges of t h e o u t r i g g e r r a f t s ( 3 ) p r o v i d e a n a t u r a l framework t o understand t h e o r i g i n of s i t e B a s : ~ e - T e - S e ( s ) ( 3 ) . The I daughter formed from such a Te s i t e choeses c l e a r l y t o a bond w i t h i t s Se(S) r a t h e r t h a n Ge neighbor and t h i s i s most l i k e l y due t o a l a r g e Van d e r Waals r e p u l s i v e i n t e r a c t i o n between non-bonded l o n e p a i r e l e c t r o n s (I-Se) which a p p a r e n t l y overwhelm t h e i o n i c i n t e r a c t i o n between I-Ge.
JOURNAL DE PHYSIQUE
I n d i c a t i o n s of s u b s t a n t i a l s t r u c t u r a l changes i n g-GeSe2 a s Se i s p r o g r e s s - i v e l y r e p l a c e d by Te become e v i d e n t from t h e s h a r p l y reduced b o t h e2qq and IB/IA n e a r x = 0 . 2 . T e , a s a d i l u t e i m p u r i t y , a p p a r e n t l y h a s t h e l a r g e s t p r o b a b i l i t y of r e p l a c i n g Se i n t h e d i m e r i s e d c o n f i g u r a t i o n where s t r a i n s due t o s u b s t i t u t i n g a l a r g e r atom can b e more e a s i l y r e l i e v e d . J u s t i f i c a t i o n of t h e s e i d e a s come from a s t a t i s t i c a l model ( 6 ) which we have d e v e l o p e d , and which r e p r o d u c e s t h e s t e e p x dependence of t h e r a t i o R = I B / I ~ i n t h e r a n g e 0 < x < 0.4.
F u r t h e r e v i d e n c e f o r t h e d i s t i n c t i v e r o l e p l a y e d by t h e p a r t i a l l y polymerized c l u s t e r s i n t h e g l a s s - f o r m i n g tendency which i s s t r o n g i n GeSe2 ( l a r g e Tx-Tg and weak i n GeTe2 ( s m a l l T,-T ) i s a s f o l l o w s . A l i n e a r e x t r a p o l a t i o n of t h e A- and B - s i t e s ~ Z Q V and TI i n F l g . g 1 f o r x > 1 shows t h a t t h e two s i t e s merge a s x + 2.
T h i s means t%zt t h e poor g l a s s - f o r m e r GeTe2 i s c l o s e t o b e i n g a COCRN. The same s i t u a t i o n a p p a r e n t l y p r e v a i l s w i t h t h e good ( p o o r ) g l a s s - f o r m e r s As2Se3(Te3) a s w e l l .
Conclusions.- We have d i s c u s s e d N Q I d a t a which d e m o n s t r a t e t h a t 1 2 9 1 Massbauer e m i s s i o n s p e c t r o s c o p y u s i n g 1 2 9 m ~ e p a r e n t i s a n o v e l and ~ a r t i c u l a r l y s u c c e s s f u l t o o l f o r p r o b i n g t h e a t o m i c s t r u c t u r e of network g l a s s e s . We have shown d i r e c t l y and s p e c i f i c a l l y t h a t t h e chemical o r d e r i s i n t r i n s i c a l l y broken even i n s t o i c h i o - m e t r i c c h a l c o g e n i d e g l a s s e s . T h i s s p e c i f i c k i n d of broken t o p o l o g i c a l symmetry h a s been shown t o a r i s e from t h e f o r m a t i o n of c l u s t e r s which c o r r e l a t e w e l l w i t h chemical t r e n d s i n t h e g l a s s - f o r m i n g tendency. According t o o u r d a t a , t h e s e c l u s t e r s p r o v i d e a much more s u i t a b l e b a s i s f o r d e s c r i b i n g t h e a t o m i c s t r u c t u r e of c h a l c o g e n i d e g l a s s e s t h a n d o e s t h e t r a d i t i o n a l random network model ( 7 ) .
Acknowledgements.- I t i s a p l e a s u r e t o acknowledge t h e encouragement and many v a l u a b l e comments of D r . J . C . P h i l l i p s d u r i n g t h e c o u r s e of t h i s work.
1 . Ovshinsky, S.R., A.I.P. Conf. P r o c . , Vol.
2,
(1976) 31.2. Bridenbaugh, P.M., E s p i n o s a , G.P., G r i f f i t h s , J.E., P h i l l i p s , J . C . , Remeika, J . P . , Phys. Rev. B
20
(1974) 4140.3. P h i l l i p s , J . C . , J Non-Cryst. S o l i d s
2
(1979) 153. and J . Non-Cryst. S o l i d s 43, (1981) 43.-
4. K i m , C.S. and Boolchand, P., Phys. Rev. B
19
(1979) 3187. and Boolchand, P . , B r e s s e r , W.J., E h r h a r t , G . J . , Phys. Rev. B3
(1981) 3669.5. S a r r a c h , D . J . , de N e u f v i l l e , J . P . , Haworth, W.L., J . Non-Cryst. S o l i d s
11
(1976) 245.
6. B r e s s e r , W . J . , e t a l . , ( m a n u s c r i p t under p r e p a r a t i o n )
7 . Nemanich, R.J., S o l i n , S.A., S o l i d S t a t e Comm.
