Valence Study of Orthorhombic and Tetragonal Ba2YCu3Oy: The Role of Oxygen Vacancies in High-Tc Superconductivity

Abstract
The valency of Cu ions in the orthorhombic and tetragonal phases of high-T c superconductor Ba2YCu3O y (6.22<y1 sites on the z=1/2 plane is strongly correlated with the high-T c (90 K) superconductivity, while the Cu valency of Cu2 sites on the basal planes affects the low-T c (50-60 K) superconductivity of the orthorhombic phase.