Incorporation of an ionic conducting alkali-metal halide into a two-dimensional copper oxide lattice through a novel stepwise intercalation route

A new lithium iodide intercalation complex, Li_0.3I _1.0Bi₂Sr_1.4La_0.1Ca _1.5Cu₂O_y, has been synthesized by applying a novel stepwise intercalation technique. According to X-ray diffraction analysis, lithium iodide species are stabilized between Bi₂O₂ double layers, leading to an increase of basal spacing of ∼3.4 Å per host layer. X-ray photoelectron and X-ray absorption spectroscopic analyses reveal that there is little charge transfer between the intercalant lithium iodide layer and host lattice. In this context, the observed minute change of T_c upon Li-I intercalation can be regarded as evidence of the negligible contribution of interlayer coupling to superconductivity. On the other hand, ac impedance analysis demonstrates that the intercalation complex of lithium iodide species shows a Li⁺ ionic conductivity in the order of ∼10^-6Ω^-1 cm^-1 at a low temperature of <70°C. In combination with an excellent electronic conductivity of the host lattice, this material can be a new type of mixed Li⁺ ionic-electronic conductor. From the viewpoint of synthesis, the present compound is the first example of the stabilization of an alkali-metal halide in the two-dimensional metal oxide lattice.