Abstract
Atomistic calculations of the energy of interaction of anion and cation vacancies with an a/2[110](110) edge dislocation in MgO have been carried out using the breathing-shell model. This is an extension of previous calculations carried out with the shell and point-ion models. Comparison of the results, obtained for a small number of defect locations around the dislocation's centre, shows only small differences among the models, indicating that for MgO the point-ion model is sufficiently accurate. A calculation of the volume of formation of Schottky defects in MgO carried out with the two shell models, using the volume derivative method, gives roughly comparable results. For Schottky defects, the relaxation volume varies from 0.36 to 0.52 molar volumes, which is close to the volume determined by Sangster and Rowell (1981), using a similar method of calculation but with a slightly different potential. An error in the program PDINT used to calculate interaction energies has been corrected, and previously published interaction energies, for an extended region surrounding the edge dislocation, have been re-evaluated. Comparison of the corrected atomistic results, with interaction energies derived from a linear isotropic elastic model (using the above estimate of the relaxation volume), now yields good agreement between the predictions of the two models at point-defect-dislocation separations of about 4b (where b is the Burgers vector of the dislocation).
At the closest distance of approach to the dislocation the elastic model always overestimates the interaction. This seems to derive mainly from inadequacies in the inhomogeneity rather than in the size-effect interaction. The maximum interaction determined with the atomistic model is for the anion vacancy at the bottom edge of the dislocation's extra planes. It is attractive and equal to 1.5 eV.