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.

LiTaO₃ crystals doped with Cr³⁺ and Nd³⁺ ions are promising for developing active nonlinear laser media. In this work, the defect structure of LiTaO₃ crystals, including those doped with Cr³⁺ and Nd³⁺, is examined. X-ray patterns of all six investigated LiTaO₃:Cr:Nd crystals are identical and correspond to a highly perfect structure. Using optical microscopy, the presence of defects of various shapes, microinhomogeneities, and lacunae was revealed. The optical absorption and Raman scattering spectra of a series of nonlinear, optical, double-doped LiTaO₃:Cr³⁺:Nd³⁺ (0.06 ≤ [Cr³⁺] ≤ 0.2; 0.2 ≤ [Nd³⁺] ≤ 0.45 wt%) crystals showed that at concentrations of doping Cr³⁺ ions less than 0.09 wt% and Nd³⁺ ions less than 0.25 wt%, the crystal structure is characterized by a low level of defects, and the optical transmission spectra characterized by narrow lines corresponding to electron transitions in Nd³⁺ ions. In this case, for the radiative transition in the cation sublattice, the existence of three nonequivalent neodymium centers is observed, and for the radiative transition, two nonequivalent centers are observed. IR absorption spectroscopy in the OH⁻-stretching vibration range revealed two main spectral regions: 3463–3465 cm⁻¹, associated with stoichiometry changes, and 3486–3490 cm⁻¹, linked to complex defects such as (V^-_Li)-OH and (Ta⁴⁺_Li)-OH. A distinct low-intensity line at ~3504 cm⁻¹ was observed only in doped crystals, attributed to (Nd²⁺_Li)-OH defects that significantly distort the oxygen-octahedral clusters due to the larger ionic radius of Nd³⁺ compared to Ta⁵⁺. In contrast, Cr-related defects cause only minor distortions. The Klauer method indicated that the highest concentration of OH⁻-groups occurs in the LiTaO₃:Cr³⁺ (0.09 wt%):Nd³⁺ (0.25 wt%) crystal, where multiple complex defects are present.

The effect of gadolinium on the defect structure and luminescent properties of LiNbO3:Gd crystals (0.003, 0.05, and 0.26 wt%) was studied using IR spectroscopy the field of OH- stretching vibrations and photoluminescence analysis. Based on IR spectral data, the threshold concentration of gadolinium has been determined. At this concentration, the defective crystal structure of the studied crystals changes; changes in the volume concentration of OH groups were also analyzed. Based on excitation spectra of LiNbO3:Gd crystals (0.003 and 0.05 wt%), the main features of the absorption bands (250–260 and 308 nm) and their causes have been identified. The dependence of the crystal luminescence intensity on the dopant concentration and selective excitation has been established.

LiNbO3:Er(0.97 wt.%) and LiNbO3:Gd(0.05 wt.%) crystals were studied by IR absorption spectroscopy in the region of OH--group stretching vibrations. It was found that the incorporation of gadolinium dopant leads to a significant deformation of the oxygen octahedron due to the large ionic radius of the Gd3+ cation and an increase in the O-O bond lengths. The IR spectrum shows a shift to the long-wavelength part of the spectrum and absorption bands with a frequency of 3524 and 3535 cm–1, corresponding to the GdLi2+-OH-GdNb2– complex defect. Calculation of the volume concentration of OH--groups showed the lowest value for LiNbO3:Er(0.97 wt.%) and the highest for the LiNbO3:Gd(0.05 wt.%) crystal, due to different mechanisms of incorporation of doping cations into the crystal structure.

We have analyzed complex defects due to the presence of hydrogen bonds in the crystal structure in nominally pure lithium niobate crystals with different Li/Nb ratio, in crystals alloyed with magnesium and zinc in a wide concentration range (LiNbO3 : Mg (0.19-5.91 mol.% MgO) and LiNbO3 : Zn (0.04-6.5 mol. % ZnO)) and in the double-alloyed crystals (LiNbO3 : Y(0.24) : Mg(0.63 wt. %) and LiNbO3 : Gd (0.25) : Mg(0.75 wt. %)), obtained by technology of direct melt alloying, and also in the double-alloyed crystal (LiNbO3 : Mg(5. 05 mol.% MgO) : Fe(0.009 mol.% Fe2O3)) grown from a charge synthesized using the technology of homogeneous alloying with magnesium and iron Nb2O5. We revealed the influence of doping impurities on the concentration of OH-groups, the type and localization of complex defects in the crystal structure. The change in the number of hydrogen atom positions in the structure of the LiNbO3 crystal allow us to judge with sufficient accuracy whether the crystal composition is stoichiometric or congruent. For doped crystals of different compositions data were obtained testifying to changes in the character of complexation of OH-groups with point defects of the cationic sublattice with formation of defects: MеLi-OH-, MеLi-MеNb-OН. A change in the mechanism of entry of the dopant cation into the structure dramatically affects the change in the properties of the crystal.The difference in the frequencies (and, correspondingly, in the values of the quasi-elastic constants of the O-H bonds) in the spectrum of a congruent crystal and doped crystals can also be contributed by differences in the electronegativity and ionic radii of the principal and doping cations.

Single crystals LiNbO3:Y(0.24):Mg:(0.63 wt. %), LiNbO3:Mg(0.5 wt. %), LiNbO3:Y(0.46 wt. %) were studied by infrared absorption spectroscopy in the area of valence vibrations of OH-groups. The absorption bands in the frequency range 3465–3488 cm–1 associated with the violation of stoichiometry in the crystal due to sharpened defects (VLi, NbLi) and formed complex defects VLi-ОН, NbLi-ОН. A part of Y3+ cations forms complex defects -ОН, -ОН- , which correspond to the absorption bands 3527, 3535 cm–1 in crystal LiNbO3:Y (0.46 wt. %). The introduction of magnesium alloying impurity into the double-doped crystal increases the effective distribution coefficient of the rare-earth impurity in the crystal and increases the luminescence of rare-earth ions by 8 times.