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)-1223 superconductor, a new compound was systematically identified under certain unsuccessful experimental conditions. It was characterized by a distinct powder diffraction pattern with cubic diamond-like extinctions and lattice parameter 8.48-8.53 angstroms, according to thermal treatment length. A comparison with other cubic phases, uncommon in superconducting systems, and in particular Ba₄CaCu₃O_8.25, together with electron microprobe analysis, allowed us to determine a composition Hg₂Re_0.5Ba₄Ca_1.5O_8.5 and supplied a reliable model for its structure. However, the peculiarity of the observed pattern, among the apparently manifold possible answers, led to an univocal interpretation in terms of the high-symmetry space group Fm3m(225), with Z = 2, ρ = 6.70 g/cm³; the reliability factor was 5.0%. Indeed this phase, singularly Cu-free and clearly not superconductive, much more than a precursor can be considered an unwanted accident in Hg(Re)-1223 preparation route: its existence was confirmed by a successful synthesis starting from the above reported stoichiometry.