Polarization Energy in Ionic Melts

Abstract

The main objective of this chapter is to emphasize the importance of polarization energy in determining the thermodynamic properties of molten salt mixtures. Firstly, consideration is given to those typical ionic compounds, such as alkali metal halides, in which each anion is symmetrically surrounded by cations, so that the anions are unpolarized in the pure salts; it is shown that the main term in the enthalpy of mixing of two molten alkali metal halides with a common anion is the polarization energy due to the dipole generated in the polarizable anion because of the different sizes of its neighboring cations. Secondly, consideration is given to those compounds, such as magnesium chloride, that are generally regarded as not being typically ionic, because they form layer lattices, in which the three cations with which each anion is in contact are all congregated around one hemisphere; it is shown that, because of the ratio of the polarizability of the anion to the cube of the Mg²⁺-Cl⁻ distance, electrostatic asymmetry is to be expected in both solid and liquid magnesium chloride, and the unsymmetrical enthalpy of mixing in a liquid mixture of an atypical and a typical ionic compound is exemplified quantitatively for the MgCl₂-CaCl₂ system.