Atomistic simulations of surface diffusion and segregation in ceramics

We demonstrate two approaches to studying processes of diffusion and segregation on the surface of oxides and ceramics. When simulating surface diffusion the activation barriers are often so high that the timescales required for direct simulation by molecular dynamics are prohibitive. We show that a combination of the Temperature Accelerated Dynamics (TAD) together with Kinetic Monte Carlo (KMC) methods can reach the timescales required without the necessity of guessing (often incorrectly) which are the dominant processes. We illustrate the power of the methods for surface diffusion and growth of rocksalt oxides. We show that, surprisingly, exchange mechanisms and correlated ion motions are important in surface and near-surface diffusion of those ionic systems. Another approach, Exchange Monte Carlo (MCX), is used to study equilibrium segregation in thin ceramic films with application to MgO/MnO. Surface concentrations as a function of temperature and film composition are determined directly from the simulations. For all compositions studied the {001} surface is Mn2+ rich; the occupancy of sites by Mn2+ decreases rapidly with depth.