Crystal structure refinement of Sr<Subscript>3</Subscript>TaGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript>

An accurate X-ray diffraction study of Sr₃TaGa₃Si₂O₁₄ (STGS) crystal (a = 8.3023(10) Å, c = 5.0853(2) Å, sp. gr. P321, Z = 1, R/wR = 0.59/0.52%, 4004 independent reflections) is performed. The use of two independent data sets obtained on diffractometers with point and 2D detectors made it possible to determine the model structure characterized by the best reproducibility of parameters. The ordered distribution of atoms over crystallographic positions and the anharmonic character of displacements of all cations and one oxygen atom are established.     

Crystal Growth and Structures of New Niobium and Tantalum Oxides: Sr<Subscript>3</Subscript>LiNbO<Subscript>6</Subscript> and Sr<Subscript>3</Subscript>LiTaO<Subscript>6</Subscript>

Abstract  

Single crystals of two new niobium and tantalum oxides, Sr₃LiNbO₆ and Sr₃LiTaO₆ were grown out of a Sr(OH)₂/LiOH·H₂O/KOH flux and characterized by single-crystal X-ray diffraction. The materials crystallize in the trigonal R-3c space group with a = 9.8029(14) ? (9.8111(11) ?), b = 9.8029(14) ? (9.8111(11) ?), c = 11.200(2) ? (11.2056(12) ?), α = β = 90°, γ = 120° for Sr₃LiNbO₆ (Sr₃LiTaO₆). The oxides exhibit the K₄CdCl₆ structure-type, and consist of alternating face-shared BO₆ octahedra (B = Nb⁵⁺ or Ta⁵⁺) and LiO₆ trigonal prisms.     

X-Ray Structure Investigation of Sr<Subscript>3</Subscript>NbGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> Langasite Family Crystal

An accurate structure analysis of Sr₃NbGa₃Si₂O₁₄ single crystals, belonging to the langasite family, has been performed. Two datasets are obtained on an Xcalibur S diffractometer equipped with a CCD detector. The structure is been refined using an averaged dataset: sp. gr. P 321, Z = 1, 295 K, sin θ/λ ≤ 1.35 Å^–1, a = 8.2797(3) Å, c = 5.0774(5) Å; the agreement factors between the model and experiment are found to be R/wR = 0.76/0.64% and Δρ_min/Δρ_max =–0.21/0.17 e/ų for 3820 independent ref lections. The Sr₃NbGa₃Si₂O₁₄ and Sr₃NbFe₃Si₂O₁₄ structures are compared, and the role of magnetic ions in the predicted P321 → P3 phase transition is analyzed.     

Ti/Zr isomorphism in wadeite: The crystal structure of the titanium-dominant K<Subscript>2</Subscript>(Ti<Subscript>0.55</Subscript>Zr<Subscript>0.45</Subscript>)Si<Subscript>3</Subscript>O<Subscript>9</Subscript> member of the series

The crystal structure of the titanium-rich mineral wadeite K₂(Ti_0.55Zr_0.45)Si₃O₉ from rischorrites of the Khibiny Alkaline Massif (Kola Peninsula, Russia) is studied by X-ray diffraction (XCalibur-S diffractometer, R = 0.0459): a = 6.8611(6) Å and c = 10.0611(9) Å; space group P6₃/m, Z = 6, D _x = 3.03 g/cm³. It is shown that the unit-cell parameters and volume of the mineral of mixed (Ti/Zr) composition are naturally intermediate between those of the terminal members of the isomorphous wadeite-based K₂ZrSi₃O₉–K₂(Ti_0.55Zr_0.45)Si₃O₉–K₂TiSi₃O₉ series. The expected correlation is due to the ionic radii of Zr⁴⁺ and Ti⁴⁺ which determine the lengths of Zr/Ti–O bonds in octahedra. The data of field observations and microscopic studies show that the Ti-dominant wadeite is formed on the basis of primary zirconium mineral in the course of a late imposed process under unique geochemical conditions.     

Structural study of new compound Bi<Subscript>2.53</Subscript>Li<Subscript>0.29</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> by the powder neutron diffraction method

A new compound of composition Bi_2.53Li_0.29Nb₂O₉ was synthesized in the course of the search for new materials with high ionic conductivity. Its crystal structure was determined from the neutron diffraction data. The new compound Bi_2.53Li_0.29Nb₂O₉ is crystallized in the orthorhombic system, sp. gr. Cmc2₁, and unit-cell parameters a = 24.849(1) Å, b = 5.4536(3) Å, and c = 5.4619(2) Å at T = 290 K (a = 24.843(2) Å, b = 5.4456(5) Å, and c = 5.4546(5) Å at T = 10 K). Within the temperature range 10–870 K, no structural phase transitions were revealed. The atomic coordinates and the thermal factors in the isotropic approximation were refined by the Rietveld method at 290 and 10 K. The data obtained were analyzed based on the calculated local balance of bond strengths.     

High-Pressure neutron diffraction study of the Nd<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> structure

The changes in the positional parameter of neodymium in Nd_1.85Ce_0.15CuO₄ under high pressures differ from those observed in Nd₂CuO₄; in fact, practically no changes in the positional parameter in the doped oxide are observed. The different behavior of the positional parameter in these materials is explained by the compensation of the interlayer charge transfer by the interlayer repulsion occurring during doping or under the action of high pressures.     

Crystal structure of the Ca<Subscript>3</Subscript>TaGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> compound

The absolute crystal structure of the Ca₃TaGa₃Si₂O₁₄ piezoelectric compound is refined using X-ray diffraction analysis. The unit cell parameters and final R factors are as follows: a = 8.112(1) Å, c = 4.9862(6) Å, space group P321, Z = 1, R = 0.98%, and R _w = 1.42%. It is shown that the configuration of the absolute crystal structure inherited from the seed material determines the positive sense of the optical activity of the crystal under investigation. The structural and acoustical characteristics of the Ca₃TaGa₃Si₂O₁₄ crystals are compared with those of the La₃Ga₅SiO₁₄ crystals.     

Electronic structure and magnetooptical properties of an La<Subscript>0.7</Subscript>Ca<Subscript>0.25</Subscript>Ba<Subscript>0.05</Subscript>MnO<Subscript>3</Subscript> single crystal

The spectral and temperature characteristics of the magnetooptical Kerr effect and the optical properties of an La_0.7Ca_0.25Ba_0.05MnO₃ single crystal are studied. The data obtained are used to calculate the components of the permittivity tensor, whose behavior is interpreted within the framework of the known models of the electronic structure of manganites.     

Crystal structure of Rb<Subscript>2</Subscript>[Ti(VO<Subscript>2</Subscript>)<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>]

The crystal structure of the new compound Rb₂[Ti(VO₂)₃(PO₄)₃] obtained by hydrothermal synthesis in the RbCl-TiPO₄-V₂O₅-B₂O₃-H₂O system (a = 13.604(2) Å, c = 9.386(2) Å, sp. gr. P6cc, Z = 4, ρ_calcd = 3.32 g/cm³) has been studied by X-ray diffraction (Xcalibur-S-CCD diffractometer, R = 0.038). It is shown that the isotypism of Rb₂[Ti(VO₂)₃(PO₄)₃] and Cs₂[Ti(VO₂)₃(PO₄)₃] is caused by the flexibility of a mixed anionic framework composed of phosphorus tetrahedra, vanadium five-vertex polyhedra, and titanium octahedra (bases of the crystal structures of these compounds). The topological correlations between the structures of titanium-vanadyl phosphates and benitoite and beryl silicates are analyzed.     

Crystal-structure refinement of Sr<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript>

The accurate X-ray diffraction study of a Sr₃Ga₂Ge₄O₁₄ crystal was performed based on two X-ray diffraction data sets collected on a diffractometer equipped with a CCD area detector (a = 8.2776(2), c = 5.0415(1) Å, sp. gr. P321, Z = 1, R/wR = 0.78/0.69%, 3645 independent reflections). The structure of Sr₃Ga₂Ge₄O₁₄ is characterized by the presence of two mixed cation sites, which is accompanied by the anharmonic motion not only of cations, but also of two oxygen atoms in general positions. The structures and electromechanical characteristics of Sr₃Ga₂Ge₄O₁₄ and Sr₃TaGa₃Si₂O₁₄ were compared. The Sr₃Ga₂Ge₄O₁₄ structure is characterized by a larger elongation of the Sr polyhedron along the a axis and, simultaneously, by the smaller unit-cell parameter a compared with Sr₃TaGa₃Si₂O₁₄, which correlates with the ratio of the piezoelectric coefficients d ₁₁. The absence of thermally stable directions in the crystals of Sr₃Ga₂Ge₄O₁₄ and Sr₃TaGa₃Si₂O₁₄ is consistent with the absence of the anomalous temperature dependence of the dielectric constant ?₃₃.     

Crystal structure of Ga<Subscript>0.5</Subscript>In<Subscript>1.5</Subscript>Se<Subscript>3</Subscript> solid solution

A solid solution of the GaIn₃Se₆ (2Ga_0.5In_1.5Se₃) composition with a hexagonal lattice (a = 7.051(3) Å, c = 19.148(2) Å, sp. gr. P6₁, z = 6, V = 824.4332(4) ų, ρ = 5.379(2) g/cm³) has been synthesized as a result of alloying Ga, In, and Se elements with a metal ratio of 1: 3. It was established that six out of nine In atoms in the lattice are located in a trigonal bipyramid, while the other three In atoms and three Ga atoms have a tetrahedral coordination.     

Crystal structure and microtwinning of monoclinic La<Subscript>3</Subscript>SbZn<Subscript>3</Subscript>Ge<Subscript>2</Subscript>O<Subscript>14</Subscript> crystals of the langasite family

The crystal structure of monoclinic La₃SbZn₃Ge₂O₁₄ crystals from the langasite family is determined by X-ray diffraction analysis [a = 5.202(1) Å, b = 8.312(1) Å, c = 14.394(2) Å, β = 90.02(1)°, sp. gr. A2, Z = 2, and R/R _w = (5.2/4.6)%]. The structure is a derivative of the Ca₃Ga₂Ge₄O₁₄-type structure (a = 8.069 Å, c = 4.967 Å, sp. gr. P321, Z = 1). The crystal studied is a polysynthetic twin with the twin index n = 2, whose monoclinic components are related by pseudomerohedry by a threefold rotation axis of the supergroup P321.     

Crystal structure of the rhombohedral phase of Fe<Subscript>3</Subscript>B<Subscript>7</Subscript>O<Subscript>13</Subscript>Br

The crystal structure of the rhombohedral phase of Fe₃B₇O₁₃Br was determined and compared with the structures of Fe₃B₇O₁₃Cl and Fe₃B₇O₁₃I. The influence of the ionic radii of the halogen atom on the structural features of the crystals of the boracite family is discussed.     

Structure and superconductivity of layered oxycarbonate Bi<Subscript>2</Subscript>Sr<Subscript>4</Subscript>Cu<Subscript>2</Subscript>CO<Subscript>3</Subscript>O<Subscript>8</Subscript> crystals

A complex study of the elemental composition, structure, and superconducting properties of single crystals of layered oxycarbonate Bi₂Sr₄Cu₂CO₃O₈ has been performed taking into consideration the growth and postgrowth annealing conditions.     

Crystal structure of non-stoichiometric copper selenides studied by neutron scattering and X-ray diffraction

Structural characteristics of non-stoichiometric copper selenides were studied by the elastic neutron and X-ray scattering techniques. Rietveld analysis was used to refine the structure of the high-temperature β-phase of the Cu_1.75Se, Cu_1.78Se, and Cu_1.83Se samples. The homogeneity ranges of the cubic phase were determined. The modification of the crystal structure accompanying the β-α phase transition was studied for Cu_1.75Se and Cu_1.98Se compounds within the 443-10 K temperature range. It was shown that the phase transition is accompanied by distortions of the fcc lattice and the ordering of copper ions.     

Low-temperature X-ray diffraction studies of [(CH<Subscript>3</Subscript>)<Subscript>2</Subscript>NH<Subscript>2</Subscript>]<Subscript>5</Subscript>Cd<Subscript>3</Subscript>Cl<Subscript>11</Subscript> crystals

[(CH₃)₂NH₂]₅Cd₃Cl₁₁ crystals are grown by the method of isothermal evaporation from saturated aqueous solutions containing dimethylamine and cadmium chlorides, [(CH₃)2NH₂]Cl and CdCl_2.5H₂O. The crystal grown are studied by the X-ray diffraction method. It is established that the crystals are orthorhombic with the unit-cell parameters at room temperature a = 18.115 ± 0.004 Å, b = 11.432 ± 0.002 Å, and c = 15.821 ± 0.003 Å. The unit-cell parameters a, b, and c of the [(CH₃)₂NH₂]₅Cd₃Cl₁₁ crystals are measured as functions of temperature in the temperature range 100–320 K. The data obtained were used to determine the thermal expansion coefficients along the main crystallographic axes. The temperature curves of the unit-cell parameters and thermal expansion coefficients showed pronounced anomalies in the vicinity of the temperatures T ₁ = 120, T ₂ = 150, and T ₃ = 180 K corresponding to the phase transitions in the [(CH₃)₂NH₂]₅Cd₃Cl₁₁ crystals. The crystals are also characterized by a pronounced anisotropy of thermal expansion.     

Single-crystal growth of trigonal DyFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> and study of magnetic properties

Fluxes with a wide stability range of trigonal DyFe₃(BO₃)₄ were selected by the successive-approximation method using direct phase probing in the Bi₂Mo₃O₁₂-B₂O₃-Dy₂O₃-Fe₂O₃ system. Crystallization parameters of the fluxes were determined. The conditions for single-crystal growth were recommended using both spontaneous crystal nucleation and seeded growth according to the Kyropoulos method. An analysis of the measured temperature and field dependences of magnetization of crystals in fields parallel or perpendicular to the threefold axis led to the conclusion that DyFe₃(BO₃)₄ undergoes a phase transition to an ordered easy-axis state and is characterized by a field-induced spin-reorientation transition with the predominant alignment of Dy³⁺ spins along the applied field.     

Synthesis and an X-ray diffraction study of Rb<Subscript>2</Subscript>[(UO<Subscript>2</Subscript>)<Subscript>2</Subscript>(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript>]

The synthesis and X-ray diffraction study of compound Rb₂[(UO₂)₂(C₂O₄)₃], which crystallizes in the monoclinic crystal system, are performed. The unit cell parameters are as follows: a = 7.9996(6) Å, b = 8.8259(8) Å, c = 11.3220(7) Å, β = 105.394(2)°, and V = 770.7(1) ų; space group P2₁/n, Z = 2, and R ₁ = 0.0271. [(UO₂)₂(C₂O₄)₃]^2? layers belonging to the AK _0.5 ⁰² T ¹¹ crystal chemical group of uranyl complexes (A = UO ₂ ²⁺ , K ⁰² = C₂O ₄ ^2? , and T ¹¹ = C₂O ₄ ^2? ) are uranium-containing structural units of the crystals. The layers are connected with outer-sphere rubidium cations by electrostatic interactions.     

Refinement of the crystal structure of rhombohedral KU<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> as a representative of orthophosphates with the NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> structure

The crystal structure of the high-temperature β modification of synthetic orthophosphate KU₂(PO₄)₃ was refined from powder X-ray diffraction data by the Rietveld method: sp. gr. \(R\bar 3c\), the unit-cell parameters a= 9.113(1) Å and c= 24.997(1) Å. The isotropic refinement converged to R _wp = 6.15, R _B = 2.14, R _F = 3.52, and S = 0.42. It was confirmed that β-KU₂(PO₄)₃ belongs to the structure type of sodium zirconium phosphate containing an actinide atom in a sixfold (octahedral) coordination formed by oxygen atoms, which is unusual for orthophosphates. The principal interatomic distances and bond angles in the structure are reported.     

X-ray and neutron diffraction studies of Rb<Subscript>4</Subscript>LiH<Subscript>3</Subscript>(XO<Subscript>4</Subscript>)<Subscript>4</Subscript><Emphasis Type="Italic">(X</Emphasis> = S,Se) single crystals

Rb₄LiH₃(SeO₄)₄ single crystals (1) are studied by the X-ray diffraction method at 180 K and Rb₄LiH₃(SO₄)₄ single crystals (2a–2c) are studied by the neutron diffraction method at 298 K (2a and (2b) and 480 K (2c). It is established that isostructural single crystals 1 and 2 (sp. gr. P4₁) have analogous systems of hydrogen bonds: chains of four XO₄ tetrahedra linked by three H bonds with the central bond (2.49 Å) being somewhat shorter than the terminal ones (2.52–2.54 Å). In the high-temperature 2c phase, the amplitudes of atomic thermal vibrations and the degree of proton disorder in the central hydrogen bond have somewhat elevated values.