Vacancy migration at the {410}/[001] symmetric tilt grain boundary of MgO: An atomistic simulation study

We present the results of atomistic simulations to evaluate diffusion pathways and hence activation energies for cation and anion vacancy migration in the MgO {410}/[001] symmetric tilt grain boundary, which can be considered as a series of dislocation pipes. The approach employed in this study is based on molecular dynamics and we found that the diffusion routes were anisotropic with diffusion down the dislocation pipes favored over diffusion between the pipes. The lowest calculated activation energies for isolated vacancies were 1.05 eV for magnesium and 1.01 eV for oxygen at 0 GPa (cf. bulk activation energies of 1.94 eV for magnesium and 2.12 eV for oxygen). The lower activation energies coupled with the enhanced defect concentrations at the interface shows that boundaries are regions of high diffusivity. However, the concentration of vacancy pairs at the interface and the high binding energy of a magnesium-oxygen pair leads to the prediction that a large component of the defects is bound, which in turn causes the activation energy for vacancy migration to approach that of the bulk. In this case the higher boundary diffusivities are the result of high defect concentrations at the boundary.