Nonstoichiometry () and High-Temperature Thermodynamic Properties of (Mg0.22Mn0.07Fe0.71)3-O4 Ferrite Spinel

Sr4-xCaxFe6-yCoyO13+ oxides crystallizing in an orthorhombic intergrowth structure in which perovskite-type SrO–((Fe, Co))O2–SrO layers alternate with ((Fe, Co))2O2.5 layers along the b axis have been synthesized. The Ca solubility decreases from x=3.2 at y=0 to x=1.6 at y=1.8. The cell parameters and volume decrease monotonically with increasing Ca Content. While the unsubstituted Sr4Fe6O13+ has 0.4 excess oxygen, the substitution of Ca and Co tends to decrease the oxygen content and average oxidation state toward, respectively, 13.0 and 3.0+. The total conductivity of Sr4-xCaxFe6-yCoyO13+ (0y0.6) increses with increasing Ca content x, which is in contrast to that found in the perovskite system Sr1-xCaxFeO3-. The conductivity versus temperature plots of Sr4-xCaxFe6O13+ exhibit a change in the sign of the slope from positive to negative at around 450°C for x1.6. While a similar transition in the perovskite system SrFeO3- is related to oxygen loss, no oxygen loss is observed up to at least 900°C in Sr4-xCaxFe6O13+. High-temperature X-ray diffraction does not reveal any structural changes up to 750°C and high values of thermoelectric power rule out the possibility of metallic behavior at T>450°C. The observed transition is attributed to a possible redistribution of the charge carriers between the perovskite and nonperovskite layers at T>450°C.