ON THE STRUCTURE OF OXIDIZED Pb₂Sr₂Y_0.5Ca_0.5Cu₃O₈. J.-E. Jorgensen, M. H. Iversen and N. H. Andersen*, Department of Chemistry, Aarhus University, DK-8000, Århus C, Denmark, *Riso National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark

Pb₂Sr₂Y_0.5Ca_0.5Cu₃0_8d is a high temperature superconductor for d= 0.0 with a critical temperature of 70 K. In contrast to the YBa₂Cu₃O_7-d compounds, superconductivity is destroyed by low temperature oxygen annealing. The stacking sequence of layers in Pb₂Sr₂Y_0.5CA_0.5CU₃0₈ is:(Y/Ca)-CuO2-SrO-PbO-Cu-PbO-SrO-CuO2-. Additional oxygen is incorporated into the PbO-Cu-PbO sandwich during low temperature annealing. A Pb₂Sr₂Y_0.5Ca_0.5Cu₃O₈ sample was oxidized at 345 deg.C for 2 hours and studied by powder neutron diffraction and electron diffraction. The electron diffraction patterns showed a 4a x 2b x c superstructure and the neutron data were analysed by the Rietveld method. The refinement was done in space group Pmmm. The additional oxygen was found to be located in the 2r (0 1/2 z) and 2p (1/2 y 1/2) positions and the refinements yielded d= 0.71(4). Comparison of the structures of oxidized and unoxidized Pb₂Sr₂Y_0.5Ca_0.5Cu₃O₈ shows that the bond valency sum for Cu in the CuO₂ layers is reduced from 2.284 to 2.189 in the oxidized sample. This reduction in bond valency sum is mainly due to an elongation of the the axial Cu-O bond which links the CuO₂ layers to the PbO-Cu-PbO sandwich. The reduction in bond valency sum is assumed to responsible for the lack of superconductivity in the oxidized sample.