Short-range lonic correlations in gold-cesium melts

Abstract

The molten gold-cesium system has received considerable attention as an example^1-3 of a liquid alloy which, though formed from metallic elements, exhibits non-metallic behaviour near stoichiometry. A stoichiometric solid compound CsAu, which has semiconducting properties and some degree of ionic binding, with gold playing the role of the negative constituent, has been known for a long time.⁴ The liquid structure of the system has been extensively studied by neutron diffraction by Martin et al. ^5-7 over a range of composition going from stoichiometry to pure cesium metal. Since the neutron scattering amplitudes of the two components are not very different, the observed diffraction pattern is mainly determined by the overall liquid structure and cannot display very strikingly the relative ordering of the constituents. Nevertheless, a broad hump is present near stoichiometry on the low-k side of the main peak in the diffraction pattern, which indicates^6,8 a marked preference for unlike nearest neighbours as in a molten ionic material.⁹ At cesium concentrations between 60 and 80 at %, on the other hand, a strong enhancement of the small-angle scattering intensity is observed, which reflects⁷ a tendency to segregation over a length scale of several Angstroms.