FE MIGRATION, MAGNETIC ORDER, AND THE SUPPRESSION OF SUPERCONDUCTIVITY IN Y1-ZCAZSR2CU2.5FE0.5O6+X

The dependence of the structural, magnetic, and superconductive proper ties of Y1-zCazSr2Cu2.5Fe0.5O6+x(0.08 less-than-or-equal-to z less-tha n-or-equal-to 0.12) on the annealing conditions has been investigated. Materials were initially prepared by conventional solid-state techniq ues at 1040-degrees-C with slow-cooling to room temperature in O2. (me thod [O]). They were then reduced at 800-degrees-C in N2 (method [N]), and reoxidized at 400-degrees-C with slow cooling to room temperature in O2 (method [NO]). The samples were characterized by X-ray diffract ion, iodometric titration, magnetic susceptibility, and Fe-57 Mossbaue r absorption spectroscopy. Samples treated by method [O] are supercond uctive with a maximum transition temperature T(c) almost-equal-to 21 K and Meissner fraction V(f) almost-equal-to 20%. The method [ NO ] tre atment modifies the Fe distribution relative to the [O] treatment and results in materials that are magnetically ordered at temperature T(N) almost-equal-to 22 K; however, modification of the Fe distribution is different from that observed in Y1-zCazBa2Cu2-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)O( x) plane; these clusters are converted upon oxidation to clusters in w hich Fe is five-fold coordinated. In Y1-zCazSr2Cu2.5Fe0.5O6+x, [N] tre atments give a smaller amount of clustering; Fe is randomly distribute d in three-fold and four-fold oxygen coordinated sites in the Cu(1)O(x ) plane and some migration to the Cu(2)O2 sheets also occurs. We prese nt a model based on relative Fe-site stabilities to interpret the data .