Defect structure of nominally pure lithium niobate crystals grown from a boron doped charge have been studied by Raman and optical spectroscopy, laser conoscopy, and photoinduced light scattering. An influence of boron dopant on optical uniformity, photoelectrical fields values, and band gap have been also studied by these methods in LiNbO₃ crystals. Despite a high concentration of boron in the charge (up to 2 mol%), content in the crystal does not exceed 10⁻⁴ wt%. We have calculated that boron incorporates only into tetrahedral voids of crystal structure as a part of groups [BO₃]³⁻, which changes O–O bonds lengths in O₆ octahedra. At this oxygen–metal clusters MeO₆ (Me: Li, Nb) change their polarizability. The clusters determine optically nonlinear and ferroelectric properties of a crystal. Chemical interactions in the system Li₂O–Nb₂O₅–B₂O₃ have been considered. Boron, being an active element, structures lithium niobate melt, which significantly influences defect structure and physical properties of a crystal grown from such a melt. At the same time, amount of defects Nb_Li and concentration of OH groups in LiNbO₃:B is close to that in stoichiometric crystals; photorefractive effect, optical, and compositional uniformity on the contrary is higher.

Physical and chemical properties have been studied in lithium niobate (LiNbO₃, LN) crystals grown by Czochralski from a boron doped melt. Optical uniformity and optical damage resistance of LiNbO₃:B crystals have been compared with control crystals of nominally pure congruent (CLN) and near-stoichiometric (NSLN K₂O) composition. LiNbO₃:B crystals structure has been studied. Studied LiNbO₃:B crystals have been grown from differently synthesized charges. The charges have been synthesized from a mixture Nb₂O₅:B-Li₂CO₃ using homogeneously doped Nb₂O₅:B precursor (sample 1, (B) = 0.0034 wt% in the charge) and by a direct solid phase synthesis from Nb₂O₅-Li₂CO₃-H₃BO₃ mixture (sample 2, (B) = 0.0079 wt% in the charge). Only traces of boron (10⁻⁵–10⁻⁴ wt%) have been detected in the samples. We have established that concentration of anti-site defects Nb_Li is lower in both LiNbO₃:B than in CLN crystals. XRD analysis has confirmed that B³⁺ cations localize in faces of tetrahedral voids O₄ of LN structure. The voids act as buffers at the anion sublattice distortion. Sample 1 has been shown to have a structure closer to NSLN K₂O crystal than sample 2. We have also shown that the chemical purity of LN crystal increases compared to the melt purity because boron creates strong compounds with impurities in the melt system Li₂O-Nb₂O₅-B₂O₃. Metals impurities thus stay in the melt and do not transfer to the crystal.

We have established that relatively simple calculations of the Coulomb interaction in the lattice of doped lithium niobate (LN, LiNbO₃) can confirm the physical properties of real crystals. We have developed a method for the double adjustment of real XRD data for calculations of Coulomb interaction in a LN cluster. The study considers two crystals doped with boron (LN:B); LN:B(1) has been grown from a charge with 0.02 mol% B₂O₃, boron has been introduced by homogeneous doping, LN:B(2) has been grown from a charge with 0.547 mol% B₂O₃, and boron has been introduced by direct solid-state doping. XRD and Rietveld method data have been obtained for these crystals. The obtained data have been used to build a model of the LN cluster; the cluster in the calculations consists of six oxygen octahedra of the LN structure. The cluster configuration has been chosen in such a way that the structure contains two tetrahedral voids. We have studied 10 variants of filling a cluster with intrinsic cations (Li, Nb), defects, and vacancies. There are 10 of them because, in addition to the basic cations in their positions, defects are present in the structure. In terms of the defects used (Nb_Li, Nb_V), we have used only those that Rietveld found for these exact LN:B crystals, and the vacancy in the niobium octahedron (V_Nb) compensates for these defects, according to the models known for LN. The energy of the Coulomb interaction between the cluster structure of a real crystal and the boron cation localized in it in different positions has been calculated for each of the configurations. Calculations have demonstrated that B is more likely to be embedded near a defect than in a regular structure. This means that boron positively influences the local substructure of doped LN crystals, not only structures the melt during crystal growth. Calculations have shown that the type and location of structural defects affect the position of boron in the structure of a LN crystal. Calculations have also shown that LN:B(1) has a more stable structure, including optical damage resistance. The photoinduced light scattering (PILS) patterns and conoscopic patterns confirm this conclusion for the studied LN:B crystals. The information obtained in this study may be useful for interpreting the defective structure of LN crystals co-doped with boron and metals (Mg, Zn, etc.). This will supplement the knowledge available in the literature regarding models that describe the structure of complexly doped LN crystals.

It is shown that B³⁺ ions can embed in trace amounts into the faces of the oxygen tetrahedra in the LiNbO₃ crystal structure (the faces bordering the lithium octahedron or the vacant oxygen octahedra) or into the oxygen plane separating oxygen-octahedral layers. By interacting with the oxygen atoms of octahedra O₆, B³⁺ ions substantially distort the anionic framework of the crystal structure and change the octahedra′s polarizability determining nonlinear optical properties of the crystal. The structural units of the cationic sublattice in B³⁺ containing crystals are ordered along the polar axis while the Li/Nb ratio increases due to the reduced concentration of Nb_Li point defects serving as deep electronic traps. It is shown that the B₂O₃ flux (an active complexing agent) significantly changes the properties of the charge and those of the melt so that the melt acquires some specific structure, while the Curie temperature of the crystals increases.

The physicochemical characteristics, optical uniformity and photorefractive properties of a series of four LiNbO₃:Er:Zn single crystals have been studied. Single crystals have been obtained by the Czochralski method. The erbium content in the crystals was ~0.5 mol % and the zinc content in the crystals was ~4.02, 4.41, 4.65, 4.66 mol %. The dopant distribution in the LiNbO₃:Er:Zn melt—crystal system has been studied, and the optimal concentration ranges of erbium and zinc in the initial melt have been determined. The ranges provide crystals with high chemical, optical uniformity and resistance to optical damage for applications in optoelectronics and laser technology.

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).