Point and small cluster defects in magnesium aluminate spinel have been studied from a first principles viewpoint. Typical point defects that occur during collision cascade simulations are cation anti-site defects, which have a small formation energy and are very stable, O and Mg split interstitials and vacancies. Isolated Al interstitials were found to be energetically unfavourable but could occur as part of a split Mg-Al pair or as a three atom-three vacancy Al 'ring' defect, previously observed in collision cascades using empirical potentials. The structure and energetics of the defects were investigated using density functional theory (DFT) and the results compared to simulations using empirical fixed charge potentials. Each point defect was studied in a variety of supercell sizes in order to ensure convergence. It was found that empirical potential simulations significantly overestimate formation energies, but that the type and relative stability of the defects are well predicted by the empirical potentials both for point defects and small defect clusters.
The energetics of the key defects that are observed to occur during simulations of radiation damage in MgO are analysed using density functional theory. The results are compared with those from the empirical potentials used to carry out the radiation damage studies. The formation energies of vacancies, interstitials, Frenkel pairs, di-vacancies and di-interstitials are calculated as a function of the increasing supercell size in order to ensure good convergence. The supercell geometries were chosen to maximise the separation distance between periodic images. Their sizes ranged from cells containing 32 atoms up to cells containing 180 atoms.Results are presented for the formation energies of the first, second and third nearest neighbour defects. Results show that the di-vacancy formation energy is in the region of 4-6 eV and that formation energies for di-interstitials are more than double this, lying in the range 12-16 eV. Comparison of the results show that empirical potentials overestimate the formation energy of di-vacancies by 1-3 eV and underestimate the formation energies of di-interstitials by about 1-2 eV. The relative stability of the defects is, however, correctly predicted by the empirical potentials. The direction and the magnitude of the displacements of the atoms surrounding the defects are in good agreement for all the systems containing interstitials. For the systems containing vacancies the direction of the displacements are in agreement but the empirical potentials predict larger displacements in all cases.
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