SUPERCONDUCTIVITY AND X-RAY PHOTOELECTRON-SPECTROSCOPY STUDIES OF BI2SR2-XLAXCACU2O8+DELTA

The crystal structure (determined from X-ray diffraction measurements) , the superconducting properties (from resistivity measurements) and e lectronic structure (from X-ray photoelectron spectroscopy investigati on) of Bi2Sr2-xLaxCaCu2O8 compounds (substituted on the strontium site ) have been studied for different lanthanum concentrations. A systemat ic increase in the lattice parameter a and a decrease in the c-axis wi th increasing x was observed. The rho-T characteristics for different x showed that T-c increases up to 91 K (x = 0.2) then falls for higher doping, indicating that the substitution induces a decrease in the ho re density. The superconductor-insulator transition occurred at a hig her doping level (x = 0.7) than in the case of a substitution for calc ium. This behaviour has been correlated to the change in the electroni c structure. This was revealed by a lowering of the Fermi level and a shift towards lower energy of the core level X-ray photoelectron spect ra Bi 4f. The different effect of the strontium substituted site in co mparison with that of the calcium site has been tentatively explained by their respective crystallographic situations giving rise to differe nt charge transfer and coupling between Bi-O and Cu-O-2 planes.