Oxygen stoichiometry and superconductivity in YBa2Cu3O6+x and Pb2Sr2Y1-xCaxO8+δ

Structural studies of the oxidation process of YBa2Cu3O6+x and Pb2Sr2Y1-xCAxO8+δ reveal that oxygen uptake does not necessarily lead to better superconducting properties. In the former compound oxygen incorporation results in a higher concentration of positive charges (electron holes) which are transferred from the square chains for the pyramidal layers. Powder neutron diffraction data on new samples prepared by the Zr-gettering technique show that the bridging oxygen position is crucial for the charge transfer. Pb2Sr2Y0.5Ca0.5O8 is superconducting at 78K. This transition is suppressed by oxygen uptake. Neutron and electron diffraction show that for δ > 0.5 the additional positive charge are localized on the Pb sublattice as Pb4+ . This localization hinders the charge transfer to the conducting CuO pyramidal layers