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

Abstract

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.