Effect of K Substitution on Local Structure of Bi1.6Pb0.4Sr2−xKxCa2Cu3O10+δ System Probed by Extended X-Ray Absorption Fine Structure

The effect of K substitution on the local atomic structure of polycrystalline Bi1.6Pb0.4Sr2−xKxCa2Cu3O10+δ samples (BSCCO, x = 0, 0.02, 0.06, and 0.10) has been investigated. Unlike in other cases of alkaline substitution, the transition temperature Tc of the K-substituted BSCCO samples decreased systematically with increasing K content. Such decreasing Tc behavior supposedly originates from changes in the local structure of the BSCCO system. Analysis of extended x-ray absorption fine structure (EXAFS) data measured at both the Cu K-edge and Sr K-edge revealed that, as the K content was increased, the Cu–O and Cu–Sr bonds systematically extended, while the Sr–O bond distance gradually reduced. The corresponding mean-square relative displacement (MSRD), which represents the average local distortion, showed large alteration in both the CuO2 and SrO planes. Local atomic displacement due to K substitution seemed to weaken the interaction between the conduction layer and spacer layer, resulting in a drastic decrease of the Tc values. Overall, these experimental results provide important information to understand the correlation between the superconductivity and local structure of K-substituted BSCCO.