CS families derived from the ReO3 structure type: An electron microscope study of reduced WO3 and related pseudobinary systems

A broad survey of some ReO₃-related CS structures has been carried out by means of an electron microscopy/diffraction study of the systems WO_x and (W⁶⁺, M⁵⁺O_x with M = V, Nb, Ta and 2.90 ? x < 3.00; MoO_x and (W, Mo)O_x; Nb(O,F)_x with 2.75 ? x ? 2.96; as well as V₂MoO₈. The observations give a broader view of the field than was previously possible. (i) They reveal the expected “swinging” of the CS planes when the compositions and/or the component ions are varied. (ii) They provide information on the degree of long-range order and the types of defect present. (iii) They throw further light on the likely mechanism by which the composition changes when a CS structure is reduced or oxidized. These topics are discussed, and an attempt is made to systematize all the observations into a coherent picture incorporating previously available information. This necessitates changing some earlier, dubious conclusions from X-ray diffraction studies.     

Electrochemical Li insertion studies on WNb12O33—A shear ReO3 type structure

Electrochemical lithium insertion studies on WNb₁₂O₃₃ synthesized by solid state reaction (SSR) are carried out in the voltage range 1.0-3.2 V. During first discharge 15.6 Li are inserted with a specific capacity of 221 mAh/g. WNb₁₂O₃₃ is also synthesized by sol-gel (SG) technique with a view to enhance the rate capability and cycling properties. The SSR and SG samples are characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and galvanostatic cycling. Electrochemical cycling performance of SG samples is superior to that of the SSR sample at high ‘C’ rates. The sample synthesized by SG method exhibits high specific capacity of 142 mAh/g after 20 cycles at 20C rate.     

Structure cristalline de SmZrF7. Relations structurales avec le type ReO3

The crystal structure of the fluorozirconate SmZrF₇ has been determined from single crystal X-ray data; the final R value is 0.028. In this compound, isotypic with all the fluorozirconates LnZrF₇ (Ln = rare earth), the symmetry of the cell is monoclinic (space group: P₂₁). The Zr atoms are surrounded by six fluorine atoms forming an almost perfect octahedron: the average distance ZrF is 2.006 Å. The Sm atoms are surrounded by a 8 fluorine atoms polyhedron, the distances SmF lie from 2.221 to 2.411 Å. The association of these two polyhedra by sharing corner forms slabs with an approximately ReO₃ type arrangement: the thickness of the slabs is two octahedra.These slabs with the composition M₂X₇ are held together so that the top fluorine atoms at the surface of a slab increase the coordination of the Sm atom of an adjacent slab. This MX_3.5 phase corresponds to a new structural type between MX₃ (ReO₃ type) and MX₄ (SnF₄ type).     

Crystal structure of [Cu(NH3)4](ReO4)2

At T = 150 K, the crystal structure of [Cu(NH₃)₄](ReO₄)₂ is studied: a = 6.5167(3) ?, b = 6.7790(3) ?, c = 7.4627(3) ?, α = 67.336(1)°, β = 80.004(1)°, γ = 70.687(1)°, V = 286.70(2) ?³, P-1 space group, Z = 1, d _x = 3.661 g/cm³. We analyze the packing of ions using the translation sublattice isolation technique.     

Elastic moduli of ReO3 by Brillouin scattering

Brillouin light scattering has been used to measure elastic stiffness constants in the metal ReO₃: c₁₁ = (71.7 ± 3.5) × 10¹¹dynes/cm², c₄₄ = (7.11 ± 0.25) × 10¹¹dynes/cm², and c₁₂ = (10.3 ± 3.4) × 10¹¹dynes/cm². These values are significantly higher than those obtained ultrasonically at room temperature by Pearsall and Coldren. Since the Brillouin study employs light which heats the scattering volume, it probes lattice forces at an effectively higher temperature. Additional data taken at lower light intensity confirms the positive acoustic velocity temperature gradient.     

Crystal structure of [Cu(en)2](ReO4)2

At T = 150 K the crystal structure of [Cu(en)₂] (ReO₄)₂ (en is ethylenediamine) is studied: a = 6.6229(1) ?, b = 14.2968(3) ?, c = 7.4859(2) ?, β = 102.415(1)°, V = 692.24(3) ?,³, P2₁/c space group, Z = 2, d _x = 3.282 g/cm³. Packing of complex cations is shown to be single layered and pseudohexagonal. Perrhenate anions are located between these layers and additionally coordinate copper atoms with Cu...O distances being 2.504(3) ?.     

[Ni(en)3](ReO4)2: Synthesis, crystal structure and thermal properties

The crystal structure of [Ni(en)₃](ReO₄)₂ (en is ethylenediamine) is studied: a = 8.3997(2)?, b = 15.6167(5)?, c = 14.2406(4)?, β = 100.378(1)°, V = 1837.46(9) OA³, P2₁/c space group, Z = 4, and d _x = 2.673 g/cm³. It is shown that packing of the complex cations can be considered as single-layer pseudohexagonal. Thermal decomposition of the salt in a hydrogen atmosphere at 550°C is used to obtain a mixture of nickel with a nanocrystalline Re_0.87Ni_0.13 solid solution (a = 2.733(2)?, c = 4.400(3)?, P6₃/mmc space group; CSR size is ∼14 nm).     

Synthesis and crystal structure of the monoclinic modification of Yb(ReO4)3(H2O)4

Ytterbium(III) tetraaquatris(tetraoxorhenate(VII)), Yb(ReO₄)₃(H₂O)₄, was prepared by the reaction of Yb₂O₃ with concentrated HReO₄ at room temperature. The colorless compound crystallizes in the monoclinic space group P2₁/n (No. 14) with four formula units per unit cell (a=730.5(1) pm, b=1484.1(5) pm, c=1311.7(2) pm, β=93.69(1)). The main feature of the crystal structure is the formation of chains _∞¹[Yb(H₂O)₄(ReO₄)₂(ReO₄)_2/2] running along [100]. This arrangement shows distorted cubic antiprisms of [Yb(H₂O)₄(ReO₄)₂(ReO₄)_2/2] interconnected via the ReO₄⁻ ligands. The chains are held together in the solid by hydrogen bonding. The compound is paramagnetic and follows the Curie-Weiss law with a magnetic moment of 4.0 μ_B at room temperature and θ=−42 K. It loses hydration water in two steps at temperatures below 400 K; decomposition begins at 850 K, forming Yb₂O₃(Re₂O₇)₂ and is complete at 1350 K leading to Yb₂O₃ as final product.     

Synthesis, characterization, and structural study of K2FeZrP3O12 with the langbeinite structure

A potassium iron zirconium phosphate, K₂FeZrP₃O₁₂, was synthesized by thermal treatment of a material obtained by the sol-gel method, mixing two aqueous solutions, the first containing KCl, FeCl₃ and ZrOCl₂, and the second, H₃PO₄. The crystal structure was refined using powder X-ray diffraction data. The unit cell is cubic, a=10.0554(3) Å, space group P2₁3. This compound is the first iron zirconium phosphate described with a langbeinite-type structure.     

Vtbrational spectra of the triclinic K3[ReO2(CN)4] single crystal

Infrared spectra of K₃[ReO₂(CN)₄] have been recorded using polarized radiation and oriented single crystals. The dichroic behaviour of the bands is analyzed and discussed on the basis of molecular geometry and the oriented molecular model. Experimentally determined dichroic ratios are used to obtain a full set of vibrational frequencies. By combining these data with those obtained from Raman spectra, the fundamental internal and lattice vibrations are assigned.     

Phenomenological and structural study of a low-temperature phase transition in the PbZrO3PbTiO3 system

The Landau-Ginsburg-Devonshire phenomenological theory has been applied to the PbZrO₃PbTiO₃ crystalline solid solution system to explore the behavior of the rhombohedral-tetragonal morphotropic phase boundary in the region below room temperature. The theory suggests that morphotropy is preserved, i.e., that the phase boundary occurs at nearly the same composition right down to 0 K. The rhombohedral (R3m)-rhombohedral (R3c) phase transition was investigated for a composition PbZr_0.6Ti_0.4O₃ using neutron diffraction. Structures in both phases were refined by the Rietveld profile fitting technique. The transition behavior in this composition was indicative of a diffuse-type phase transition, with a transition temperature somewhere between 250 and 300 K. The diffuse nature of this transition is perhaps due to short-range ordering of Zr and Ti. However, powder neutron diffraction is not ideal for determining critical behavior; therefore, it is difficult to make a quantitative conclusion in this respect. Values of the spontaneous polarization were obtained from the (Zr/Ti) shifts, and compared to those deduced from phenomenological theory.     

Synthesis, electron diffraction, XRD and DSC study of the new elpasolite-related oxyfluoride, Tl3MoO3F3

Tl₃MoO₃F₃, a previously unreported member of the A₂BM^VIO₃F₃ family of elpasolite-related oxyfluorides, has been prepared by the reaction of TlF with MoO₃ at 655°C. DSC shows two major polymorphic phase transitions at 42°C and 130°C, respectively. Electron diffraction and XRD studies of the complex room temperature polymorphic form of this material indicate that the spacegroup symmetry is monoclinic P1a1 with a superstructure unit cell given by , when expressed with respect to the underlying ideal elpasolite-type parent structure. This superstructure, while related, is not isomorphous to that recently reported for K₃MoO₃F₃. The existence of a shared subset of strong type satellite reflections suggests the existence of a common intermediate superstructure. A highly structured, three-dimensional continuous diffuse intensity distribution is observed in Tl₃MoO₃F₃ and Rb₂KMoO₃F₃. This suggests that a particular pattern of local O/F ordering and associated Mo ion shifts, recently shown to be responsible for the existence of this diffuse distribution in the case of K₃MoO₃F₃, may be common to the entire family of elpasolite-related A₂BM^VIO₃F₃ compounds.     

Neutron powder diffraction study of tetragonal Li7La3Hf2O12 with the garnet-related type structure

We have successfully synthesized a polycrystalline sample of tetragonal garnet-related Li-ion conductor Li₇La₃Hf₂O₁₂ by solid state reaction. The crystal structure is analyzed by the Rietveld method using neutron powder diffraction data. The structure analysis identifies that tetragonal Li₇La₃Hf₂O₁₂ has the garnet-related type structure with a space group of I4₁/acd (no. 142). The lattice constants are a=13.106(2) Å and c=12.630(2) Å with a cell ratio of c/a=0.9637. The crystal structure of tetragonal Li₇La₃Hf₂O₁₂ has the garnet-type framework structure composed of dodecahedral La(1)O₈, La(2)O₈ and octahedral HfO₆. Li atoms occupy three types of crystallographic site in the interstices of this framework structure, where Li(1) atom is located at the tetrahedral 8a site, and Li(2) and Li(3) atoms are located at the distorted octahedral 16f and 32g sites, respectively. These Li sites are filled with the Li atom. The present tetragonal Li₇La₃Hf₂O₁₂ sample exhibits bulk Li-ion conductivity of σ_b=9.85×10⁻⁷ S cm⁻¹ and grain-boundary Li-ion conductivity of σ_gb=4.45×10⁻⁷ S cm⁻¹ at 300 K. The activation energy is estimated to be E_a=0.53 eV in the temperature range of 300-580 K.     

Strain energy due to {001} crystallographic shear planes in materials possessing the ReO3 structure

The elastic strain energy in a structure of the ReO₃ type containing ordered arrays of {001} CS planes has been calculated. The values obtained are for the elastic strain energy of the matrix between CS planes and also the relaxation energy of the ions in the CS planes themselves. Interactions from all the CS planes in the crystal are summed and not just those from nearest neighbors. The extent to which the CS planes can be considered to transmit the forces which strain the crystal is considered by including a variable parameter, α, in the calculations, which is related to the type of chemical bonding present in the CS planes. The results are compared with experimental observations in the WO₃Nb₂O₅ and NbO₂F systems. It is concluded that the value of α is high for WO₃ doped with Nb₂O₅ and low for NbO₂F in accord with the expectations of chemical bonding. The results also support the view that elastic strain energy is important in influencing the microstructures observed in crystals containing CS planes.     

Synthesis and structure analysis of tetragonal Li7La3Zr2O12 with the garnet-related type structure

We have successfully synthesized a high-purity polycrystalline sample of tetragonal Li₇La₃Zr₂O₁₂. Single crystals have been also grown by a flux method. The single-crystal X-ray diffraction analysis verifies that tetragonal Li₇La₃Zr₂O₁₂ has the garnet-related type structure with a space group of I4₁/acd (no. 142). The lattice constants are a=13.134(4) Å and c=12.663(8) Å. The garnet-type framework structure is composed of two types of dodecahedral LaO₈ and octahedral ZrO₆. Li atoms occupy three crystallographic sites in the interstices of this framework structure, where Li(1), Li(2), and Li(3) atoms are located at the tetrahedral 8a site and the distorted octahedral 16f and 32g sites, respectively. The structure is also investigated by the Rietveld method with X-ray and neutron powder diffraction data. These diffraction patterns are identified as the tetragonal Li₇La₃Zr₂O₁₂ structure determined from the single-crystal data. The present tetragonal Li₇La₃Zr₂O₁₂ sample exhibits a bulk Li-ion conductivity of σ_b=1.63×10⁻⁶ S cm⁻¹ and grain-boundary Li-ion conductivity of σ_gb=5.59×10⁻⁷ S cm⁻¹ at 300 K. The activation energy is estimated to be E_a=0.54 eV in the temperature range of 300–560 K.     

Geometry and electronic structure of CBr3. Comparison with CF3 and CCl3

Ab initio UHF geometry investigation of the CBr₃ radical and the SW X calculations for its ionization potential, electron affinity and electronegativity are reported. A comparison is made with our previous results for the CCl₃ and CF₃ radicals.     

Structure and properties of rhombohedral CePd3Ga8: A variant of the cubic parent compound with BaHg11 structure type

Single crystals of a new intermetallic gallide, R-CePd₃Ga₈, have been synthesized from excess molten gallium. Single-crystal X-ray diffraction reveals that R-CePd₃Ga₈ crystallizes in the R-3m space group with a=b=c=8.4903(10) Å and α=β=γ=89.993(17). R-CePd₃Ga₈ is a variant of the cubic BaHg₁₁ structure type with three structural units: a Ce-centered polyhedron, a distorted cube of Pd₂Ga₆ and a Pd-centered cuboctahedron. The distortions of these units are compared to undistorted analogous units in intermetallic compounds with BaHg₁₁ structure type. Field and temperature-dependent magnetization measurements on R-CePd₃Ga₈ reveal a paramagnetic material with strong antiferromagnetic correlations and a magnetization consistent with Ce³⁺. Electrical resistance measurements indicate Kondo behavior between localized Ce³⁺ magnetic moments.     

High-pressure syntheses of TaS3, NbS3, TaSe3, and NbSe3 with NbSe3-type crystal structure

Transition metal trichalcogenides TaSe₃, TaS₃, NbSe₃ and NbS₃ were prepared under the reaction conditions of 2 GPa, 700°C, 30 min. NbSe₃ is exactly the same as that obtained in the usual sealed-tube method. The other products are modifications of each usual phase. They have crystal structures very similar to that of NbSe₃. The lattice parameters are a = 10.02Å, b = 3.48 Å, c = 15.56 Å, β = 109.6° for TaSe₃, a = 9.52 Å, b = 3.35 Å, c = 14.92 Å, β = 110.0° for TaS₃, and a = 9.68 Å, b = 3.37 Å, c = 14.83 Å, β = 109.9° for NbS₃. In spite of the similarity in their crystal structures, these high-pressure phases show a variety of electrical transport properties. TaSe₃ is a superconductor having T_c at 1.9 K. TaS₃ is a semiconductor with two transitions at 200 and 250 K. NbS₃ is a semiconductor with E_a = 180 MeV.     

A comparative study of species N3 and P3

Calculations on linear and bent structures of N₃ and P₃ show that these species are quite different. N₃ is linear, P₃ is bent almost to a D_3h geometry. The symmetry of P₃ in D_3h is ²E″ but the Jahn-Teller distortion is very small, ≈4°. The many-body expansion of the energy of P_n clusters appears to be only slowly convergent.     

A kinetics study of the reaction of SO2 with CH3O2

The reaction ot CH₃O₂ with SO₂ has been studied using the flash photolysis/ultraviolet absorption technique. In contrast to previous measurements, no reaction could be detected over the temperature range 298–423 K. An upper limit of 5 × 10^?17 cm³ molecule^?1 s^?1 has been determined for the reaction rate constant.