Calculation of the Temperature Dependences of the Densities of Liquid Rubidium and Cesium Halides with Allowance for the Charge-Dipole Contribution to the Melt Pressure

The temperature dependences of the densities of molten rubidium and cesium halides are described for the first time in terms of a statistical theory of liquid. The equation of state used in this work allows us to take into account the contribution of charge-dipole interactions between ions in melts using a charged hard sphere model. This version of the equation of state is obtained using a thermodynamic perturbation theory in combination with the virial theorem, which connects expressions for various contributions to internal energy and fluid pressure. The calculation results are shown to be qualitatively and quantitatively consistent with experimental data on the temperature dependences of the densities of molten rubidium and cesium halides. The errors in calculating the densities range from 1 to 10% when only standard tabular values of ion radii and polarizabilities are used. We also present and analyze the main trends in changes in the hard sphere, Coulomb, and charge-dipole contributions to the pressure of the molten salts as functions of temperature and chemical composition. These contributions are found to decrease when the melts are heated and when the cation and anion radii in the salt compositions increase. The contribution caused by the interactions of point ion charges and induced dipoles in the melts under consideration is shown to be about 10% of the Coulomb contribution. Thus, it increases in the melt density, which, in turn, causes better agreement of the calculation results with the available experimental data.