Yu. F. Shepelev

Crystal structures of the partially K-, Rb-,and Cs-exchanged forms of NaX zeolite in both the hydrated and the dehydrated (400 °C) states

Crystal structures of the hydrated forms of three partially K-, Rb-, and Cs-exchanged NaX [Si/Al = 1.11(2)] with the exchange levels of 0,57(6), 0.50(5), and 0.48(4), respectively, and of the K and Cs dehydrated forms have been determined using X-ray single-crystal data. For the dehydrated forms, data collection was carried out at 400 °C. The structures were refined in the cubic space group Fd3 to final R -factors of 0.031 (233 reflections), 0.045 (360) and 0.063 (656) for the hydrated and 0.078 (229) and 0.059 (376) for the dehydrated forms, respectively. Analysis of the cation distribution in the hydrated forms shows that K ions penetrate into all zeolite cavities, whereas Rb ions diffuse into the sodalite cage, but not into the hexagonal prism, and Cs ions are located only in the supercage. Dehydration of the zeolites is accompanied by a migration of unexchanged sodium cations into the hexagonal prism. Destruction of the dehydrated Rb-exchanged crystal after the 6 h-long exposure at 400 °C seems to be caused by a slow migration of the Rb ions into the prism up to a marked level. In d -CsNaX. Cs ions have been observed in the sodalite cage and, in minor amounts (about 1 per unit cell), in the prisim. Obviously, such a small concentration of Cs in the prism cannot cause a loss of crystallinity, as does the Rb in d-RbNaX. The cation content of site II in the partially exchanged X zeolites is strongly influenced by the kind of the exchanged cations.

Crystal structure of a partially lithium-exchanged X zeolite in hydrated (25°C) and dehydrated (275°C) states

Abstract The crystal structures of a partially Li-exchanged X zeolite Li 62 Na 30 Al 92 Si 100 O 384 ·200 H 2 O (h-LiNaX) and its dehydrated form (d-LiNaX) have been determined using single-crystal X-ray diffraction data. The dehydrated form was measured at 275°C. The crystal structures were refined by the least-squares method in the cubic space group Fd 3 [ a = 24.82(1) A , h-LiNaX; a = 24.75(1) A , d-LiNaX] with 385 and 190 reflections to final R-factors of 0.067 and 0.065 for the hydrated and dehydrated forms, respectively. In h-LiNaX, the Li ions are located in site I′ and displaced from the plane passing through the coordination oxygen atoms by 0.6(1) A. In d-LiNaX, in addition to site I′, the Li + ions occupy site II at the center of the six-membered rings. The large distortions of the zeolite framework occurring upon dehydration are related to the cation redistribution and strong interactions between Li ions and oxygen-framework atoms.

Causes of modulation and hole conductivity of the high-Tc superconductor Bi2Sr2CaCu2O8+x according to X-ray single-crystal data

The one-dimensionally modulated structure of the high-Tc superconductor Bi2Sr2CaCu2O8+x with a0=5.407(2), b0=5.412(3), c0=30.1771(8) AA, q=0.210(2)b0* has been analysed in the commensurate approximation using the conventional three-dimensional space group Pnnn and utilizing single-crystal X-ray diffraction data. A reliable model (R=0.07, Rw=0.079 for 1011 unique reflections) has been obtained. The extra oxygen atoms have been found inside and between the Bi-O-Bi chains of the BiO layer. The modulation displacement functions of the atoms have been obtained by means of harmonic analysis of the modulation displacements. It has been shown that the topology of Bi-O-Bi chains. The modulation of the structure and the form of the modulation displacement functions can be explained by the shifts of the oxygen atoms in the BiO layer correlated according to the antiferroelectrical law. The cationic vacancies in the Sr sites provide the hole conductivity in the copper-oxygen plane and lead to the formation of the percolation picture in this plane.

Crystal Structures of Na2ZnP2O7, K2ZnP2O7, and LiKZnP2O7 Phases in the M2O–ZnO–P2O5 Glass-Forming System (M = Li, Na, and K)

The crystal structures of Na2ZnP2O7 and K2ZnP2O7 are determined from X-ray powder diffraction data. The structure of LiKZnP2O7 is determined using a single-crystal sample. In the alkali zinc phosphates studied, the Zn2+ cation is coordinated by four oxygen atoms of the phosphate tetrahedra. In the first two compounds, the [ZnP2O7]2– anion has the form of a sheet consisting of Zn and P oxygen-containing tetrahedra that involves only five-membered rings. In the LiKZnP2O7 structure, the zinc phosphate anion of the same composition has the form of a three-dimensional skeleton with a three-dimensional system of channels. These channels are linked via six-membered, eight-membered, and ten-membered rings. The atomic coordinates and interatomic distances are reported.

Structure of bismuth oxoborate Bi4B2O9 at 20, 200, and 450°C

Single crystals of bismuth oxoborate Bi4B2O9 have been grown by slowly cooling the melt of a stoichiometric Bi2O3 + H3BO3 mixture. The structure of the borate (monoclinic space group P21/c, a = 11.107 Å, b = 6.629 Å, c = 11.044 Å, β = 91.04°, Z = 4) has been studied at 20, 200, and 450°C. The structure is described not only in terms of full BiO6− and BiO7 polyhedra but also in terms of truncated BiO3− and BiO4− polyhedra and BO3 triangles, as well as oxo-centered OBi3 triangles and OBi4 tetrahedra. It is shown that both the B-O and Bi-O bond lengths are practically unaffected by temperature. Only the angles between polyhedra change with temperature, being responsible for the strong anisotropy of Bi4B2O6 thermal expansion, which was measured by high-temperature powder X-ray diffraction: α11 = 20, α22 = 15, α33 = 6 × 10−6 °C−1, and μ = (c, α33) = −19°.

The crystal structure of a new 84 K superconductor, Bi4Sr4CaCu3O14+x

Abstract The crystal structure of a new superconductor in the Bi-Sr-Ca-Cu oxide system of nominal composition Bi4Sr4CaCu3O14 has been studied by single crystal X-ray diffraction. A modulation was observed along the b-axis. The structure of the fundamental unit cell has been determined in orthorhombic space group Pbmm, a = 5.411 (2), b = 5.417 (3), c = 27.75 (1) A, Z = 2. The final R-factor is 0.067 for 167 unique reflections collected up to 2θ = 60° with Ag Kα radiation. The essential feature of the structure is the existence of two kinds of copper-oxygen layers with square-pyramidal (CuO3-layer) and square-dipyramidal (CuO4-layer) oxygen coordinations of the Cu atoms. Two perovskite-like blocks [Sr-CuO3-Ca-CuO3-Sr] and [Sr-CuO4-Sr] are alternated by double [BiO]2 layers. The bonds in the BiO planes form chains Bi-O-Bi extending along the b-axis.

Structure of dipotassium (nitrilo-κN-triacetato-κ3O,O'',O'''')oxoperoxovanadate(V)–water (1/2)

K 2 [V(C 6 H 6 NO 6 )O(O 2 )].2H 2 O, M r =401.28, orthorhombic, Pnam, a=7.597 (5), b=12.951 (3), c=13.131 (5) A, V=1291.9 (1) A 3 , Z=4, D x =2.06 g cm -3 , λ(Mo Kα)=0.71069 A, μ=14.4 cm -1 , F(000)=808, T=295 K, R=0.019 for 1820 unique reflections. Vanadium is surrounded by a sevenmembered coordinating pentagonal bipyramid formed by a tetradentate nitrilotriacetate ligand, a bidentate peroxo group and a vanadyl oxygen. The bipyramid is symmetrical relative to the mirror plane passing through the V atom, the apical O atoms, the equatorial N atom and the midpoint between the peroxo O atoms. The nitrilotriacetate ligand forms three glycinate rings with the V atom, one of which coincides with the mirror plane, the two others being symmetrical relative to the plane

Synthesis and crystal structure of strontium borate Sr4B14O25

The new compound Sr4B14O25 (4SrO · 7B2O3) corresponding to an oxide ratio of 4: 7 has been identified and synthesized in the SrO-B2O3 system. The crystal structure of the compound has been determined (space group Cmc21, a = 7.734(5) Å, b = 16.332(5) Å, c = 14.556(5) Å, Z = 4, 702 F(hkl), R = 0.078). The borate anions form a three-dimensional framework consisting of borate groups of two types: three-ring structures (2□, Δ) and BO3 triangles. Layers formed by 14-membered rings composed of boron-oxygen tetrahedra and triangles packed within the layer according to the herringbone pattern can be distinguished in the framework. The strontium atoms are located on the mirror symmetry planes between these layers. The compound is metastable and decomposes, on long-term storage, into strontium di- and metaborate.

Crystal structure of the K2(Zn3P4O14) compound

Single crystals of potassium zinc phosphate, namely, K2(Zn3P4O14), are grown by crystallization of a glass of the related compound. The crystal structure of the compound is determined using single-crystal X-ray diffraction.

Thermal vibrations and the structure of quasi-two-dimensional R2CuO4 crystals (R=La, Pr, Nd, Sm, Eu, and Gd)

Thermal vibrations of ions in R2CuO4 crystals (R=La, Pr, Nd, Sm, Eu, Gd) were studied by x-ray diffractometry. A comparative analysis of thermal displacements of the copper and rare-earth ions permitted a conclusion as to the main interactions responsible for the structural state of the CuO2 sheets and of a crystal as a whole. The structural properties were found to correlate with the magnitude of the ionic radius and with the ground state of the rare-earth ions.