Basing on the well established correlations between the formation energy of Schottky defects and the melting temperature as well as the heat of fusion, a vacancy model of melting of alkali halides has been proposed, according to which:
- melting starts when the concentration of Schottky defects in the solid phase reaches a critical value of 0.0017, the same for all the alkali halides,
- the melting process is a process of generation of additional vacancies at the cost of the heat of melting, in the result of which the concentration of Schottky defects increases by Δc = 0.15.
The proposed model permits to predict the changes in many physical properties (molar volume, slope and shape of the melting curve, etc.) on passing through the melting point as well as to determine the melting curve of alkali halides. The predictions of the proposed vacancy model of melting of alkali halides agree well with the experimental data available in the literature.