Theoretical study of defect structures in pure and titanium-doped alumina

First-principles plane-wave pseudopotential calculations were performed to study electronic structures and formation energies of point defects in pure and Ti-doped Al₂O₃. In the case of pure Al₂O ₃, the Schottky defects were found to be most stable, and the reaction energy was comparable with experiment. As compared to the Schottky defects in pure Al₂O₃, however, substitutional Ti ³⁺ or Ti⁴⁺-related defects exhibited smaller formation energies. In particular, substitutional Ti³⁺ ions showed the smallest formation energy in the relatively reduced atmosphere. The substitutional Ti³⁺ induces an electron-occupied defect level in the band gap of Al₂O₃, which can give rise to the electronic conductivity of Ti-doped Al₂O₃ observed experimentally.