Manifestation of structural features of LiNbO3:Zn and LiNbO3:Mg crystals in their IR absorption spectra in the stretching region of OH- groups

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 (ae2.0 mol % ZnO) and the LiNbO3:Mg crystals near their first and second concentration thresholds (ae3.0 and 5.5 mol % MgO) decrease markedly, indicating ordering of the position of the OH- groups in the structure of the crystals.