AuBa2(Y1-x,Ca-x)Cu2O7: A new superconducting gold cuprate with T-c above 80 K

We have synthesised by the high pressure-high temperature technique a new layered gold cuprate of chemical formula AuBa2(Y1-x,Ca-x)Cu2O7 (0 less than or equal to x less than or equal to 0.4) (hereafter denoted as Au-1212). The structural properties of this compound are investigated by means of electron microscopy and powder X-ray diffraction. The average structure (orthorhombic Pmmm, a = 3.8298(5) Angstrom, b = 3.8420(5) Angstrom, c = 12.111(1) Angstrom for nominal composition x = 0.4) is very similar to that of YBa2Cu3O7, with Au3+ cations in square planar coordination sharing corners to form chains running along the b axis. Electron microscopy reveals the presence of a superstructure with double b parameter, which is attributed to a long range ordered zig-zag conformation of the square chains. For x = 0, the formal copper valence is 2 + and the compound is insulating. Doping of the conducting CuO2 planes is achieved by the partial substitution of yttrium by calcium, up to x approximate to 0.4. Upon substitution, the compound becomes superconducting with a maximum T-c at similar to 82 K for x approximate to 0.4.