Oxygen Deficiencies in Titanium Oxide Clusters as Models for Bulk Defects

TD-DFT calculations were performed on neutral TinO2n, TinO2n-1, and TinO2n-2 clusters, where n ≤ 7. Our calculations show that the TinO2n clusters are closed shell systems containing empty d orbitals and that the partially filled d orbitals of the sub-oxide clusters have a profound effect on their structural, electronic, and topological properties. The low energy photoexcita-tions of TinO2n 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 photoabsorption, the localization of the hole is accompanied by a local bond elongation, i.e., polaron formation, whereas d-electrons are generally delocalized among the cluster. Several of the compact of the TinO2n-2 structures contain higher symmetry which is reflected in their relative stability in the experimental cluster distribution. In particular, the tetrahedral symmetry of the optimized ground state structure for Ti4O6 inhibits charge carrier localization and therefore contains higher stability.