Relationship between Oxygen Deficiency and Superconductivity in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-<i>y</i></sub> Perovskite

Abstract

Ten YBa2Cu3O7-y (YBCO) perovskite samples with varying oxygen deficiency (0.15<y<1) were prepared by changing the quenching temperature or annealing conditions. With increasing quenching temperature, the transition temperature (Tc) for superconductivity decreased. This is accompanied by increasing oxygen deficiency and changes in the lattice constants. The oxygen deficiency of the sample annealed under nitrogen atmosphere at 700°C was more than that of the sample quenched at 950°C. There were no differences observed between the oxygen deficiency, lattice constants, nor superconductivity of the sample annealed under oxygen atmosphere and that cooled slowly from 350°C (0.5°C/min) in air. Annealing under high oxygen pressure caused destruction of the YBCO structure and disappearance of the superconductivity. Considering all the experimental results obtained in this study, YBCO perovskite can be classified into three types: Ortho-I (y<0.2), Ortho-II (0.2<y<0.5), and Tetra (0.5<y<1). Ortho-I type shows superconductivity with a Tc of around 90K, while the Tc onset of Ortho-II is below 70K. Although oxygen deficiency and lattice constants change continuously, a discontinuous Tc change is observed when YBCO changes from Ortho-I to Ortho-II. Tetragonal type is semiconductive and is not superconductive above 4K. The small difference in the oxygen deficiency between Ortho-I and Ortho-II indicates that it is necessary to minimize the oxygen deficiency in order to obtain a uniform material with a Tc of 90K. However, annealing under oxygen does not modify the superconductivity of samples cooled slowly.