Aggregates of O vacancies (F centers) represent a particular form of point defects in ionic crystals. The combination of two O vacancies was considered: the M center, in the bulk and on the surface of MgO by means of cluster model calculations. Both neutral and charged forms of the defect M and M+ were taken into account. The ground state of the M center was characterized by the presence of two doubly occupied impurity levels in the gap of the material; in M+ centers the highest level was singly occupied. For the ground-state properties, a gradient corrected density functional theory approach was used. The dipole-allowed singlet-to-singlet and doublet-to-doublet electronic transitions were determined by means of explicitly correlated multireference second-order perturbation theory calculations. These were compared with optical transitions determined with the time-dependent density functional theory formalism. The results showed that bulk M and M+ centers gave rise to intense absorptions at about 4.4 and 4.0eV, respectively. Another less intense transition at 1.3eV had also been found for the M+ center. On the surface the transitions occurred at 1.6eV (M+) and 2eV (M). The results were compared with recently reported electron energy loss spectroscopy spectra on MgO thin films.

Ground- and Excited-State Properties of M-Center Oxygen Vacancy Aggregates in the Bulk and Surface of MgO. D.Domínguez-Ariza, C.Sousa, F.Illas, D.Ricci, G.Pacchioni: Physical Review B, 2003, 68[5], 054101 (9pp)