Abstract
Inhomogeneous superconductivity with TC above 90 K was recently reported in PrBa2Cu3O7-y single crystals grown by TSFZ method. The possible metal-insulator transition is suggested to be due to the variation of the oxygen parameter y. To verify this suggestion, we tried some different sample preparation routes including direct quenching, vacuum annealing, flowing-oxygen annealing, and high oxygen pressure annealing on the as-sintered bulk samples to control the parameter y, especially in the high (y ~ 0.1) and low (y ~ 0.8) oxygen content regions. Structural analysis indicates that there is an orthorhombic to tetragonal phase transition around y = 0.5, while at y ~ 0 and y ~ 0.9 the system becomes unstable and impurities reveal in X-ray diffraction patterns. Electrical resistivity measurements indicate that with oxygen content decreases the system becomes more insolating, which is resulted from the reduction of conducting hole concentration. Magnetic susceptibility data show that the anomalous Pr ordering temperature TN(Pr) decreases monotonically with decreasing oxygen content, regardless of the structural transition. This variation of TN(Pr) is believed closely correlated with the wave function overlap between Pr-4f and O-2pp orbital in the CuO2 plane. The result shows that the metal-insulator transition, or even superconductivity, may occur at fully oxygenated state (y ~ 0). Sample preparation with more carefully controlling the annealing pressure, temperature and time may be crucial for the observation of superconductivity in PrBa2Cu3O7-y.