Calculation of the Heat Capacities of Molten Cesium Halides Using Thermodynamic Perturbation Theory

Abstract—Because of great difficulties in conducting high–temperature experiments to measure the thermal effects in molten salts, the information accumulated to date on the temperature dependences of the heat capacities even for the simplest subclass of salt melts, namely, alkali metal halides, cannot make an answer about the existence and reliability of trends in decreasing or increasing the heat capacities of melts with temperature. Most data on the heat capacities of molten halides are presented in handbooks as temperature-independent quantities. Therefore, to reveal trends in temperature-induced changes in the heat capacities of melts, it is advisable to turn to theoretical analysis methods. In this work, we develop a version of a thermodynamic perturbation theory and apply it to describe the temperature dependences of the heat capacities of a number of halide melts. The model of taking into account charge–dipole interactions using a system of comparing charged hard spheres, which was tested earlier in calculating the enthalpies of alkali-halide melts, is applied to calculate the isobaric heat capacities of molten cesium fluoride, chloride, bromide, and iodide in the temperature range 200 K above their melting temperatures. A combination of a mean spherical model of charged hard spheres of different diameters and the first correction caused by point-charge-induced dipoles to the interionic interaction of molten salts is shown to be a good basis for qualitative and quantitative agreement with experimental data on heat capacities within a few percent. In addition, the proposed model is found to predict a weak monotonic decrease in the heat capacities of melts upon heating in all cases of cesium halides.