Fe ion distribution and superconductivity of FeSr2RECu2O6+y (REÂ =Â Eu, Er, Gd, Nd and Y)

An investigation was carried out into the distribution of Fe ions between Cu(1) and Cu(2) sites for FeSr2RECu2O6+y (RE = Eu, Er, Gd, Nd and Y), in addition to the structural, magnetic and electric properties. After a series of annealing steps to induce superconductivity, all samples exhibited a Ba2YCu3O6-type tetragonal crystal structure and the lattice constants increased with RE ion radius. Zero resistivity was observed for FeSr2YCu2O6+y, FeSr2ErCu2O6+y and FeSr2GdCu2O6+y. 57Fe Mössbauer study revealed that for all samples except FeSr2NdCu2O6+y, about 10% of the Fe ions were present at Cu(2) sites. The separate three annealings are effective for FeSr2RECu2O6+y except FeSr2NdCu2O6+y for the suppression of Fe ion substitution at Cu(2) site and the carrier doping to CuO2 plane.