A computational study of point defects and diffusion in enstatite

In order to contribute to the understanding of diffusion processes in enstatite (Mg2Si2O6), we have used atomistic simulation techniques to study point defects in this mineral. We present results for a variety of Scottky and Frenkel defects of all atomic species present in it. We have furthermore calculated the activation energy for magnesium diffusion. We break diffusion down into a succession of hops between neighboring sites. Each hop is associated with a migration energy barrier. By making a series of hops, the diffusing ion may cross the unit cell. The maximum migration energy necessary to attain movement in a direction is the activation energy for diffusion in that direction. The defects correspond to diffusing species in their minimum energy configuration and in the activated state. The migration energy is found by subtracting the energy of the initial, minimum energy, state from that of the activated state. The activated state is found utilizing transition-state search algorithm based on the rational function optimization (RFO) procedure. This method is capable of locating saddle points on an energy surface from an arbitrary starting point. The defect energy calculations and the RFO transition-state searches were performed using the generalized Mott-Littleton approach available in the computer code GULP (Gale and Rohl, 2003). In order to ensure that the appropriate saddle points were found, an initial investigation of the energy surface was carried out via the technique of adiabatic mapping. This technique involves fixing the diffusing ion in a series of positions on a three-dimensional grid and minimizing the energy of the rest of the structure. A trial transition state is selected from this grid and used as an initial configuration for the RFO transition-state search.