Structural and Magnetic Properties of Oxygen-Deficient Perovskites (Sr,Ca)FeOx and Sr(Fe,Co)Ox*

A wide variety of compounds crystallize into perovskite-related structures, making them versatile laboratories for a variety of inquiries and applications. Oxygen-deficient perovskites, in particular, have attracted interest for use in fuel cells and related applications due to high oxygen mobility. We have studied oxygen-deficient perovskites based on SrFeO$_x$ using x-ray diffraction and Mossbauer spectroscopy. While SrFeO$_x$ exhibits cubic Pm-3m symmetry, CaFeOx has a more distorted structure (orthorhombic Pnma), with gradual reduction in lattice symmetry in doped (Sr,Ca) intermediaries. SrFeO$_x$ exhibits magnetic order below 50K, while CaFeO$_x$ is ordered (with a possible paramagnetic volume fraction) at 300K. Oxygen contents x of the air-annealed samples ranged from 2.3 to 2.65; interestingly, synthesis under 1 atm of flowing O$_2$ did not result in a significant change in oxygen content. Somewhat different behavior is observed for the B-site doped series Sr(Fe,Co)O$_x$. In this case, cubic symmetry is maintained down to 10\% Fe composition, with a sudden reduction thereafter. While the structure of air-annealed SrCoO$_x$ is consistent with orthorhombic symmetry, it is not consistent with previously reported results; the reason for this discrepancy remains unknown.