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.     

Solid-state synthesis in the system Na0.8NbyW1−yO3 with 0?y?0.4: A new phase, Na0.5NbO2.75, with perovskite-type structure

Series of compounds in the system Na_xNb_yW_1−yO₃ were prepared according to the appropriate molar ratio of Na₂WO₄, WO₃, WO₂ and Nb₂O₅ with x=0.80 and 0.0?y?0.4 at 600 °C in evacuated silica glass tubes. These compounds were investigated by X-ray powder diffraction, optical microscopy, microprobe analysis, Raman and optical microspectroscopy. A y-dependent separation into three distinct coloured crystallites with cubic perovskite-type structures is observed: (i) red-orange crystallites with composition Na_xWO₃ with slightly decreasing x (i.e. 0.8-0.72) with increasing nominal y, (ii) bluish solid solution of composition Na_xNb_yW_1−yO₃ and (iii) white crystallites of a new phase having defect perovskite-type structure with composition Na_0.5NbO_2.75.     

A green route for microwave synthesis of sodium tungsten bronzes NaxWO3 (0<x<1)

A green route has been developed for microwave synthesis of sodium tungsten bronzes Na_xWO₃ (0<x<1) from Na₂WO₄, WO₃ and tungsten powder. The hybrid microwave synthesis was carried out in argon atmosphere using CuO powder as the heating medium. Tungsten powder is used as the reducing agent instead of the alkali metal iodides previously used for the microwave synthesis of oxide bronzes. The prepared samples were characterized by powder X-ray diffraction, energy-dispersive X-ray analysis and scanning electron microscopy, and their phase constitutions, crystal structures and morphologies are in consistence with that in the literature. This synthesis method is simple, green and atom economic, and promising for preparation of other oxide bronzes and related compounds.     

Phase relations,dopant effects,structure, and high electrical conductivity in the Na2WO4Na2MoO4 system

The x, T-phase diagram of the binary system Na₂WO₄Na₂MoO₄ has been redetermined at ambient pressure, taking into account the influence of hysteresis effects. Thermodynamic calculations, based upon transition entropies as determined by precision DSC (differential scanning calorimetry), indicate that the system is almost ideal with respect to the high-temperature phases.As anion dopes, Na₂SO₄ and Na₂CrO₄ give a metastable extension of the β-phase of Na₂WO₄ at decreasing temperature, involving some 40°C at 0.01 mole fraction of dopant. Cation dopes like Li₂WO₄ and K₂WO₄ behave quite differently.The electrical conductivity through the phase diagram is high in the α-phase (σ ～ 10^?2 mho cm^?1) almost regardless of composition. The anomalous high conductivity of the β-phase decreases with increasing molybdate content. In pure Na₂MoO₄ an anomaly occurs at the α-α₂ transition, resembling the behavior of Na₂WO₄ at the β-α transition. The (highest) α₂-phase is hexagonal, ($\text{P6}{}_3\text{mmc}$, showing large anisotropic thermal vibrations. The α-phase is orthorhombic (Fddd) as is the β-phase (probably Pbn2₁).     

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.     

The preparation,phase relationships,and Eu-151 Mössbauer spectroscopy of europium tungsten bronzes and related phases

Crystal chemistry and phase relations for the bronze-forming region of the EuWO system have been investigated. A bronze Eu_xWO₃ is stable up to 1000°C when x ? 0.125 and in the region 0.085 ? x ? 0.125 the symmetry is cubic. A tetragonal bronze exists at x = 0.05, and an orthorhombic bronze with a structure closely related to the orthorhombic form of WO₃ exists below x = 0.01. Mössbauer spectra at room temperature and at 80 K indicate that in all these phases the europium is highly ionized as Eu(III) with no electron localization to give (EuII) even at low values for x. The decomposition products of the bronzes have been established, and the Mössbauer parameters for the highly nonstoichiometric tungstates Eu_xWO₄ were determined. Both Eu(II) and Eu(III) resonances were obtained, and a cation vacancy model for Eu_xWO₄ was found to fit the data best. In conformity with the foregoing data, a sample of composition “Eu₂W₂O₇” was found not be be a pyrochlore but to comprise a mixture of Eu₆WO₁₂, Eu_xWO₄, and W. The phase relationships for the europium bronze system Eu_xWO₃ are compared with those of other ionic bronzes Na_xWO₃, Li_xWO₃, and Al_xWO₃.     

Oxidation state of tungsten in the NaxWO3bronzes

X-ray photoemission spectra of W4f electrons in vacuum-cleaved cubic Na_xWO₃ covering the entire composition range give no evidence for the existence of multiple W valences states in the bulk. Strong oxidation effects are observed in air exposed surfaces.     

A combined computational and experimental study of methane activation during oxidative coupling of methane (OCM) by surface metal oxide catalysts

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na₂WO₄ catalytic active sites for CH₄ heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO₄ catalytic active sites resulting in surface WO₅ sites with retarded kinetics for C–H scission. On the other hand, dimeric Mn₂O₅ surface sites, identified and studied via ab initio molecular dynamics and thermodynamics, were found to be more efficient in activating CH₄ than the poisoned surface WO₅ sites or the original WO₄ sites. However, the surface reaction intermediates formed from CH₄ activation over the Mn₂O₅ surface sites are more stable than those formed over the Na₂WO₄ surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO_x, in agreement with the experimental findings in the literature on Mn-promoted catalysts. Consequently, the Mn-promoter does not appear to have an essential positive role in synergistically tuning the structure of the Na₂WO₄ surface sites towards CH₄ activation but can yield MnO_x surface sites that activate CH₄ faster than Na₂WO₄ surface sites, but unselectively.

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not necessarily enhance the activity of the surface Na₂WO₄ catalytic active sites for CH₄ heterolytic dissociation during OCM.     

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.     

Heat capacity measurement of cubic lithium tungsten bronzes from 200 to 800°K

Heat capacities of three cubic lithium tungsten bronze samples (Li_xWO₃) with x values of 0.363, 0.437, and 0.478 were measured from 200 to 800°K. Heat capacities per gram-atom at the same temperature of Li_0.363WO₃ and Li_0.437WO₃ were equal within experimental error and also equal to those of Na_0.485WO₃, Na_0.698WO₃, and Na_0.794WO₃, regardless of the difference of the composition. λ-type heat capacity anomalies were observed around 330, 460, and 590°K in Li_0.363WO₃ and around 330 and 460°K in Li_0.437WO₃ and Li_0.478WO₃, showing the existence of second-order phase transitions. The entropy increments of the transitions were obtained as 1.36, 0.45, and 1.68 J mole^?1 K^?1 for Li_0.363WO₃, 1.09 and 0.59 J mole^?1 K^?1 for Li_0.437WO₃, and 1.42 and 0.50 J mole^?1 K^?1 for Li_0.478WO₃.     

Structural relaxation of PbO–WO3–P2O5 glasses

The structural relaxation of three compositional series of PbO–WO₃–P₂O₅ glasses with composition (0.5 ? x/2)PbO·xWO₃·(0.5 ? x/2)P₂O₅, x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5; 0.5PbO·xWO₃·(0.5 ? x)P₂O₅, x = 0, 0.1, 0.2, and 0.3; and (0.5 ? x)PbO·xWO₃·0.5P₂O₅, x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5 was studied by thermomechanical analysis. The structural relaxation was studied in the transformation region using the Tool–Narayanaswamy–Moynihan’s and Tool–Narayanaswamy–Mazurin’s models. The relaxation function of Kohlrausch Williams and Watts was used. The parameters of both models were calculated by nonlinear regression analysis of thermodilatometric curves measured by thermomechanical analyzer under the constant load. Both models very well describe the experimental data. It was found that the modulus is increasing with increasing amount of WO₃ in all glasses. On the contrary, the width of the spectrum of relaxation times is decreasing with increasing amount of WO₃ in all studied glasses. Both models possess the values of metastable melt thermal expansion coefficient equal to their experimental value.     

TeO2-WO3 Glasses: Infrared, XPS and XANES Structural Characterizations

Transparent (1−x)TeO₂-xWO₃ glasses with 0≤x≤0.325 mol were synthesized by the fast quenching technique. Several complementary techniques as infrared, X-ray photoelectron and X-ray absorption spectroscopies were used to approach the structure of these tungsten oxide-tellurite glasses. Special attention was paid to the oxidation state and the coordination state of tungsten atoms. The structural results show that (1−x)TeO₂-xWO₃ glasses present characteristic tellurium environments which vary with their chemical composition while tungsten ions always adopt an octahedral configuration.     

Rationally designed/constructed MnOx/WO3 anode for photoelectrochemical water oxidation

The activity of WO₃ photoanode could be improved efficiently after loading MnO_x by photodeposition. The maximum photocurrent density of composite photoanode is achieved with a deposition time of 3 min, which is higher than that of pristine WO₃ photoanode around 40%.     

An outline of the structure of new layered bismuth lanthanum tungstate,Bi2−xLaxWO6 (x = 0.4–1.1)

An outline of the structure of a continuous solid-solution series Bi_2?xLa_xWO₆ with x = 0.4–1.1 (space group P2/c and Z = 8) has been determined from a lattice imaging method of electron microscopy. A high-resolution lattice image of Bi_1.4La_0.6WO₆ selected as representative of the series showed that the structure consists of a regular stacking of Bi_1.4La_0.6O₂ layers interleaved with WO₄ layers. A structural model of Bi_2?xLa_xWO₆ was proposed and atomic coordinates were estimated on the basis of the model. The structural relations between Bi_2?xLa_xWO₆ and Bi₂WO₆ were discussed.     

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.     

Über nichtstöchiometrische Wolfram-Verbindungen Synthese und Gitterkonstanten von Verbindungen der Mischkristallreihe WO3?NaWO3

On Nonstoichiometric Tungsten Compounds. Synthesis and Lattice Constants of WO₃? NaWO₃ Mixed Crystal Compounds In the range of temperature from 850 to 1300°C sodium tungsten bronzes with the formula Na_xWO₃ were prepared from a stoichiometric mixture of W, WO₃ and Na₂WO₄. The variation of lattice constant with nominal bronze composition was determined.     

Composition-dependent microhardness and its relationship to bonding in sodium tungsten bronzes

The technique of microhardness measurements using diamond indenters is outlined and assessed for its potential use in quantifying bonding changes and studying reactions in nonstoichiometric crystals. Results are presented for both Vickers and Knoop hardness values on {001} and {011} crystal planes of cubic sodium tungsten bronzes, Na_xWO₃, with x in the range 0.5 to 0.75. The Knoop data show that in only one direction, 〈110〉 on {001}, is the hardness sensitive to changes in composition. Hardness in the 〈110〉 directions and the degree of anisotropy increase as the sodium content of the bronze increases. All the crystal faces examined showed marked anisotropic behavior, with 〈110〉 being about 50% harder than 〈100〉 on {001} faces, while on {011} planes hardness increases in the sequence $\text{〈100〉:〈21}\text{1}\text{〉:〈11}\text{1}\text{〉 ≈ 〈01}\text{1}\text{〉}$. Hardness results from isomorphous and isoelectronic ReO₃ are considered with the Na_xWO₃ data to show the dominant role played by Na⁺WO₃ matrix interactions in determining the properties of these materials. The results are discussed in terms of current bonding theories for bronzes.     

Ca0.8-2x(YbxTb0.1Na0.1+x)2xWO4近红外下转换荧光粉水热合成及发光性质

一种在近红外光谱(NIR)区域高效的量子剪裁现象已在Ca_0.8-2x(Yb_xTb_0.1Na_0.1+x)_2xWO₄(x=0~0.2)荧光粉中得到证实,该量子剪裁通过吸收紫外线光子发射近红外光子,能量传递包括两个协同过程,分别是WO₄^2-基团到Yb³⁺离子和WO₄^2-基团到Tb³⁺离子再到Yb³⁺离子,Yb³⁺离子的掺杂浓度对荧光粉在可见光和近红外光谱的发光,荧光寿命和量子效率的影响已进行了详细得研究。经计算,量子效率最大达到135.7%。铽与镱共掺钨酸钙的近红外量子剪裁,通过吸收太阳光谱的1个紫外光到2个1000nm光子(2倍光子数增加)的下转化机制实现高效率硅太阳能电池的途径。     

Bi2−xLnxWO6: a novel layered structure type related to the Aurivillius phases

The crystal structure of Bi_0.7Yb_1.3WO₆ (a representative of the isomorphous series Bi_2−xLn_xWO₆; 0.3<x<1.3, for most lanthanides) has been determined. Contrary to previous suggestions, this structure type (space group A2; a=8.1070(3) Å, b=3.7048(2) Å, c=15.8379(8) Å, β=103.548(4)°) contains layers of stoichiometry WO₄, containing WO₆ octahedra sharing both edges and corners. These layers alternate with fluorite-like (Bi/Yb)₂O₂ sheets; this is a novel and unexpected arrangement and contrasts dramatically with the purely corner-sharing octahedral WO₄-layer in the parent Aurivillius phase Bi₂WO₆.     

Na0.33WO3溶胶及薄膜的制备与光致变色性质研究

无机光致变色材料在图像显示、光记录、信息存储和光转换方面有着巨大的潜在应用前景,引起了材料工作者的广泛重视[1￣3]。氧化钨是一种重要的无机光致变色材料,目前对氧化钨的研究多以无定