Abstract
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 cation site
ordering, unique to this compound, is observed in the tetrahedral layers (which may be
implicated in the b-axis doubling). A lesser degree of cation site ordering occurs in the
octahedral sites. As well, this material shows an unique arrangement of tetrahedral
chains, and a new space group for the brownmillerite family. The magnetic structure is G-type antiferromagnetic, with preferred orientation of magnetic moments parallel to the
c-axis between 3.8K to 100K. A spin re-orientation occurs in the range of 100K to 225K,
above which and up to 510K the orientation of magnetic moments switches to along the
a-axis. This spin re-orientation has not been seen for any brownmillerite material before.
The neutron diffraction data indicate different site specific ordering temperatures at about
450(5)K and 520(5)K. The refined ordered moments at 3.8K are somewhat smaller than
expected for Fe3+ and Co3+(high spin) but are similar to those found in Sr2FeCoO5. There is evidence for spin canting from isothermal magnetization data that shows well pronounced
hysteresis and remnant magnetization at 5K and 200K.
