Oxygen Deficiencies in Titanium Oxide Clusters as Models for Bulk Defects

TD-DFT calculations were performed on neutral Ti_nO_2n, Ti_nO_2n–1, and Ti_nO_2n–2 clusters, where n ≤ 7. Calculations show the Ti_nO_2n clusters are closed shell systems containing empty d orbitals and that the partially filled d orbitals of the suboxide clusters have a profound effect on their structural, electronic, and topological properties. The low energy photoexcitations of Ti_nO_2n clusters are all O-2p to Ti-3d transitions, while the open-shell suboxide clusters are all characterized by d–d transitions that occur at a much smaller optical gap. Upon low energy photoabsorption, the localization of the hole is accompanied by a local bond elongation, i.e., polaron formation, whereas d-electrons are generally delocalized around the cluster. The properties of the clusters, including the oxygen binding energies and structures, were calculated to account for the variation in relative populations found in experimental cluster distributions. Several Ti_nO_2n–2 clusters contain higher symmetry which is reflected in their relative stability. In particular, the tetrahedral symmetry of Ti₄O₆ inhibits charge carrier localization and therefore exhibits higher stability.