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

Abstract

Interstitial molybdenum–tungsten, vanadium–tungsten and vanadium–molybdenum–tungsten oxynitrides in the solid solution series Mo 1− z W z (O x N y ) and V 1− z W z (O x N y ) ( z=0, 0.2, 0.4, 0.5, 0.6, 0.8, 1), and V 1− u− z Mo u W z (O x N y ) ( 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 (NH 4VO 3, (NH 4) 6Mo 7O 24·4H 2O and (NH 4) 6W 12O 39·18H 2O). A study of the influence of the preparative variables on the outcomes of this procedure is presented. Compounds in the Mo 1− z W z (O x N y ) series are prepared as single phases by ammonolysis of the respective freeze-dried precursors (during 2 h) 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 V 1− z W z (O x N y ) and V 1− u− z Mo u W z (O x N y ) series are prepared as single phases by ammonolysis (during 2 h) of crystalline precursors (as resulting from thermal treatment in air at 873 K, during 12 h, 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 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 magnetic measurements.