Defect chemistry of Ti and Fe impurities and aggregates in Al ₂O₃

We report a theoretical evaluation of the properties of iron and titanium impurities in sapphire (corundum structured α-Al₂O ₃). Calculations using analytical force fields have been performed on the defect structure with the metals present in isolated, co-doped and tri-cluster configurations. Crystal field parameters have been calculated with good agreement to available experimental data. When titanium and iron are present in neighbouring face and edge-sharing orientations, the overlap of the d-orbitals facilitates an intervalence charge transfer (Fe^III/Ti ^III → Fe^II/Ti^IV) with an associated optical excitation energy of 1.85 eV and 1.76 eV in the respective configurations. Electronic structure calculations based on density functional theory confirm that Fe^III/Ti^III is the ground-state configuration for the nearest-neighbour pairs, in contrast to the often considered Fe^II/Ti^IV pair. Homonuclear intervalence charge transfer energies between both Fe^III/Fe^II and Ti ^IV/Ti^III species have also been calculated, with the energy lying in the infra-red region. Investigation of multiple tri-clusters of iron and titanium identified one stable configuration, Ti^III-(Ti ^IV/Fe^II), with the energy of electron transfer remaining unchanged. This journal is