Intralayer cation ordering in a brownmillerite superstructure: Synthesis, crystal, and magnetic structures of Ca₂FeCoO₅

The synthesis, crystal, and magnetic structures and the bulk magnetic properties of Ca₂FeCoO₅, a brownmillerite type oxide, are presented. The crystal structure, solved and refined from single crystal X-ray and powder neutron diffraction data, is described in Pbcm with cell parameters, a = 5.3652(3) Å, b = 11.0995(5) Å, c = 14.7982(7) Å. Thus, one axis, b in this setting, is doubled in comparison with the standard brownmillerite structure description giving rise to two sets of octahedral and tetrahedral sites. Aided by the strong scattering contrast between Fe and Co for neutrons, a nearly perfect intralayer cation site ordering, not observed for any brownmillerite before, is detected in the tetrahedral layers. There is a lesser degree of cation site ordering in the octahedral sites. Overall, the Fe/Co site ordering is of the NaCl type both within and between the tetrahedral and octahedral layers. There are also both intra- and interlayer ordering of tetrahedral chain orientations. The left- and right-handed orientations alternate within each tetrahedral layer as well as between the closest tetrahedral layers. The occurrence of the rare Pbcm space group in Ca ₂FeCoO₅ is not consistent with a recently proposed structure-field map for brownmillerite oxides. The magnetic structure is G-type antiferromagnetic, with preferred orientation of magnetic moments parallel to the longest axis between 3.8 K to 100 K which switches to the shortest axis between 225 K and 550 K. The neutron diffraction data indicate different site specific ordering temperatures at about 450(5) K and 555(5) K. The refined ordered moments at 3.8 K are somewhat smaller than expected for Fe³⁺ and Co³⁺(high spin) but are similar to those found in Sr ₂FeCoO₅. There is evidence for spin canting from isothermal magnetization data that shows well pronounced hystereses and remnant magnetizations at 5 K and 200 K.