Crystal structure of the Fe_1-xSr₂YCu_2+xO_6+δ magnetic superconductor

FeSr₂YCu₂O_6+δ with a tetragonal Ba₂YCu₃O_6+δ-type structure exhibits superconductivity around 60 K when it is properly annealed in a reducing atmosphere, and subsequently, in an oxidizing atmosphere. This compound exhibits atomic order of Cu and Fe along the c-axis, and superconductivity on the CuO₂ plane competes with antiferromagnetic order on the FeOδ plane. In an Fe_1-xSr₂YCu_2+xO_6+δ solid solution (0 ≤ x ≤ 0.3), although it would be expected that the substitution of Cu for Fe causes suppression of magnetism and an increase of T_c, T_c, in fact, decreases with increasing x. Neutron powder diffraction indicates that the oxygen content, given by 6+δ, decreases with increasing x. The decrease of 6+δ causes a decreasing charge carrier concentration on the CuO₂ plane, and as a result T_c decreases. (author)