Calculations of the geometry and optical properties of FMg centers and dimer (F2-type) centers in corundum crystals

Results of the defect calculations in α-Al2O3 (corundum) crystals using the semiempirical method of intermediate neglect of differential overlap and large, 65-atom-stoichiometric quantum clusters are reported. The geometry and the electronic density distribution are simultaneously and self-consistently optimized for both the ground and excited states of the electronic FMg, FMg- single centers and two kinds of dimer (F2, F2+, F22+) centers where two oxygen vacancies belong to the same basic oxygen triangle and are next nearest neighbors on adjacent basal planes, respectively. Their optical absorption and luminescence energies are compared with available experimental data; absorption bands are predicted. Positions of dimer-center local levels within the gap of the perfect crystal are calculated.