Rigid Ion Potential Research Articles

Anharmonic effects in the phonon spectra of sodium chloride

Results are reported of a series of molecular-dynamics "experiments" on solid NaCl at temperatures of 80, 302, 954, and 1153 K. Attention is focused primarily on the computation of the dynamical structure factor and its one-phonon approximation; comparison of the two allows the isolation of contributions from multiphonon and interference terms. Anharmonic effects are analyzed in terms of perturbation theory and the theory is found to give satisfactory results for the phonon frequencies at temperatures up to 80% that of melting. Most of the calculations are carried out for a simple rigid-ion potential, but the effects of polarization and its incorporation in molecular-dynamics calculations are briefly discussed.

Interionic potentials in alkali halides and their use in simulations of the molten salts

Abstract After an outline of work on rare-gas systems, which serves as a target for parallel work on alkali halides, and an initial brief survey of those parts of this parallel work for which results have been obtained, interionic potential models for alkali halides are considered in some detail. The rigid ion potentials of Fumi and Tosi are discussed and then a major part of the section is devoted to deriving a new set of polarizable ion potentials, which incorporate the ideas behind the lattice dynamical shell model. Extensions which include many-body terms in the potentials are considered briefly and finally the information which can be obtained from alkali halide diatomic molecules is discussed. In the third section methods of computer simulation for ionic liquids are outlined, concentrating on the molecular dynamics method, and some of the properties which can be obtained by analysing the ion trajectories are listed. Results from simulations, including some new work on LiF, NaCl and RbI, are reviewed.

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.

Effects of polarization on some static and dynamic properties of molten NaI

Pair interaction potentials for NaI are obtained by fitting to solid state data at 0K and to some properties of the diatomic molecule. In order to examine the effects of polarization on properties of molten NaI, molecular dynamics simulations are performed with rigid ion and shell model potentials the shell model potentials being fitted to the same data as are used for the rigid ion potential with the additional parameters being determined from dielectric constant data. Two shell models are considered; one is fitted to diatomic molecule properties and the other is more in line with empirical ionic radii. Simulations with 216 ions have been carried out for these potentials and the following results are presented: radial distribution functions velocity and force autocorrelation functions and mean square displacements.

The structure of molten NaCl from a simulation model which allows for the polarization of both ions

Two-ion-polarizable shell-model interionic potentials for NaCl are derived from solid-state data including ionic radii. Radial distribution functions for the melt are obtained from these potentials by a molecular dynamics simulation and the results are compared with recently reported measurements. Significant improvements over previous calculations based on rigid-ion potentials are found.

Transport properties of molten alkali halides

The transport properties of molten alkali halides have been calculated in molecular dynamics experiments based on rigid-ion pair potentials of the Huggins-Mayer form with parameters proposed by Tosi and Fumi. The coefficients of shear viscosity and electrical conductivity are obtained by computing the response of the system to a small applied field of the appropriate type, and the extension of the method to the calculation of current-current correlation functions of acoustic and optic character is also considered. Comparison with experimental data shows that the electrical conductivities are very well reproduced, but systematic discrepancies are found for diffusion coefficients and viscosities.

Simulation of molten NaI including polarization effects

A comparison is made of the radial distribution functions for NaI calculated by molecular dynamics simulation from consistent rigid ion and shell model interionic potentials. Substantial differences are observed.