The FeSr 2 YCu 2 O 6þ compound with a Ba 2 YCu 3 O 6þ -type structure exhibits superconductivity around 50 K, only when it is properly annealed in N 2 atmosphere and subsequently in O 2 atmosphere. The compound without N 2 -annealing, however, does not exhibit superconductivity. This compound consists of a sequence of alternating CuO 2 double sheets with nanometer-scale separation as those in high-T c superconductors, and FeO sheets similar to that in the SrFeO 3À magnetic compound. Neutron diffraction study shows that both N 2 -annealing and subsequent O 2 -annealing cause ordering of Cu and Fe, and that both O 2 -annealing and subsequent oxidization under high-O 2 pressure supply the charge. However, the charge was distributed not only to the CuO 2 sheet but also the FeO sheet. The bond valence sum calculation indicates that charges distributed to the FeO sheet more than to the CuO 2 sheet in the superconducting sample.
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