Crystal Structure of Magnetic Superconductor FeSr₂YCu₂O_6+δ

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The FeSr₂YCu₂O_6+δ compound with a Ba₂YCu₃O_6+δ-type structure exhibits superconductivity around 50 K, only when it is properly annealed in N₂ atmosphere and subsequently in O₂ atmosphere. The compound without N₂-annealing, however, does not exhibit superconductivity. This compound consists of a sequence of alternating CuO₂ double sheets with nanometer-scale separation as those in high-T_c superconductors, and FeO_δ sheets similar to that in the SrFeO_3-δ magnetic compound. Neutron diffraction study shows that both N₂-annealing and subsequent O₂-annealing cause ordering of Cu and Fe, and that both O₂-annealing and subsequent oxidization under high-O₂ pressure supply the charge. However, the charge was distributed not only to the CuO₂ sheet but also the FeO_δ sheet. The bond valence sum calculation indicates that charges distributed to the FeO_δ sheet more than to the CuO₂ sheet in the superconducting sample.