CALCULATION OF THE MELTING POINTS OF ALKALI HALIDES USING THE THERMODYNAMIC PERTURBATION THEORY

Abstract

A model for calculating phase equilibria between a liquid and a crystal is proposed, which makes it possible to evaluate the melting points of ionic compounds. The dependence of the melting temperatures of alkali halides on the cation-anion composition can be described in terms of ionic radii and polarizabilities using thermodynamic perturbation theory for the molten phase. The chemical potential of the crystal phase contains the Born-Mayer formula for the electrostatic part of the energy and the Debye formula for the vibration contribution. The full system of equations describing the equilibrium between liquid and solid includes not only the equality of chemical potentials, but also contains the equation of state to calculate the equilibrium density of melts at the crystallization point. One more equation of the system is necessary for the self-consistent computation of the characteristic Blum’s screening parameter within the mean spherical model of the ionic mixture. On this basis, the melting points of fluorides, chlorides, bromides and iodides of lithium, sodium, potassium, rubidium and cesium have been calculated. It has been shown that the combination of the reference mean-spherical model of charged hard spheres with different diameters and the perturbation due to the charge-induced dipoles into the chemical potentials of molten salts is a good basis for quantitative agreement with experimental data on the melting temperatures within a few percent. Moreover, the regularities of the change in the melting temperatures reduced to the Coulomb energy at the maximum contact of the cation and anion, as well as depending on the difference in the ionic radii of the salts, are discussed.