Order and disorder in heavily Fe-substituted high-Tc superconductors

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******
â—‹Takashi Mochiku* Yoshiaki Hata** Tuerxun Wuernisha*** Kazuhiro Mori**** Toru Ishigaki***** Takashi Kamiyama*** Hiroki Fujii* Hiroshi Yasuoka** Kazuo Kadowaki****** Kazuto Hirata*

Although most of high-Tc 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-Tc superconductor FeSr2YCu2O6+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 FeSr2NdCu2O6+d with larger lattice size than FeSr2YCu2O6+d, exhibiting superconductivity is dependent on lattice size in this system.