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Due to the Raman spectra structure changes were searched in a LiNbO3:Zn (0.04÷5.84 mol %), LiNbO3:Mg(5.1 mol %) and LiNbO3:В (0.08÷0.18 mol %) single crystals series with the varying of the dopant concentration. Optical homogeneity and photorefractive properties were researched by photoinduced light scattering. Analysis of IR absorption spectra of LiNbO3:Zn (0.04–4.46 mol % ZnO) and LiNbO3:Mg (0.19–5.91 mol % MgO) single crystals in the stretching region of OH– groups has been used to gain insight into composition-dependent structural changes in the crystals. The results demonstrate that, characteristically,the OH– groups occupy different sites in the doped and congruent LiNbO3 crystals and have different quasielastic O–H bond constants in their structure. In stoichiometric lithium niobate crystals, all of the OH– sites and quasi-elastic O–H bond constants are identical. At threshold Zn and Mg dopant concentrations, the frequencies, widths, and intensities of the observed lines change sharply. The linewidths in the spectra of the LiNbO3:Zn crystals near their first concentration threshold (≈2.0 mol % ZnO) and the LiNbO3:Mg crystals near their first and second concentration thresholds (≈3.0 and 5.5 mol % MgO) decrease markedly, indicating ordering of the position of the OH– groups in the structure of the crystals.
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