Isothermal–isobaric molecular dynamics simulation of polymorphic phase transitions in alkali halides

Abstract

As the first application of the isothermal–isobaric molecular dynamics technique to a system with Coulombic forces, the pressure-induced face-centered-cubic to body-centered-cubic transition of some alkali halides is studied assuming Born–Mayer–Huggins potentials between the ions. The long-range forces are handled by the cubic harmonic expansion of the Ewald summation. The property that any periodic boundary condition of orthorhombic symmetry can be uniformly treated in this expansion allows the independent fluctuation of each box length in the course of the simulation of the isobaric ensemble. This reduces stresses during phase transitions from one crystalline state to another and yields less defects. By the appropriate change in the external pressure, the fcc–bcc transition takes place reversibly only in the presence of a considerable amount of defects. Otherwise, quite large hysteresis is found. It is shown that the [100], [010], and [001] directions of the original fcc lattice become the [ 1/2 1/2 1̄], [1̄01], and [111] directions in the bcc lattice, respectively. This confirms the Watanabe–Tokonami–Morimoto mechanism supported also by most of the experimental observations.