Calculation of the Melting Temperatures of Alkali Metal Halides Using the Thermodynamic Perturbation Theory

• Published in: Russian metallurgy Metally Vol. 2023; no. 8; pp. 977 - 985
• Main Authors: Davydov, A. G., Tkachev, N. K.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 01.08.2023; Springer Nature B.V
• Subjects: Anions; Bromides; Cations; Cesium; Chemical potential; Chemistry and Materials Science; Dipoles; Equations of state; Fluorides; Formulas (mathematics); Halides; Lithium; Materials Science; Melt temperature; Melting; Metal halides; Metallic Materials; Perturbation theory; Phase equilibria; Point charge; Rubidium; Solidification point; Thermodynamics; charged hard-sphere model; polarizability; melting temperature; thermodynamic perturbation theory; alkali metal halides; ionic radii; induction interaction
• ISSN: 0036-0295; 1555-6255; 1531-8648
• DOI: 10.1134/S0036029523080074

A model is proposed to describe liquid–crystal phase equilibria in order to calculate the melting temperatures of ionic compounds. The dependence of the melting temperatures of alkali metal halides on the cation–anion composition of a salt can be described in terms of ionic radii and polarizabilities when the thermodynamic perturbation theory is used for a molten phase. For the chemical potential of a crystalline phase, the Born–Mayer formulas for electrostatic energy and the Debye formula for taking into account the contribution of vibrations are used. The complete system of equations describing the liquid–solid equilibrium includes not only the equality of chemical potentials, but also self-consistency using an equation of state for calculating the equilibrium melt density at the solidification point. Another equation of the system is derived using the mean spherical model of an ion mixture for self-consistent finding of characteristic Blum’s screening parameter. On this basis, the melting temperatures of fluorides, chlorides, bromides, and iodides of lithium, sodium, potassium, rubidium, and cesium are calculated. A combination of the model of charged hard spheres of different diameters, which is taken as a reference in the mean spherical approximation, and the first correction due to the ion dipoles induced by the point charge of another ion to the chemical potential of a liquid salt is shown to be a good basis for quantitative agreement with the experimental data on the melting temperatures within a few percent. In addition, we also discusses the laws of changing the melting temperature reduced to the Coulomb energy at the minimum cation–anion distance and its dependence on the difference in the ionic radii of salts.