Fe ion distribution and superconductivity of FeSr₂RECu₂O_6+y (RE = Eu, Er, Gd, Nd and Y)

Highlights: • FeSr₂EuCu₂O_6+y, FeSr₂ErCu₂O_6+y, FeSr₂GdCu₂O_6+y, FeSr₂NdCu₂O_6+y and FeSr₂YCu₂O_6+y were synthesized and the distribution of Fe ions between Cu(1) and Cu(2) sites was determined. • After the three-step annealing, zero resistivity was observed for FeSr₂YCu₂O_6+y, FeSr₂ErCu₂O_6+y and FeSr₂GdCu₂O_6+y. • ⁵⁷Fe Mössbauer study revealed that for all sample except FeSr₂NdCu₂O_6+y about 10% of the Fe ions were present at Cu(2) and the annealing are effective for the suppression of Fe ion substitution at Cu(2) site and the carrier doping to CuO₂ plane. - Abstract: An investigation was carried out into the distribution of Fe ions between Cu(1) and Cu(2) sites for FeSr₂RECu₂O_6+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 FeSr₂YCu₂O_6+y, FeSr₂ErCu₂O_6+y and FeSr₂GdCu₂O_6+y. ⁵⁷Fe Mössbauer study revealed that for all samples except FeSr₂NdCu₂O_6+y, about 10% of the Fe ions were present at Cu(2) sites. The separate three annealings are effective for FeSr₂RECu₂O_6+y except FeSr₂NdCu₂O_6+y for the suppression of Fe ion substitution at Cu(2) site and the carrier doping to CuO₂ plane