First-principles phase diagram calculations for the system NaCl–KCl: The role of excess vibrational entropy

First principles phase diagram calculations were performed for the system NaCl–KCl. Plane-wave pseudopotential calculations of formation energies were used as a basis for fitting cluster expansion Hamiltonians, both with and without an approximation for the excess vibrational entropy (SVIB). Including SVIB dramatically improves the agreement between calculated and experimental phase diagrams: experimentally, the consolute point is {XC = 0.348, TC = 765 K}Exp; without SVIB, it is {XC = 0.46, TC ≈ 1630 K}Calc; with SVIB, it is {XC = 0.43, TC ≈ 930 K}Calc.