Simple thermodynamic model for the specific heat of the fluorite crystalsPbF2,CaF2, andSrCl2

The excess specific heat for ${\mathrm{SrCl}}_2$, ${\mathrm{PbF}}_2$, and ${\mathrm{CaF}}_2$ fluorite crystals has been computed by means of statistical mechanics using a model derived from the Welch and Dienes phenomenological model for phase transitions. The enthalpy is written in a form that partly takes the long-range interactions into consideration. It is shown that the transition temperature is always attained for low defect concentrations, as experimentally found by Schröter and Nöltig. The calculations are in quantitative agreement with available experimental data.