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)-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 Å, according to thermal treatment length. A comparison with other cubic phases, uncommon in superconducting systems, and in particular Ba4CaCu3O8.25, together with electron microprobe analysis, allowed us to determine a composition Hg2Re0.5Ba4Ca1.5O8.5 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.