Polarization effects in the simulation of lead (II) fluoride

An interionic potential to describe the interactions in PbF2 is constructed from ab initio calculations. The potential is based upon formal ionic char ges and includes polarization effects arising from the induced dipoles on b oth anions and cations. The cation polarization effects are shown to be cru cial to explain observable differences between PbF2 and alkaline-earth fluo rides of comparable cation size (SrF2 and BaF2). In particular, the lower t ransition pressure between the beta- (fluorite) and alpha- (cotunnite) phas es and the qualitative difference between the shapes of the phonon dispersi on for PbF2 and the alkaline earths are reproduced. Simulations show a tran sition to a superionic conducting state in the beta-phase, though at a temp erature slightly higher than that observed experimentally. No ionic conduct ion is observed in the alpha-phase at comparable temperatures, in agreement with experiment. The pattern of diffuse neutron scattering predicted by th e simulations in the superionic domain is shown to reproduce the distinctiv e distribution of intensity observed experimentally.