CHARGE LOCALIZATION IN OXIDIZED PB2SR2Y0.5CU3O8+8 STUDIED BY ELECTRONAND NEUTRON POWDER DIFFRACTION

Oxidized Pb2Sr2Y0.5Ca0.5Cu3O8+delta was studied by electron diffractio n and neutron powder diffraction. The electron diffraction diagrams sh owed a doubling along the b-axis and a quadruplication along the a-axi s indicating that the excess oxygen is incorporated into the structure in an ordered way. The oxygen content was determined from refinement of the neutron data and delta = 1.2(1) was obtained. Calculation of bo nd valency sums for the cations shows that the bond valency sum for Cu in the CuO2 layers in Pb2Sr2Y0.5Ca0.5Cu3O8 decreases when the compoun d is oxidized, thereby explaining the lack of superconductivity in the oxidized compound. (C) 1998 Elsevier Science B.V.