Nano-engineering via intercalation: Layer-by-layer interstratification of high-T_c superconducting and superionic materials Ag_xI _yBi₂Sr₂Ca_n-1Cu_nO _2n+4 (n=1, 2, and 3)

Through intercalation, we have successfully synthesized new materials having layer-by-layer stacking of superconducting and superionic conductor, Ag_xI_yBi₂Sr₂Ca_n-1Cu _nO_2n+4 (n equals 1, 2 and 3). According to powder x-ray diffraction analysis, the lattice expansions along the c-axis upon intercalation of Ag-I are approximately 7.3 angstrom independent on various x values. Superconducting properties were maintained after the intercalation with slight T_c depressions. In order to investigate the evolution of electronic and crystal structures of host compound upon Ag-I intercalation and to determine the intracrystalline structure of the intercalated silver iodide, extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge structure (XANES) analyses have been performed. According to the nonlinear curve fitting for the Ag K-edge EXAFS spectra, the coordination number for silver in the Ag-I intercalate has been determined to be 4, which is in good agreement with the previous results for Ag⁺ ionic conductors. This strongly suggests that our new compounds may also have AG⁺ ionic conductivity, which has been measured by the a.c. impedance spectroscopy and the pulsed method. All the intercalates have been proved to be mixed conductors with substantial ionic contributions. For example, the ionic conductivity of Ag_xI _yBi₂Sr₂CaCu₂O₈ (x equals 1.09) is about 10^-1.5 (ohm cm)^-1 at 270 degrees C, which is comparable to those of other Ag⁺ superionic conductors such as AG⁺-(beta) - alumina and Py₅Ag₁₈I₂₃ (Py equals C₅H₅HN), with an ionic transference number of 0.40.