A new Cu-free cubic phase as competitor in the synthesis of Hg(Re)-1223 superconductor

During the attempts to improve the sealed-quartz tube synthesis of Hg(Re)-1 223 superconductor, a new compound was systematically identified under cert ain unsuccessful experimental conditions. It was characterized by a distinc t powder diffraction pattern with cubic diamond-like extinctions and lattic e parameter 8.48-8.53 Angstrom, according to thermal treatment length. A co mparison with other cubic phases, uncommon in superconducting systems, and in particular Ba4CaCu3O8.25, together with electron microprobe analysis, al lowed us to determine a composition Hg2Re0.5Ba4Ca1.5O8.5, and supplied a re liable model for its structure. However, the peculiarity of the observed pa ttern, among the apparently manifold possible answers, led to an univocal i nterpretation in terms of the high-symmetry space group Fm3m(225), with Z = 2, rho = 6.70 g/cm(3); the reliability factor was 5.0%. Indeed this phase, singularly Cu-free and clearly not superconductive, much more than a precu rsor can be considered an unwanted accident in Hg(Re)-1223 preparation rout e: its existence was confirmed by a successful synthesis starting from the above reported stoichiometry. (C) 2000 Elsevier Science Ltd. All rights res erved.