Oxygen Stoichiometry And Superconductivity In Yba2cu3o6+X And Pb2sr2y1-Xcaxo8+Delta

Structural studies of the oxidation process of YBa 2 Cu 3 O 6+x and Pb 2 Sr 2 Y 1−x Ca x O 8+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 to 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. Pb 2 Sr 2 Y 0.5 Ca 0.5 O 8 is superconducting at 78 K. 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 Pb 4+ . This localization hinders the charge transfer to the conducting CuO pyramidal layers.