Defect chemistry of Ti and Fe impurities and aggregates in Al₂O₃
2014
Bristow, Jessica K. | Tiana, Davide | Parker, Stephen C. | Walsh, Aron
We report a theoretical evaluation of the properties of iron and titanium impurities in sapphire (corundum structured α-Al₂O₃). 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 (Feᴵᴵᴵ/Tiᴵᴵᴵ → Feᴵᴵ/Tiᴵⱽ) 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 Feᴵᴵᴵ/Tiᴵᴵᴵ is the ground-state configuration for the nearest-neighbour pairs, in contrast to the often considered Feᴵᴵ/Tiᴵⱽ pair. Homonuclear intervalence charge transfer energies between both Feᴵᴵᴵ/Feᴵᴵ and Tiᴵⱽ/Tiᴵᴵᴵ 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, Tiᴵᴵᴵ–(Tiᴵⱽ/Feᴵᴵ), with the energy of electron transfer remaining unchanged.
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