Effects of polarization on equilibrium and dynamic properties of ionic systems

Molecular-dynamics calculations are reported for systems of ions in which the contribution to the interionic forces arising from polarization is taken explicitly into account. Two models of polarization are considered: a modification of that used for alkali-halide molecules by Tosi and Doyama and a simplified version of the shell model of lattice dynamics. In each case the effects of polarization are superimposed on an underlying rigid-ion pair potential of the type proposed by Tosi and Fumi. Calculations of the phonon spectra show that use of either model leads to agreement with experimental data on solid NaCl, which is considerably better than that achieved with the Tosi-Fumi potential alone; good results are also obtained for solid KI. Calculations for molten KI based on the shell model show that the effect of polarization in the liquid state is to increase significantly the diffusion coefficients, particularly that of the positive ion, and decrease the shear viscosity, but the effect on electrical conductivity is almost negligible. These results are broadly consistent with the known adequacy of the rigid-ion-model to account for the transport coefficients of molten alkali halides. Polarization is also shown to lead to a reduction in the characteristic frequencies and a considerably increased damping of collective longitudinal-optic-type modes in the liquid. The effect on other collective modes is less pronounced.