Crystal structures of Hg-Sr-Ca-Cu-O superconductors with enhanced flux pinning: Hg1-xRexSr2Can-1CunO2n+2+delta(n=2,3; x approximate to 0.2-0.25)

The crystal structures of Hg1-xRexSrxCan-1CunO2n+2+delta (n=2,3; x approximate to 0.2-0.25) (synthesized at high pressure) were refined using neutron powder diffraction and the Rietveld analysis technique. Superconductivity was not observed in the as-prepared n=2 compound while a T-c of 107 K was measured for the n=3 sample. Postannealing in evacuated tubes results in creating superconductivity in the n=2 sample with a T-c of 90 K and in enhancing it up to 120 K in the n=3 sample. The structures of both as-prepared and vacuum-annealed samples were investigated. In both compounds, Re substitutes at the Hg site. Four additional oxygen atoms are incorporated into the (Hg, Re) plane for each Re atom providing an octahedral environment for Re. Thus, for a composition of x approximate to 0.25, all available oxygen sites in the (Hg, Re) plane are filled. These oxygen atoms shift off the ideal position to form bonds of appropriate length (1.88 Angstrom) with Re. Vacuum-annealed samples show oxygen deficiency in the (Hg, Re) layer suggesting that the actual Re content is below the solubility limit. The enhanced flux pinning for these chemically substituted compounds could result from the observed shortening of the blocking layer by 0.8-0.9 Angstrom and/or from increased interlayer coupling through the blocking layer by possible improvement in electrical conduction. Additionally, Hg-rich regions and/or planar defects (faults) separating domains where the Re-O-6 octahedra are ordered in different ways could act as pinning centers.