Superconductivity and magnetic order in RBa2Cu3Oz

Mössbauer studies of⁵⁷Fe in RBa_2?y K _y (Cu_1?x Fe _x )₃O_z, 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 YBa_1.5K_0.5(Cu_0.95Fe_0.05)₃O_6.1 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) YBa_1.5K_0.5(Cu_0.95Fe_0.05)₃O_6.5 one Cu(2) subsite remains magnetic (T _N=440 K). The implications of these findings on the valencies of the Cu ions are discussed.