Crystal structure of magnetic superconductor FeSr2YCu2O6+δ

The FeSr2YCu2O6+delta compound with a Ba2YCu3O6+delta-type structure exhibits superconductivity around 50K, 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 CuO2 double sheets with nanometer-scale separation as those in high-T, superconductors, and FeOdelta sheets similar to that in the SrFeO3-delta 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 CuO2 sheet but also the FeOdelta sheet. The bond valence sum calculation indicates that charges distributed to the FeOdelta sheet more than to the CuO2 sheet in the superconducting sample.