Люминесцентные свойства нестехиометрических кристаллов ниобата лития различного состава и генезиса (обзор)

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.

A NEW METHOD OF THE STOICHIOMETRY INCREASE OF THE LITHIUM NIOBATE NONLINEAR-OPTICAL CRYSTAL

Experimental and theoretical data on the influence of В2О3 flux on the crystal-melt system, the structural features, and the optical properties of a crystal of lithium niobate are summarized. The Gibbs energies of the borate impurities formation (Al4B2O9, CaB2O4, CaB4O7, Ca2B2O5, Ca3B2O6, PbB2O4) in a congruent composition charge of lithium niobate are calculated. It was found that the element boron, as an active complexing agent, in the composition of the В2О3 flux aligns the distribution coefficients of lithium (KLi) and niobium (KNb). Also, the element boron is able to prevent the transition of trace amounts of impurity metals into the structure of a lithium niobate crystal. Boron increases the ordering of structural units of the cation sublattice and distorts the anionic framework of the crystal. This is due to the fact that boron is embedded in the tetrahedral voids faces of the crystal structure in trace amounts (4∙10-4 mol.%). This leads to changes in bond lengths O-O of the oxygen octahedra O6, thereby changing polarizability oxygen-octahedral cluster NbO6, determining nonlinear optical and ferroelectric properties of the crystal.

Metal–organic frameworks based on polynuclear lanthanide complexes and octahedral rhenium clusters

A series of MOFs comprising tetrahedral lanthanide complexes, isonicotinate linkers, and the octahedral cluster anion [Re6S8(CN)6]4– demonstrate structure-directing effects of the rhenium cluster.

ACTIVE-ILMENITE SURFACE STRUCTURE INFLUENCE ON ACID-ASSISTED BALL MILLING

Active-ilmenite powder derived from natural ilmenite sand was prepared by the ball milling process with acid-solution and Deionized (DI) water. Morphology and particle size of active-ilmenite product in acid/DI-assisted ball milling process were monitored by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). Surface atomic component and chemical bonding were investigated by X-ray photoelectron spectroscopy (XPS). Meanwhile, bulk chemical oxidation and fine structure of active-ilmenite were studied by X-ray absorption near edge structure (XANES) and extended X-ray absorption ne-structure spectroscopy (EXAFS) to confirm the oxidation state and local active species structure at surface. Active-ilmenite by acid-assisted ball milling process is a distinctive method for the preparation of active-ilmenite product with high active surface. Moreover, the distortion of TiO6 and FeO6 octahedral cluster on the sample surface was detected in all milled samples with acid-assisted ball milling process. The presence of Fe[Formula: see text], Fe[Formula: see text] ions and miniature sulfate was also detected on the sample surface by milled product with acid-assisted method.

Beryllium–beryllium double-π bonds in the octahedral cluster of Be2(μ2-X)4 (X = Li, Cu, BeF)

Double πBe–Be bonds formed by the help of s1-type electron donating ligand.