Crystal structure and charge localization in Pb 2Sr 2Y 1− xCa xCu 3O 8 for x = 0.0–0.5

Abstract

Neutron powder diffraction studies of Pb 2Sr 2Y 1- x Ca xCu 3O 8 samples for x = 0.0-0.5 have shown that Ca doping causes a positive charge transfer to the CuO 2 as well as to the PbO layers. The bond-valency sum for the Cu ions in the CuO 2 layer increases as a function of x, while the bond-valency sum for the Pb ions increases up to x = 0.4 and is then saturated. The axial Cu-O bond length shows a maximum at x = 0.4, and the shortest axial Cu-O bond length was found for x = 0.5. The x = 0.5 sample was found to become superconducting with an onset temperature of 79 K. The distance between the Cu ions in the CuO 2-(Y, Ca)-CuO 2 sandwich is increased by 0.14 Å by going from x = 0.0 to 0.5.