`Covalent' effects in `ionic' liquids

Abstract

Many binary systems (and their mixtures) which might be expected to be `ionic', from electronegativity considerations, are found to exhibit pronounced `covalent effects' in their condensed-phase structure and dynamical properties. Recent work, involving both electronic structure calculations and computer simulation, has suggested that the interactions arise and are describable within the ionic model - provided that many-body effects, whose origin is the change in an ion's properties caused by interaction with its environment, are included. In systems where they are substantial, the many-body effects promote remarkably rich changes in the intermediate-range structure of an ionic liquid. AlCl3 becomes molecular, BeCl2 a `living polymer' of extended chains, and the distinctive intermediate-range order (IRO) of the three-dimensional-network, glass-forming systems ZnCl2, BeF2 and SiO2 is reproduced. The structural changes have considerable dynamical consequences: for ZnCl2 the slow structural relaxation, leading to the glass transition, may be traced back to the relaxation of the IRO. On shorter timescales (higher frequency) these liquids exhibit spectroscopic bands usually assigned to quasi-molecular units. The formation and dissociation of these units is crucial in ionic conduction, and other transport properties.