Browsing by Author "Barnes, PRF"

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    Orthorhombic superstructures within the rare earth strontium-doped cobaltate perovskites: Ln1−xSrxCoO3−δ (Ln=Y3+, Dy3+–Yb3+; 0.750≤x≤0.875)
    (Elsevier, 2007-08) James, M; Avdeev, M; Barnes, PRF; Morales, L; Wallwork, KS; Withers, RL
    A combination of electron, synchrotron X-ray and neutron powder diffraction reveals a new orthorhombic structure type within the Sr-doped rare earth perovskite cobaltates Ln1−xSrxCoO3−δ (Ln=Y3+, Dy3+, Ho3+, Er3+, Tm3+and Yb3+). Electron diffraction shows a C-centred cell based on a 2√2ap×4ap×4√2ap superstructure of the basic perovskite unit. Not all of these very weak satellite reflections are evident in the synchrotron X-ray and neutron powder diffraction data and the average structure of each member of this series could only be refined based on Cmma symmetry and a 2√2ap×4ap×2√2ap cell. The nature of structural and magnetic ordering in these phases relies on both oxygen vacancy and cation distribution. A small range of solid solution exists where this orthorhombic structure type is observed, centred roughly around the compositions Ln0.2Sr0.8CoO3−δ. In the case of Yb3+ the pure orthorhombic phase was only observed for 0.850≤x≤0.875. Tetragonal (I4/mmm; 2ap×2ap×4ap) superstructures were observed for compositions having higher or lower Sr-doping levels, or for compounds with rare earth ions larger than Dy3+. These orthorhombic phases show mixed valence (3+/4+) cobalt oxidation states between 3.2+ and 3.3+. DC magnetic susceptibility measurements show an additional magnetic transition for these orthorhombic phases compared to the associated tetragonal compounds with critical temperatures > 330K. © 2007, Elsevier Ltd.
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    Sol-gel tungsten oxide/titanium oxide multilayer nanoheterostructured thin films: structural and photoelectrochemical properties
    (American Chemical Society, 2007-12-20) Luca, V; Blackford, MG; Finnie, KS; Evans, PJ; James, M; Lindsay, MJ; Skyllas-Kazacos, M; Barnes, PRF
    Multilayer structures of alternating thin titanium and tungsten oxide layers having dimensions of similar to 20 nm have been fabricated from titanium alkoxide and various tungstate precursor solutions using the dip coating technique. Single, double, and triple layer titanate and tungstate thin films were deposited on silicon substrates, and these films were initially annealed at 400°C. Structural and microstructural aspects of the films were investigated using a variety of techniques, including X-ray reflectometry, grazing incidence X-ray absorption spectroscopy (GIXAS), cross-sectional transmission electron microscopy (TEM), and secondary ion mass spectrometry. The dimensions of the films and the character of the interfaces were principally gauged by cross-sectional TEM and X-ray reflectometery. All films were continuous on a local scale and had relatively low surface roughness. At the treatment temperature of 400°C, only the tungsten oxide component showed appreciable crystallinity. The multilayer films had relatively diffuse interfaces, even after annealing in air at this temperature. At these temperatures, easily measurable diffusion of tungsten into the titanium oxide component was observed, whereas the diffusion of titanium into the tungsten oxide component occurred to a lesser degree. At higher temperatures, interdiffusion of components was found to be significant. TEM, X-ray diffraction, and Ti K-edge GIXAS measurements indicated that annealing at 400°C generated films in which the titanate component remained amorphous while the tungstate component crystallized in the tetragonal modification Of WO3, which is normally stable only at high temperatures. Grazing incidence X-ray absorption spectroscopy allowed the degree of distortion of the tungsten oxygen polyhedra to be monitored as a function of depth into the film. The photoelectrochernical activity of the multilayer film electrodes was investigated, and the activity for water photo-oxidation was assessed. The photoelectrochernical response was greatest when crystalline WO3 was bounded on both sides by amorphous TiO2 layers. In this bounded state, WO3 had unique structural characteristics. © 2007, American Chemical Society