Electrical conductivity and high resolution electron microscopy studies of WO3−x crystals with 0 ≤ x ≤ 0.28

WO_3?x crystals with 0 ≤ x ≤ 0.28 have been studied by means of HREM and electrical conductivity measurements. Semiconducting behavior with andE_a of the order 0.06 eV was observed for crystals which, according to the HREM study, contained {102}CS planes (x ? 0.03). Plots of conductivity vs1–T for WO₃ and WO_3?x containing disordered {102}CS planes are also presented. Metallic behavior was found for crystals with ordered {103}CS planes (x ? 0.11), for W₁₂O₃₄ (x = 0.167), and for W₁₈O₄₉ (x = 0.28).     

The electrostatic interaction energy between crystallographic shear planes in reduced tungsten trioxide

The contribution to the internal energy of slightly reduced WO₃ crystals containing CS planes due to electrostatic interactions between ions in the CS plane and ions in the surrounding crystal matrix or in neighboring CS planes has been investigated theoretically. Three CS plane geometries have been studied, {102}, {103}, and {001}. Using simple assumptions about the charge distribution in the CS planes, numerical values for these interaction energies have been estimated. It was found that the interaction energy between a CS plane and the surrounding matrix was negligible compared to the repulsive (coulomb) interaction energy between a pair of CS planes. The magnitude of this repulsive energy was in the order {103} < {102} < {001}. The possible significance of these results in controlling the microstructure of crystals containing CS planes is discussed.     

The elastic strain energy of crystallographic shear planes in ReO3-related oxides. I. The formation energy of isolated CS planes

The formation energy of isolated CS planes in the ReO₃ structure type has been estimated. The CS planes considered are {102}, {103}, {104}, {105}, {106}, {107}, and {001}. The major components of the formation energy were considered to be the loss of oxygen from the crystal and the elastic strain energy of the matrix surrounding the CS plane so formed. In addition, the internal energy of the CS plane itself was also large and of importance. It was found that {102} CS planes have the lowest formation energy, but {001} CS planes are only slightly less favorable. These results are compared with the experimental data available for the materials NbO₂F and WO₃.     

Crystallographic shear in the Nb2O5WO3 and Ta2O5WO3 systems

Crystallographic shear (CS) phases occurring in the Nb₂O₅WO₃ and Ta₂O₅WO₃ systems near to WO₃ were characterized by X-ray diffraction and high-resolution transmission electron microscopy. The Nb₂O₅WO₃ samples were heated at 1600K. They contained ordered {104} and {001} CS planes and wavy CS which were composed of intergrowths of {104} and {001} CS segments. The composition range over which the {104} CS series extended was from (Nb,W)O_2.954 i.e., (Nb,W)₆₅O₁₉₂, to (Nb,W)O_2.942, i.e., (Nb,W)₅₂O₁₅₃. The composition range over which the {001} CS series extended was from (Nb,W)O_2.9375, i.e., (Nb,W)₁₆O₄₇ to (Nb,W)O_2.875, i.e., (Nb,W)₈O₂₃. The Ta₂O₅WO₃ samples were prepared at 1593, 1623, and 1672K. At lower temperatures ordered {103} CS phases were found, with a composition range extending between (Ta,W)O_2.960, i.e., (Ta,W)₅₀O₁₄₈, to (Ta,W)O_2.944, i.e., (Ta,W)₃₆O₁₀₆. At 1673K ordered {103} CS phases occurred, as did wavy CS composed of intergrowths of {103} and {104} CS segments.     

Polaron interaction energies in reduced tungsten trioxide

Consideration of the properties of reduced tungsten trioxide suggest that the mobile charge carriers are polarons. As it is uncertain how the presence of polarons will influence the microstructures of the crystallographic shear (CS) planes present in reduced tungsten trioxide we have calculated both the polaron-CS plane and polaron-polaron interaction energy for a variety of circumstances. Three CS plane geometries were considered, {102}, {103}, and {001} 1CS plane arrays, and the nominal compositions of the crystals ranged from WO_2.70 to WO_3.0. The polarons were assumed to have radii from 0.6 to 1.0 nm and the polaron-CS plane electrostatic interaction was assumed to be screened. The results suggest that for the most part the total interaction energy is small and is unlikely to be of major importance in controlling the microstructures found in CS planes. However, at very high polaron densities the interaction energy could be appreciable and may have some influence on the existence range of CS phases.     

Notes on phases occurring in the binary tungsten-oxygen system

The phases occurring in the binary tungsten-oxygen system in the composition region WO₃WO₂ have been clarified by electron microscopy and powder X-ray diffraction in the temperature range from 723 to 1373 K. There are five structure types in the binary system, besides WO₃, viz., the {102} CS structures, the {103} CS structures, W₂₄O₆₈, W₁₈O₄₉, and WO₂. The {102} and {103} CS structures, and W₂₄O₆₈ structures, were always disordered and true equilibrium was not achieved even after 5 months of heating at 1373 K. The lowest temperature for the formation of the CS phases was of the order of 873 K, and the disordered W₂₄O₆₈ structure formed at somewhat higher temperatures. The formation of the latter phase was also slower than the formation of the CS phases. The results suggest that elastic strain energy is of importance in controlling the microstructures found in the nonstoichiometric regions.     

Structure and “oxidation behavior” of W24O70, a new member of the {103} CS series of tungsten oxides

Reduced tungsten trioxide crystals WO_3?x, formed by vapor transport from a preparation with bulk composition WO_?2.90, have been studied by X-ray diffraction and electron microscopy. A single-crystal X-ray investigation showed the existence of the ordered {103} CS-structure W₂₄O₇₀, a new member of the homologous series W_nO_3n?2. Electron diffraction patterns of crystal fragments, with a few exceptions, showed the presence of the W₂₄O₇₀ phase (composition WO_2.917). Lattice images, however, indicated a fairly ordered {103} CS-phase, W₂₄O₇₀, intergrown with slabs of WO₃ giving gross compositions of the examined crystals in the range WO_2.93WO_2.96. The wide WO₃ slabs were probably formed by an oxidation process during the preparation.     

Reversibility of electrosurface transfer through eutectic interfaces of MeWO4|WO3 (Me ? Ca, Sr, Ba)

It is found that electrosurface transition (EST) through eutectic interfaces of (?/+)WO₃|MeWO₄(+/?) induced by electric field is a reversible process. In the case of the (?/+) polarity, nominally, in the ??direct experiment??, macro amounts of WO₃ from a W ₃ ^(?) brick are drawn in the (+) direction onto the inner surface of MeWO₄ forming a two-phase {ie1070-1} composite. Simultaneously, nonequivalent countertransport of Me ²⁺ within the W ₃ ^(?) brick occurs, which changes the color of W ₃ ^(?) from the natural hue to dark-green. Intercalation of Me ²⁺ into W ₃ ^(?) is proved by several spectroscopic methods. The key role in the EST phenomenon belongs to a nonautonomous electrolytic phase of MeW-s formed on the contact interface with WO₃|MeWO₄. The composition of MeW-s is close to W/Me ?? 2. As a result of EST, the cell acquires a more complicated structure: {fx1070-1} where |///| are interface regions occupied by the MeW-s phase. At the cathodic boundary of subcell {fx1070-2} the following process occurs: {fx1070-3}, The process at the anodic boundary is: {fx1070-4}. Ultimately, WO₃ is transported in the (+) direction (into the composite) and Me ²⁺ penetrates under the effect of the gradient in chemical potential into W ₃ ^(?) forming a dark-green Me _x WO₃ phase with its front reaching the (?) Pt electrode. After the end of the ??direct experiment??, the cell polarity was changed to (+/?) and the ??reverse experiment?? was carried out. Now, on the cathodic boundary | ₄ of subcell {fx1070-5} anions (WO₄)^2? are generated that are discharged on boundary ₃ | to oxide WO₃ that is intercalated into the right boundary of MeWO_{4 ? (3)}, where the rightmost composite region {ie1070-2} is formed. Thus, the mass of W ₃ ^(?) decreases; it becomes dark-green (see above) and the mass of the MeWO₄ disk continues growing and now its structure is as follows {ie1070-3}. It is important that the left W ₃ ⁽⁺⁾ disk that was dark-green after the ??direct experiment?? gradually becomes lighter in the ??reverse experiment?? up to its natural pale green color, i.e., Me ²⁺ is deintercalated from it: Me ²⁺: Me _x WO₃ + 1/2O₂ ?? xMe ²⁺ + 2e + WO₃. It is found that dependences of variations of disk masses ??m(Q) practically coincide for the ??direct?? and ??reverse?? experiments.     

Accommodation of oxygen loss in WO3 equilibrated with CO + CO2 buffers

Tungsten trioxide reduced at about 1270 K by means of controlled atmospheres (P₀₂ = 3.7 · 10^?8 to 1.7 · 10^?13 atm) was studied by high resolution transmission electron microscopy, electron diffraction, and X-ray powder diffraction. The accommodation of oxygen loss in the parent WO₃ lattice in the range WO₃ to WO_2.72 was clarified. The results indicate a solid state mechanism. Intergrowth has been found to take place between several of the structural types that occur in this composition range. The intergrowth features include directional changes in shear plane arrays (“swinging shear planes”). Details of the structural variation with the oxygen content are reported. Ordered shear planes on {102} directions were found to stabilize the orthorhombic WO₃ parent lattice at room temperature. W₂₄O₆₈ has been prepared in a fairly well-defined state.     

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.     

The elastic strain energy of crystallographic shear planes in ReO3-related oxides. II. CS plane arrays

The elastic strain energy of the matrix lying between pairs of crystallographic shear (CS) planes in the ReO₃ type structure has been calculated as a function of CS plane spacing and CS plane type. The CS planes considered are {104}, {105}, {106}, {107}, and {001}, and in addition the results for {102} and {103}, reported previously are included. These results are used to discuss and explain the relative stabilities of differing {10m} CS plane arrays and also the relative stabilities of members of the homologous series generated by ordered arrays of these CS planes. The microstructures of arrays of CS planes that may occur in reduced binary or ternary tungsten oxides, which are slightly distorted variants of the ReO₃ type, and in NbO₂F which has the ReO₃ structure are also considered.     

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.     

Oxidation of bismuth-tungsten bronzes

The oxidation of bismuth-tungsten bronzes at 600 and 950°C has been studied using high-resolution electron microscopy at 200 and 500 kV. At the lower temperature, a topotactic transformation to lamellae of Bi₂WO₆ in a WO₃ matrix was observed but at higher temperature larger crystals were produced, primarily of Bi₂W₂O₉ but with some disordered intergrowths.     

Electron microscopy studies of W18O49. 2. Defects and disorder introduced by partial oxidation

W₁₈O₄₉ was oxidized in air at about 500K for different intervals of time. Defects of various kinds, related to structures of higher oxides, were observed. These were a coherent intergrowth of W₁₂O₃₄, {102}, and {103} crystallographic shear, and WO₃-type structures. A new type of TTB structure was also observed as a defect. Its formation mechanism is proposed and discussed.     

The nature and the mechanism of ion transfer in tungstates Me2+{WO4} (Ca, Sr, Ba) and Me 2 3+ {WO4}3 (Al, Sc, In) according to the data acquired by the tubandt method

The Tubandt method of electrolysis is used for studying the nature of ionic carriers in ceramics of tungstates Me²⁺{WO₄} (Ca, Sr, Ba) and Me ₂ ³⁺ {WO₄}₃ (Al, Sc, In) which are solid electrolytes. These compounds have the salt-like islet structure with isolated tetrahedrons {WO ₄ ^2? } and are crystallized in the allied structural types of scheelite (CaWO₄) for Me²⁺ and Sc tungstate (Sc₂{WO₄}₃) for Me³⁺. The electrolysis is carried out in 2- or 3-section cells (?)Pt|M ₂ ^n/n+ {WO₄}|Me ₂ ^n/n+ {WO₄}|Pt(+) in air atmosphere at the temperature of ～900°C and cell voltage of 4 and 300 V. All experiments without exception demonstrate a decrease in the mass of the cathodic section of cells. This points to the negative charge of ionic mass carriers and their transfer towards the Pt⁽⁺⁾ electrode. The cathodic briquette mass loss Δm ^(?) depends linearly on the charge passed through a cell. In all experiments with MeWO₄ tungstates, the anodic disk mass remains constant. The electrolysis of Me₂(WO₄)₃ cells is always accompanied by an increase in the anolyte mass Δm ⁽⁺⁾; however, in all experiments, Δm ^(?) > Δm ⁽⁺⁾. All data on mass variation and the results of studying the composition of nearelectrode electrolyte layers by XRD and SEM methods correspond to the condition $t_{WO_4^{2 - } } > t_C $ (C is the cation), i.e., {WO ₄ ^2? } anions pertain to the major ionic carriers. The transport number $t_{WO_4^{2 - } } $ is calculated based on the Faraday law from Δm ^(?). It is shown that the second ionic carrier with the mobility even higher than that of {WO ₄ ^2? } is the O^2? ion. For middle values of transport numbers, their ratio is shown to be $t_{O^{2 - } } $ (0.5–0.8) > (0.2–0.5) $t_{WO_4^{2 - } } $ . No results that would confirm the involvement of Me²⁺ and Me³⁺ ions in conduction are obtained.     

An electron microscope study of the FeOFe2O3TiO2 system and of the nature of iron-doped rutile

An electron microscope study of FeOFe₂O₃TiO₂ reveals a wide range of unsuspected CS behavior. At 1300°C, isolated {132} faults (MX_1.995) coexist with aggregated {121} CS planes (MX_1.97). At lower temperatures, no aggregated faults occur and isolated faults swing towards {011}; these are often stepped and are accompanied by a high dislocation density. Above 1400°C, elements of (121) and (132) intergrow, forming intermediate high-index CS structures whose indices depend on oxygen/metal ratio, $\text{Fe}{}^{\mathrm{3+}}\text{Fe}{}^{\mathrm{2+}}$ ratio, and temperature. At a given temperature, and in air, there is a range of oxygen/metal ratios where the CS plane swings continuously from (132) to (121); the width of this range increases with increasing temperature (1.97-1.93 at 1500°C). The observations suggest a mechanism for transforming from rutile to α-PbO₂-derived CS structures. Pseudobrookite is incoherent with rutile, coexisting with slightly reduced rutile (MX_1.995) below about 1200°C but with ordered CS structures above this. Wavy domain boundaries and a new superstructure appear in beam-heated pseudobrookite.     

Enthalpy of formation of aqueous tungstate ion and of crystalline tungstic acid (H2WO4)

Calorimetric measurements of the enthalpy of reaction of WO₃(c) with excess OH^?(aq) have been made at 85°C. Similar measurements have been made with MoO₃(c) at both 85 and 25°C, to permit estimation of ΔH°=?13.4 kcal mol^?1 for the reaction WO₃(c)+2OH^?(aq)=WO^2?₄(aq)+H₂O(liq) at 25°C. Combination of this ΔH° with ΔH°_f for WO₃(c) leads to ΔH°_f=?256.5 kcal mol^?1 for WO^2?₄(aq). We also obtain ΔH°_f=?269.5 kcal mol^?1 for H₂WO₄(c). Both of these values are discussed in relation to several earlier investigations.     

Phase relations and exsolution phenomena in the system NiOTiO2

The phase relations in the system NiOTiO₂ were studied between 1000 and 1600°C using quenched powder specimens, DTA runs, and single crystal diffusion couples. Quenching experiments establish the stable phases TiO₂ (rutile), NiTiO₃ an ilmenite structure type, Ni_2(1+x)Ti_1?xO₄ (x ≥ 0.16), a cation-excess spinel, and Ni_1?2xTi_xO (rocksalt structure type). DTA runs reveal the existence of an additional nonstoichiometric ilmenite phase Ni_1?2xTi_1+xO₃ (x ≤ 0.03) above 1260°C. In quenched (1500, 1450°C) or slowly cooled single crystal diffusion couples, mutual oriented exsolutions occur in the rutile crystal and in the ilmenite diffusion zone. Orientation relations are: {101}_rutile{1120}_ilmenite; ∥010〉_rutile∥00.1〉_ilmenite. The cation-excess spinel decomposes below 1375°C into oriented intergrowth of NiTiO₃ (ilmenite) and NiO: {111}_NiO{0001}_NiTiO3; ∥110〉_NiO∥21.0〉_NiTiO3.     

Phase transitions in tungsten trioxide at low temperatures

Capacitance and electrical resistivity measurements have been made on stoichiometric and on oxygen-deficient tungsten trioxide crystals from 4.2 to 300°K. X ray oscillation and rotation photographs were made on single crystals of both materials near 200°K and near 300°K. Capacitance and resistivity anomalies identify phase transitions near 40, 65, 130, 220, and 260°K in stoichiometric WO₃. Resistivity anomalies occur near 80, 130, 220, and 260°K in oxygen-deficient tungsten trioxide. Capacitance measurements suggest that the transformation at 130°K of a low-temperature phase to a high-temperature phase of stoichiometric WO₃ is associated with a doubling of thec-parameter of the unit cell. Resistivity measurements establish probable conduction mechanisms in each phase of stoichiometric and of oxygen-deficient tungsten trioxide, and show that oxygen-deficient tungsten trioxide undergoes a semiconductor-to-metal transition near 220°K. Electronic phenomena that do not appear to be associated with structural phase transformations are observed near 16°K in stoichiometric WO₃.