(189cg) MD Simulation of a Magnesium Oxide Grain Boundary

Magnesium oxide is a ceramic compound with a melting temperature that makes it a desirable refractory material. It is also used as a protective coating on plasma displays, and as a diffusive barrier when treating radioactive waste. Diffusion through grain boundaries in similar materials is orders of magnitude faster than bulk diffusion, but is difficult to resolve experimentally.

We investigate how grain boundaries influence the transport properties of systems of MgO polycrystals consisting of parallel, layered MgO crystals. Simulations were run for a period of 100 ns. The systems were analyzed using an inherent structure approach, by removing thermal displacements from the system in post-processing. This allowed us to look for the unbalanced bonding at the grain boundary interface, and identify grain boundary atoms by their 6th nearest-neighbor distance. The mean squared displacement (MSD) was used to calculate self-diffusion coefficients of bulk and grain atoms. We demonstrate thermally induced grain mobility correlated with sliding and rotation of neighboring crystals. We also show the diffusivity as a function of distance from the center of the grain boundary, by obtaining diffusivities from MSD within planes normal to the boundary interface, to obtain an effective grain boundary width. We show atomic migration between the crystalline bulk and grain boundaries, and the effect of that migration on the effective grain boundary width. We show that magnesium self-diffusion is much faster than oxygen self-diffusion, and obtain values for the self-diffusion coefficient of magnesium.