STRUCTURE, SUPERCONDUCTIVITY AND XPS STUDIES OF THE BI2.1SR1.93CA0.97-XYXCU2O8+Y SYSTEM

A systematic study of the effect of Y doping at the Ca site in the hig h-T(c) superconductor Bi2212 has been made. Starting with an off-stoic hiometric composition Bi2.1Sr1.93(Ca0.97-xYx)Cu2O8+y (BiY2212) by powd er X-ray diffraction, resistivity, AC and DC susceptibility and XPS me asurements have shown the single-phase nature for all values of x. The decrease in T(c) observed systematically by both resistivity and AC s usceptibility measurements with increasing Y concentration, and the su bsequent metal-semiconductor cross-over, is explained as due to the de crease in the average valence of the copper brought about by the exces s charge introduced by Y. The Cu 2p core level shows that the relative intensity of the satellite, the energy separation between main line a nd satellite, and the binding energy of the main line vary in a system atic way. A decrease in charge-transfer energy and an increase in the hybridisation strength is predicted for higher x with the configuratio n interaction model. The presence of internal strain in this supercond uctor makes the BVS calculation not applicable.