6533b829fe1ef96bd128ac6b

RESEARCH PRODUCT

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)

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Abstract Interstitial polymetallic oxynitrides in the solid solution series V 1− u − z Cr u Mo z (O x N y ), V 1− u − z Cr u W z (O x N y ) and Cr 1− u − z Mo u W z (O x N y ) ( u , z  = 0.2, 0.33, 0.4, 0.6, u  +  z z Cr z Mo z W z (O x N y ) ( z  = 0.25) composition, can be obtained by ammonolysis of precursors resulting from the freeze-drying of aqueous solutions of the simple metal salts NH 4 VO 3 , (NH 4 ) 2 CrO 4 , (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and (NH 4 ) 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 V 1− u − z Cr u Mo z (O x N y ) 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 V 1− u − z Cr u W z (O x N y ) and the Cr 1− u − z Mo u W z (O x N y ) 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 V z Cr z Mo z W z (O x N y ) 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.