Electron paramagnetic resonance and electron-nuclear double resonance were used to characterize Cu2 ions substituting for Ti 4 ions in nominally undoped TiO2 crystals having the rutile structure. Illumination at 25K with 442nm laser light reduces the concentration of Cu2 ions by more than a factor of 2. The laser light also reduces the electron paramagnetic resonance signals from Fe3 and Cr3 ions and introduces signals from Ti3 ions. Warming in the dark to room temperature restores the crystal to its pre-illuminated state. Monitoring the recovery of the photo-induced changes in the Cu2 ions and the other paramagnetic electron and hole traps as the temperature was raised from 25K to room temperature provides evidence that the Cu2 ions have an adjacent doubly ionized oxygen vacancy. These oxygen vacancies serve as charge compensators for the substitutional Cu2 ions and lead to the formation of electrically neutral Cu2-VO complexes during growth of the crystals. The Cu2-VO complexes act as electron traps and convert to non-paramagnetic Cu-VO complexes when the crystals were illuminated at low temperature. Complete sets of spin-Hamiltonian parameters describing the electron Zeeman, hyperfine, and nuclear electric quadrupole interactions for both the 63Cu and 65Cu nuclei were obtained from the electron paramagnetic resonance and electron-nuclear double resonance data. This study suggested that other divalent cation impurities in TiO2 such as Co2 and Ni2 might also have an adjacent oxygen vacancy for charge compensation.

Oxygen Vacancies Adjacent to Cu2 Ions in TiO2 (Rutile) Crystals. Brant, A.T., Yang, S., Giles, N.C., Iqbal, M.Z., Manivannan, A., Halliburton, L.E.: Journal of Applied Physics, 2011, 109[7], 073711