A New Approach to the Evaluation of Thermodynamic Properties of Nonideal Solid Solutions by the Molecular Dynamics Method: A Mixing Model of Alkali Halide Solid Solution

We present a new approach to evaluating the intracrystalline thermodynamic properties of solid solutions by combining the molecular dynamics (MD) method and a chemical thermodynamic approach. The coordination type of ions in alkali halide solid solutions (A, B)X is classified into 7 types Xn(n=0–6) for halide ions and into 13 types An(n=0–12) and Bn(n=0–12) for cations, depending on the kind and the coordination number of the nearest cation. Entropy of mixing for a nonideal solid solution was formulated by using the configurational entropy of coordination types. The equilibrium concentration of coordination types is expressed by equilibrium reactions among coordination types. The equilibrium constants in the solid solution of the system NaCl–KCl were then calculated by the MD method and the excess free energies of mixing were obtained by calculation. The asymmetric nature of the solvus curve of the system NaCl–KCl is well demonstrated and the temperature of the apex of the solvus agrees well with experimental data.