Structure of the Fe-doped high-temperature superconductors Y1Ba2Cu3Oy and Bi2Sr2Ca1Cu2Oy

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Abstract
The high‐temperature superconductors YBa2(Cu1−xFex)3Oy and Bi2Sr2Ca1(Cu1−xFex)2Oy have been examined by resistivity measurements, x‐ray diffraction, and Mössbauer experiments in order to understand the influence of Fe on the structure of the perovskites. The concentration dependence of the Fe substitution and the results of the annealing experiments are consistent with the following model which explains the observed structural changes with Fe composition in YBa2Cu3Oy. For low Fe content, the Fe atoms mainly substitute on the Cu(I) sites leading only to a slight decrease of Tc, but to an attraction of more oxygen ions, which are introduced into the (001) plane. The Fe ions with five‐ or sixfold coordination act like local symmetry points inducing the formation of short orthogonal chains crossing at the Fe ions. This leads to the orthorhombic‐to‐tetragonal transition at an Fe concentration of 2.3%. For higher Fe content, Tc more rapidly decreases, probably due to a partial substitution of Fe also on the Cu(II) sites of the perovskite until the superconductivity vanishes near 16% Fe. The three occuring Mössbauer doublets are assigned to different (four‐, five‐, and sixfold) oxygen environments of the Fe ions. Similar environments occur in Bi2Sr2Ca1Cu2Oy with 20% Fe, although it is known that no Cu‐O chains exist in this structure.