Orthorhombic to tetragonal transition and appearance of superconductivity in Nd0.7Pr0.3Ba2 − kSrkCu3O6+z (0≤k≤1)

Preparation, X-ray diffraction and ac susceptibility of Nd0.7Pr0.3Ba2 − kSrkCu3O6+z (0≤k≤1) are reported. An orthorhombic to tetragonal transition was observed for k≥0.75. Superconductivity occurred only for k≥0.25. When the samples were heated in argon prior to oxygen annealing both the orthorhombicity (for 0≤k ≤0.75) and Tc increased (by 10 K for k = 1). Though a weakening of Pr-f hybridization with CuO bands due to Sr substitution may explain the data, a detailed knowledge of oxygen defect chemistry will be required to understand the behavior of Pr in LnBa2Cu3O6+z(Ln = rare earths).