The Orthorhombic to Tetragonal Transition under Controlled Oxygen Content in High-T_c Superconductor YBa₂Cu₃O_y

Abstract

The orthorhombic to tetragonal phase transition in YBa 2 Cu 3 O 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.