The energetics and electronic structures of a series of neutral and charged O and Mg vacancy defects (F, F+, F2+, V, V-, V2-, P) were calculated by using stationary total-energy functionals. It was found that, contrary to the case of a charge compensation model for anion-cation defect pairs in ionic materials, it was energetically unfavorable for an isolated neutral O vacancy (F center) to transfer electrons to an isolated neutral Mg vacancy (V center) and form isolated F2+ and V2- centers. Charge compensation was unfavorable because additional electrons at the V center introduced new occupied states in the gap, and increased the energy of the defect. This result was consistent with the interpretation of spectroscopic data for MgO in which the ground-state defects were either neutral or singly-charged. The computed formation energies of both F and V centers were larger than the cohesive energy of MgO per formula unit, but the binding energy of the defects in the P center configuration was 12.16eV. The attraction between the F and V centers was enhanced when the defects carried a net charge. The position of the vacancy defect state in the fundamental energy gap of MgO was found to be in qualitative agreement with a model for optical absorption and emission, and was used as a simple model for the formation energies of the defects. The contribution of the band-structure energy to the stationary functional was found to account for more than 90% of the defect energy. This component of the defect formation energy was calculated directly by using a recursion method, rather than as the difference between the total energies of an 8000-atom cluster, with and without the defect.

Stability of Vacancy Defects in MgO: the Role of Charge Neutrality. A.Gibson, R.Haydock, J.P.LaFemina: Physical Review B, 1994, 50[4], 2582-92