Defects and dislocations in MgO: atomic scale models of impurity segregation and fast pipe diffusion

Abstract

Dislocations are known to influence the formation and migration of point defects in crystalline materials. We use a recently developed method for the simulation of the cores of dislocations in ionic materials to study the energy associated with the formation of point defects close to the core of a ½<110>{10} edge dislocation in MgO. These are then compared with the energies for the same point defects in otherwise perfect MgO. It is found that all of the defect species are bound to the dislocation core, with binding energies of between 1.5 and 2.0 eV. Vacancies are found to be most stable when they remove under-coordinated ions at the tip of the extra half plane, while the impurities are most stable within the dilatational stress field below the glide plane. By mapping the distribution of energies for point defects around the dislocation line we reveal the coupling between the effective point defect size and the stress field associated with the dislocation. We also examine the energy barrier to diffusion of vacancies along the dislocation line and find that vacancy migration along the dislocation line will be substantially enhanced compared to migration through the dislocation-free crystal structure. Activation energies are 0.85-0.92 of the barrier in the perfect crystal, demonstrating the importance of pipe diffusion along extended defects for low temperature mobility in ionic materials.