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LASER-RAMAN STUDY OF THE STRUCTURE OF SILVER DOPED As40S60 GLASS
A. Firth, A. Owen, P. Ewen
To cite this version:
A. Firth, A. Owen, P. Ewen. LASER-RAMAN STUDY OF THE STRUCTURE OF SIL- VER DOPED As40S60 GLASS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-903-C4-906.
�10.1051/jphyscol:19814196�. �jpa-00220823�
JOURNAL DE PHYSIQUE
CoZZoque C4, suppZe'ment au nO1O, Tome 4 2 , octobre 1981 page C4-903
LASER-RAMAN STUDY OF THE STRUCTURE OF SILVER DOPED AsqOSsO GLASS
A.P. F i r t h , A . E . Owen and P . J . Ewen
Department of EZectricaZ Engineering, University o f Edinburgh, King's Bui Zdings, Edinburgh EH9 3JL, Scot Land, U. K.
A b s t r a c t . - The Raman s p e c t r a of Ag doped a-As2S3 g l a s s with compositions along t h e pseudo-binary l i n e As2S3-P.g2S i s presented. Small amounts of Ag have l i t t l e e f f e c t on t h e s t r u c t u r e of a-As2S3 whereas g l a s s e s containing l a r g e r amounts (> 21 atm %) c o n t a i n s t r u c t u r a l u n i t s found i n corresponding c r y s t a l l i n e forms.
I n t r o d u c t i o n . - This paper i s concerned with t h e s t r u c t u r e of Ag-As-S g l a s s e s with a view t o understanding t h e s t r u c t u r a l changes which r e s u l t from t h e photodoping of Ag i n t o a-As2S3.
Glass formation i n t h e Ag-As-S system has been s t u d i e d by Kawamoto e t a1 (1) and Golovach e t a1 ( 2 ) . Their r e s u l t s a r e i n general agreement and i n d i c a t e two g l a s s forming regions. Glasses c l o s e t o t h e composition of a-As2S3, and an i s l a n d - shaped region lying on t h e pseudo-binary As2S3-Ag2S centred on t h e composition AgAsS2.
In t h i s study t h e Raman s p e c t r a of g l a s s e s within t h e two regions was s t u d i e d and an attempt a t i d e n t i f y i n g t h e s t r u c t u r a l u n i t s p r e s e n t i n t h e g l a s s e s was made.
Experimental.- All t h e amorphous samples were bulk g l a s s e s . The Ag containing g l a s s e s were prepared by r a p i d cooling from t h e m e l t and t h e a-AsZS3 was a commer- c i a l sample of high o p t i c a l q u a l i t y . Glassy As2S3 has a deep red-colour and s l i g h t l y t r a n s p a r e n t ; with t h e a d d i t i o n of small amounts of Ag t h e r e i s l i t t l e e f f e c t on transparency but t h e colour becomes a deeper red. The g l a s s containing 20 a t % Ag was only very s l i g h t l y t r a n s p a r e n t whereas t h e g l a s s e s containing 25 and 30 a t % were opaque. For comparative purposes, s p e c t r a on some c r y s t a l l i n e samples a r e a l s o reported. C r y s t a l l i n e AgAsS2 was prepared by slow cooling of i t s melt while c r y s t a l l i n e samples of Ag40A~16S44 (which i s c l o s e t o p r o u s t i t e ) were obtained even on r a p i d quenching. The stoichiometry of each sample was checked by e l e c t r o n microprobe a n a l y s i s and i t a c c u r a t e t o within
+ 4
a t%.
0
All t h e samples were e x c i t e d with red l i g h t of wavelength 6764 A from a Kr-ion l a s e r . A Coderg computer-controlled T800 t r i p l e spectrometer w i t h a cooled RCA-C3 1034 A p h o t o m u l t i p l i e r o p e r a t i n g i n t h e photon-counting mode was used f o r d e t e c t i o n . The s p e c t r a were recorded a t room temperature using back r e f l e c t i o n and a s p e c t r a l s l i t width of = 3 cm-1 was used. To avoid t h e damaging e f f e c t o f
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814196
C4-904 JOURNAL DE PHYSICUE
focused r a d i a t i o n only s l i g h t focusing from a c y l i n d r i c a l l e n s was used. The beam power was t y p i c a l l y 100 mW and t o remove plasma l i n e s a prism f i l t e r was used.
Results and Discussion.- Figure 1 shows t h e p o l a r i s a t i o n unanalysed Stokes-Raman s p e c t r a of s i x g l a s s e s whose compositions l i e along t h e pseudo-binary l i n e
As2S3
-
Ag2S. There a r e two groups of t h r e e s p e c t r a , corresponding t o compositions which l i e within t h e two regions of g l a s s formation ( t h e s e a r e t h e bottom t h r e e and top t h r e e r e s p e c t i v e l y ) . I t i s c l e a r t h a t t h e s p e c t r a l c h a r a c t e r i s t i c s of t h e two regions of g l a s s formation a r e very d i f f e r e n t .The s p e c t r a of t h e two g l a s s e s containing small amounts of A9 a r e very s i m i l a r t o the spectrum of a-Asps3 which i n d i c a t e s t h a t small amounts of Ag have no s i g n i f i - c a n t e f f e c t on t h e s t r u c t u r e of a-As2S3. The only d e t e c t a b l e changes i n t h e un- analysed s p e c t r a a s t h e Ag content i s increased a r e t h a t t h e sho l d e r a t 315 cm-'
r
d i s a p p e a r s , t h e main peak a t 338 cm-l s h i f t s s l i g h t l y t o 341 cm- and t h e band a t 490 cm-1 i n c r e a s e s s l i g h t l y i n i n t e n s i t y .
In c o n t r a s t , t h e s p e c t r a of t h e g l a s s e s containing l a r g e amounts of Ag ( t o p t h r e e s p e c t r a ) d i f f e r considerably from t h e a-As2S3 spectrum. The main high- frequency band (between
-
300 and 400 cm-') of t h e s e g l a s s e s has a broad asymmetric p r o f i l e , t h e r e i s a shoulder a t-
350 cm-1 and a broad peak above 369 cm-1. As t h e Ag content i s increased from 21 t o 30 a t % t h i s peak becomes more pronounced and s h i f t s from 369 t o 376 cm-l. Between 100 and 300 cm-1 t h e Ag21A~29S50 spectrum- -
e x h i b i t s s t r u c t u r e s i m i l a r t o t h a t i n t h e a-As2S3 spectrum but i n t h e Ag25A~25S50 and Ag30A~22S48 s p e c t r a t h i s region i s f e a t u r e l e s s . However, a l l t h r e e s p e c t r a e x h i b i t increased s c a t t e r i n g i n t h e region 200-300 cm-l, r e l a t i v e t o t h e a-As2S3 spectrum. Also, a l l t h r e e s p e c t r a contain a broad band a t 468 cm-I; t h i s band i s most pronounced i n t h e Ag25A~25S50 spectrum.
Figure 2 shows t h e VH p o l a r i s e d Raman s p e c t r a corresponding t o t h e s i x compositions of Figure 1. The l e t t e r s VH i n d i c a t e t h a t t h e i n c i d e n t l i g h t was v e r t i c a l l y p o l a r i s e d and t h a t t h e analysed l i g h t was h o r i z o n t a l l y p o l a r i s e d r e l a t i v e
F I G U E I THE W N A l l S t D S I N E S R L M N SPtClRA OF C O I I P O S I T I M I M T L I E ILPK;
itf PItU00-BINAQI L I N L I ~ S ~ - ~ ~ S T M SP[CIRA L l h W2W.lY3 TO 1% H I G H 1 ff € It 338 ~e BANl O F a - 1 ~ ~ 5 ~
t o t h e s c a t t e r i n g p l a n e . The VH s p e c t r a d i f f e r c o n s i d e r a b l y f r o m t h e unanalysed s p e c t r a w i t h t h e e x c e p t i o n o f t h e Ag25A~25S50 spectrum. The main d i f f e r e n c e s a r e i n t h e shape o f t h e 300-400 cm-' band and i n t h e i n t e n s i t y o f t h e l o w - f r e q u e n c y boson peak r e l a t i v e t o t h i s band. F o r example, t h e VH spectrum o f a-As2S3 has a peak a t 315 cm-' c o r r e s p o n d i n g t o a s h o u l d e r i n t h e unanalysed spectrum b u t does n o t have a peak a t 338 cm-l. I t f o l l o w s t h a t t h e 315 and 338 cm-1 bands a r e de- p o l a r i s e d and p o l a r i s e d r e s p e c t i v e l y and hence t h e 315 cm-l bands r e s u l t s f r o m an asymmetric v i b r a t i o n and t h e 338 cm-l band f r o m a symmetric v i b r a t i o n .
I n t h e case o f t h e c o m p o s i t i o n s c o n t a i n i n g s m a l l amounts o f Ag a Raman band forms a t ?38 cm-1 and outgrows t h e peak a t 315 cm-1 i n t h e a-As2S3 spectrum as t h e
- .
Ag c o n t e n t i n c r e a s e s . The VH s p e c t r a o f t h e c o m p o s i t i o n s Ag21A~29S50 and Ag30As22S48 have broad peaks a t 329 and 335 cm-' r e s p e c t i v e l y , c o r r e s p o n d i n g t o s h o u l d e r s i n t h e unanalysed s p e c t r a . The broad peaks a t 369 and 376 cm-' i n t h e r e s p e c t i v e unanalysed s p e c t r a a r e a b s e n t f r o m t h e VH s p e c t r a and hence a r e p c l a r i s e d and a r i s e f r o m symmetric v i b r a t i o n s . The VH spectrum o f Ag25A~25S50 has a s i m i l a r p r o f i l e t o t h e unanalysed spectrum and t h e d e p o l a r i s a t i o n spectrum o f t h i s m a t e r i a l shows v e r y l i t t l e s t r u c t u r e . T h i s suggests t h a t a ' m o l e c u l a r ' model may n o t be a p p r o p r i a t e f o r t h i s c o m p o s i t i o n .
F o r t h e c o m p o s i t i o n s c o n t a i n i n g s m a l l amounts o f Ag t h e changes t h a t o c c u r i n t h e s p e c t r a as t h e Ag c o n t e n t i n c r e a s e s a r e s i m i l a r t o t h o s e r e p o r t e d f o r b i n a r y As-S g l a s s e s as t h e S c o n t e n t i s i n c r e a s e d by a s m a l l amount above 60 a t % ( 3 ) . Hence t h e changes observed f o r t h e Ag-As-S c o m p o s i t i o n s may s i m p l y r e f l e c t t h e i n c r e a s e i n t h e S:As r a t i o t h a t o c c u r s as one proceeds f r o m As2S3 t o Ag2S a l o n g t h e pseudo-binary l i n e . T h i s would i m p l y t h a t t h e Ag has v e r y l i t t l e e f f e c t on t h e Raman spectrum. A l t e r n a t i v e l y , i t s e f f e c t may be masked by t h e s p e c t r a l changes r e s u l t i n g f r o m t h e i n c r e a s e i n t h e S:As r a t i o . F u r t h e r e x p e r i m e n t s a r e p r e s e n t l y b e i n g c a r r i e d o u t t o d e t e r m i n e t h e c o n t r i b u t i o n o f t h e change i n t h e S:As r a t i o t o t h e s p e c t r a o f t h e Ag-As-S g l a s s e s .
I n t h e case o f t h e h e a v i l y doped g l a s s e s t h e s p e c t r a l changes o c c u r r i n g w i t h i n c r e a s i n g Ag c o n t e n t can be a t t r i b u t e d t o t h e appearance o f degenerate modes o f t h e As2S3 pyramid u n i t ( 4 ) as a r e s u l t o f t h e asymmetry produced i n t h i s u n i t b y t h e
i n c o r p o r a t i o n o f Ag atoms i n t h e As-S network. The Ag atoms may d i s p l a c e S atoms a t t h e c o r n e r s of t h e pyramids o r , more l i k e l y , f o r m S-Ag-S b r i d g e s between pyramids; such b r i d g e s a r e observed i n t h e s t r u c t u r e o f t h e c r y s t a l s s q i t h i t e ( 5 )
C4-906 JOURNAL DE PHYSIQUE
(AgAsS?) and p r o u s t i t e ( 6 ) (Ag3AsS3). The i n c o r p o r a t i o n of Ag atoms between t h e pyramids d i s t o r t s t h e pyramid geometry and s p l i t s t h e degenerate modes of t h e pyramid u n i t . The S-Ag-S bridges a r e a l s o expected t o c o n t r i b u t e t o t h e s p e c t r u
1 !'
and a r e probably r e s p o n s i b l e f o r t h e 'ncreases s c a t t e r i n g between 200 and 300 cm-
.
S t r e t c h i n g f r e q u e n c i e s around 185 cm- ( 7 ) a r e expected f o r t h i s bridge ' m o l e c u l e ' . The o r i g i n of t h e band a t 468 cm-' i n t h e s p e c t r a of t h e heavily doped samples has not y e t been determined, although we note t h a t t h i s frequency i s approximately double t h a t of t h e c e n t r e of t h e region of increased s c a t t e r i n g . I t i s i n t e r e s t i n g t h a t t h e band i s most i n t e n s e i n t h e Ag25A~25S50 spectrum. This may be r e l a t e d t o t h e absence of s t r u c t u r e i n t h e d e p o l a r i s a t i o n spectrum of
Ag25A~25S50, which i n d i c a t e s t h a t t h e i n t e r - u n i t bonding i s strong f o r t h i s p a r t i c u l a r g l a s s . This composition a l s o has a c r y s t a l l i n e form, s m i t h i t e , AgAsS2.
The Raman s p e c t r a of a- and c-Ag25A~25S50 a r e compared i n Figure 3 and s i m i l a r l y a comparison between a-Ag30As22S48 and c-kg40A~16S44
-
which i s c l o s e t o t h estoichiometry of p r o u s t i t e Ag3AsS3 - i s shown i n Figure 4 . These two f i g u r e s show t h a t corresponding c r y s t a l l i n e and amorphous phases produce Raman s p e c t r a w i t h a s i m i l a r p r o f i l e . From t h i s i t may be deduced t h a t a-Ag25As75S50 i s formed from
- - - - - -
s t r u c t u r a l u n i t s found i n s m i t h i t e and t h a t a-Ag30A~22S48 c o n t a i n s s t r u c t u r a l u n i t s found i n p r o u s t i t e .
References
1. Kawamoto, Y. e t a l . J . Amer. Ceram. Soc. 56 (1973), 289.
2. Golovach, 1.1. e t a l . Fizika i khimya s t e k l a 3 (1977), 463.
3. Ewen, P.J. P h . D . T h e s i s , University of Edinburgh (1978).
4. F i r t h , A . P . M.Sc. T h e s i s , University of Edinburgh (1980).
5. S t r u c t u r e Reports 29, 25.
6. S t r u c t u r e Reports 31A, 19.
7 . Byer, H . H . e t a l . Ferro e l e c t r i c s 5 (1973), 207.
