Synthesis, Crystal Chemistry, and Electrical Properties of the Intergrowth Oxides Sr4−xCaxFe6−yCoyO13+δ

Sr4−xCaxFe6−yCoyO13+δ 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+δ has δ≈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+δ (0≤y≤0.6) increases with increasing Ca content x, which is in contrast to that found in the perovskite system Sr1−xCaxFeO3−δ. The conductivity versus temperature plots of Sr4−xCaxFe6O13+δ exhibit a change in the sign of the slope from positive to negative at around 450°C for x≥1.6. While a similar transition in the perovskite system SrFeO3−δ is related to oxygen loss, no oxygen loss is observed up to at least 900°C in Sr4−xCaxFe6O13+δ. 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.