X-Ray and Electron Microscopy Studies of Rare-Earth Tungsten Bronzes

Rare-earth tungsten bronzes, RE_xWO₃, of perovskite tungsten-bronze (PTB) type, formed by conventional solid-state synthesis, have been studied by X-ray powder diffraction, electron diffraction in combination with microanalysis, and high-resolution transmission electron microscopy (HRTEM). The X-ray patterns indicated cubic PTB type structure with a≈3.83 Å (subcell), while the electron microscopy study indicated a lowering of the subcell symmetry and a complex superstructure. The upper phase composition limit, x≤0.25, for the RE_xWO₃ bronzes with PTB-related structures was established from the microanalysis study. Ordered, disordered and microtwinned superstructures were revealed by electron diffraction patterns and HRTEM images taken along 〈110〉_p. The superstructure is due to filling of the interstices in the WO₃ structure with the rare-earth ions. A hypothetical model of the superstructure based on the contrast features in the HRTEM images has been deduced. The relationships between the RE_xWO₃ bronzes formed by solid-state synthesis under high- and ambient-pressure conditions are presented.     

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).     

An electron microscope study of some nonstoichiometric tungsten oxides

Large crystals of WO₃ have been reduced to a composition of approximately WO_2.91 at 3 different temperatures, 950, 1000, and 1070°C. After reduction the crystals were examined by optical microscopy and transmission electron microscopy. The crystals were faulted in a variety of ways and rarely perfectly ordered. Large crystals heated at 1070°C supported oxygen loss by formation of {103} CS planes while crystals heated at 950°C contained {102} CS planes. At 1000°C {102} and {103} CS planes coexisted. It was found that the way in which the WO₃ structure accommodated oxygen loss was a function of composition and of temperature. In all experiments, some vapour transport also took place, resulting in the growth of needle shaped crystals. These were always members of the W_xO_3n?2 homologous series of oxides, and contained {103} CS planes, irrespective of the formation temperature.     

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.     

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.     

Planar defects in β-alumina

Sodium β-alumina crystals were elaborated by melting of a mixture of Na₂CO₃ and Al₂O₃ or by PbO flux evaporation and were studied by transmission electron microscopy. They exhibit regular planar defects lying in the {11.0} prismatic planes. These defects are described as antiphase boundaries for the cationic sublattice with fault vectors $\text{1}\text{2}\text{〈10.0〉}$ (such faults do not affect the anionic sublattice). As a consequence it would be interesting to study precisely the structure of the sodium β cationic lattice in the vicinity of the melting point.     

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.     

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.     

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.     

Defect generation of rutile-type SnO2 nanocondensates: Imperfect oriented attachment and phase transformation

The rutile-type SnO₂ nanocondensates as condensed by Nd-YAG laser ablation on Sn target under oxygen background gas were characterized by analytical electron microscopy to have {110}, {100} and {101} facets, which are beneficial for {∼hkl} vicinal attachment to form edge dislocations, faults and twinned bicrystals. The {011}-interface relaxation, by shearing along 〈011〉 directions, accounts for a rather high density of edge dislocations near the twin boundary thus formed. The rutile-type SnO₂ could be alternatively transformed from orthorhombic CaCl₂-type structure (denoted as o) following parallel crystallographic relationship, ()_r//()_o; [111]_r//[111]_o, and full of commensurate superstructures and twins parallel to (011) of both phases.     

Structural relationships and mechanisms for the stoichiometry change from MX3 (YF3-type) through MX2 (fluorite-type) to M2X3 (C-type sesquioxide)

Structures produced by inducing stoichiometry changes in crystalline fluorides and oxide-fluorides of yttrium by pyrohydrolysis have been studied by X-ray powder diffraction and electron microscopy. The structures of YF₃, various fluorite-related intermediates and the ultimate product of hydrolysis, Y₂O₃, are all closely related. The pyrohydrolysis is topotactic; the anion sublattice remains intact and vacancies and oxygen substitute for fluorine on the anion sublattice in an ordered, cooperative way to produce fully ordered product structures. A ‘unit slip’ mechanism, involving the most favourable slip systems for a primitive cubic sublattice, 〈001〉{110} and 〈001〉{110}, is postulated as a possible mechanism for the process.     

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.     

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.     

Facet‐Dependent Electrical Conductivity Properties of Silver Oxide Crystals

Ag₂O cubes, truncated octahedra, rhombic dodecahedra, and rhombicuboctahedra were synthesized in aqueous solution. Two tungsten probes were brought into contact with a single particle for electrical conductivity measurements. Strongly facet‐dependent electrical conductivity behaviors have been observed. The {111} faces are most conductive. The {100} faces are moderately conductive. The {110} faces are nearly non‐conductive. When electrodes contacted two different facets of a rhombicuboctahedron, asymmetrical I–V curves were obtained. The {111} and {110} combination gives the best I–V curve expected for a p‐n junction with current flowing in one direction through the crystal but not in the opposite direction. Density of states (DOS) plots for varying number of different lattice planes of Ag₂O match with the experimental results, suggesting that the {111} faces are most electrically conductive. The thicknesses of the thin surface layer responsible for the facet‐dependent properties of Ag₂O crystals have been determined.     

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.     

K1.4P4W14O50: An odd-m member (m = 7) of the monophosphate tungsten bronze series KxP4O8(WO3)2m

The crystal structure of K_xP₄W₁₄O₅₀ (x = 1.4) has been solved by three-dimensional single crystal X-ray analysis. The refinement in the cell of symmetry $\text{A2}\text{m}$, with a = 6.660(2) Å, b = 5.3483(3) Å, c = 27.06(5) Å, and β = 97.20(2)°, Z = 1, has led to R = 0.036 and R_w = 0.039 for 2436 reflections with $\text{σ(I)}\text{I}\text{≤ 0.333}$. This structure belongs to the structural family K_xP₄O₈(WO₃)_2m, called monophosphate tungsten bronzes (MPTB), which is characterized by ReO₃-type slabs of various widths connected through PO₄ single tetrahedra. This bronze corresponds to the member m = 7 of the series and its framework is built up alternately of strands of three and four WO₆ octahedra. The structural relationships with the P₄O₈(WO₃)_2m series, called M′PTB, are described and the possibility of intergrowth between these two structures is discussed.     

Structured diffuse scattering and polar nano-regions in the Ba(Ti1−xSnx)O3 relaxor ferroelectric system

The observation via electron diffraction of relatively sharp G±{001}* sheets of diffuse intensity arising from the large amplitude excitation of inherently polar, transverse optical modes of distortion in Ba(Ti_1−xSn_x)O₃ (BTS), 0.1?x?0.5, samples, both at room temperature as well as liquid nitrogen temperature, shows that the polar nano-regions (PNRs) in these relaxor ferroelectric materials correspond to the same highly anisotropic 〈001〉 chain dipoles as are characteristic of the normal ferroelectric end member BaTiO₃ itself. The correlation length along the chain of these 1-d PNRs can, in principle, be determined from the width of the observed {001}* diffuse sheets in reciprocal space and is estimated to be at least 5 nm even for the higher x samples. The distribution of the substitutional Sn ions thus appears to have only a minor effect upon the correlation length along the 〈001〉 chain dipole directions. It is suggested that the role of the dopant Sn ions is not to directly induce PNRs but rather to set up random local strain fields preventing the condensation of long wavelength homogeneous strain distortions of the unit cell thereby suppressing transverse correlations of the 〈001〉 chain dipoles and the development of long-range ordered ferroelectric state/s.     

Low-temperature heat capacity of some alkali metal tungstates

Heat capacity measurements have been made on the two triclinic tungstates, Li_0.2WO₃ and Na_0.33WO₃ from 1 to 60 K. In addition to the normal Debye term the data show a large contribution which can be fit to a single Einstein mode associated with the oscillation of the alkali ions in the holes formed by the corner bonding of six WO₆ octahedra. The Einstein characteristic temperatures obtained are 71 ± 2 and 78 ± 2 K for Li_0.2WO₃ and Na_0.33WO₃, respectively. The results are compared with those reported earlier for the hexagonal tungsten bronzes.     

Bronzes with a tunnel structure KxP4O8(WO3)2m: The tenth member of the series—KP8W40O136

The crystal structure of KP₈W₄₀O₁₃₆, the tenth member of the series K_xP₄O₈(WO₃)_2m, has been resolved by three-dimensional single-crystal X-ray analysis. The space group is $\text{P2}{}_1\text{c}$ and the cell parameters are a = 19.589(3) Å, b = 7.5362(4) Å, c = 16.970(3) Å and β = 91.864(14)°. The framework is built up from ReO₃-type slabs connected through pyrophosphate groups. The structure is compared to those of the other members of the series: although the ReO₃-type slabs show a different type of tilting of the WO₆ octahedra, the dispersion of WO distances is always higher for the octahedra linked to one or two P₂O₇ groups and decreases in proportion as W is farther from these groups. The perovskite cages of the slabs are described and compared to those encountered in the structures of WO₃ and of the bronzes A_xWO₃.     

Conductivity of composite materials based on Me2(WO4)3 and WO3 (Me = Sc,In)

Composites {Me₂(WO₄)₃ ? xWO₃} (Me = Sc, In) (x = 0.5–99%) are synthesized and characterized by XRD and electron microscopy methods and also by the density and specific surface measurements. Temperature dependences of the total conductivity of composites are measured. The contributions of σ_tot and σ_el are assessed by the $\sigma (a_{O_2 } )$ and EMF methods. The concentration dependences of conductivity and activation energy are plotted based on the σ_tot and σ_ion data. It is shown that (a) in the interval x = 0–30 vol % WO₃ (0–70 mol %), the conductivity is independent of composition and the ionic component prevails; (b) in the interval x = 60–94.5 vol % (90–99 mol %), the electron conductivity prevails and increases with the increase in x; (c) in the x interval of 30–60 vol % WO₃ (70–90 mol %), the conductivity is mixed, i.e., electron(n-type)-ionic; the latter region represents the transition interval from ionic to electron conductivity as x increases. These data are compared with the results obtained earlier for MeWO₄-WO₃ composites (Me = Ca, Sr, Ba). As regards the structural topology, the {Me₂(WO₄)₃ ? xWO₃} composites pertain to the randomly distributed type. It is shown that in contrast to {Me^IIWO₄ · xWO₃} composites, the composites under study do not form the nonautonomous interface phase with the high ionic conductivity. The possible reasons for the observed differences in the topology and the conduction type of composites based on MeWO₄ and Me₂(WO₄)₃ are analyzed.