New family of Au-based superconductors AuBa2Can-1CunO2n+3 (n=3, 4)

AuBa2Ca2Cu3O9 (Au-1223-Ba) and AuBa2Ca3Cu4O11 (Au-1234-Ba), two high-T-c su perconductors, were synthesized by high-pressure/high-temperature at 6 GPa and 1250-1300 degreesC. These phases are the third and fourth members of th e AuBa2Can-1CunO2n+3 series. The Au-1223-Ba and Au-1234-Ba phases crystalli ze in an orthorhombic primitive system with the lattice parameters a = 3.81 82(4) Angstrom, b = 3.8555(4) Angstrom, and c = 15.445(2) Angstrom, and a = 3.8266(3) A, b = 3.8505(3) A, and c = 18.494(1) A, respectively. The Au-12 34-Ba phase showed bulk superconductivity below 99 K. The superconducting t ransition of the as-grown Au-1223-Ba sample was very broad with a transitio n temperature of approximate to 30 K and with a small superconducting volum e fraction. The volume fraction increased after the sample was postannealed at 300 degreesC in O-2 atmosphere.