Preparation and topotactical oxidation of ScVO3 with bixbyte structure:: a low-temperature route to stabilize the new defect fluorite ScVO3.5 metastable phase

ScVO3 has been prepared by controlled reduction of a ScVO4 precursor under an H-2/N-2 flow at 1250 degreesC. The crystal structure of this material has been studied at room temperature by Rietveld refinement of high-resolution neutron powder diffraction (NPD) data. Sc3+ and V3+ are distributed at random over the metal sites of a C-M2O3 bixbyite-type structure, space group Ia (3) over bar, a = 9.6182(1) Angstrom. The thermal analysis of ScVO3 in an air flow shows two subsequent oxidation processes, with a final reversal to ScVO4 above 600 degreesC. An intermediate phase of composition ScVO3.5, containing V4+ cations, can be isolated by isothermal annealing at 350 degreesC in air. This metastable phase has been identified by X-ray diffraction (XRD) as a fluorite-type oxide (space group Fm (3) over barm, a = 4.947(2) Angstrom), also showing a random distribution of Sc and V cations over the metal positions. The Rietveld refinement of the ScVO3.5, structure from powder XRD data in a fluorite structural model yields abnormally high thermal factors for the oxygen atoms, suggesting oxygen mobility in this metastable material.