Atomic ordering in FeSr2LnCu2O6+δ system (Ln = Nd, Y and Er)

FeSr2YCu2O6divided bydelta exhibits superconductivity around 50 K, when it is properly annealed in N-2 atmosphere and subsequently in O-2 atmosphere under ambient and high pressure. Neutron powder diffraction study shows that N-2-annealing causes ordering of Cu and Fe, and that O-2-annealing and oxidizing under high pressure supply the charge on the CuO2 sheets. Recently, we have found a superstructure in the sample N-2-annealed within a particular temperature range. Electron diffraction study indicates that it has an orthorhombic structure with the unit cell of root2a x root2a x 2c as compared with the tetragonal FeSr2YCu2O6+delta,5 Structure with the unit cell of a x a x c. Neutron powder diffraction study indicates that it has the CoSr2YCu2O7-type structure with the FeO4 tetrahedron, due to not only ordering of Cu and Fe but also oxygen ordering on the FeOdelta sheet, which is consistent with the electron diffraction study. We have also obtained the similar superstructure in FeSr2ErCu2O6divided bydelta, but the situation is different in FeSr2NdCu2O6+delta which does not exhibit superconductivity even after O-2-annealing and oxidizing. Atomic ordering in the FeSr2YCu2O6+delta system is influenced by the size of the ionic radius in the Y site, which is effective on the occurrence of superconductivity. (C) 2003 Elsevier B.V. All rights reserved.