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 NbLi 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.

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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:В crystals structure has been studied. Studied LiNbO3:В 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 NbLi is lower in both LiNbO3:B than in CLN crystals. XRD analysis has confirmed that В3+ cations localize in faces of tetrahedral voids О4 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.

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