Distortion of ReO6 octahedron in the Hg0.82Re0.18Ba2Ca2Cu3O8+d superconductor

Rhenium (Re) LIII edge X-ray absorption spectroscopy was used in order to determine rhenium valence and the local oxygen coordination in Hg0.82Re0.18Ba2Ca2Cu3O8+d polycrystalline samples prepared with three different oxygen content. Thermoelectric power measurements confirmed small increments of charge carrier number as a function of oxygen content. The X-ray absorption near-edge spectroscopy (XANES) analysis showed a valence variation for Re (+6.8, +6.9 and +7.0) in these samples. The extended X-ray absorption fine structure (EXAFS) analysis indicated that the oxygen local order around Re atoms in these Hg0.82Re0.18Ba2Ca2Cu3O8+d samples can be described as a distorted ReO6 octahedron with two different Re–O bound lengths. The valence evaluated by the bound length in the distorted ReO6 octahedron is in agreement with the XANES measurements. Moreover, the distorted ReO6 octahedron and the Cu–Opl angle built a scenario which can justify the high intrinsic term value found in the optimal doped sample under external hydrostatic pressure.