THE BISMUTH OXYCARBONATE BI2SR4CU2CO3O8 - A NEW SUPERCONDUCTOR WITH A TC(ONSET) OF 30-K

A new high-T(c) superconductor, Bi2SrCu2CO3O8, with a T(c onset) of 30 K, has been synthesized. The XRD and HREM studies of this oxycarbonate show that it exhibits an orthorhombic incommensurate structure with a=5.466 angstrom, b=qX5.46 angstrom, c=39.50 angstrom and q=1/k= 8.85 to 9. A structural model is proposed which corresponds to a regular intergrowth of undulating 2201-type lavers Bi2Sr2CuO6, with Sr2CuO2CO3-type layers, so that it represents the n=n'=1 member of the intergrowth series (Bi2Sr2CuO6)n. (Sr2CuO2CO3)n'. Though it is characterized by a low Meissner volume fraction of about 5%, this new phase is of great interest for the understanding of superconductivity in these compounds, since the two components of this structure are either not superconducting, for Sr2CuO2CO3, or exhibit a lower T(c) of 22 K for Bi2Sr2CuO6; moreover, the fact thal no superconductivity has been observed in the Bi-Sr-Cu-O system above 22 K shows that tbe introduction of CO3 groups in this system enhances superconductivity. The possible synthesis of new high-T(c), superconductors Bi2nA4nCu3n-1CO3)11n-3 (A=Sr, La ) is discussed.