CATION DISTRIBUTION, THERMODYNAMIC AND KINETICS CONSIDERATIONS IN NANOSCALED COPPER FERRITE SPINELS. NEW EXPERIMENTAL APPROACH BY XPS AND NEW RESULTS BOTH IN THE BULK AND ON THE GRAIN BOUNDARY

Abstract

In nanoscaled substituted ferrites, the knowledge of the cation distribution in the spinel structure from the bulk onto the grain boundary allows a complete understanding of the specific reactivity towards oxygen pressure and its correlation with physical properties. To distinguish the particularities due to the grain size, two different grain sized copper ferrites with almost the same copper concentration are studied at temperatures below 500°C. New X-ray photoelectron spectroscopy (XPS) investigations on the grain boundary and previous results in the bulk allow for differentiating composition and cation distribution in not only just-prepared materials, but also in oxidized ones. The simulation of the oxidation process in cation-deficient spinel, which takes into account the mechanical stresses, is experimentally verified. Moreover, the coercivity as a function of the temperature under oxygen pressure is not only related to the cation distribution and the point defect concentration, but also to the composition gradient from the bulk onto the grain boundary. With regard to the temperature and the oxygen partial pressure, the thermodynamic states of the single phase ferrites are not only in relation to the deviation from stoichiometry δ, but also to the chemical and mechanical gradients occurring from the bulk onto the grain boundary. A single phase ferrite is at thermodynamic equilibrium and said to be stable when no chemical gradient occurs, and thus when the ion distribution is homogeneous. Variations of the deviation from the stoichiometry occurring from the oxygen pressure or temperature are reversible. Metastable single phase ferrites are also chemically homogeneous. Contrary to the previous case, the slightest gas pressure or temperature variation induces irreversible oxidation or reduction. Usually, only nanoscaled ferrites are metastable at high oxygen pressure after complete oxidation in cation deficient spinels. When the grains are not chemically homogeneous, single nanosized phase ferrite is out of equilibrium and unstable.