Freeze-dried precursor-based synthesis of new vanadium-molybdenum oxynitrides

Interstitial vanadium molybdenum oxynitrides in the solid solution series V1 - 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 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 this series are prepared as single phases by nitridation at 1038 K, followed by fast cooling of the samples. As for the VN and Mo2N individual nitrides, all the V1 - zMoz(OxNy) compounds in this series have the rocksalt crystal structure, in which the metal atoms are in a fcc arrangement, with N and O atoms occupying octahedral interstitial positions. Cell parameters increase regularly with z, as can be expected taking into account the sizes of V and Mo atoms. The materials have been characterized by X-ray powder diffraction, elemental analysis, scanning electron microscopy and thermogravimetry under oxygen flow. V1 - zMoz(OxNy) grains are aggregates of nanometric spherical particles with typical diameters of ca. 20 nm. Their stability in oxygen atmosphere is low but increases with the Mo content.