Crystal structure data on a new cubic phase acting as antagonist in the synthesis of Hg(Re)-1223 superconductor

During the attempts to improve sealed-quartz tube synthesis of Hg(Re)-1223 superconductor, a new compound was systematically identified in certain unsuccessful experimental conditions. It was characterized by a well distinct powder diffraction pattern with cubic diamond-like extinctions and lattice parameter 8.48-8.53 Å. The composition was determined as Hg₂Re_0.5Ba₄Ca_1.5O_8.5 and an univocal interpretation was found in terms of the high-symmetry space group Fm3m(225), with Z=2, (=6.70 g/cm³; the reliability factor was 5.0%. This phase, Cu-free and not superconductive, rather than a precursor can be considered as an unwanted accident in Hg(Re)-1223 preparation route. Its existence was confirmed by a successful synthesis starting from the above reported stoichiometry.