PbY1-xCaxBa2Cu3O7F2: synthesis, structure and superconductivity

A new phase, PbYBa2Cu3O7F2, has been synthesized and characterized. The material is obtained from the fluorination of PbYBa2Cu3O7 and retains structural relationships to this phase and PbYBaSrCu3O7+δ. The unit cell is tetragonal: space group I4mm, a = 3.8666(2) A, c = 29.532(2) A. Fluorine inserts into the structure in the vicinity of the linear Cu2 species present in PbYBa2Cu3O7, and additional structural rearrangement converts the coordination around Cu2 to square pyramidal (four equatorial F and one apical O). Although oxidation of PbYBa2Cu3O7+δ via oxygen incorporation has previously been reported to result in almost complete cation disorder between the Pb and Cu2 cations, low temperature fluorination occurs with no loss of order. PbYBa2Cu3O7F2 is superconducting (Tc∼30K), and Ca-doping on the Y sites results in an increase in Tc to 50 K and an enhanced superconducting volume fraction