Synthesis and Stability Region of Stoichiometric Nanocrystalline Vanadium−Iron Spinel Powders

Vanadium−iron spinels with nanometric sizes have been synthesized by a soft chemistry route. This way of elaboration consists of a coprecipitation followed by thermal treatments at low temperatures. The last thermal annealing is performed under a reducing atmosphere in order to obtain the exact oxygen stoichiometry (four oxygen atoms for three metal atoms in AB 2 O 4 ). Because of the low temperatures used, very low oxygen partial pressures (between 10 - 20 and 10 - 30 Pa) have to be applied. For this purpose, a reducing setup has been used. It consists of H 2 /N 2 /H 2 O gas mixtures, creating a reducing atmosphere with low oxygen partial pressure. This atmosphere can be applied to a sample in a thermobalance, so the mass variations are correlated with the oxygen content (or oxygen stoichiometry) in the sample. Thermal annealing under low oxygen partial pressure allows one to obtain stoichiometric spinels of Fe 3 - x V x O 4 (0 ≤ x ≤ 2).