Structure and superconductivity of the HgBa2Ca3Cu4O10+delta phase

A bulk superconducting HgBa2Ca3Cu4O10+delta phase (Hg-1234) was obtained by a high-pressure high-temperature technique in a polyphasic sample containing approximate to 15% of impurities. The lattice parameters of the as-synthesized phase were found to be a = 3.8524(4) Angstrom, c = 18.942(5) Angstrom, with a T-c,T-onset of 116 K. Subsequent heat treatments showed that only strong reducing or oxidizing conditions allow one to obtain a major variation of lattice parameters and of T-c,T-onset, while heat treatments in 1 bar oxygen do not produce any significant change. The structural model refined by X-ray Rietveld powder diffraction showed a large deficiency of the Hg site [0.79(1)], relatively high thermal parameters for Hg and Ba, and an unusual shea Hg-O-ap distance. All these features can be explained by the presence of carbon replacing Hg in the as-prepared Hg-1234 phase.