Synthesis of new molybdenum-tungsten, vanadium-tungsten and vanadium-molybdenum-tungsten oxynitrides from freeze-dried precursors

Interstitial molybdenum-tungsten, vanadium-tungsten and vanadium-molybdenum-tungsten oxynitrides in the solid solution series Mo1-zWz(OxNy) and V1-zWz(OxNy) (z = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1), and V1-u-zMouWz(OxNy) (u, z = 0.2, 0.33, 0.4, 0.6; u + z < 1), have been obtained by ammonolysis of precursors resulting from the freeze-drying of aqueous solutions of the metal salts (NH4VO3, (NH4)(6)Mo(7)O(24)(.)4H(2)O and (NH4)(6)W(12)O(39)(.)18H(2)O). A study of the influence of the preparative variables on the outcomes of this procedure is presented. Compounds in the Mo1-zWz(OxNy) series are prepared as single phases by ammonolysis of the respective freeze-dried precursors (during 2h) at different temperatures between 973 and 1023 K, optimised for each composition, followed by slow cooling of the samples (except for the Mo-only containing phase, in which fast cooling has been used). Compounds in the V1-zWz(OxNy) and V1-u-zMouWz(OxNy) series are prepared as single phases by ammonolysis (during 2h) of crystalline precursors (as resulting from thermal treatment in air at 873 K, during 12 It, of the freeze-dried precursors) at 1073 K, followed by slow cooling of the samples. All the compounds in these series have the rock-salt crystal structure, in which the metal atoms are in an fee 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 magnetic measurements. (C) 2004 Elsevier Inc. All rights reserved.