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 NbLi 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

Influence of Doping Technology on the Stoichiometry and Features of the Localization of B³⁺ Cations in LiNbO₃:B Single Crystals

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 (NbLi, NbV), we have used only those that Rietveld found for these exact LN:B crystals, and the vacancy in the niobium octahedron (VNb) 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

Boron Influence on Defect Structure and Properties of Lithium Niobate Crystals

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