Spin-crossover structural transition in the layered perovskite Gd₂SrCo₂O₇

The structure of the double-layer Ruddlesden-Popper phase Gd2 Sr Co2 O7 has been determined in the temperature range 300-973 K using powder neutron diffraction. The room-temperature structure is tetragonal and displays small tilts of the oxygen sublattice that are described in P 42 /mnm symmetry instead of the idealized I4/mmm structure. A structural transition at T≈580 K is characterized by reduction of unit cell symmetry and a marked axial elongation of the Co O6 octahedra; this behavior is associated with a spin crossover at the octahedrally coordinated cobalt (III) ions, most likely from an intermediate to a high spin configuration.