VALENCE STATES OF COPPER IN COPPER FERRITE SPINELS CUXFE3-XO4 (0-LESS-THAN-X-LESS-THAN-OR-EQUAL-TO-1) FINE POWDERS - EVIDENCE OF COPPER INSERTION

The oxidation in cation deficient spinels of copper iron spinels CuxFe 3-xO4 (0 < x less than or equal to 1) synthesized by soft chemistry wi th a grain size < 50 nm has been investigated by thermal measurements (DTG and DSC), For stoichiometric synthesized spinels when the copper content determines the number of oxidizable cations 1-x per mole of fe rrite, three oxidation phenomena corresponding to Cu+ at B-sites(130 d egrees C), Fe2+ at B-sites (180 degrees C) and Cu+ at A-sites (240 deg rees C) have been found in close relation with the cation-oxygen dista nce of each oxidizable cation. For nonstoichiometric synthesized spine ls CuxFe3-xO4+delta with delta < 0 when the oxidation reveals a mass g ain larger than that calculated with 1-x, the presence of additional i nterstitial Cu ions has been envisaged, The oxidation temperature of t hese interstitial ions (> 300 degrees C) is higher than that for Cu+ i ons at A-sites suggesting that interstitial copper ions also essential ly reside in A-sites. (C) 1997 Elsevier Science B.V.