Partial crystallization and melting of an Se sublattice in the superionic conductor Ag₂Se: a molecular dynamics study

The partial crystallization and melting of the Se sublattice in the superionic conductor Ag₂Se are investigated by means of a constant-pressure and constant-temperature molecular dynamics simulation. The effective interionic potential of Rino et al. (1988) is used. The transition temperature is estimated to be about 1000 K, which is reasonable in comparison with the actual transition temperature, 1170 K. The atomic structures of the Se sublattice in the crystallization and melting processes are analysed by the method of the Voronoi polyhedron. The defect formation process before melting is studied in detail from a microscopic point of view. It is shown that, in the superionic phase, the repulsive force acting on an Se ion from its neighbouring Ag ions behaves as the restoring force maintaining the Se BCC sublattice, and that an opportunity to create the defect is given when the restoring force weakens.