Molecular dynamics simulations of ionic crystal films

Abstract

Thin crystalline films of 6 to 18 ionic (100) planes thick are simulated by the computer simulation technique of molecular dynamics. They are found to be stable up to the normal melting temperatures. Model LiCl, NaCl, KCl, and RbCl parameters are used in a rigid ion approximation and are compared with a NaCl simulation using a point polarizable ion model. It is concluded that previous static lattice and lattice dynamics calculations have probably overestimated the influence of polarization forces in producing a surface contraction. The Coulomb point charge interactions were evaluated using isolated laminae boundary conditions. Surface ion relaxation is less than several percent of the regular lattice interplanar spacing. It is shown to be quite sensitive to the potential interaction parameters. The rigid ion mean square amplitudes of vibration are ∼20% smaller than experimental scattering data. Velocity autocorrelation functions and their derived power spectra are evaluated for motion which is parallel and perpendicular to the surface plane. They provide insights into the dynamical coupling of sublattice vibrations which are manifest, at a more resolved level, in the surface phonon spectra. Surface energies and stress decrease by 10% to 25% on equilibrium from the regular laminas and show a fascinating temperature dependence.