Effect of the preparation method and grinding time of some mixed valency ferrite spinels on their cationic distribution and thermal stability toward oxygen

Abstract

The reactivity in oxygen of several mixed valency ferrite spinels, namely Fe 3O 4, FeCr 2O 4, Fe 3− x Ti x O 4 and Fe 3− x Mo x O 4 has been examined by DTG analysis as a function of the preparation and the grinding histories. For all samples a low temperature preparation method or a prolonged grinding time which induced small particles (<0.3 μm) permits the formation of a γ-spinel phase with a high content of defects. In contrast, for spinels with large crystallite sizes (>0.5 μm), which have been unground or annealed at high temperatures, the DTG curves show that the oxidation starts at higher temperatures and that the defect spinel phases cannot be retained during the whole range of oxidation temperature. This fact has been attributed to the presence of stress induced by the lattice parameter gradient that promotes the formation of nuclei of the α-rhombohedral phase from the superficial γ-defect phase.