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1.
Series of compounds in the system NaxNbyW1−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 NaxWO3 with slightly decreasing x (i.e. 0.8-0.72) with increasing nominal y, (ii) bluish solid solution of composition NaxNbyW1−yO3 and (iii) white crystallites of a new phase having defect perovskite-type structure with composition Na0.5NbO2.75. 相似文献
2.
G.K. Wertheim 《Chemical physics letters》1979,65(2):377-379
It is shown that the complex W 4f structure of the metallic NaxWO3 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. 相似文献
3.
A green route has been developed for microwave synthesis of sodium tungsten bronzes NaxWO3 (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. 相似文献
4.
The x, T-phase diagram of the binary system Na2WO4Na2MoO4 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, (, showing large anisotropic thermal vibrations. The α-phase is orthorhombic (Fddd) as is the β-phase (probably Pbn21). 相似文献
5.
It is argued that the mixed-valence interpretation of the X-ray photoelectron spectrum of the 4f levels of tungsten in Nax WO3 is more plausible than those based on final-state effects. 相似文献
6.
C.S. Dimbylow I.J. McColm C.M.P. Barton N.N. Greenwood G.E. Turner 《Journal of solid state chemistry》1974,10(2):128-136
Crystal chemistry and phase relations for the bronze-forming region of the EuWO system have been investigated. A bronze EuxWO3 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 EuxWO4 were determined. Both Eu(II) and Eu(III) resonances were obtained, and a cation vacancy model for EuxWO4 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, EuxWO4, and W. The phase relationships for the europium bronze system EuxWO3 are compared with those of other ionic bronzes NaxWO3, LixWO3, and AlxWO3. 相似文献
7.
X-ray photoemission spectra of W4f electrons in vacuum-cleaved cubic NaxWO3 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. 相似文献
8.
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 COx, 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 MnOx 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. 相似文献
9.
The crystal structures of two sodium tungsten bronzes, Na0.33WO3 and Na0.48WO3, have been determined by three-dimensional single-crystal X-ray analysis. They were found to crystallize in the tetragonal space groups 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 Na0.33WO3 and 8.4% for Na0.48WO3. 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 [] 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. 相似文献
10.
Heat capacities of three cubic lithium tungsten bronze samples (LixWO3) 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 Li0.363WO3 and Li0.437WO3 were equal within experimental error and also equal to those of Na0.485WO3, Na0.698WO3, and Na0.794WO3, regardless of the difference of the composition. λ-type heat capacity anomalies were observed around 330, 460, and 590°K in Li0.363WO3 and around 330 and 460°K in Li0.437WO3 and Li0.478WO3, 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 Li0.363WO3, 1.09 and 0.59 J mole?1 K?1 for Li0.437WO3, and 1.42 and 0.50 J mole?1 K?1 for Li0.478WO3. 相似文献
11.
The activity of WO3 photoanode could be improved efficiently after loading MnOx 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%. 相似文献
12.
Mária Chromčíková Marek Liška Magdaléna Lissová Petr Mošner Ladislav Koudelka 《Journal of Thermal Analysis and Calorimetry》2013,114(3):947-954
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. 相似文献
13.
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. 相似文献
14.
The reaction of the cubic sodium bronzes, NaxWO3, 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. 相似文献
15.
An outline of the structure of a continuous solid-solution series Bi2?xLaxWO6 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 Bi1.4La0.6WO6 selected as representative of the series showed that the structure consists of a regular stacking of Bi1.4La0.6O2 layers interleaved with WO4 layers. A structural model of Bi2?xLaxWO6 was proposed and atomic coordinates were estimated on the basis of the model. The structural relations between Bi2?xLaxWO6 and Bi2WO6 were discussed. 相似文献
16.
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 NaxWO3 were prepared from a stoichiometric mixture of W, WO3 and Na2WO4. The variation of lattice constant with nominal bronze composition was determined. 相似文献
17.
一种在近红外光谱(NIR)区域高效的量子剪裁现象已在Ca0.8-2x(YbxTb0.1Na0.1+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WO42-基团到Yb3+离子和WO42-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。 相似文献
18.
19.
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, NaxWO3, 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 . Hardness results from isomorphous and isoelectronic ReO3 are considered with the NaxWO3 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. 相似文献
20.
Peter S Berdonosov Valeriy A Dolgikh Ronald I Smith 《Journal of solid state chemistry》2004,177(7):2632-2634
The crystal structure of Bi0.7Yb1.3WO6 (a representative of the isomorphous series Bi2−xLnxWO6; 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. 相似文献