Influence of oxygen defects on the structure and magnetic properties of Sr1-xBixCoO3-y (0.1 <= x <= 0.2) supercell perovskites

The synthesis and structural characterization of perovskite-related Sr1-xBixCoO3-y compounds in the compositional range 0.05 <= x <= 0.25 is reported. The as-prepared x = 0.10, 0.15, and 0.20 materials adopt I4/mmm supercell structures based on layers of CoO6 and CoO6-x oxygen-deficient layers alternating along the c axis. Analysis of neutron powder diffraction data reveals that the oxygen vacancies are located on two crystallographic sites in the xy plane, resulting in tetrahedral, square-pyramidal, and occasional octahedral cobalt environments. All three as-prepared phases are antiferromagnetically ordered at room temperature and display increasing cobalt moments with increasing Bi content, i.e., 2.14(3), 2.32(2), and 2.64(2) mu(B) for x = 0.10, 0.15, and 0.2), respectively, at 10 K. Upon annealing under flowing oxygen, the samples undergo an exothermic phase transition to the cubic perovskite structure, indicating that the superstructure is metastable. The oxygen-annealed x = 0.15 sample exhibits a low-temperature spin glass phase consistent with competition between ferromagnetic and antiferromagnetic couplings. The relationship to the isostructural Ln(1-x)Sr(x)CoO(3-delta) supercell perovskites, Ln = Y, Eu-Ho, is discussed.