A new cointercalated superconducting bismuth cuprate, (HgI₂)_0.5Bi_1.85Pb_0.35Sr_1.9Ca_2.1Cu_3.1O(10+δ)

Intercalation of HgI₂ into high-T(c) superconducting Bi_1.85Pb_0.35Sr_1.9Ca_2.1Cu_3.1O(10+δ) (Bi2223) compound is achieved by a stepwise reaction technique in which the iodine intercalate is used as a secondary host material. While the vapor transport reaction is ineffective in intercalating mercuric salts directly into Pb-doped or single crystalline Bi-based cuprates, the present stepwise reaction method can provide a useful way of intercalating larger molecules into Bi₂O₂ double layers of Bi-based cuprates. According to Hg L(III)-edge extended X-ray absorption fine structure analyses, it is found that the intercalated mercuric iodide is stabilized as a linear molecule with additional free iodine species in the interlayer space of Bi2223. Taking into account the fact that no free iodine can be found in the HgI₂ intercalated Bi₂Sr₂CaCu₂O(8+δ) polycrystal prepared by the direct intercalation method; the present experimental finding indicates that the intracrystalline structure of mercuric iodides in Bi₂O₂ double layers can be modified by controlling the synthetic route. The linear geometry of intercalated mercuric iodide has been further confirmed by performing ex-situ X-ray diffraction measurements on the deintercalation products, which clarify that the partial deintercalation of mercuric iodide leads to the formation of an intermediate phase with a parallel orientation of HgI₂ molecules with respect to the basal plane.