Oxygen vacancies in lithium tantalate

Lithium tantalate crystals turn ``black'' when annealed above approximately 850 \ifmmode^\circ\else\textdegree\fi{}C in an argon atmosphere. The resulting optical absorption spectrum has a distinct peak at 460 nm but also extends through the entire visible and near ultraviolet. We attribute this absorption to oxygen vacancies containing two electrons. Optical bleaching near 77 K converts the initial absorption spectrum to a different spectrum having a peak near 570 nm. Electron-spin-resonance results show that the 77-K bleach also produces ${\mathrm{Ta}}^{4+}$ ions. Both ${\mathrm{Ta}}^{4+}$ ions and oxygen vacancies (containing only one electron) con- tribute to the absorption spectrum after the bleach.