PbBa2Y1-xCaxCu307+delta:synthesis, structure and superconductivity

A new phase, PbBa2YCu3O7+delta has been synthesized and characterized. Although its structure is similar to that of the chemically related PbBaSrYCu3O7+delta phase, neutron powder diffraction data have revealed significant structural differences. The unit cell is orthorhombic, with space group Fmm2, a = 5.4712(1) Angstrom, b = 5.4597(1) Angstrom, c = 27.8943(4) Angstrom. Although non-superconducting in this form, Ca-doping on the Y sites induces a transition from orthorhombic to tetragonal symmetry and the material becomes superconducting with T-c = 90 K. For both Undoped and Ca-doped materials, the oxygen content increases significantly (to similar to8.3 per formula unit) when heated at 450 degreesC in oxygen. Oxidation causes partial cation disorder within the structure and results mainly in oxidation of Pb and Cu ions, which are not in the superconducting blocks. Superconductivity is also observed in Ca-doped oxidized samples.