Intracrystalline structure of molecular mercury halide intercalated in high-T(c) superconducting lattice of Bi₂Sr₂CaCu₂O(y)

X-ray absorption spectroscopic studies have been systematically carried out for the new high-T(c) superconducting intercalation compounds, (HgX₂)_0.5Bi₂Sr₂CaCu₂O(y) (X = Br and I), with a hybridized crystal lattice consisting of superconducting layer and insulating one. From the Hg L(III)-edge EXAFS analysis, it is found for the first time that the intercalated mercuric halide is stabilized as a linear molecule with the bond distance (Hg-X) of 2.46 Å for the HgBr₂ intercalate and 2.65 Å for the HgI₂ one, respectively. These are cross-confirmed not only by micro-Raman studies but also by one-dimensional electron density mapping based on (00l) X-ray reflections, which is well consistent with the EXAFS fitting results. The present I L(I)- and Br K-edge XANES results indicate a partial electron transfer from the host lattice of Bi₂Sr₂CaCu₂O(y)to the intercalant HgX₂ layer with the partial oxidations of Bi₂O₂ and CuO₂ layers. And the latter was also observed clearly from the Bi L(III)-edge XANES and Cu K-edge EXAFS analyses, which are in good agreement with the I L(I)- and Br K-edge XANES results. It is, therefore, concluded that the charge transfer between host and guest is mainly responsible for the T(c) evolution upon intercalation, and the T(c) of this compound is surely related to the variation of hole concentration rather than electronic coupling along the c-axis.