NEW SUPERCONDUCTING VANADOMERCURY CUPRATES HG0.8V0.2BA2CAM-1CUMO2M+2+DELTA

Two superconducting vanadomercury cuprates, Hg0.8V0.2Ba2CuO4+delta and Hg0.8V0.2Ba2CaCu2O6+delta have been synthesized for the first time. The XRD and HREM study of these compounds show that they exhibit a very regular layer stacking characteristic of the 1201 and 1212 structures, respectively. Nevertheless, they differ from the pure mercury based cuprates, by a significant decrease of the c parameters: a=3.8864(1) Angstrom, c=9.3387(3) Angstrom for the sample with nominal composition Hg0.8V0.2Ba2CuO4.3 and a=3.8694(2) Angstrom, c=12.5003(7) Angstrom for the sample with nominal composition Hg0.8V0.2Ba2CaCu2O6.(3). The performances of these superconductors are similar to the pure mercury-based cuprates prepared under high pressure, in spite of their structural differences, showing that the presence of vanadium on the mercury sites does not affect superconductivity. Both as-synthesized oxides exhibit a narrow transition with a T-c midpoint of 90 K and 115 K, respectively, and a high superconducting volume fraction of 70% at 70 K. The study of the influence of oxygen non-stoichiometry upon superconductivity shows that the as-synthesized cuprate with nominal composition Hg0.8V0.2Ba2CuO4.3 is nearly optimized, whereas the 1212 cuprate corresponding to the nominal composition Hg0.8V0.2Ba2CaCu2O6.3 can be optimized by oxygen annealing, leading to a T-c midpoint of 124 K. The magnetization measurements versus H, at various temperatures, show that their pinning properties are similar to those of the pure ''Ba-Hg'' cuprates that exhibit the same critical temperatures.