Synthesis and characterization of Sr 0.75Y 0.25Co 1-xM xO 2.625+δ ( M=Ga, 0.125⩽ x⩽0.500 and M=Fe, 0.125⩽ x⩽0.875)

The effect of replacing Co ³⁺ by Ga ³⁺ and Fe ³⁺ in the perovskite-related tetragonal phase Sr _0.75Y _0.25CoO _2.625 with unit cell parameters: a=2 a_p, and c=4 a_p (314 phase) has been investigated. The 314 phase is formed by Sr _0.75Y _0.25Co _1-xM _xO _2.625+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 Sr _0.75Y _0.25Co _0.75Ga _0.25O _2.625 and Sr _0.75Y _0.25Co _0.5Fe _0.5O _2.625+δ were refined using neutron powder diffraction data. It was found that the oxygen content of Sr _0.75Y _0.25Co _0.5Fe _0.5O _2.625+δ is higher than in Fe-free 314 phase, so that δ corresponds to 0.076, whereas δ=0 in Sr _0.75Y _0.25Co _0.75Ga _0.25O _2.625+δ. Magnetization measurements on the unsubstituted Sr _0.7Y _0.3CoO _2.62 and Ga-substituted Sr _0.75Y _0.25Co _0.75Ga _0.25O _2.625 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 Sr _0.75Y _0.25Co _0.5Fe _0.5O _2.625+δ compound is G-type antiferromagnetic at room temperature, whereas Sr _0.75Y _0.25Co _0.75Ga _0.25O _2.625 does not exhibit magnetic ordering at room temperature.