First-principles study of defect energetics in titanium-doped alumina

First-principles plane-wave pseudopotential calculations were performed to study electronic structures, structural relaxation, and energetics of point defects in Ti-doped Al₂O₃. Substitutional and interstitial Ti ions with charge compensating intrinsic defects were considered, and their formation energies were evaluated under various atomic chemical potentials. It was found that substitutional Ti⁴⁺ ions with charge compensating Al vacancies were most stable in the oxidized condition. In contrast, as oxygen chemical potentials decreased, the formation energy of substitutional Ti³⁺ decreased to have the smallest value in the relatively reduced conditions. However, in the intermediate range of oxygen potentials, substitutional Ti³⁺ and Ti⁴⁺ exhibited similar formation energies, indicating that these Ti defects can coexist in a particular reduction environment.