Superconducting Materials Center, National Institute for Materials Science^* Department of Applied Physics, National Defense Academy, Japan^** Institute of Materials Structure Science, High Energy Accelerator Research Organization, Japan^*** Research Reactor Institute, Kyoto University, Japan^**** Quantum Beam Science Directorate, Japan Atomic Energy Agency, Japan^***** Institute of Materials Science, University of Tsukuba^******
○Takashi Mochiku^* Yoshiaki Hata^** Tuerxun Wuernisha^*** Kazuhiro Mori^**** Toru Ishigaki^***** Takashi Kamiyama^*** Hiroki Fujii^* Hiroshi Yasuoka^** Kazuo Kadowaki^****** Kazuto Hirata^*

Although most of high-T_c superconductors with light substitution of transition metal exhibit superconductivity, the heavily substituted compounds do not exhibit superconductivity even after annealing under high oxygen pressures, which is considered to promote superconductivity due to charge introduction. However, superconductivity was discovered around 50 K in heavily Fe-substituted high-T_c superconductor FeSr₂YCu₂O_6+d, which was annealed in an N₂ atmosphere and subsequently in an O₂ atmosphere. Neutron powder diffraction study shows that N₂-annealing causes ordering of Cu and Fe, and that O₂-annealing supplies the charge on the CuO₂ sheets. We have also found a superstructure in the samples N₂-annealed within a particular temperature range. It has the CoSr₂YCu₂O₇-type superstructure with the FeO₄ tetrahedron, due to not only atomic ordering of Cu and Fe but also oxygen ordering. The formation of the FeO₄ tetrahedron is important for exhibiting superconductivity in heavily Fe-substituted compounds, because Cu can be not substituted for Fe in the FeO₄ tetrahedron and the formation of the FeO₄ tetrahedron promotes atomic ordering of Cu and Fe to exhibit superconductivity. However, since the formation of the FeO₄ tetrahedron is not promoted by N₂-annealing in Nd-substituted compound FeSr₂NdCu₂O_6+d with larger lattice size than FeSr₂YCu₂O_6+d, exhibiting superconductivity is dependent on lattice size in this system.