Monte Carlo Simulations of Superionic CaF2

A simple version proposed in our previous paper for superionic conductors is applied to superionic CaF 2 . That is, the model potential between ions is composed of only the soft-core and the Coulomb potentials. Monte Carlo calculations were performed with a fine-grained lattice for 324- and 96-ion systems at several temperatures for a constant density. It is found that as the temperature increases the fractional number of anions in the tetrahedral locations shows a drop in the magnitude in a narrow temperature range, while the fraction of anions in the octahedral locations shows a featureless smooth increase, the increase of the diffusion is gradual, and the transition is like second-order (no latent heat). A rather largely system-size-dependent result on the fluctuations of potential energies is remarked in the superionic phase.