Heterostructured Bi-Based Cuprate High-T_c Superconductors with Unusually Coordinated Metal Halides

Attempts have been made to prepare various high-T_c superconducting intercalation compounds, M-X-Bi₂Sr₂Ca_n-1Cu_nO _2n+4+δ (M=metal; X=halogen; n=1, 2, and 3), with the synthetic strategy of hard-soft-acid-base interaction. The chimie douce route such as vapor transport and ionic diffusion is found to be effective in intercalating soft Lewis acidic guest species (I^-₃, Ag_xI^x-w_w, HgI^δ-₂, HgBr^δ-₂, and AuI^δ-₃) into the Lewis basic Bi₂Sr₂Ca_n-1Cu_nO _2n+4+δ lattice. According to the X-ray diffraction and magnetic property measurements on these intercalates, it becomes clear that the basal increment upon intercalation (3.25-7.35 Å) is governed by the interlayer geometry of the intercalant, while the accompanying T_c change depends on the charge transfer between host and guest. The extended X-ray absorption fine structure and Raman studies reveal that mercuric halides are intercalated as linear molecules as in vapor state, while silver iodide forms a two-dimensionally extended ionic-conducting channel. In the case of new gold iodide intercalate, unique trigonal planar AuI₃ species is identified as the first example of an isolated gold iodide molecule.