Fe migration, magnetic order, and the suppression of superconductivity in Y1-zCazSr2Cu2.5Fe0.5O6+x

The dependence of the structural, magnetic, and superconductive properties of Y1-zCazSr2Cu2.5Fe0.5O6+x(0.08⩽z⩽0.12) on the annealing conditions has been investigated. Materials were initially prepared by conventional solid-state techniques at 1040°C with slow cooling to room temperature in O2 (method [O]). They were then reduced at 800°C in N2 (method [N]), and reoxidized at 400°C with slow cooling to room temperature in O2 (method [NO]). The samples were characterized by X-ray diffraction, iodometric titration, magnetic susceptibility, and 57Fe Mössbauer absorption spectroscopy. Samples treated by method [O] are superconductive with a maximum transition temperature Tc≈21 K and Meissner fraction Vf≈20%. The method [NO] treatment modifies the Fe distribution relative to the [O] treatment and results in materials that are magnetically ordered at temperature TN≈22 K; however, modification of the Fe distribution is different from that observed in Y1-zCazBa2 Cu2-yFeyO6+x. In the latter material, [N] treatments give, in addition to migration of Fe to the Cu(2)O2 sheets, Fe clusters in threefold trigonal sites in the Cu(1)Ox plane; these clusters are converted upon oxidation to clusters in which Fe is five-fold coordinated. In Y1-zCazSr2Cu2.5Fe0.5O6+x, [N] treatments give a smaller amount of clustering; Fe is randomly distributed in three-fold and four-fold oxygen coordinated sites in the Cu(1)Ox plane and some migration to the Cu(2)O2 sheets also occurs. We present a model based on relative Fe-site stabilities to interpret the data.