Local Structure in Orthorhombie and Tetragonal Ba2YCu3O7-y: The Role of Oxygen Vacancies for High Tc Superconductivity

Cu valency and local structure in the orthorhombic and tetragonal phases of high T c superconductor Ba2YCu3O7-y (0.03<y<0.64) have been studied by the near-edge and extended X-ray absorption fine structure (EXAFS). Oxygen vacancies on the z = 1/2 plane modify both Cu valency and local structure. The near-edge spectra show that Cu ions at Cu1 sites in tetragonal Ba2YCu3O7-y are monovalent. From the temperature dependence of the Cu-O distance and its mean relative displacement, it is suggested that the breathing mode Cu-O stretching vibration is slightly suppressed in the tetragonal phase, which may degrade superconducting properties.