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.