E. L. Belokoneva

Multipole analysis of the electron density in triphylite, LiFePO4, using X-ray diffraction data

The electron density distribution and 3d-orbital electron occupancies for the Fe atom in synthetic triphylite, LiFePO 4 , have been analysed using single-crystal X-ray diffraction data measured at T=298 K with Mo Kα (λ=0.71069 A) radiation to a resolution corresponding to (sin θ max /λ)=1.078 A -1 . Measurements of 3265 reflections gave 944 unique data [R int (I)=0.036] with I>2σ(I). For an atomic multipole density model fitted by least-squares methods R(F)=0.0174 for all unique reflections. The Fe atom 3d-orbital occupancies have been derived from the multipole population coefficients using point-group-specific relations

X-ray and Electron Diffraction Study of MgO

Precise X-ray and high-energy transmission electron diffraction methods were used for the study of electron density and electrostatic potential in MgO crystals. The structure amplitudes were determined and their accuracy estimated using ab initio Hartree-Fock structure amplitudes as criteria. The electrostatic potential distributions, reconstructed using Fourier series from both X-ray and electron diffraction data, are in satisfactory mutual agreement and are similar to the theory. They, however, suffer from restricted experimental resolution and, therefore, the reconstruction of the electrostatic potential via an analytical structural model is preferable. The model of electron density was adjusted to X-ray experimental structure amplitudes and those calculated by the Hartree-Fock method. The electrostatic potential, deformation electron density and the Laplacian of the electron density were calculated with this model. The critical points in both experimental and theoretical model electron densities were found and compared with those for procrystals from spherical atoms and ions. A disagreement concerning the type of critical point at (,,0) in the area of low, near-uniform electron density is observed. It is noted that topological analysis of the electron density in crystals can be related with a close-packing concept.

Spectroscopic studies of Ln2Ca3B4O12-Nd3+ (Ln=Y, La, Gd) crystals

Abstract In this work, the neodymium doped double borate single crystals Ln 2 Ca 3 B 4 O 12 -Nd (Ln=La, Gd, Y) were Czochralsky grown. Their structure, spectroscopic characteristics, and lifetimes were studied. The distribution coefficients of neodymium ions into the borate lattice Ln 2 Ca 3 B 4 O 1 2 -Nd (Ln=La, Gd, Y) were determined. They are close to 1 and slightly increase in the series La, Gd, Y in accordance with the change in the melting temperature. Using the Judd–Ofelt method we determined the intensity parameters: Ω 2 =4.56×10 −20 cm 2 and 4.59×10 −20 cm 2 ; Ω 4 =7.29×10 −20 cm 2 and 7.56×10 −20 cm 2 , and Ω 6 =9.87×10 −20 cm 2 and 10.76×10 −20 cm 2 for La 2 Ca 3 B 4 O 12 -Nd and Gd 2 Ca 3 B 4 O 12 -Nd, respectively,. The microparameters and selfquenching rate in the La 2 Ca 3 B 4 O 12 -Nd and Gd 2 Ca 3 B 4 O 12 -Nd crystals were calculated. It was shown that the neodymium selfquenching could be described within the framework of the static model, which, probably, corresponds to the structural properties. The possible application of the studied crystals as active laser media, including also the laser-diode pumped ones was concluded.

Potassium bromo-borate K3[B6O10]Br—A new nonlinear optical material

A new polar hexaborate, K3[B6O10]Br (space group R3m), is synthesized under hydrothermal conditions. The radical anion is composed of macallisterite hexaborate blocks 6[3Δ + 3T] formed by boron triangles and boron tetrahedra. The blocks are joined together through vertices into a [B6O10]∞∞∞2− zeolite-like framework of a new type. Large K+ cations and Br− anions are located in extended channels that are aligned parallel to rhombohedral translations of the crystal lattice. The structure of the sublattice formed by large ions consists of BrK6 bromo-centered octahedra, which are linked together through vertices into a hexagonally distorted perovskite framework. The perovskite framework is inserted into the boron-oxygen framework so that these two frameworks do not intersect. The nonlinear optical properties of powdered samples of the K3[B6O10]Br compound are investigated using the second harmonic generation method. It is found that the K3[B6O10]Br compound exhibits a high nonlinear optical activity that exceeds the activities of the majority of borate compounds and is characteristic of haloid borates of the hilgardite family.

High-temperature crystallization and X-ray characterization of Y2SiO5, Y2Si2O7 and LaBSiO5

The laser crystals of chromium-doped yttrium oxyorthosilicate (YSO) were grown from melts by Czochralski technique. The crystals of YSO, yttrium pyrosilicate and stillwellite-like lanthanum borosilicate were obtained from high-temperature solutions. Lithium and potassium di- and trimolybdates were used as fluxes for silicate systems. In the case of borosilicate crystals, the choice of fluxes was based on potassium trimolybdate with an excess of potassium fluoride. The composition of grown crystals was studied by electron microprobe analysis, and structural characteristics were determined for all the single crystals.

Critical points in a crystal and procrystal

The critical points in the model electron density distributions of LiF, NaF, NaCl, and MgO crystals, constructed from accurate X-ray diffraction data, are determined. For LiF and MgO they are compared with those obtained from a Hartree–Fock electron density calculation. Both experiment and theory show the same type of critical points on the bond lines. The topological features in areas between structural units, where the electron density is low and near-uniform, turn out to be model dependent and cannot be established well with the data available. Topological analysis of procrystals (hypothetical systems consisting of spherical atoms or ions placed on the same sites as atoms in real crystal) show that (3, −1) critical points, usually connected with bonding interaction, are observed on interatomic lines in these nonbonded systems as well.

Crystal Growth and Structural Refinements of the Y2SiO5, Y2Si2O7 and LaBSiO5 Single Crystals

The laser crystals of chromium doped yttrium oxyorthosilicate (YSO) were grown from the melt by Czochralski technique. The crystals of YSO, yttrium pyrosilicate and stillwellite-like lanthanum borosilicate were obtained from high temperature solutions. Lithium and potassium di- and trimolybdates were used as fluxes in the growth experiments on silicate crystals. In the case of borosilicate crystals, the choice of fluxes was based on the potassium trimolybdate with an excess of potassium fluoride. The composition of grown crystals was studied by electron microprobe analysis, and structural characteristics were determined for all the single crystals.

Acentric polyborate, Li3[B8O12(OH)3], with a new type of anionic layer and Li atoms in the cavities.

Single crystals of Li3[B8O12(OH)3] were synthesized under hydrothermal conditions using a complex initial composition that included SiO2. The crystal structure is acentric, space group P62c, with a = 8.9301(2) Å and c = 15.9962(4) Å, and Z = 4. New double three-membered rings of hexaborate blocks and monoborate triangles are connected into a polyborate anionic layer. Although the new structure is similar to double ring silicate Na3Y[Si6O15], detailed comparison of the anionic groups demonstrates principal differences between silicates and borates. Sodium silicate was proposed as an intrinsic fast ion conductor, and for the new borate Li ionic conductivity is very probable, as Li atoms in it are in positions similar to the Na atoms in the silicate.

Comparative crystal chemical analysis of borophosphates and borosilicates

A comparative crystal chemical analysis of mixed borophosphate and borosilicate anionic radicals revealed the main difference between them: the condensation of [PO4] tetrahedra does not proceed via common vertices, whereas the sharing of vertices is possible for the [SiO4] tetrahedra. In all the borophosphates considered, phosphorus is found only in the formally isolated tetrahedra. The central part of the heterogeneous radicals is formed by tetrahedra of lower charge ions (B+3), and the tetrahedra that incrust the central part have higher valences (Si+4, P+5). In borosilicates, silicon forms its own constructions up to complex layers and frameworks because its charge (Si+4) is lower than that of phosphorus (P+5). The higher charge of phosphorus impedes two-dimensional and three-dimensional condensation. As a result, borophosphates form a variety of soro and chain radicals and only few layer and framework constructions.

New hilgardite-group polyborate Pb2[B5O9]Br with high optical nonlinearity

New anhydrous lead borate Pb2[B5O9]Br (sp. gr. Pnn2) was synthesized by the hydrothermal method. The second harmonic generation from polycrystalline samples of Pb2[B5O9]Br is characterized by a higher signal than that observed in powdered LiB3O5. The crystal structure of the new hilgardite-group compound was refined by the Rietveld method. Analysis of the known orthorhombic polar varieties of hilgardites, including the new compound, showed that their boron-oxygen frameworks are occupied by the Pb2+, Ca2+, Eu2+, and Ba2+ cations; the Cl−, Br−, and OH− anions; and water molecules in combinations determined by their sizes.