Phase transformations of ammonium tungsten bronzes

This article discusses the formation and structure of ammonium tungsten bronzes, (NH₄) _x WO_3−y . As analytical tools, TG/DTA-MS, XRD, SEM, Raman, XPS, and ¹H-MAS NMR were used. The well-known α-hexagonal ammonium tungsten bronze (α-HATB, ICDD 42-0452) was thermally reduced and around 550 °C a hexagonal ammonium tungsten bronze formed, whose structure was similar to α-HATB, but the hexagonal channels were almost completely empty; thus, this phase was called reduced hexagonal (h-) WO₃. In contrast with earlier considerations, it was found that the oxidation state of W atoms influenced at least as much the cell parameters of α-HATB and h-WO₃, as the packing of the hexagonal channels. Between 600 and 650 °C reduced h-WO₃ transformed into another ammonium tungsten bronze, whose structure was disputed in the literature. It was found that the structure of this phase—called β-HATB, (NH₄)_0.001WO_2.79—was hexagonal.     

Electronic structure of cubic tungsten subnitride W<Subscript>2</Subscript>N in comparison to hexagonal and cubic tungsten mononitrides WN

A full potential FLAPW-GGA method is used to study for the first time the electronic structure of cubic tungsten subnitride W₂N, and its equilibrium lattice parameter, density, energy of cohesion, coefficients of low temperature heat capacity and Pauli paramagnetic susceptibility are calculated. It is discussed in comparison to the similar values of hexagonal and cubic tungsten mononitrides WN.     

Ein neues Syntheseverfahren für die Wolframbronze Cs0,29WO3

A Novel Synthetic Access to the Tungsten Bronze Cs_0.29WO₃ and its Crystal Structure The hexagonal tungsten bronze Cs_0.29WO₃ was obtained in form of black, prismatic crystal by the reduction of WO₃ with molten cesium iodide at 700°C. Its crystal structure was determined by X-ray diffraction (399 unique observed reflexions, R = 0.058). Crystal data: a = 741.2(3), c = 760.0(5) pm, space group P6₃22, Z = 6. It corresponds to the known structure of hexagonal tungsten bronzes, having tungsten atoms displaced from the octahedra centres by 11.9 pm and with three different W? O bond lengths (198, 191, 187 pm). The WO₆ octahedra are slightly titled mutually.     

Thermal stability of hexagonal tungsten trioxide in air

We studied the thermal stability of different hexagonal tungsten trioxide, h-WO₃ samples, which were prepared either by annealing hexagonal ammonium tungsten bronze, (NH₄)_0.33−xWO_3−y, or by soft chemical synthesis from Na₂WO₄. The structure and composition of the samples were studied by powder XRD, SEM-EDX, XPS and ¹H-MAS NMR. The thermal properties were investigated by simultaneous TG/DTA, on-line evolved gas analysis (TG/DAT-MS), SEM and in situ powder XRD. The preparative routes influenced the thermal properties of h-WO₃ samples, i.e. the course of water release, the exothermic collapse of the hexagonal framework and the phase transformations were all affected.     

仲钨酸铵水热法制备WO3及其表征

仲钨酸铵水热法制备WO3及其表征;仲钨酸铵（APT）;正交设计;WO3;水热     

Synthesis of Mesoporous Tungsten Trioxide with Crystalline Pore Wall at Low Hydrothermal Temperature

Well-ordered hexagonal mesoporous tungsten trioxide with crystalline pore walls were synthesized at low hydrothermal temperature by using cationic quaternary ammonium gemini surfactants as structure-directing agents and sodium tungstate dihydrate (Na₂WO₄·2H₂O) as a precursor. The effects of alkyl chain length of gemini surfactants, hydrothermal temperature and molar ratio of tungsten to gemini surfactants have been investigated in detail. The strong self-assembly ability of gemini surfactants, strong electrical interaction between gemini surfactants and tungsten trioxide, and solvent extraction strategy contributed together to the coexistence of WO₃ mesostructures and crystalline pore walls.     

A novel scandium fluoride, [C2N2H10]0.5[ScF4], with an unprecedented tungsten bronze-related layer structure

[C(2)N(2)H(10)](0.5)[ScF(4)] exhibits isolated anionic layers of corner-linked ScF(6) octahedra enclosing 3-, 4-, 5- and 6-membered rings, with features reminiscent of both hexagonal and tetragonal tungsten bronze-type structures.     

Preparation and characterization of WO3 from ammonium paratungstate via hydrothermal method

Tungsten trioxide powder has been prepared from ammonium paratungstate via hydrothermal method using orthogonal and mono-level design of experiments. The effects of preparation process on particle size, specific surface area, crystal form and crystalline morphology of the tungsten trioxide was investigated by TEM and XRD etc. It was found that the optimum conditions of the preparation are hydrothermal crystallization for 8 h at 180°C, followed by vacuum drying at 45°C and calcination at 500°C for 2 h. The blank reference experiment shows that hydrothermal crystallization treatment favors the formation of hexagonal tungsten trioxide, and the tungsten trioxide powder sample prepared by this method has a high degree of crystallinity. __________ Translated from Chinese Journal of Applied Chemistry, 2005, 22(8) (in Chinese)     

Ba0.15WO3: A bronze with an original pentagonal tunnel structure

The structure of a new barium tungsten bronze, Ba_0.15WO₃, has been established by X-ray diffraction and high-resolution microscopy studies. This bronze is orthorhombic, space group Pbm2 or Pbmm, with a = 8.859(3) Å, b = 10.039(8) Å, and c = 3.808(2)Å. The “WO₃” framework is built up from corner-sharing WO₆ octahedra forming pentagonal tunnels where the barium ions are located. Structural relationships with hexagonal tungsten bronze and tetragonal tungsten bronze structures are discussed.     

X-Ray Investigation of Tungsten Carbides Synthesized by Cumulative Explosion

High-temperature tungsten carbides — cubic modification -WC_1-x and hexagonal modification W₂C_1-x — were obtained under conditions of cumulative explosion. Conical linings with an opening of 30° prepared from mixtures of finely disperse powders of tungsten and graphite were used for synthesis. The known crystal structures of tungsten carbides are analyzed and their nomenclature is refined. The presence of nitrogen and oxygen in the reaction zone considerably changes the phase composition. Phases with unit cell parameters substantially deviating from literature data have been obtained.     

Crystal structures of some niobium and tantalum oxides. VIII. The 5Rb2O:14.6Ta2O5 phase—A tunnel structure

Rb₁₀Ta_29.20O₇₈ crystallizes in the hexagonal system with unit-cell dimensions (from single-crystal data) a = 7.503(4)Å, c = 36.348(4)Å, and space group $\text{P6}{}_3\text{mmc}\text{, z = 1}$. The structure was solved using three-dimensional Patterson and Fourier techniques. Of the 666 unique reflections measured by counter techniques, 515 with I ? 3σ(I) were used in the least-squares refinement of the model to a conventional R of 0.057 (R_ω = 0.039). The structure of Rb₁₀Ta_29.20O₇₈ consists of layers of corner-sharing groups of six edge-shared octahedra separated by layers of single octahedra and double hexagonal tungsten bronze-like layers, these layers being perpendicular to the hexagonal c-axis. Nine-coordinate tricapped trigonal prismatic sites between the hexagonal tungsten bronze-like layers are partially occupied by Ta(V) ions.     

Structure of small defects in nonstoichiometric WO3−x

Structural models are derived for the hitherto “unresolved” small defects which should occur in the nonstoichiometric phase WO_3?x. The mechanisms of aggregation or interaction of these small defects to produce extended defect structures (i.e., crystallographic shear planes and pentagonal and/or hexagonal tungsten bronze-type columns) are discussed next. Linear defects, consisting of two pairs of edge-shared octahedra, are proposed. These readily explain the large number of electron microscope observations of precipitation and dissolution phenomena reported for reduced WO_3?x and doped tungsten trioxide specimens.     

The effect of K+ ion exchange on the structure and thermal reduction of hexagonal ammonium tungsten bronze

This paper discusses the changes in the structure and thermal reduction of nanosize hexagonal ammonium tungsten bronze (HATB), (NH₄)_0.33−xWO_3−y, which were caused by K⁺ ion exchange (doping) and studied by XRD, XPS, ¹H-MAS NMR, FTIR, SEM and TG/DTA-MS. Comparison of the cell parameters of undoped and doped HATB revealed that both a and c cell parameters decreased after the ion exchange reaction, which showed that smaller K⁺ ions partly replaced the larger NH₄⁺ ions in the hexagonal channels of HATB. After the reaction, from the hexagonal channels less NH₃ evolved, which also supported the incorporation of K⁺ ions into the hexagonal channels.     

Polyporous C@WC_(1-x) composite and its electrocatalytic activity for p-nitrophenol reduction

The polyporous carbon supported tungsten carbide(polyporous C@WC_1-x) composite was prepared using hexagonal silica MCM-41 as the hard template by raw material solution impregnation,mechanical milling and simultaneous reduction and carbonization by temperature programming in mixture gas（CH₄/H₂）.The structure and morphology of polyporous C@WC_1-x composite were studied via X-ray diffraction,transmission electron microscopy and so on.The electrocatalytic property was tested for the electroreduction of p-nitrophenol（PNP） in neutral media.Results revealed that the composite is consisted of polyporous carbon and nanocrystalline WC_1-x,possessing good electrocatalytic activity in the reaction of PNP reduction.     

Electronic structure of tungsten aluminum carbides W<Subscript>2</Subscript>AlC and WAlC<Subscript>2</Subscript>

The full-potential FLAPW-GGA method was used for the first time to calculate the electronic structure of hexagonal tungsten aluminum carbides W₂AlC and WAlC₂ and their equilibrium structural parameters, density, cohesion energies, formation energies, low-temperature heat capacity coefficients, and Pauli paramagnetic susceptibility. These characteristics are discussed in comparison with analogous parameters for the initial binary carbides WC and Al₄C₃.     

Electrochromic and structural characteristics of mesoporous WO<Subscript>3</Subscript> films prepared by a sol-gel method

The preparation of electrochromic films of mesoporous tungsten trioxide from tungstic acid and tungstic hexaethoxide precursors with the addition of an organic stabiliser via a sol-gel method is reported. These films have been structurally characterised and both the film morphology and crystalline composition of the films were found to be significantly dependent on the temperature at which the films were annealed and upon the choice of precursor. Films annealed at lower temperatures consisted of amorphous and hexagonal tungsten trioxide, whereas films annealed above 500 °C comprised solely of monoclinic WO₃. The electrochromic activity of the films was found to be equally dependent on method of preparation, and both the composition and the structure of the WO₃ films were shown to clearly influence the colouration efficiency of the films.Dedicated to Zbigniew Galus on the occasion of his 70^th birthday.     

One-dimensional WO3 and its hydrate: One-step synthesis,structural and spectroscopic characterization

We report on a one-step hydrothermal growth of one-dimensional (1D) WO₃ nanostructures, using urea as 1D growth-directing agent and a precursor free of metals other than tungsten. By decreasing the pH of the starting solution, the size of the nanostructures was reduced significantly, this development being accompanied by the realization of phase pure hexagonal WO₃ nanorods (elimination of monoclinic impurity phase) and a red shift in optical absorption edge. Surface analyses indicated the presence of reduced tungsten species in the WO₃ nanostructures, which increased two-fold in a hydrated WO₃ phase obtained with further decrease in pH. We suggest that oxygen vacancies are responsible for this defect state in WO₃, while protons are responsible or contribute significantly to the same in the hydrated phase.     

Remarkable rare-earth metal silicide oxides with planar Si6 rings

New rare-earth silicide oxides, La10Si8O3 (1) and Ce10Si8O3 (2), were synthesized through high-temperature reactions of the pure elements under controlled oxygen atmosphere conditions. The remarkable silicides crystallize in a unique crystal structure (space group P6/mmm; a = 10.975(3) A (La) and 10.844(1) A (Ce); c = 4.680(1) A (La) and 4.561(1) A (Ce)) that features a 3-D framework of corner-shared O-centered (La/Ce)6 octahedra, reminiscent of hexagonal tungsten bronzes, with planar Si6 rings enclosed within its hexagonal channels. Band structure calculations indicate the compounds are metallic, with optimized La-Si bonds, and a benzene-like [Si6]6- anion. Compound 1 exhibits temperature independent paramagnetism. Compound 2 exhibits Curie-Weiss paramagnetism, and an antiferromagnetic ordering below 7 K.     

One-Pot Hydrothermal Synthesis of Ternary 1T-MoS2/Hexa-WO3/Graphene Composites for High-Performance Supercapacitors

A new ternary composite of 1T-molybdenum disulfide, hexagonal tungsten trioxide, and reduced graphene oxide (M-W-rGO) is synthesized by using a one-pot hydrothermal process. The synergetic effect of 1T-MoS₂ and hexa-WO₃ nanoflowers improves the electrochemical performance for supercapacitors by inducing additional active sites and hexagonal tunnels, respectively, which lead to high storage capacity and easy transfer of electrolyte ions. The ternary M-W-rGO composite has a high specific capacitance of 836 F g⁻¹ at 1 A g⁻¹, which is nearly twice that of binary composites of M-rGO and W-rGO with high capacitance retention of 86.35 % after 3000 cycles at a high current density of 5 A g⁻¹. This study provides a new ternary composite that can be used as an electrode material for high-performance supercapacitors.     

Structure and Thermal Stability of La0.10WO3+y; A Hexagonal Tungsten Bronze Related Phase Formed at High Pressure

The structure and thermal stability of a hexagonal tungsten bronze (HTB) related compound, La_xWO_3+y with x≈0.10 and y≈0.15, has been studied by X-ray diffraction, thermal analysis, and electron microscopy. The structure was refined by the Rietveld method from X-ray powder diffractometer data of a La_0.10WO₃ sample prepared at T=1250°C and P=25 kbar, which consisted of two tungsten bronze related phases in 1:1 proportion. The unit cell dimensions are as follows: La_0.108WO_3+y (y≈0.16), a=7.40890(5), and c=3.79329(4) Å (HTB-related structure); La_0.091WO₃, a=3.82458(6) Å (cubic perovskite tungsten bronze (PTB) structure). The lanthanum atoms in La_0.108WO_3+y are located on the hexagonal axis and statistically distributed on two sites close to the tungsten atom plane. Thermal stability studies of the La_0.10WO₃ sample in an argon atmosphere under ambient pressure conditions revealed that the HTB-related compound is metastable, decomposing to the stable PTB-type structure and WO₃. It was also found from the TG experiments in argon and oxygen that additional oxygen atoms (y) are present in the structure, thus forming a lanthanum tungsten oxide of the above composition. The electron diffraction and microanalysis studies confirmed that crystals of the HTB- and PTB-type structures were formed, with a lanthanum content of x≈0.1.