Semi-empirical supercell calculations for free- and bound-hole polarons in α-Al₂O₃ crystal

Two different parametrizations of the semi-empirical method of the intermediate neglect of the differential overlap (INDO) are applied to the calculations of the small-radius hole polarons in the corundum (α-Al₂O₃) crystal. The 80-atom supercell has been used for the study of the atomic and electronic structure of a free small-radius hole polaron (the self-trapped hole, STH) and a hole polaron bound by a Mg impurity (the so-called V_Mg centre), respectively. Both parametrizations indicate that the two-site (quasi-molecular) configurations of both kinds of polaron have the lowest energy (which does not exclude the existence of one-site polarons also characterized by considerable relaxation energies). For V_Mg centres the lower energy is calculated for the defect configuration where the Mg-ion substitution is nearest to the hole polaron. Experimental ENDOR data on V_Mg defects are discussed in the light of the calculations.