Growing, Structure and Optical Properties of LiNbO₃:B Crystals, a Material for Laser Radiation Transformation

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.