Polymorphism and structure of Nd<Subscript>2</Subscript>MоO<Subscript>6</Subscript> single crystals

The tetragonal and monoclinic modifications of Nd₂MoO₆ single crystals have been investigated by X-ray diffraction. The analysis has been on laboratory sources (2D CCD diffractometers: Xcalibur S (Shared Research Center, Institute of Crystallography, Russian Academy of Sciences) and BRUKER Smart APEX 2 (Shared Research Center, Institute of General and Inorganic Chemistry, Russian Academy of Sciences)) and on a synchrotron radiation source (ESRF, Grenoble, PILATUS@SNBL diffractometer). Local displacements of all atoms in the oxymolybdate structure have been revealed for the first time using electron density difference maps. Possible violation of the translational periodicity in the distribution of Mo and Nd atoms over types is indirectly confirmed, and statistical disorder of oxygen atoms is revealed. The results of the study confirm the previous suggestion that polymorphic transformations occur in oxymolybdates at a very low rate and, apparently, continuously in a wide temperature range. With an increase in temperature, layers undergo ordering according to the cation type as a result of the phase transition from the monoclinic to the tetragonal phase.     

Growth from gas cavities in a molten salt and properties of superconducting Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>8+δ</Subscript> single crystals

Perfect single crystals of the high-temperature superconductor Bi₂Sr₂CaCu₂O_8+δ with the superconducting transition temperature T_C = 72–85 K (depending on the crystallization conditions) are obtained by the method of free growth in gas cavities formed in a KCl solution-melt. The specific features of the growth process are in the formation of an enclosed growth gas cavity in a (previously synthesized) blend of a specified phase composition dissolved in KCl and the free crystal growth in this cavity. The combination of growth and high-temperature annealing in the same process made it possible to obtain uniform (ΔT_C = 1.5 K) single crystals with stable superconducting properties. Annealing of the grown single crystals in oxygen or in air in the temperature range 400–850°C confirmed that the crystals with maximum values of T_C are optimally doped.     

Growth of single crystals of the high-pressure phase of α-PbO<Subscript>2</Subscript>

The technique and results of hydrothermal growth of single crystals of the high-pressure phase of α-PbO₂ are described. Dark brown crystals of predominantly prismatic habit with characteristic sizes of 550 × 100 × 150 μm³ were obtained at p = 4 GPa by cooling an aqueous solution in the temperature range t = 600–300°C.     

Peculiarities of the behavior of the W–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system in a controlled reducing atmosphere

The W–Al₂O₃ system at T = 2400 K and standard pressure (controlled Ar + H₂ atmosphere) has been calculated by stochastic simulation. It is shown that the presence of hydrogen leads to the formation of aluminum hydrides, hydrogen oxides, and aluminum hydroxides; the compounds from the two latter groups (except for water) can interact directly with tungsten. The main chemical reactions occurring in the system are determined, based on which a conclusion about the cyclic character of the processes is drawn. Some recommendations on the composition and pressure of controlled atmosphere for growing sapphire crystals are given.     

X-ray diffraction study of Ba<Subscript>3</Subscript>TaFe<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> single crystal—a promising langasite-type multiferroic

Ba₃TaFe₃Si₂O₁₄ single crystals (sp. gr. P321, Z = 1), promising langasite-type multiferroics, have been grown by floating zone melting. An accurate X-ray diffraction study of Ba₃TaFe₃Si₂O₁₄ single crystal has been performed using two datasets, obtained independently for two different orientations of the same sample on a diffractometer equipped with a CCD area detector at 295 K. Structure refinement is performed based on an averaged dataset: a = 8.5355(1) Å, c = 5.2332(1) Å, sp. gr. P321, Z = 1; the R factors of model structure refinement were found to be R/wR = 1.02/1.23% for 4552 independent reflections. Disordering asymmetry is revealed for the magnetic Fe ion in the 3f site and the Ba cation in the 3e site.     

Investigation of the structure and properties of K<Subscript>9</Subscript>H<Subscript>7</Subscript>(SO<Subscript>4</Subscript>)<Subscript>8</Subscript> · H<Subscript>2</Subscript>O single crystals

The features of the conductivity of K₉H₇(SO₄)₈ · H₂O single-crystal samples in the temperature range of superprotonic phase transition have been investigated. The K₉H₇(SO₄)₈ · H₂O crystal structure is determined and refined taking into account hydrogen atoms by X-ray diffraction analysis at a temperature of 295 K: monoclinic symmetry, sp. gr. P2₁/c, Z = 4, a = 7.059(1), b = 19.773(1), c = 23.449(1) Å, β = 95.33(1)°, R ₁/wR ₂ = 2.71/1.71. The structural data obtained suggest that the occurrence of high conductivity in K₉H₇(SO₄)₈ · H₂O crystals with an increase in temperature is related to the diffusion of crystallization water and motion of K ions, as well as to the transformation of the system of hydrogen bonds and protonic motion. The stabilization of the high-temperature superprotonic phase and its supercooling to low temperatures are due to the presence of channels for the motion of K ions and slow backward diffusion of water in the crystal.     

Optical properties and refractive indices of Gd<Subscript>3</Subscript>Al<Subscript>2</Subscript>Ga<Subscript>3</Subscript>O<Subscript>12</Subscript>:Се<Superscript>3+</Superscript> crystals

Crystals of cerium-doped gadolinium–gallium–aluminum garnet have been grown by the Czochralski method. The transmission and reflection spectra of these crystals in the wavelength range of 250–800 nm have been obtained by optical spectroscopy. Refractive indices are calculated based on the measured Brewster angles, the experimental results are approximated using the Cauchy equation, and a dispersion dependence is obtained.     

Crystal structure of the cubic β<Subscript>ms</Subscript>-phase of a La<Subscript>1.82</Subscript>Bi<Subscript>0.18</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystal at 33 K

Single crystals of the cubic β_ms-phase of La_1.82Bi_0.18Mo₂O₉ have been characterized for the first time by precision X-ray diffraction at 33 K. The structure of a crystal determined at T = 33 K is identical to the structure studied at room temperature. It is confirmed that the La, Mo1, and O1 atoms deviate from their positions on the threefold axis in the high-temperature β-phase; part of lanthanum atoms is substituted by bismuth atoms, which are located on the threefold axis; part of the molybdenum atoms return to the position on the threefold axis.     

Growth of ultrathin twin-free <Emphasis Type="Italic">b</Emphasis>-oriented YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 – <Emphasis Type="Italic">x</Emphasis></Subscript> films

Twin-free b-oriented YBa₂Cu₃O_{7 – x} films with a thickness less than 40 nm have been epitaxially grown on (100)SrLaGaO₄ crystals. Based on the temperature dependence of resistance, the onset temperature of the transition to the superconducting state is found to be 90 K; the transition width is 4 K. The film growth has been performed in two stages. A (100)PrBa₂Cu₃O_{7 – x} buffer layer was previously grown on a (100)SrLaGaO₄ substrate by rf magnetron sputtering in an Ar–O₂ gas mixture at a continuous and monotonic increase in temperature from 660 to 830°C. The main YBa₂Cu₃O_{7 – x} film was grown on the buffer layer surface by pulsed laser deposition in an oxygen medium at a fixed temperature (800°C). The above processes were implemented in different chambers, which were connected by a vacuum channel for transporting samples. Both films were grown in situ, without contacting atmosphere in all growth stages. An X-ray diffraction study has shown that the YBa₂Cu₃O_{7 – x} films are single-crystal and free of precipitates of other phases and domains of other orientations.     

Non-isothermal crystallization kinetics of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>–CaO–SiO<Subscript>2</Subscript> glass containing nucleation agent P<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript>

Fe₂O₃–CaO–SiO₂ glass ceramics containing nucleation agent P₂O₅/TiO₂ were prepared by sol-gel method. The samples were characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The activation energy and kinetic parameters for crystallization of the samples were calculated by the Johnson-Mehi-Avrami (JMA) model and Augis-Bennett method according to the results of DSC. The results showed that the crystallization mechanism of Fe₂O₃–CaO–SiO₂ glass, whose non-isothermal kinetic parameter n = 2.3, was consistent with surface crystallization of the JMA model. The kinetics model function of Fe₂O₃–CaO–SiO₂ glass, f(α) = 2.3(1–α)[–ln(1–α)]^0.57, was also obtained. The addition of nucleation agent P₂O₅/TiO₂ could reduce the activation energy, which made the crystal growth modes change from onedimensional to three-dimensional.     

Structural investigation of anion-deficient manganites La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3 − δ</Subscript>

The crystal structure of the anion-deficient manganites La_0.7Sr_0.3MnO_{3 ? δ} (δ = 0, 0.10, 0.15, 0.20) is investigated using high-resolution neutron diffraction. At room temperature, the crystal structure of the stoichiometric manganite La_0.7Sr_0.3MnO₃ and the anion-deficient manganite La_0.7Sr_0.3MnO_2.9 is satisfactorily described in rhombohedral space group R \(\bar 3\) c. The anion-deficient manganite La_0.7Sr_0.3MnO_2.85 is characterized by two perovskite phases with space groups R \(\bar 3\) c and 14/mcm. The crystal structure of the La_0.7Sr_0.3MnO_2.8 manganite corresponds to the structure of a perovskite phase with space group I4/mcm. It is established that the phase separation in the crystal structure of the La_0.7Sr_0.3MnO_{3 ? δ} manganites at a temperature of 293 K is associated with a nonuniform distribution of oxygen vacancies.     

Structure of Nd<Subscript>5</Subscript>Mo<Subscript>3</Subscript>O<Subscript>16 + δ</Subscript> Single Crystals Doped with Tungsten

The structure of tungsten-doped Nd₅Mo₃O_{16 + δ} single crystals has been investigated by X-ray diffraction analysis, energy-dispersive X-ray spectroscopy (EDXS) microanalysis, and extended X-ray absorption fine-structure (EXAFS) spectroscopy. Tungsten atom, which partially replaces molybdenum atoms in the structure, is found to be located close (at a distance of 0.57 Å) to the Mo atom site. The interstitial oxygen atom О3 shifts from the crystallographic site with coordinates (0.5, 0.5, 0) to the site with coordinates (0.43, 0.48, 0.04) and complements the tungsten coordination to octahedral. The decrease in the unit-cell parameter and conductivity with an increase in the tungsten concentration in the sample is shown to be related to the fact that the interstitial oxygen atom О3 (responsible for conductivity) is in the coordination environment of tungsten atom.     

Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>–In<Subscript>2</Subscript>O<Subscript>3</Subscript> thin films on sapphire substrates: Synthesis and ultraviolet photoconductivity

The structure and electrical and optical properties of β-Ga₂O₃–In₂O₃ thin films on sapphire substrates with different orientations have been investigated. The samples have been prepared by annealing of gallium–indium metallic films on sapphire substrates in air at different gallium-to-indium ratios in the initial mixture. The photoconductivity of these structures in the solar-blind ultraviolet spectral region has been examined.     

Structural Analysis of Sr<Subscript>3</Subscript>NbFe<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> Single Crystal from the Langasite Family and Finding of Negative Thermal Expansion in the Range of 83–110 K

Iron-containing Sr₃NbFe₃Si₂O₁₄ single crystals from the langasite family, which are interesting for researchers due to their magnetic ordering at T _N = 26 K, have been grown by the floating zone melting method. Accurate X-ray diffraction analysis is performed at 293 and 90.5 K using the data collected on a CCD diffractometer. To compensate for systematic errors, two data sets are collected at each temperature. The structure is refined based on averaged data set: sp. gr. P321, Z = 1, sin θ/λ ≤ 1.35 Å^–1; a = 8.2609(4) Å, c = 5.1313(3) Å at 293 K and a = 8.2344(6) Å, c = 5.1243(6) Å at 90.5 K; the agreement factors are R/wR = 1.18/1.03% and Δρ_min/Δρ_max =–0.57/0.25 e/Å3 for 3583 independent reflections at 293 K and R/wR = 1.18/1.13% and Δρ_min/Δρ_max =–0.54/0.23 e/Å3 for 3638 reflections at 90.5 K. Negative thermal expansion in the direction of the cell c axis is revealed in the range of 83–110 K.     

Refinement of the composition and structure of YBaCo<Subscript>4−<Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>7+δ</Subscript> crystals

YBa(Co_{4 ? x} Al _x )O_{7 + δ} (114Y) crystals have been grown in the Y-Ba-Co-O system by spontaneous crystallization from a slowly cooled nonstoichiometric melt. To change the oxygen content, the crystals were isothermally annealed in air in the range of 280–490°C. The crystals grown were characterized by scanning electron microscopy and powder X-ray diffraction. According to the data of an X-ray spectroscopic quantitative microprobe analysis, the average compositions of “as-grown” and oxygen-saturated crystals were Y_1.04Ba₁Co_3.54Al_0.50O_7.8 and Y_1.02Ba₁Co_3.55Al_0.51O_8.4, respectively. The refinement of the crystal structure after saturation on an automatic Bruker X8APEX diffractometer with a CCD detector (MoKα radiation, graphite monochromator, θ_max = 32.54°, sp. gr. P6₃ mc, a = 6.2746(9), c = 10.257(3) Å, V = 349.71(13) ų, Z = 2, d _calcd = 5.220 g/cm³) reveals the location of Al in two independent positions of Co atoms and yields the general formula of the compound as YBaCo_3.26Al_0.74O₇. Problems related to the difference in the compositions obtained by different methods are discussed.     

Electrostatic model of the LiТаO<Subscript>3</Subscript> ferroelectric

Possible combinations of the electronic polarizability tensor components for oxygen and tantalum ions have been established based on the analysis of the refraction coefficients of LiТаO₃ crystal in the far-IR region. The local electric field on structurally nonequivalent ions in the LiТаO₃ unit cell was calculated within the modified point-multipole model. A combination of the electronic polarizability tensor components for oxygen and tantalum ions and their effective charges is established, which provides a good correspondence of the calculation results with the experimental data on LiТаO₃ crystals obtained by ⁷Li nuclear magnetic resonance and ¹⁸¹Та nuclear quadrupole resonance and with the known value of the crystal spontaneous polarization.     

Na<Subscript>3</Subscript>Tb<Subscript>3</Subscript>[Si<Subscript>6</Subscript>O<Subscript>18</Subscript>] · H<Subscript>2</Subscript>O,a synthetic analogue of microporous mineral gerenite

Crystals of a new silicate, Na₃Tb₃[Si₆O₁₈] · H₂O, space group \(P\bar 1\), are obtained under hydrothermal conditions. The formula of the compound is determined in the course of structure solution. The silicate is a synthetic analogue of the gerenite mineral (Ca_1.21Na_0.57)(Y_2.24Dy_0.68)Si₆O₁₈ · 2H₂O, whose structure contains six-membered rings formed by SiO₄ tetrahedra. The [Si₆O₁₈] rings are connected by TbO₆ octahedra into a mixed microporous framework with voids filled by Na atoms and water molecules. The new silicate differs from gerenite by the occupation of the Ca position by Na atoms and population of the pores sandwiched between six-membered rings. By virtue of conditions of hydrothermal synthesis in the absence of Ca and excess of Na in the system, an additional Na position appears in the void. It is populated statistically, and in gerenite it was occupied by water molecules only. In the new structure, the position of water is split into two statistically populated positions. The inclusion of Na atoms in additional positions in framework pores and their high thermal vibrations are indicative of ion-exchange properties of the structure. Possible paths of ion exchange are discussed.     

Crystal structure and properties of K<Subscript>2</Subscript>[OsO<Subscript>2</Subscript>(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>2</Subscript>] · 2H<Subscript>2</Subscript>O

The synthesis and X-ray diffraction study of the compound K₂[OsO₂(C₂O₄)₂] · 2H₂O are performed. The compound crystallizes in the triclinic crystal system, space group P \(\bar 1\), a = 6.545(1) Å, b = 6.835(2) Å, c = 7.595(2) Å, α = 85.76(2)°, β = 65.33(2)°, γ = 71.14(2)°, and Z = 1. The osmium atom is located at the center of symmetry and has a distorted octahedral coordination formed by oxygen atoms: two oxygen atoms of the osmyl group occupy the apical positions [Os-O, 1.730(2) Å], and four oxygen atoms of the oxalate ions lie in the equatorial plane. The K⁺ cation is surrounded by ten oxygen atoms located at different K-O distances in the range from 2.787(2) to 3.158(2) Å. The assignment of the absorption bands in the IR spectrum of K₂[OsO₂(C₂O₄)₂] · 2H₂O is performed. The electronic absorption spectra of the compound are recorded in different solvents, and the thermal behavior in air is studied.     

New crystals of the CsHSO<Subscript>4</Subscript>–CsH<Subscript>2</Subscript>PO<Subscript>4</Subscript>–H<Subscript>2</Subscript>O system

Cs₆H(HSO₄)₃(H₂PO₄)₄ crystals, grown for the first time based on an analysis of the phase diagram of the CsHSO₄–CsH₂PO₄–H₂O ternary system, have been investigated by structural analysis using synchrotron radiation. The atomic structure of the crystals is determined and its specific features are analyzed.     

AErP<Subscript>2</Subscript>O<Subscript>7</Subscript> (A = Rb,Cs) and HEuP<Subscript>2</Subscript>O<Subscript>7</Subscript>·3H<Subscript>2</Subscript>O: Crystal Structures,Vibrational Studies and Thermal Behaviours

Abstract  

New diphosphates AErP₂O₇, A = Rb (1), Cs (2), and HEuP₂O₇·3H₂O (3) have been prepared via soft chemistry route from evaporation of aqueous solution. Their crystal structures have been solved by single crystal diffraction data. The compounds (1) and (2) crystallize in the monoclinic space group P2₁/c with the following unit cell dimensions: (1) a = 7.7248(4) ?, b = 10.9342(6) ?, c = 8.6716(3) ?, β = 105.434(4)° and Z = 4; (2) a = 7.9192(4) ?, b = 10.8606(4) ?, c = 8.7684(5) ?, β = 104.317(5)° and Z = 4. Compound (3) crystallises in the triclinic space group P − 1, with the unit cell parameters: a = 6.4036(5) ?, b = 6.8753(6) ?, c = 9.7964(8) ?, α = 81.706(7)°, β = 80.218(7)°, γ = 88.381(7)°. Crystal structure of (1) and (2) consists of 3D open framework built from [ErO₆] octahedra sharing corners with [P₂O₇] units, leading to the formation of channels running along c direction in which A⁺ [A = Rb(1), Cs(2)] ions are located. The compound (3), on the other hand, exhibits a lamellar structure, in which edge-sharing polyhedra of eightfold coordinated europium form chains along [010]. These chains are joined in turn to each others using diphosphates bridges. A network of O–H···O hydrogen bonds reinforces the cohesion of the structure of (3). The diphosphate groups in (1), (2) and (3) adopts an eclipsed configuration. IR and Raman spectra of the three new phosphates show usual signals related to the P₂O₇ diphosphate group and the water molecule. TG and DTA studies were carried out on crystals of (3). The data are correlated to the crystal structures ones.