Superionic and superconducting nanohybrids with heterostructure, Ag_xI_wBi₂Sr₂Ca_n-1Cu _nO_y (0.76 ≤ x ≤ 1.17, n = 1, 2, and 3)

New mixed conducting hybrid systems, Ag_xI_wBi₂Sr₂Ca_n-1Cu _nO_y (n = 1, 2, and 3), have been developed successfully by intercalating the superionic conducting Ag-I layer into the superconducting Bi₂Sr₂Ca_n-1Cu_nO_y lattice. Although the Ag-I intercalation gives rise to a remarkable basal increment of ∼7.3 Å, which is twice as large as that of the iodine intercalate (Δd = 3.6 Å), it has little influence on the superconducting property with only a slight T_c depression. Systematic X-ray absorption near edge structure (XANES)/extended X-ray absorption fine structure (EXAFS) studies clearly reveal the charge transfer between host and guest, indicating that a change in hole concentration of the CuO₂ layer is the main origin of T_c evolution upon intercalation. According to the ac impedance and dc relaxation analyses, the Ag_xI_wBi₂Sr₂Ca_n-1Cu _nO_y compounds possess fast ionic conductivities (σ_i = 10^-1.4-10^-2.6 Ω^-1 cm^-1 at 270°C) with the activation energies of 0.22 ± 0.02 eV, which are similar to those of other two-dimensional Ag⁺ superionic conductors. A more interesting finding is that these intercalates exhibit both high electronic and ionic conductivities with ionic transference numbers of t_i = 0.02-0.60, due to their interstratified heterostructures consisting of a superionic conducting silver iodide layer and a metallic host layer. A close relationship between local crystal structure and ionic conductivity has been elucidated from the detailed Ag K-edge EXAFS analyses, where a reasonable pathway for Ag⁺ ionic conduction is suggested along with the intracrystalline structure of the Ag-I sublattice.