XPS STUDIES ON THE SUPERCONDUCTOR-INSULATOR TRANSITION IN BI2-XPBXSR2CA1-YYYCU2O8+DELTA SYSTEM

Changes in electronic structure of the Bi-O and Cu-O planes due to the simultaneous doping of Pb at the Bi site and Y at the Ca site in the Bi-2212 system have been studied using core-level photoemission. The r are-earth doping at the Ca site causes an increase in the valence stat e of Bi and a decrease in average Cu valence. Incorporation of Pb into the system results in a retarded rate of increase of Bi valence. Pb, which seems to exist in two valence states in this system, may also be involved in a redox-type electron exchange mechanism with Bi and Cu. The results of Bi 4f, Pb 4f and Cu 2p core-level spectra of Pb-doped a nd Pb-free compounds are discussed.