Nature of Ti Interstitials in Reduced Bulk Anatase and Rutile TiO₂

The preferred charge and spin state of an interstitial (Ti int ) atom in reduced bulk rutile or anatase Ti 1+ x O 2 has been investigated with spin-polarized hybrid density functional theory (DFT) calculations. A neutral Ti int placed in an interstitial cavity of titania spontaneously transforms into a Ti 3+ ion with nearly one electron localized on the 3d shell; the remaining three electrons are donated to the lattice Ti ions. Spin polarization is essential to obtain a correct description of the electronic structure of the defect.