A defect structure model of LiNbO₃:Sc₂O₃

We have systematically studied the behaviour of the absorption bands due to the O-H bond-stretching vibration and its polarization characteristics in LiNbO₃:Sc₂O₃ with the use of well characterized crystals. It has been found that the O-H bond-stretching vibrational frequencies, ν(OH), have a strong correlation with Nb concentration in the crystals as in the case of MgO-doped LiNbO₃. The values of ν(OH) shift to the higher-frequency region when the Sc concentration in the crystal exceeds about 23 mol%. The magnitude of the frequency shift is smaller and the polarization dependence of the absorption bands due to ν(OH) is weaker in Sc₂O₃-doped LiNbO₃ than in MgO-doped LiNbO₃. These features are attributed to the difference of the charge between Sc³⁺ and Mg²⁺. We have also proposed an ideal defect structure model for Sc₂O₃-doped LiNbO₃, which is based on the Li-site vacancy model as an intrinsic defect structure model. The observed behaviour of ν(OH) is consistently explained by the proposed defect structure model. This supports the justification of the extrinsic defect structure model based on the Li-site vacancy model for Sc₂O₃-doned LiNbO₃ as well as for MgO-doped LiNbO₃.