Hydrogen donors and Ti3+ ions in reduced TiO2 crystals

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to identify and characterize the neutral hydrogen donor in TiO2 crystals having the rutile structure. These spectra are best observed near 5 K. The neutral donors are present without photoexcitation in crystals that have been slightly reduced at high temperature in a nitrogen atmosphere. The same defects can be photoinduced at low temperature in oxidized crystals. The neutral hydrogen donor in this lattice consists of a substitutional Ti3+ ion adjacent to a substitutional OH– molecular ion. The axis of the OH– molecule lies in the basal plane with the hydrogen ion extending out from the oxygen in a direction perpendicular to the Ti-O bonds. Spin-Hamiltonian parameters are obtained from the angular dependence of the EPR and ENDOR spectra (principal values are 1.9732, 1.9765, and 1.9405 for the g matrix and –0.401, + 0.616, and –0.338 MHz for the 1H hyperfine matrix). The principal axis associated with the + 0.616 MHz principal value is in the basal plane 22.9° from a [110] direction and the principal axis associated with the –0.338 MHz principal value is along the [001] direction. Our results show that interstitial Ti3+ ions are not the dominant shallow donors in slightly reduced TiO2 (rutile) crystals.