Superconductivity and magnetic order in RBa2Cu3Oz

Mossbauer studies of57Fe in RBa2−yK y (Cu1−xFe x )3Oz, with R=Y and Pr;y=0 and 0.5;x=0.01, 0.05 and 0.1 andz between 5.9 and 7.1, have been performed. A minority of the iron ions enter the Cu(2) site and reveal its magnetic order. In nonsuperconducting YBa1.5K0.5(Cu0.95Fe0.05)3O6.1 two distinctly inequivalent magnetic iron sites are observed, probably corresponding to iron in the Cu(2) site with different Ba−K neighbours. In superconducting (T c =60 K) YBa1.5K0.5(Cu0.95Fe0.05)3O6.5 one Cu(2) subsite remains magnetic (TN=440 K). The implications of these findings on the valencies of the Cu ions are discussed.