Statistical Thermodynamic Analysis of the Effect of Chemical Composition on Changes in the Melting Temperatures of Alkali Metal Halides

An interpretation is proposed for the dependence of the melting temperatures of an entire subclass of alkali metal halides on the chemical composition, based on an analysis of changes in different contributions to the internal energy of salts in the molten and crystalline phases with variation in the sum of the radii of their cations and anions. The expression for calculating the energy of liquid salt melts includes the contribution from charge–dipole interactions between ions, which is considered in a work based on thermodynamic perturbation theory with a basis in the form of a model of charged hard spheres. The Born–Mayer formula is used for the energy of the crystalline phase in the electrostatic part, while Debye’s formula is employed to consider the contribution from vibrations. An explanation is given for the lower values of the reduced melting temperatures of lithium and sodium halides, relative to other salts. It is shown that deviations of the reduced melting temperatures of lithium and sodium halides depending on the sum of ionic radii can be explained by Coulomb and translational contributions to the energy in the molten state, along with Madelung and Born contributions in the crystalline phase.