Phase decomposition study in CoxFe3-xO4 iron cobaltites:: Synthesis and structural characterization of the spinodal transformation

CoxFe3-xO4 iron cobaltites (1.21 < x < 2.35) have been prepared in the form of submicronic particles from mixed oxalic precursors heat treated at 900 and 600 degrees C. The samples obtained at 900 degrees C are made of a single spinel phase in which CO2+ and Fe3+ ions area distributed in both octahedral and tetrahedral sites and Co3+ ions are only located in the octahedral sublattice. For heat treatments carried out at 600 degrees C two spinel phases are formed. according to the phase diagrams. Pure Co1.7Fe1.3O4 oxide annealed at 700 degrees C progressively transformed into two iron-rich and cobalt-rich spinel phases, respectively. More than 120 h of heat treatment is required to get the complete decomposition of the pristine oxide. At the beginning this oxide is submitted to a spinodal transformation. Alternating regions of iron-rich and cobalt-rich spinels are then created. The periodicity of the nanostructuration of the material is estimated to be close to 5 nm. (C) 2007 Elsevier Masson SAS. All rights reserved.