Synthesis and characterization of Sr0.75Y0.25Co1−xMxO2.625+δ (M=Ga, 0.125⩽x⩽0.500 and M=Fe, 0.125⩽x⩽0.875)

The effect of replacing Co3+ by Ga3+ and Fe3+ in the perovskite-related tetragonal phase Sr0.75Y0.25CoO2.625 with unit cell parameters: a=2ap, and c=4ap (314 phase) has been investigated. The 314 phase is formed by Sr0.75Y0.25Co1−xMxO2.625+δ, with x⩽0.375 for M=Ga and x⩽0.625 for M=Fe. High-resolution transmission electron microscopy and electron diffraction revealed frequent microtwinning in the iron-containing compounds, in contrast to the Ga-substituted 314 phases. Diffraction experiments and electron microscope images indicated that at higher Fe contents, 0.75⩽x⩽0.875, a disordered cubic perovskite structure forms. The crystal structures of Sr0.75Y0.25Co0.75Ga0.25O2.625 and Sr0.75Y0.25Co0.5Fe0.5O2.625+δ were refined using neutron powder diffraction data. It was found that the oxygen content of Sr0.75Y0.25Co0.5Fe0.5O2.625+δ is higher than in Fe-free 314 phase, so that δ corresponds to 0.076, whereas image in Sr0.75Y0.25Co0.75Ga0.25O2.625+δ. Magnetization measurements on the unsubstituted Sr0.7Y0.3CoO2.62 and Ga-substituted Sr0.75Y0.25Co0.75Ga0.25O2.625 compounds indicate the presence of a ferromagnetic-like contribution to the measured magnetization at 320 and 225 K, respectively, while replacing Co by Fe leads to the suppression of this contribution. A neutron diffraction study shows that the Sr0.75Y0.25Co0.5Fe0.5O2.625+δ compound is G-type antiferromagnetic at room temperature, whereas Sr0.75Y0.25Co0.75Ga0.25O2.625 does not exhibit magnetic ordering at room temperature.