The oxidation state of tungsten in NaxWO3 bronzes as determined by X-ray photoelectron spectroscopy

The XPS spectrum of the 4f levels of W in Na_xWO₃ bronzes is explained by assuming the presence of three oxidation states (W⁶⁺, W⁵⁺ and W⁴⁺). Tetravalent tungsten is formed by dismutation of W⁵⁺. The partial reduction of W⁶⁺ and W⁵⁺. during the recording of XPS spectra is briefly considered.     

Composition dependence of elasticity in NaxWO3

Single crystal Na_xWO₃ elastic constants have been measured for x = 0.522, 0.628, 0.695, and 0.74 using Brillouin light scattering. The elasticity of Na_xWO₃ is found to be similar to that of ReO₃. Calculations indicate that an observed decrease in c₁₁ with increasing sodium concentration results from perturbation associated with an increasing lattice constant of strong, covalent WO bonds.     

The reaction of cubic sodium tungsten bronzes,NaxWO3, with metallic iron

The reaction of the cubic sodium bronzes, Na_xWO₃, with powdered iron metal has been studied by heating samples in vacuo and also at high pressure. Evidence for reaction is found at unexpectedly low temperatures. The reaction is an overall reduction which proceeds via an increase in the sodium content of the bronze phase up to some temperature-dependent limiting composition for which x < 1. The existence of this limit, its temperature dependence, and the identity of the other products of reduction have been explained in terms of the partial oxygen pressure of the system. The course of the reduction has been followed through the evolution of the bronze lattice parameter and a reaction mechanism is postulated. No evidence of significant incorporation of iron into a stable cubic sodium bronze phase has been found.     

Crystal structure studies of tetragonal sodium tungsten bronzes,NaxWO3. I. Na0.33WO3 and Na0.48WO3

The crystal structures of two sodium tungsten bronzes, Na_0.33WO₃ and Na_0.48WO₃, have been determined by three-dimensional single-crystal X-ray analysis. They were found to crystallize in the tetragonal space groups $P\overline{4}{2}_{1}m(a=12.097,c=3.754\AA ,Z=10)$ and P4/mbm (a = 12.150,c = 3.769Å, Z = 10), respectively. The structures were solved by standard Patterson and Fourier techniques and refined by full-matrix least-squares to final conventional discrepancy indices of 8.9% for Na_0.33WO₃ and 8.4% for Na_0.48WO₃. In general, the oxygen atoms were found to be either twofold or fourfold disordered, suggesting that the WO₆ octahedra do not have axes exactly aligned parallel to the crystallographicc-axis. The structure found can be viewed as a composite of two kinds of domain structures. These domain structures would require a doubling of thec-axis along with selection of newa- andb-axes along the [1 1 0] and [$[\overline{1}10]$] directions. There exist pentagonal and tetragonal sites in both these sodium tungsten bronzes for sodium atoms occupancy. In Na _xWO₃, x = 0.48, all the pentagonal sites are filled and 40% of the smaller tetragonal sites are also occupied. As x decreases to 0.33 though, only the pentagonal sites are occupied. A relation between the x value and the Na _xWO₃ crystal structures is postulated, extrapolating from the results found in these structure determinations.     

What does core-electron spectroscopy tell us about the initial state of NaxWO3?

It is shown that the complex W 4f structure of the metallic Na_xWO₃ obtained by X-ray photoemission cannot be attributed to discrete initial states. The relationship between the observed final-state satellite energy and the binding energy of WO₃ is elucidated.     

Thermochemistry of hydrogen tungsten bronze phases HxWO3

The enthalpies of formation of two hydrogen tungsten bronze phases H_0.35WO₃ and H_0.18WO₃ have been determined by solution calorimetry. Values obtained for formation from H₂(g) and WO₃(s) at 298.15 K were H_0.35WO₃(s), ?9.6 ± 0.8 kJ mole^?1 and H_0.18WO₃(s), ?4.8 ± 0.6 kJ mole^?1. The stabilities of these phases towards decomposition, disproportionation and oxidation are discussed.     

The stability of the perovskite-type zirconium tungsten bronze ZrxWO3

It has been found that a perovskite-related zirconium tungsten bronze Zr_xWO₃ (with 0 < x ? 0.08) forms readily at temperatures between 973 and 1573° K. Prolonged heating causes the bronze to decompose to other oxide products at all the temperatures investigated. These results are summarized in phase diagrams. Possible reasons for the decomposition of the bronze are discussed.     

Properties of compounds of type NaxSi46 and NaxSi136

A qualitative account is given of the electrical and magnetic properties of compounds of the type Na_xSi₁₃₆. It is suggested that a metal-insulator transition takes place with decreasing x. It is conjectured that in sintered powders the resistance is due to amorphous contacts between grains, leading to a resistivity proportional to $\text{A}\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$.     

On the interpretation of the X-ray photoelectron spectrum of the 4f levels of tungsten in Nax WO3 bronzes

It is argued that the mixed-valence interpretation of the X-ray photoelectron spectrum of the 4f levels of tungsten in Na_x WO₃ is more plausible than those based on final-state effects.     

Preparation and properties of the systems Fe2−xCrxWO6, Fe2−xRhxWO6, and Cr2−xRhxWO6

Solid solutions of the end members Fe₂WO₆, Cr₂WO₆, and Rh₂WO₆ have been prepared and their crystallographic and magnetic properties studied. All solid solutions crystallize with the trirutile structure, and their magnetic behavior is characterized by the existence of antiferromagnetic interactions and effective molar Curie constants corresponding to those expected from contributions of the spinonly moments of high-spin Fe³⁺, Cr³⁺, and diamagnetic low-spin Rh³⁺ ions. Fe₂WO₆ crystallizes with the tri-α-PbO₂ structure and is antiferromagnetic and conducting. The random rutile Rh₂WO₆ is conducting, and the difference between its magnetic and electric properties and those of the inverse trirutile Cr₂WO₆ are discussed in terms of possible interactions between Cr³⁺(3d) or Rh³⁺(4d) orbitals and W⁶⁺(5d) orbitals.     

Inelastic neutron scattering spectra of the hydrogen tungsten bronze H0.4WO3

Inelastic neutron scattering spectra of metallic H_0.4WO₃ show only the vibrations that would be expected of a metal hydroxide. The diffusion coefficient of the proton could not be detected from quasi-elastic scattering with the best available neutron spectrometer setting an upper limit to its value of 10^?7 cm²/sec. Both these results confirm that this material is correctly described as a hydroxide.     

Les conducteurs ioniques NaxYxZr1−xS2

Results about new ionic conductors Na_xY_xZr_1?xS₂ are presented. The electronic conductivity normally induced by sodium intercalation layered chalcogenides has been suppressed by performing at the same time a substitution of zirconium by yttrium in the slabs of the host structure. Electrical measurements using the complex impedance technique lead to the activation energy of the ionic conduction. This energy was found to present a marked minimum around x = 0.50. A NMR study showed quadrupole coupling to prevail in relaxation process. Electrical field gradient is maximum for x = 0.50. The activation energy of the local motion remains practically constant throughout the composition range. The above results are compared to others obtained from compounds having the same structure.     

Electrochemical intercalation and deintercalation of NaxMnO2 bronzes

Various Na_xMnO₂ bronzes have been electrochemically deintercalated. Na_0.40MnO₂ has a channel structure which is maintained for a large intercalation range (0.30 ≤ × ≤ 0.58). In order to explain the upper intercalation limit, an ordered sodium distribution between two types of Na⁺ sites is proposed. Na_0.70MnO₂ and α-NaMnO₂ have lamellar structures of P2 and 0′3 types. During intercalation the original P2 type is maintained for 0.45 ≤ × ≤ 0.85 while two reversible structural transitions are observed from α-NaMnO₂. A similar behavior occurs during the deintercalation of the high-temperature β-NaMnO₂ variety. In each case of the structural transition the double octahedral layers remain unchanged. Electronic localization (increased by Mn³⁺ Jahn—Teller effect) tends to trap the Na⁺ ions and therefore increases the relaxation time of the investigated materials.     

Thermochemistry of ammonium tungsten bronze, (NH4)0.25WO3

The enthalpy of formation of ammonium tungsten bronze, (NH₄)_0.25WO₃(s), at 298.15 K has been determined by solution calorimetry. The value obtained for formation from NH₃(g), H₂(g) and WO₃(s) was ?25.7 ± 0.8 k¹ mol^?1. The stability of the bronze towards decomposition and oxidation is discussed.     

Structural chemistry of Sr3Cr2WO9, Ca3Cr2WO9, and Ba3Cr2WO9

We have studied the preparation and crystallographic structure of three perovskite-type compounds: Sr₃Cr₂WO₉, cubic, the lattice parameter of which is a = 7.812Å; Ca₃Cr₂WO₉, tetragonal, the lattice parameters of which are a = 5.408 Å and c = 7.635Å; and Ba₃Cr₂WO₉, hexagonal, the lattice parameters of which are a = 5.691 Å and c = 13.957Å. We have compared these three structures and shown the relationship between the dimensions of the alkaline-earth metal and the existence of the different structures.     

铂-氢钨青铜电极的制备及对氢氧化的电催化

采用电化学还原法在表面改性的碳布上,通过改变催化剂沉积顺序及氢钨青铜沉积时间制备铂-氢钨青铜复合催化剂,所得电极作为质子交换膜燃料电池(PEMFC)阳极。利用X射线衍射(XRD)、热重分析(TG)、扫描电子显微镜(SEM)、循环伏安(CV)及单电池极化性能测试研究了催化剂的组成、沉积量、分散性及其对氢氧化的电催化活性。实验结果表明,氢钨青铜沉积时间及催化剂沉积顺序对电极催化性能有显著影响,当氢钨青铜沉积时间为10 min,先沉积氢钨青铜、后沉积铂所得Pt/HxWO3电极对氢氧化具有最佳的催化活性。适量的氢钨青铜才能与铂形成较好的协同催化效应。     

抗CO中毒的Pt-HxWO3电沉积制备及其对甲醇氧化的催化作用

用循环伏安法在玻碳电极上制备出铂-氢钨青铜(Pt-HxWO3)催化剂, 用扫描电子显微镜(SEM)、Raman光谱和循环伏安法研究了催化剂的组成、结构及其对甲醇氧化的催化作用. 结果表明, Pt-HxWO3的活性与制备溶液中铂与钨原子比及酸度有关, 与相同条件下制备的纯铂(Pt)催化剂相比, Pt-HxWO3对甲醇的氧化有更强的催化活性. 当制备溶液中铂和钨的原子比为1:8时, 得到的催化剂Pt-HxWO3中铂与钨原子比为4:1, 其催化活性最佳, 甲醇氧化电流是Pt作为催化剂时的1.7倍. Pt-HxWO3的强催化活性归因于其抗CO中毒能力, CO在Pt-HxWO3上氧化起始电位提前了50 mV.     

Valence state of the Fe ions in Sr1−yLayFeO3

The Sr²⁺_1?yLa³⁺_yFeO₃ system with 0.1 ≦ y ≦ 0.6 has been studied mainly by the Mössbauer effect. The results are discussed referring to the Ca_1?xSr_xFeO₃ system. The following four kinds of electronic phases have been observed: the paramagnetic and the antiferromagnetic average valence phases and the corresponding mixed valence phases. Two kinds of Fe ions coexist, in general, in the mixed valence phases. In the antiferromagnetic mixed valence phase, typically at 4 K, the magnetic hyperfine field and the center shift each takes a wide range of value depending on the composition, while a beautiful correlation is kept between them. The extreme values are close to those expected for Fe³⁺ and Fe⁵⁺. The appropriate chemical formulas are, therefore, Ca_1?xSr_xFe^(3+Δ)+_0.5Fe^(5?Δ)+_0.5O₃ and $\text{Sr}{}_{\mathrm{1?y}}\text{La}{}_{\mathrm{y}}\text{Fe}{}^{\mathrm{(3+\delta )+}}{}_{\mathrm{<mtext>(1+y)</mtext><mtext>2</mtext>}}\text{Fe}{}^{\mathrm{(5?\delta )+}}{}_{\mathrm{<mtext>(1?y)</mtext><mtext>2</mtext>}}\text{O}{}_3$.     

Oxidation process of MoOxCy to MoO3: kinetics and mechanism

A non-isothermal kinetic study of the oxidation of “carbon-modified MoO₃” in the temperature range of 150-550°C by simultaneous TGA-DTA was investigated. During the oxidation process, two thermal events were detected, which are associated with the oxidation of carbon in MoO_xC_y and MoO₂ to MoO₃. The model-free and model-fitting kinetic approaches have been applied to TGA experimental data. The solid state-kinetics of the oxidation of MoO_xC_y to MoO₃ is governed by F1 (unimolecular decay), which suggests that the reaction is of the first order with respect to oxygen concentration. The constant (Ea)_α value (about 115±5 kJ/mol) for this first stage can be related to the nature of the reaction site in the MoO₃ matrix. This indicates that oxidation occurs in well-defined lattice position sites (energetically equivalent). On the other hand, for the second stage of oxidation, MoO₂ to MoO₃, the isoconversional analysis shows a complex (Ea)_α dependence on (α) and reveals a typical behavior for competitive reaction. A D2 (two-dimensional diffusion) mechanism with a variable activation energy value in the range 110-200 kJ/mol was obtained. This can be interpreted as an inter-layer oxygen diffusion in the solid bulk, which does not exclude other simultaneous mechanism reactions.     

Solid state reactions in the system Nb2O5·WO3: An electron microscopic study

Lattice imaging electron microscopy has been used to study the mechanism of solid state reactions of the type: A_s → B_s + C_s, in which the product B is able to intergrow coherently with the starting material A, but the product C cannot do so. C must be formed by a fully reconstructive, heterogeneous process; formation of B is only partially reconstructive, and essentially homogeneous. Reactions were the reversible phase reactions in the system Nb₂O₅WO₃: disproportionation of the (5 × 4)₁ block structures of 8Nb₂O_5·5WO₃, to form (4 × 4)₁ blocks of 7Nb₂O_5·3WO₃ as coherent product, and that of 9Nb₂O_5·8WO₃ (with (5 × 5)₁ blocks), forming (5 × 4)₁ blocks of 8Nb₂O_5·5WO₃ as coherent product. The coherent product structure is formed in isolated rows of blocks, or small packets of such rows, running across each crystal. The reaction does not work in progressively from some surface initiating step, with an interface between unchanged and converted material, but represents a block-by-block conversion, linearly propagated. Nb₂O₅ and WO₃ must be abstracted, in appropriate stoichiometric ratio, from each block but must ultimately reach and react at the surface, to form the incoherent product (a pentagonal tunnel network structure, in both cases). Some homogeneous transport process involving lattice diffusion must be invoked. Domains of highly anomalous structure, regarded as relicts of transient conditions, are occasionally observed. From reactions at relatively low temperatures, these have structures that can be regarded as partially ordered nonstoichiometric solid solutions; after prolonged heating, and at higher temperatures they form well ordered strips of metastable block structures. Both types represent strong, spontaneous fluctuations of composition, which impose a corresponding structure locally. These fluctuations may be associated with the transport of WO₃ and Nb₂O₅ away from the locus of reaction. Evidence about the mechanism of the reactions, the role of dislocations and the nature of cooperative processes is considered.