A MODEL FOR THE STRUCTURE OF SOME SEMICONDUCTING LIQUID ALLOYS : EVIDENCE FOR IONIC BONDING

Abstract
Several liquid metallic alloys exhibit a transition from a metallic to a "semiconducting" state as a function of concentration. At the particular stoichiometric composition where the transition occurs the electronic structure of the alloy is very different in character from that of the pure metallic constituents. For example in CsAu there is much evidence to suggest that this is a fully ionized melt consisting of Cs⁺ and Au^- ions while for other alloys such as Mg₃Bi₂, Cs₃Sb and Li₄Pb the situation is more uncertain and it has been suggested that these alloys may form covalently bonded "molecules" or complexes. Diffraction experiments can play a useful role in the elucidation of the bonding in such alloys. The partial structure factors of a covalently bonded system will exhibit features which are not observed for ionic systems and vice versa. In order to interpret neutron and X-ray diffraction data on these liquid semiconductors we have calculated the partial structure factors assuming an ionic model applies. More specifically, we have modelled the interatomic potentials by charged hard-spheres and computed the structure factors within the mean-spherical-approximation. The amount of charge transfer from one species to another and the diameters of the spheres can be varied. By comparing our calculated total neutron and X-ray scattered intensities with the experimental data we can examine the validity of the ionic picture. We find strong evidence for fully ionic bonding in CsAu and for pronounced charge transfer in Li₄Pb and Mg₃Bi₂. We argue that the large low-angle peak which is observed in both the neutron and X-ray data in Li₄Pb arises from molten salt-like charge ordering. A similar charge-ordering effect gives a good account of the structure observed in Mg₃Bi₂. For Cs₃Sb the ionic model may be less realistic. A preliminary account of the calculations and comparison with experiment for CsAu has been published. A more detailed paper is in preparation.