Interatomic force modelling of local coordination and extended structure in ionic-covalent liquids

Abstract

Liquid-state studies on families of stoichiometric compounds, and on mixtures that are stable in the liquid phase over large ranges of composition, offer a unique opportunity to explore trends in the evolution of bonding between the ionic, metallic and covalent canonical forms. The present work is primarily focused on the evolution from mainly ionic to increasingly covalent bonding character in polyvalent metal halides. After a review of strongly ionic strontium chloride in its liquid and hot-solid states, the formation and stability of local tetrahedral configurations of halogens around polyvalent metal ions are discussed, such ‘complexes’ being evaluated by ionic models both in vacuo and in liquid mixtures of alkaline-earth and alkali halides. The problem of how local tetrahedral configurations connect via strongly angle-dependent forces to yield intermediaterange order in liquid structure is then introduced by a discussion of structural data on liquid zinc chloride. An effort to develop appropriate interatomic force models, based on extensions of the well known bond-charge model for crystalline germanium, thereby allowing the use of those theoretical techniques that are standard for pair-potential systems, is illustrated by results for the liquid structure of germanium near equilibrium freezing and at strong supercooling.