The OH − absorption spectra of low doped lithium niobate crystals

The OH− absorption spectra of low doped lithium niobate (LiNbO3) crystals have been investigated. Though no apparent band shift is observed in these absorption spectra, their shapes are quite different. In order to analyze the information on the defect structure underlying these OH− absorption bands, the normalization and difference methods were employed. It was found that although the doping concentrations are under the thresholds the doping ions have apparent affect to the site occupation of OH− ions. The OH− vibrations related to MgLi+ (Mg2+ occupying Li-site) and InLi2+ are 3483 and 3484 cm−1 in LiNbO3:Mg and LiNbO3:In crystals, respectively. The absorption peak of LiNbO3:Ti (2.5 mol%) crystal at 3487 cm−1 is mainly related to TiLi3+–OH− and the 3489 cm−1 peak of LiNbO3:Mg (5.0 mol%), Ti (10.0 mol%) related to MgLi+–OH−, TiNb−–OH− and TiLi3+–OH−. Doping with Na improves the peak intensity near 3466 cm−1 and induces a new absorption peak at 3470 cm−1. The absorption bands of LiNbO3 crystals codoped with trivalent ions are associated with the co-effect of the doped ions and have some different characteristics from mono-doped crystals.