PbY₁₋xCaxBa₂Cu₃O₇F₂:synthesis, structure and superconductivity

A new phase, PbYBa2Cu3O7F2, has been synthesized and characterized. The material is obtained from the fluorination of (PbYBaCu3O7)-Cu-2 and retains structural relationships to this phase and PbYBaSrCu3O7+delta. The unit cell is tetragonal: space group 14mm, a = 3.8666(2) Angstrom, c = 29.532(2) Angstrom. Fluorine inserts into the structure in the vicinity of the linear Cu-2 species present in PbYBa2CuO7, and additional structural rearrangement converts the coordination around Cu-2 to square pyramidal (four equatorial F and one apical O). Although oxidation of PbYBa2CuO7+delta via oxygen incorporation has previously been reported to result in almost complete cation disorder between the Pb and Cu-2 cations, low temperature fluorination occurs with no loss of order. PbYBa2CuO7F2 is superconducting (T-c similar to 30 K), and Ca-doping on the Y sites results in an increase in T-c to 50 K and an enhanced superconducting volume fraction.