The optical absorption of undoped and Fe-doped crystals which had been grown under various O partial pressures was investigated. A 2.9eV band which was found in undoped crystals formed only during growth, and was proportional to the fourth root of the O partial pressure. This band consisted of 3 broad superposed absorption bands, and was attributed to a hole localized on an O ion. A 2.5eV band which was found in crystals that had been grown in reduced atmospheres was identified as being an O vacancy, rather than an anti-site Nb, by means of density measurements and optical bleaching studies. It was found that the concentration of O vacancies was very sensitive to the presence of Fe impurities in the as-grown crystals. On the basis of these results, it was proposed that the present material was best formulated as: [Li1-xNbx/5][Nb]O3+, where x and  represented the deviations from stoichiometry.

T.Suzuki: Journal of Crystal Growth, 1996, 163, 403-10