Superconductivity in Y(Ba1-yLay)2Cu3O9-δ

The superconducting properties of the Y(Ba1-yLay)2Cu3O9-δ phase (note La solely substituting Ba) are studied over the entire homogeneity range, viz., 0.00 < y < 0.36(2) and 1.69(2) < δ < 3.00(2) for well-defined samples prepared by the citrate gel method and annealed at controlled partial pressures of O2. Superconductivity (for Tc > 5 K, by ac susceptibility) is observed over a large range of compositions. In terms of oxygen content, the lower limit for superconductivity varies from 9-δ ∼ 6.3 for y = 0.00 to 9-δ ∼ 7.05 for y = 0.20. Tc increases with increasing oxygen content (referring to the oxygen saturation limit 9-δ = 6.95(1) + y for 320C at PO2 = 100 kPa), irrespective of the linkage of the copper-oxygen square chains when 9-δ exceeds 7. For y = const., Tc is hence correlated with the overall hole concentration p = (5 - 2δ - 2y)/3 per (Cu-O)p+, and these correlations are in turn related to deformation of the structure. For p = const., Tc decreases with increasing y, and a hole transfer from the Cu(2) to Cu(1) coordination sphere appears to be composed of: (1) a large contribution due to an increased number of oxygens bonded to Cu(1), (2) a smaller contribution associated with a decrease in orthorhombic deformation, and (3) an even smaller contribution [about half and of opposite sign to (2)] attached to decreased tetragonal deformation. The effects of La substitution are compared with those of Sr substitution, application of external pressure, and change in oxygen content. The occurrence of superconductivity is correlated with fractional occupancy of a single, non-bonding orbital leading to a metallic state, and this picture is discussed for various oxide systems