Freeze-dried precursor-based synthesis of new polymetallic oxynitrides, V1−u−zCruMoz(OxNy),V1−u−zCruWz(OxNy), Cr1−u−zMouWz(OxNy) (u, z = 0.2, 0.33, 0.4, 0.6, u + z < 1), and VzCrzMozWz(OxNy) (z = 0.25)

Interstitial polymetallic oxynitrides in the solid solution series V1−u−zCruMoz(OxNy), V1−u−zCruWz(OxNy) and Cr1−u−zMouWz(OxNy) (u, z = 0.2, 0.33, 0.4, 0.6, u + z < 1), as well as the tetrametallic VzCrzMozWz(OxNy) (z = 0.25) composition, can be obtained by ammonolysis of precursors resulting from the freeze-drying of aqueous solutions of the simple metal salts NH4VO3, (NH4)2CrO4, (NH4)6Mo7O24·4H2O and (NH4)6W12O39·18H2O. A study of the influence of the preparative variables on the outcomes of this procedure is presented. Compounds in the V1−u−zCruMoz(OxNy) series have been prepared as single phases by direct ammonolysis of the respective freeze-dried precursors during 2 h at 1173 K, followed by fast cooling of the samples. In contrast, the preparation of compounds in both the V1−u−zCruWz(OxNy) and the Cr1−u−zMouWz(OxNy) series requires the intermediate formation of crystalline precursors. These last compounds result from thermal treatment in air at 973 K, during 12 h, of the corresponding freeze-dried powders. Then, they were subjected to ammonolysis, during 2 h, at 1123 K (V containing series) or 1173 K (Mo containing series), followed by slow cooling of the samples. The tetrametallic VzCrzMozWz(OxNy) compound also has been prepared as single phase by ammonolysis of the crystalline intermediate at 1073 K, during 2 h, followed by slow cooling. All the compounds in these series have the rock-salt crystal structure, in which the metal atoms are in a face-centred cubic arrangement, with non-metal atoms occupying octahedral interstitial positions. The materials have been characterized by X-ray powder diffraction, elemental analysis, thermogravimetric analysis under oxygen flow and scanning electron microscopy.