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Submitted on 1 Jan 1980
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A MODEL FOR THE STRUCTURE OF SOME
SEMICONDUCTING LIQUID ALLOYS : EVIDENCE
FOR IONIC BONDING
A. Copestake, R. Evans, M.M. Telo da Gama
To cite this version:
JOURNAL DE PHYSIQUE Cozzoque 68, suppz6ment au n08, Tome 41, aoct 1980, page
Cg-321
A MODEL FOR THE STRUCTURE OF SOME SEMICONDUCTING LIQUID ALLOYS
:
EVIDENCE FOR
IONIC BONDING
A . Copestake, R. Evans and M.M. Telo Da Gama
H.H. WiZZs Physics Laboratory, University o f BristoZ, BristoZ BS8
ITL,
U.K.A b s t r a c t
S e v e r a l liquid m e t a l l i c alloys exhibit a t r a n s i t i o n f r o m a m e t a l l i c to a 'semiconducting' s t a t e a s a function of concentration. At the p a r t i c u l a r s t o i c h i o m e t r i c composition w h e r e the t r a n s i t i o n o c c u r s the e l e c t r o n i c s t r u c t u r e of the alloy i s v e r y different in c h a r a c t e r f r o m that of the p u r e m e t a l l i c constituents. F o r example in CsAu t h e r e i s much evidence1 to suggest that t h i s i s a fully ionized m e l t consisting of CS' and AU- ions while f o r other a l l o y s such a s Mg Bi
3 2'
Cs3Sb and L i 4 P b t h e situation i s m o r e u n c e r t a i n and i t h a s been suggested that t h e s e a l l o y s may f c r m covalently bonded 'molecules' o r complexes. Diffraction e x p e r i m e n t s can play a u s e f u l r o l e
2
in the elucidation of the bonding in such a l l o y s
.
The p a r t i a l s t r u c t u r e f a c t o r s of a covalently bonded s y s t e m will exhibit f e a t u r e s which a r e not o b s e r v e d f o r ionic s y s t e m s and vice v e r s a . In o r d e r to i n t e r p r e t neutron and X-ray diffraction data on t h e s e liquid s e m i c o n d u c t o r s we have calculated the p a r t i a l s t r u c t u r e f a c t o r s a s s u m i n g an ionic model applies. M o r e specifically, we have modelled the i n t e r a t o m i c potentials by charged h a r d - s p h e r e s and computed the s t r u c t u r e3
f a c t o r s within t h e mean-spherical-approximation
.
The amount of c h a r g e t r a n s f e r f r o m one s p e c i e s to another and the d i a m e t e r s of t h e s p h e r e s can be varied. By comparing o u r calculated total neutron and X - r a y s c a t t e r e d i n t e n s i t i e s with the e x p e r i m e n ~ a l d a t a we can examine the-
validity ,of t h e ionic picture. We find s t r o n g evidence for fully ionic bonding in CsAu and f o r pronounced c h a r g e t r a n s f e r in Li4Pb and Mg3Bi2. We a r g u e that the l a r g e low-angle peak which i s o b s e r v e d in both the neutron and X - r a y d a t a 4 in L i P b a r i s e s f r o m molten s a l t - l i k e c h a r g e 4 ordering. A s i m i l a r c h a r g e - o r d e r i n g effect gives a good account of t h e s t r u c t u r e observed 5
in Mg3BiZ. F o r Cs3Sb the ionic model m a y be l e s s r e a l i s t i c .
A p r e l i m i n a r y account of t h e calculations and c o m p a r i s o n with e x p e r i m e n t 6 f o r CsAu has 7
been published
.
4 m o r e detailed p a p e r i s in preparation. 1. F o r a review :;ee y'. Hensel, 1979, Adv.Phys.,28,
555.2. J. E. Enderby, 1978, in 'The Metal Non-Metal Transition in Disordered Systems' ed.
L. R.
F r i e d m a n and D. P. Tunstall (University of Edinburgh P r e s s ) p. 425. 3. A Waisman and J.L.
Lebowitz, 1970, J. Chem. P h y s . ,52,
4307; 1972,Ibid,
E ,
3086, 3093.4. H. R u p p e r s b e r g and H. Egger, 1975, J. Chem. Phys., 2 , 4 0 9 5 . 5. A. Boos and S. Steeb, 1977, Phys. Lett.,
LA,
333.6. W. Martin, W. Freyland, P. L a m p a r t e r and S. Steeb, 1980, Phys. and Chem. of Liquids (to be published).
7. R. Evans and M. M. Telo Da Gama, 1980, PhiL Mag., B 4L 351.
