We have re-examined the infrared absorption bands due to the O-H bond-stretching vibration and the polarization characteristics in undoped and MgO-doped LiNbO3 using well-characterized crystals. It has been found that the O-H bond stretching vibrational frequency nu (OH) has a strong correlation with Nb concentration in the crystals. We have also determined the position where hydrogen enters using Novak's empirical relationship between the values of nu (OH) and the length of the hydrogen bond and the structure analysis data for the undoped crystals. On the basis of those results and the polarization characteristics, we have examined the intrinsic and the extrinsic defect structure models in LiNbO3. It has been clarified that the behaviour of nu (OH) reflects the defect structures. The behaviour of nu (OH) supports the Li-site vacancy model as the intrinsic defect structure model, and the corresponding extrinsic defect model. A brief discussion is also given of the behaviour of nu (OH) in crystals simultaneously doped with two kinds of impurity.
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