Anomalous Thermoelectric Power Behaviors of Iodine Intercalated (Bi,Pb)₂Sr₂Ca_n-1Cu_nO _2n+4+δSuperconductors (n=2 and 3)

Thermoelectric powers (TEPs) of the pristine (Bi,Pb)₂Sr₂Ca_n-1Cu_nO _2n+4+δ(n=2 and 3) compounds and their iodine intercalates were measured as a function of temperature, in order to investigate the effect of intercalation on the conduction mechanism of the host compounds. From the comparative measurements, it is revealed that the iodine intercalation gives rise to different evolution of TEP depending upon the host lattice, i.e., a downward shift for Bi₂Sr₂CaCu₂O_8+δand an upward shift for Bi_2-xPb_xSr₂Ca₂Cu₃O _10+δ. In order to elucidate an origin of such contrasting TEP behaviors in both intercalates, the evolution of electronic structure upon intercalation has been examined by performing the systematic micro-Raman and X-ray absorption near edge structure (XANES) analyses. According to the Raman and I L_I-edge XANES studies, it is found that there is a partial electron transfer from host lattice to guest layer that is the same for both intercalates, which is in contrast with their opposite shifts of TEP. The present Cu K- and Bi L_III-edge XANES results also indicate the cooxidation of CuO₂and BiO layers for both intercalates, whereas the Pb L_III-edge XANES analysis makes it clear that the Pb^+IVion in Bi_2-xPb_xSr₂Ca₂Cu₃O _10+δis completely reduced to Pb^+IIion upon intercalation, leading to a decrease of hole contribution to TEP and consequently to an upward shift of TEP. From the present experimental findings, it becomes clear that a simple application of the universal plot of TEP (300 K) vs hole density may result in an inaccurate estimation of the oxidation state of CuO₂layer.