We have established that luminescence in lithium niobate crystals both congruent and near-stoichiometric (R ≈ 1) is due to point defects in the cationic sublattice and intraconfigurational transitions in the oxygen-octahedral NbO₆ clusters. We have also determined that the main contribution to the luminescence in the visible and near IR regions is made by luminescence centers with the participation of Nb_Li defects: the Nb_Li-Nb_Nb bipolaron pair and the Nb_Li-O defect in a congruent crystal. The minimum intensity of bipolaron luminescence has been observed in stoichiometric crystals obtained using different technologies. Weak luminescence of the Nb_Li-Nb_Nb bipolaron pair indicates a small number of Nb_Li defects in the crystal structure. The number of Nb_Li defects in the crystal structure indicates a deviation of the crystal composition from stoichiometry.

We proposed and investigated a refinement of technology for obtaining Mg-doped LiNbO₃ (LN) crystals by co-doping it with B. LN:Mg (5.0 mol%) is now the most widely used material based on bulk lithium niobate. It is suitable for light modulation and transformation. We found that non-metal boron decreases threshold concentrations of the target dopant in many ways. In addition, we earlier determined that the method of boron introduction into the LN charge strongly affects the LN:B crystal structure. So we investigated the point structural defects of two series of LN:Mg:B crystals obtained by different doping methods, in which the stage of dopant introduction was different. We investigated the features of boron cation localization in LN:Mg:B single crystals. We conducted the study using XRD (X-ray diffraction) analysis. We have confirmed that the homogeneous doping method introduces an additional defect (Mg_V) into the structure of LN:Mg:B single crystals. Vacancies in niobium positions (V_Nb) are formed as a compensator for the excess positive charge of point structural defects. According to model calculations, boron is localized in most cases in the tetrahedron face common with the vacant niobium octahedron from the first layer (V_Nb^IO₆). The energy of the Coulomb interaction is minimal in the LN:Mg:B crystal (2.57 mol% MgO and 0.42 × 10⁻⁴ wt% B in the crystal); it was obtained using the solid-phase doping technology. The solid-phase doping technology is better suited for obtaining boron-containing crystals with properties characteristic of double-doped crystals (LN:Mg:B).

A brief review of the features of the defect structure and studies of the luminescent properties of nonlinear optical lithium niobate crystals of various compositions and genesis was given. It was established that the electron- hole pair \({\text{Nb}}_{{{\text{Nb}}}}^{{{\text{4 + }}}}\)–O^– in the oxygen-octahedral cluster NbO₆ emitted in the short-wavelength region of the visible spectrum (400–500 nm), while point defects (V_Li and \({\text{Nb}}_{{{\text{Nb}}}}^{{{\text{4 + }}}}\)–\({\text{Nb}}_{{{\text{Li}}}}^{{{\text{4 + }}}}\) bipolarons)—in the long-wavelength region (500–620 nm). At the ratio of Li/Nb ≈ 1 the luminescence was extinguished in the visible region of the spectrum due to decreasing the intrinsic luminescence centers. It was shown that the presence of polaron luminescence in the near-IR region (700–1050 nm) was due to the small polarons \({\text{Nb}}_{{{\text{Li}}}}^{{{\text{4 + }}}}\) and impurity ions Cr³⁺ localized in lithium and niobium octahedra. The energy transfer between the luminescence centers in the visible and near-IR spectral regions was detected. Moreover, luminescence in near-IR regions was dominant. Doping of LiNbO₃ crystals with zinc and magnesium at ZnO < 4.46 mol % and MgO < 5.29 mol % led to decreasing luminescence of intrinsic defects (V_Li, \({\text{Nb}}_{{{\text{Nb}}}}^{{{\text{4 + }}}}\)–\({\text{Nb}}_{{{\text{L}}{{{\text{i}}}^{{{\text{4 + }}}}}}}^{{}}\)). However, there was an increase of the contribution of the short-wave spectrum component at higher dopant concentrations because of the introduction of Zn and Mg into the origin positions of Nb ions.

A brief review of the features of the defect structure and studies of the luminescent properties of nonlinear optical lithium niobate crystals of various compositions and genesis was given. It was established that the electron-hole pair NbNb4+-O- in the oxygen-octahedral cluster NbO6 emitted in the short-wavelength region of the visible spectrum (400-500 nm), while point defects (VLi and NbNb4+-NbLi4+ bipolarons) - in the long-wavelength region (500-620 nm). At the ratio of Li/Nb≈1 the luminescence was extinguished in the visible region of the spectrum due to decreasing the intrinsic luminescence centers. It was shown that the presence of polaron luminescence in the near-IR region (700-1050 nm) was due to the small polarons NbLi4+ and impurity ions Cr3+ localized in lithium and niobium octahedra. The energy transfer between the luminescence centers in the visible and near-IR spectral regions was detected. Moreover, luminescence in near-IR regions was dominant. Doping of LiNbO3 crystals with zinc and magnesium at ZnO<4.46 mol.% and MgO<5.29 mol.% led to decreasing luminescence of intrinsic defects (VLi, NbNb4+-NbLi4+). However, there was an increase of the contribution of the short-wave spectrum component at higher dopant concentrations because of the introduction of Zn and Mg into the origin positions of Nb ions.

The concentration changes in the photoluminescence spectra of LiNbO₃:Zn crystals (0.004–6.5 mol % ZnO) are investigated. It is observed that, as the zinc concentration increases from 0.004 to 1.42 mol % ZnO, the intensities of the luminescence bands caused by V_Li, Nb_Nb, and Nb_Nb–Nb_Li defects decrease. As the composition of the crystal approaches the second concentration threshold (≈7.0 mol % ZnO), the luminescence halo shifts by ≈0.41 eV towards the high energy range of the spectrum and the intensities of luminescence centers with maxima at 2.66 and 2.26 eV, which are caused by the possible appearance of Zn_Li point defects, increase. It is shown that a LiNbO₃:Zn crystal (4.69 mol % ZnO) obtained by the t-echnology of homogeneous doping contains a greater number of luminescence centers of different natures than the congruent crystal and zinc-doped crystals obtained by the technology of direct doping of the melt. A Li-NbO₃:Zn crystal (4.52 mol % ZnO) exhibits the quenching of main defects (V_Li, Nb_Nb, Zn_Li) due to an increase in the number of nonradiative transitions as compared to other LiNbO₃:Zn crystals in the concentration range [ZnO] = 4.46–6.50 mol %.

Дефекты и некоторые физические свойства номинально чистых и легированных цинком кристаллов ниобата лития