The Orthorhombic to Tetragonal Transition under Controlled Oxygen Content in High-Tc Superconductor YBa2Cu3Oy

Abstract

The orthorhombic to tetragonal phase transition in YBa₂Cu₃O _y (y=6.9-Δ y) compounds under fixed oxygen content and its relation to superconductivity were investigated by means of X-ray diffraction and resistivity measurement. In the quenching method, the phase transition was observed at about Δ y=0.62 (800°C quenching temperature). In the region Δ y=0.62–0.69, the high-temperature tetragonal phase transformed into the low-temperature orthorhombic phase by slow cooling, but at Δ y>0.69 such a transformation was not observed. No essential difference in T _c was observed between different phases having the same oxygen content in the region Δ y>0.5.