THE ROLE OF DOPANTS ON THE FORMATION AND DEFECT STRUCTURES OF THE M-TYPE OF CALCIUM FERRITE

The formation mechanisms of M-type calcium ferrite were further assessed by doping Y2O3, BaCO3 and SrCO3. It was found that the ionic radius, rather than the valence is the predominant factor in the formation of the magnetoplumbite phase of calcium ferrite. The CaO/sd2Fe2O3 phase which played a precursor role in the formation of the M-type calcium ferrite was further verified. The defect structures based on the substitution of Ca2+ by La3+, the charge compensation by Fe2+, and release of oxygen were supported by DTA-TGA and conductivity data. The conductivity was assumed to, occur through a hopping mechanism as it increased with increasing temperature. The estimated values of the activation energy based on the small-polaron conduction were 0.35 to 0.40 eV in the high temperature region and 0.027 to 0.054 eV in the low temperature region. Moreover, the preexponential factor in the conductivity equation is an exponential function of the fraction of the M phase in the specimen. © 1990 Chapman and Hall Ltd.