Influence of host lattice on the chemical bonding nature of guest species in high-T_c superconducting I-Bi₂Sr_1.5-xLa _xCa_1.5Cu₂O_y nanohybrids

The influence of the host lattice on the chemical bonding nature of guest iodine species in high-T_c superconducting I - Bi₂Sr _1.5-xLa_xCa_1.5Cu₂O_y nanohybrids has been examined by performing combinative X-ray photoelectron spectroscopic (XPS) and X-ray absorption spectroscopic (XAS) analyses. The iodine species were found to possess two kinds of oxidation states, negatively charged iodine (I^δ-) and highly oxidized heptavalent iodine (I^+VII). The concentration of heptavalent iodine species is significantly increased by replacing Sr^+II ions with La ^+III ions, indicating that excess oxygen in the host lattice oxidizes the intercalated iodine into heptavalent iodine species. From mass spectroscopic analysis for disintercalated species, it is certain that the heptavalent iodine exists as a periodate IO₄^- cluster in easily removable sites such as the edge or surface of crystallite. The I L _I-edge XAS analyses reveal that even in heavily La-substituted compounds the content of the negatively charged iodide species is still greater than that of the periodate species. The degree of charge transfer between host and guest is also found to depend on La content, which makes it possible to explain the T_c behavior of iodine intercalates with respect to La substitution. The present study provides an intriguing example for the possible control of the bonding nature of guest species through a modification of the composition of the host lattice.