Synthesis of new vanadium–chromium and chromium–molybdenum oxynitrides by direct ammonolysis of freeze-dried precursors

Abstract

Interstitial vanadium–chromium and chromium–molybdenum oxynitrides in the solid solution series V1 − zCrz(OxNy) and Cr1 − zMoz(OxNy) (z = 0.0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) have been obtained by direct ammonolysis of precursors resulting from the freeze-drying of aqueous solutions of the appropriate metal salts. A study of the influence of the preparative variables on the outcomes of this procedure is presented. Compounds in the V1 − zCrz(OxNy) series are prepared as single phases by nitridation at 1073 K, followed by fast cooling of the samples. Compounds in the Cr1 − zMoz(OxNy) series are prepared as nearly single phases by nitridation at different temperatures, optimized for each composition, followed by fast cooling of the samples. As for the VN, CrN and Mo2N binary nitrides, all the V1 − zCrz(OxNy) and Cr1 − zMoz(OxNy) compounds in these series have the rock salt crystal structure, in which the metal atoms are in a fcc arrangement, with non-metal atoms occupying octahedral interstitial positions. The materials have been characterized by X-ray powder diffraction, elemental analysis, scanning electron microscopy and thermogravimetry under oxygen flow. V1 − zCrz(OxNy) and Cr1 − zMoz(OxNy) grains are aggregates of nanometric spherical particles with diameters typically around 20 nm. Surface oxidation must account to a large extent for the oxygen content of these compounds.