Modelling of point defects in α-al203

The semiempirical method of the Intermediate Neglect of Differential Overlap (INDO), in the program SYM-SYM, has been used for calculations on perfect and imperfect corundum crystals. For the perfect crystal the periodical Large Unit Cell (LUC) model was used while the Molecular Cluster (MC) model was used in defect calculations. By means of the MC model, we have investigated the optical properties of electronic centers (F+ F, F-, FMg, F-Mg) in corundum. Calculated optical properties of these defects are compared with experimental values and new bands are predicted to exist in the absorption spectrum of Mg-doped corundum. The energy levels of F-type and Mg-impurity related centers are found to lie in the gap between the upper valence band and conduction band, as for similar centers in MgO and alkali halides.