Restoration of superconductivity in YSrBaCu2.6Fe0.4O6+z

X-ray diffraction, iodometric titration, resistivity and SQUID magnetometry measurements have been performed on samples of YSrBaCu2.6Fe0.4O6+z in which yttrium has been partially replaced by calcium. The calcium-free compound is semiconducting but superconductivity is restored for x>0.05. The normal-state resistivity decreases and Tc increases with increasing Ca concentration. The effective valence of copper decreases with increasing calcium content and there is also some loss of oxygen. The observed behaviour is quite similar to that previously reported for samples which were doped with Al but values of Tc are generally lower with Fe doping. This is thought to be due primarily to the fact that the Fe partially occupies the Cu(2) sites in the superconducting Cu-O planes.