Structure, Energetics, and Spectra for the Oxygen Vacancy in Rutile: Prominence of the Ti-H_O-Ti Bond

Under reducing conditions, rutile TiO2 develops O vacancies (VO) coupled to Ti3+ centers. It is favorable for H atoms to enter this system, either forming OH groups or occupying vacancy sites (denoted HO) that bond to two Ti atoms next to the vacancy. OH defects are well documented by the presence of infrared modes at ∼3300 cm-1, while HO is relatively underinvestigated. We report the energies, geometries, and vibrational frequencies of hydrogen defects in rutile predicted from quantum mechanics calculations, focusing on the coexistence of OH and HO. We find that HO is more stable than OH by 1.42 eV, leading to an infrared mode at ∼1200 cm-1. Introducing a second H forms an OH bond with an infrared mode at ∼3300 cm-1. These results suggest that assessments of hydrogen storage in mantle phases of rutile and similar minerals based on OH bands may significantly underestimate H concentrations.