Fluorination of mercury-based high-temperature superconductors

There are numerous reports of cationic substitutions on the mercurocuprate very high-temperature superconductors of general formula HgBa2Can-1CunO2n+2+δ. Such investigations have been carried out in an attempt to mimic, within chemical substitution at room pressure, the spectacular effect of external pressure, which has raised the superconducting transition temperature, Tc, to record-high values above 160 K. In the present work, we report the post-synthetic incorporation of elemental fluorine as an interstitial anion into the first five congeners of the mercurocuprate family of superconductors. The effect of fluorine incorporation by this route appears to mimic the more usual oxygenation process in giving maximum Tc values of 96, 128 and 135 K for the single, double and triple CuO2-layer materials, respectively, while the Tc of the five CuO2-layer compound are increased to an extent only previously observed by high-pressure oxygenation. We comment upon a derived linear correlation of Tc with the crystal a-lattice parameter and compare this behaviour against that of high-pressure studies. Interestingly, it appears that intrinsic 'chemical factors' dictating the average copper oxidation state have the more marked effect upon Tc than the application of external hydrostatic pressure. © 1997 Elsevier Science B.V