Molecular dynamics of ionic solid and liquid surfaces

Abstract

Molecular-dynamics computer simulations of model alkali halide solids and melts were performed with the aim of elucidating the role of electrostatic interactions in the determination of bulk and interphasial behavior. Simplified pair potentials of the $n:1\mathrm{}$ form are shown to compare favorably with the results of the more complicated Born-Mayer-Huggins potential, and have great corresponding states capability. New lattice-summation techniques for obtaining the long-range Coulomb correction are also discussed, which are markedly different from the usually adopted Ewald formulas. The properties of solid and liquid surface states were examined under the influence of vacuum and rigid-wall constraints. Both boundaries have noticeable effects on the material to a depth of 10-20 Å into the bulk of the material. Charge separation in the vacuum interphase of model molten LiCl was not observed. The problems associated with definitions of pressure and diffusion coefficients in the interphase are discussed and results are presented. Attempts were made to produce surfaces which would be polar under steady-state conditions. An applied electric field in the direction perpendicular to the surface plane for a melt/vacuum interface did not induce any observable charge separation. It was necessary to apply an electric field equivalent to four times the rootmean-square force at equilibrium before ion desorption into the adjacent vacuum was observed. This took the form of infrequent departures of ion pairs. Slow surface structural evolution was demonstrated by applying an electric field parallel to the surface. The interphasial ionic mobilities were smaller there than in the bulk. The highly polar (111) model NaCl solid surface was unstable with respect to an, as yet, undetermined reconstructed form.