Study of the crystal structure of the high Tc superconductor Hg1–xRexBa2Ca2Cu3O8+δ by using EXAFS, XANES and XRD

Abstract

Hg based superconducting ceramics present the highest critical temperatures ever reported, particularly the phase 1223 with the highest Tc so far obtained –135 K. However, these compounds are obtained by a complex synthesis route and suffer degradation at ambient atmosphere. This disadvantage may be overcome by doping with an appropriate amount of Re. Samples of nominal composition Hg0.82Re0.18Ba2Ca2Cu3O8+δ having different oxygen contents were studied by X ray absorption and diffraction techniques. Using XANES we determined the Re valence = +7 in an octahedral distorted coordination. We determined the distances of Re–O bonds (1.85 Å and 2.10 Å) by EXAFS. XRD Rietveld fits showed segregation of two superconducting phases with distinct lattice parameters. The main phase is associated with Hg, Re–1223 with higher oxygen content, and the secondary superconducting phase is related to undoped Hg–1223.