Structure of the "third phase" found on annealing the Hg-1223 superconductor precursor mixture Ba2Ca2Cu3O7 with Re addition

The recognition of the "third phase" appearing (besides BaCuO2 and ca2CuO3) after thermal treatments in the precursor mixture Re0.2Ba2Cu3O7 to prepare Hg(Re)-1223 superconductor has caused many troubles: some features of XRD powder patterns indicated that a new unsuspected phase ids forme. Such a phase was identified as ba4CuRe2O12, duly synthesised and structurally characterised: G.S. R3-m, a=5.779 A, c=28.30 A, Z=3, ro=7.165 g/cm3. it is a member of the already known rhomboedral family A4MRe2O12, but with cation vacancies and a multiplicity of possible Cu-Re vicariances, generating a number of polytypes that only suitably combined provide a satisfactory description of the observed pattern (RB=7.6%). The real structure seems to oscillate with outstanding easiness among these modifications differing solely in site occupancies.