Synthesis, crystal chemistry, and electrical properties of the intergrowth oxides Sr4-xCaxFe6-yCoyO13+δ

Sr4-xCaxFe6-yCoyO13+delta oxides crystallizing in an orthorhombic intergrowth structure in which perovskite-type SrO-(Fe, Co)O-2-SrO layers alternate with (Fe, Co)(2)O-2.5 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+delta has delta approximate to 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+delta (0 less than or equal to y less than or equal to 0.6) increases with increasing Ca content x, which is in contrast to that found in the perovskite system Sr1-xCaxFeO3-delta. The conductivity versus temperature plots of Sr4-xCaxFe6O13+delta exhibit a change in the sign of the slope from positive to negative at around 450 degrees C for x greater than or equal to 1.6. While a similar transition in the perovskite system SrFeO3-delta is related to oxygen loss, no oxygen loss is observed up to at least 900 degrees C in Sr4-xCaxFe6O13+delta. High-temperature X-ray diffraction does not reveal any structural changes up to 750 degrees C and high values of thermoelectric power rule out the possibility of metallic behavior at T > 450 degrees C. The observed transition is attributed to a possible redistribution of the charge carriers between the perovskite and nonperovskite layers at T > 450 degrees C. (C) 1999 Academic Press.