FLUORINATION OF MERCURY-BASED HIGH-TEMPERATURE SUPERCONDUCTORS

There are numerous reports of cationic substitutions on the mercurocup rate very high-temperature superconductors of general formula HgBa2Can -1CunO2n+2+delta, Such investigations have been carried out in an atte mpt to mimic, within chemical substitution at room pressure, the spect acular effect of external pressure, which has raised the superconducti ng transition temperature, T-c, to record-high values above 160 K. In the present work, we report the post-synthetic incorporation of elemen tal 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 oxygenatio n process in giving maximum T-c values of 96, 128 and 135 K for the si ngle, double and triple CuO2-layer materials, respectively, while the T-c of the five CuO2-layer compound are increased to an extent only pr eviously observed by high-pressure oxygenation. We comment upon a deri ved linear correlation of T-c with the crystal a-lattice parameter and compare this behaviour against that of high-pressure studies. Interes tingly, it appears that intrinsic 'chemical factors' dictating the ave rage copper oxidation state have the more marked effect upon T-c than the application of external hydrostatic pressure. (C) 1997 Elsevier Sc ience B.V.