Transport coefficients and the Stokes–Einstein relation in molten alkali halides with polarisable ion model

Abstract

A polarisable ion model is parameterised for the whole series of molten alkali halides by using first-principles calculations based on density functional theory. Viscosity, electrical conductivity and thermal conductivity are determined using molecular dynamics simulations via the Green–Kubo formulae and confronted to experimental results. The calculated transport coefficients are generally in much better agreement than those obtained with the empirical Fumi–Tosi potentials. The inclusion of polarisation effects significantly decreases the viscosity and thermal conductivity, while it increases the electrical conductivity. The individual dynamics of the ions is analysed using the Stokes–Einstein relation. The anion behaviour is always well represented using the slip boundary condition, while for cations there is an apparent shift from slip to stick condition when the ionic radius decreases. This difference is interpreted by subtle changes in their coordinating environment, which are maximised in the case of Li+ cation.