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

✍ Scribed by S. Guggilla; T. Armstrong; A. Manthiram

• Publisher: Elsevier Science
• Year: 1999
• Tongue: English
• Weight: 210 KB
• Volume: 145
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1999.8261

✦ Synopsis

Sr 4؊x Ca x Fe 6؊y Co y O 13؉ 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 Sr 4 Fe 6 O 13؉ 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 Sr 4؊x Ca x Fe 6؊y Co y O 13؉ (04 4y4 40.6) increases with increasing Ca content x, which is in contrast to that found in the perovskite system Sr 1؊x Ca x FeO 3؊ . The conductivity versus temperature plots of Sr 4؊x Ca x Fe 6 O 13؉ exhibit a change in the sign of the slope from positive to negative at around 4503C for x5 51.6. While a similar transition in the perovskite system SrFeO ؊ B is related to oxygen loss, no oxygen loss is observed up to at least 9003C in Sr 4؊x Ca x Fe 6 O 13؉ . High-temperature X-ray di4raction does not reveal any structural changes up to 7503 3C and high values of thermoelectric power rule out the possibility of metallic behavior at T' '4503 3C. The observed transition is attributed to a possible redistribution of the charge carriers between the perovskite and nonperovskite layers at T' '4503 3C.