Point defects and deviation from stoichiometry in (Znx−y/4Mn1−x−3y/4Fe2+y)1−δ/3O4

The deviation from stoichiometry, δ, in the spinel (Znx−y/4Mn1−x−3y/4Fe2+y)1−δ/3O4 with x=0.25 was measured thermogravimetrically at 1200 °C as a function of the oxygen activity, aO2 and the cationic composition, y. Point defect thermodynamic modeling of the oxygen activity dependence of δ suggests that cation vacancies are the majority ionic defects at high oxygen activity and that cation interstitials dominate at lower activities. For the spinel with x=0.25 and y=0.08 the deviation from stoichiometry was measured also at 900, 1000 and 1100 °C. The variation of the majority defect population with temperature for the latter spinel composition, which is typical for technical soft ferrites, is discussed and enthalpies for defect formation reactions are estimated. The upper limit of the stability field of this ferrite at 1200 °C is determined. Implications of the variation of the phase stability field and the deviation from stoichiometry, δ, for the sintering regimes of soft ferrites are addressed.