MIXED-VALENCE DEFECT FERRITES - A NEW FAMILY OF FINE POWDERS AND THIN-FILMS OF SPINEL FERRITES

When highly divided spinel ferrites become reactive enough with oxygen , to allow the oxidation of the Fe2+ ions at low temperature and of su bstitute cations too, when these cations are capable of different vale nce states. We prepared fine particles of spinel ferrites substituted by Mn, Mo, Cu, by ''chimie douce'', especially from oxalate precursors and used them to reveal and to study the oxido-reduction phenomena oc curring in these finely divided materials. It was shown that the oxida tion created a new family of spinel ferrites : the mixed-valence defec t ferrites, having specific characteristics and properties. The ferrit es of this type can be fine powders prepared at low temperature, or gr ound ferrites obtained at high temperature, or thin films.