POINT-DEFECTS AND CATION TRACER DIFFUSION IN (COXFE1-X)3-DELTA-O4 SPINELS

New data consisting of values for the deviation from stoichiometry, delta, and for cation tracer diffusion coefficients, D(Me)* (Me=Co, Fe, and Mn), in (CoxFe1-x)3-deltaO4 have been obtained experimentally. Delta and D(Me)* have been measured at 1200-degrees-C as a function of oxygen activity, a(O2), and at different cationic compositions which cover essentially the entire spinel phase stability region at this temperature. All delta versus a(O2) curves measured are S-shaped. From the oxygen activity dependence of the deviation from stoichiometry it is concluded that cation vacancies are the majority point defects at high a(O2) while cation interstitials are dominant at low a(O2). All D(Me)* versus a(O2) plots for different cationic compositions show a distinct minimum, which indicates that the high temperature diffusion process is governed by cation vacancies at high a(O2) and by interstitials at low a(O2). By combining information on point defects and cation tracer diffusion, mean mobilities of cations and the cation diffusion mechanism in this system are discussed.