Evolution of high T_c superconductivity of Bi₄Sr_3-xLa_xCa₃Cu₄O_y upon iodine intercalation

X-ray photoelectron spectroscopic study has been performed to investigate the chemical bonding nature of the iodine intercalated in the lattice of Bi₄Sr_3-xLa_xCa₃Cu₄O_y, which is strongly correlated with the evolution of superconductivity. X-ray diffraction analyses indicate the formation of first staged iodine intercalates, I₂Bi₄Sr_3-xLa_xCa₃Cu₄O_y, whose superconducting transition temperatures were changed depending upon La substitution rate(x). X-ray photoelectron spectroscopic results exhibit the coexistence of the negative-charged iodine(I^δ-) and the highly oxidized periodate(I^+VII) with a different ratio, depending upon the amount of La substitution. The periodate species seems to increase as the increase of the La substitution rate(x), which has led to a conclusion that the excess oxygen may oxidize the intercalated iodine to a highly oxidized periodate. Remarkable depression of periodate concentration during Ar⁺-ion sputtering underlines that such a highly oxidized species are surely enriched on the surface and/or the grain boundary. T_c evolution upon iodine intercalation will be discussed in view of charge transfer and enhancement of two dimensionality.