CRYSTAL-STRUCTURE AND CHARGE LOCALIZATION IN PB2SR2Y1-XCAXCU3O8 FOR X= 0.0-0.5

Neutron powder diffraction studies of Pb2Sr2Y1-xCaxCu3O8 samples for x = 0.0-0. 5 have shown that Ca doping causes a positive charge transfe r to the CuO2 as well as to the PbO layers. The bond-valency sum for t he Cu ions in the CuO2 layer increases as a function of x, while the b ond-valency sum for the Pb ions increases up to x = 0.4 and is then sa turated. The axial Cu-0 bond length shows a maximum at x = 0.4, and th e shortest axial Cu-0 bond length was found for x = 0.5. The x = 0.5 s ample was found to become superconducting with an onset temperature of 79 K. The distance between the Cu ions in the CuO2-(Y, Ca)-CuO2 sandw ich is increased by 0.14 angstrom by going from x = 0.0 to 0.5.