solid compound semiconductor Cs3 Sb suggests a similar type of bonding, namely by ... predominantly ionic bonding has been given by a ... In the following, a summary of the relations used ... term in (6) vanish if both kinds of atoms have the.
Neutron Diffraction Study on the Structure of Liquid Cs-Sb Alloys P. Lamparter, W. Martin, and S. Steeb Max-Planck-Institut f ü r Metallforschung, Institut f ü r Werkstoffwissenschaften. Stuttgart
W. Freyland
Institut f ü r Physikalische C h e m i e der Universität M a r b u r g
Z. Naturforsch. 38 a, 3 2 9 - 3 3 5 (1983); received N o v e m b e r 11, 1982 By neutron d i f f r a c t i o n experiments t h e total structure factors and the total p a i r correlation functions of liquid C s - S b alloys containing 85, 75, 65, a n d 50 at% Cs, respectively, were determined. The structural results c o n f i r m t h e n o n metallic p r o p e r t i e s of Cs-Sb melts. T h e correspondence of the nearest n e i g h b o u r a t o m i c a r r a n g e m e n t in liquid Cs 7 5 Sb 2 5 and in the solid c o m p o u n d s e m i c o n d u c t o r C s 3 S b suggests a similar type of bonding, n a m e l y by valence b o n d s and ionic forces simultaneously. T h e stability of this c o m p o u n d in the molten state leads to a microsegregation tendency between c o m p o u n d f o r m i n g regions and excess Cs in the concentration range from p u r e Cs up to 25 at% Sb, which is established by a small angle scattering effect. Proceeding f r o m Cs 7 5 Sb 2 5 to Cs 5 oSb 5 o, a continuous c h a n g e in the structure takes place. Covalently bonded Sb chains are f o r m e d just as f o u n d in the corresponding solid c o m p o u n d s A S b (A = alkali metal). An additional d i f f r a c t i o n peak in f r o n t of the m a i n p e a k of the structure factors within this c o m p o s i t i o n range implies the f o r m a t i o n of rather large m o l e c u l a r clusters in the alloys.
Introduction A number of liquid alloy systems f o r m e d from metallic constituents is known, which exhibit non metallic properties near certain stoichiometric compositions. In liquid Au 5 oCs 5 o [1], Li 80 Pb 2 o [2], and Mg 6 6 7 Bi 3 3 3 [3] e.g. the electrical conductivity drops by orders of magnitude compared to that of the pure elements. This metal-non metal transition clearly requires a change in the bonding characteristics to occur in these concentration ranges. In the last few years the nature of the chemical bonding in such semiconducting alloys was the subject of various experimental and theoretical studies (see e.g. [ 4 - 6 ] ) , emphasizing the question whether it is ionic due to charge transfer between the components or whether it is covalent and thus forming molecular units in the alloy. For the case of the Au-Cs system evidence for predominantly ionic bonding has been given by a variety of investigations [1, 5, 7 - 1 0 ] . Concerning the Cs-Sb system, however, which exhibits a steep metal-non metal transition at the stoichiometric composition Cs 3 Sb, from recent investigations of the Reprint requests to Prof. Dr. S. Steeb, MPI f ü r Metallforschung, Institut f ü r Werkstoffwissenschaften, S e e s t r a ß e 92, 7000 Stuttgart 1.
electronic properties it was concluded that liquid Cs-Sb alloys are not essentially ionic, but that also covalent bonding takes place [7, 11], T h e aim of the present study was to obtain information on the type of bonding in liquid Cs-Sb alloys by investigation of the atomic scale structure by means of neutron diffraction experiments. Even though from a diffraction experiment the nature of the chemical bonding obviously cannot be seen directly, this is closely related to the atomic structure of a liquid alloy. Basic equations In the following, a summary of the relations used in the present study will be given. For a comprehensive review see e.g. [12], According to the F a b e r Ziman definition [13] the total structure factor SFZ(Q)
is o b t a i n e d f r o m t h e c o h e r e n t l y
scattered
intensity per atom / (/?): cAbl (b> 2cAbAbB + (by
Cnbl (b)2 GAB
(R) •
j 4T:R2Q(R)
R"
is related to the partials by (6): SBT(0 = ^
+
W
0
(C?)
+
2(b){bA-bB) (b2)
5NC(Ö).
(6)
• S N N ( 0 describes the correlations between n u m b e r density fluctuations, S c c ^ ) those between concentration fluctuations, and S N C ( 0 the cross correlations which are present for the case of d i f f e r e n t atomic sizes in the binary alloy. It should be noticed that the second and the third term in (6) vanish if both kinds of atoms have the same scattering length. T h a t m e a n s that concentration fluctuations do not contribute explici t l y to the total scattering, b u t only implicitely to S N N ( 0 v i a the cross correlations S , N C ( 0 . This can easily be seen f r o m the t h e r m o d y n a m i c limit of (6) (Q - 0), for bA = bB- see e.g. [14] - : SBT(0) = W O )
(3)
- Q o X i k B T +
