SINGLE-CRYSTAL STUDY OF THE 1201-SUPERCONDUCTOR HG0.8BI0.2BA2CUO4+DELTA

Single crystals of the ''1201'' mercury based cuprate have been grown for the first time. Their structure determination allows the classical Jahn-Teller distortion of the octahedral copper layer to be confirmed and gives evidence for the presence of oxygen at the level of the Hg/ Bi layer leading to the formula Hg0.8Bi0.2Ba2CuO4.3. The [Hg0.8Bi0.2Od elta](infinity) layers can then be described as built up from HgO2 sti cks that coexist with BiO6 octahedra, bismuth being nearly pentavalent . The magnetic study of these crystals shows that they exhibit superco nductivity up to 75 K (T-c(midpoint = 62 K)), with 100% superconductin g volume fraction at low temperature. Moreover the M(B) loops exhibit fishtail shapes which were not present for measurements on ceramics. T he fishtail and irreversibility links of (Hg, Bi)-1201 are studied for the H parallel to c geometry. These data are compared with those of T l-single crystals; it appears that (Hg, Bi)-1201 is quite similar to T l monolayers and differs greatly from Tl bilayers. This underlines the influence of structural anisotropy in both Hg- and Tl-cuprates on the ir superconducting properties.