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

In nanoscaled substituted ferrites, the knowledge of the cation distri bution in the spinel;structure from the bulk onto the grain boundary a llows a complete understanding of the specific reactivity towards oxyg en pressure and its correlation with physical properties. To distingui sh the particularities due to the grain size, two different grain size d copper ferrites with almost the same copper concentration are studie d at temperatures below 500 degrees C. New X-ray photoelectron spectro scopy (XPS) investigations on the;grain boundary and previous results in the bulk allow for differentiating composition and cation distribut ion in not only just-prepared materials, but also in oxidized ones. Th e simulation of the oxidation process in cation-deficient spinel, whic h takes into account the mechanical stresses, is experimentally verifi ed. Moreover, the coercivity as a function of the temperature under ox ygen pressure is not only related to the cation distribution and the p oint defect concentration, but also to the composition gradient from t he bulk onto the grain boundary. With regard to the temperature and th e oxygen partial pressure, the thermodynamic states of the single phas e ferrites are not only in relation to the deviation from stoichiometr y delta, but also to the chemical and mechanical gradients occurring f rom the bulk onto the grain boundary. A single phase ferrite is at the rmodynamic 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 pre ssure or temperature are reversible. Metastable single phase ferrites are also chemically homogeneous. Contrary to the previous case, the sl ightest gas pressure or temperature variation induces irreversible oxi dation or reduction. Usually, only nanoscaled ferrites are metastable at high oxygen pressure after complete oxidation in cation deficient s pinels. When the grains are not chemically homogeneous, single nanosiz ed phase ferrite is out of equilibrium and unstable. (C) 1998 Elsevier Science Ltd. All rights reserved.