Synthesis, crystal chemistry, and electrical properties of the intergrowth oxides Sr{sub 4{minus}x}Ca{sub x}Fe{sub 6{minus}y}Co{sub y}O{sub 13+{delta}}
- Univ. of Texas, Austin, TX (United States). Texas Materials Inst.
Sr{sub 4{minus}x}Ca{sub x}Fe{sub 6{minus}y}Co{sub y}O{sub 13+{delta}} oxides crystallizing in an orthorhombic intergrowth structure in which perovskite-type SrO-(Fe, Co)O{sub 2}-SrO layers alternate with (Fe, Co){sub 2}O{sub 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{sub 4}Fe{sub 6}O{sub 13+{delta}} has {delta} {approx} 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{sub 4{minus}x}Ca{sub x}Fe{sub 6{minus}y}Co{sub y}O{sub 13+{delta}} (0 {le} y {le} 0.6) increases with increasing Ca content x, which is in contrast to that found in the perovskite system Sr{sub 1{minus}x}Ca{sub x}FeO{sub 3{minus}{delta}}. The conductivity versus temperature plots of Sr{sub 4{minus}x}Ca{sub x}Fe{sub 6}O{sub 13+{delta}} exhibit a change in the sign of the slope from positive to negative at around 450 C for x {ge} 1.6. While a similar transition in the perovskite system SrFeO{sub 3{minus}{delta}} is related to oxygen loss, no oxygen loss is observed up to at least 900 C in Sr{sub 4{minus}x}Ca{sub x}Fe{sub 6}O{sub 13+{delta}}. 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. 20 refs., 14 figs.
- Sponsoring Organization:
- Petroleum Research Fund (United States); Welch (Robert A.) Foundation, Houston, TX (United States)
- OSTI ID:
- 679233
- Journal Information:
- Journal of Solid State Chemistry, Vol. 145, Issue 1; Other Information: PBD: Jun 1999
- Country of Publication:
- United States
- Language:
- English
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