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

Abstract

ScVO₃ has been prepared by controlled reduction of a ScVO₄ precursor under an H₂/N₂ flow at 1250 °C. 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, a = 9.6182(1) Å. The thermal analysis of ScVO₃ in an air flow shows two subsequent oxidation processes, with a final reversal to ScVO₄ above 600 °C. An intermediate phase of composition ScVO_3.5, containing V⁴⁺ cations, can be isolated by isothermal annealing at 350 °C in air. This metastable phase has been identified by X-ray diffraction (XRD) as a fluorite-type oxide (space group Fmm, a = 4.947(2) Å), also showing a random distribution of Sc and V cations over the metal positions. The Rietveld refinement of the ScVO_3.5 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.