Abstract
A study of precise oxygen concentration dependence of the superconducting volume fraction in Ba2YCu3Oy (6.22≤y≤6.96) revealed the phase diagram of this superconducting system. Three characteristic phases are found based on the oxygen concentration: (1) a nonsuperconducting tetragonal phase for 6.2<y<6.4, (2) a low-Tc (Tc= 55∼70 K) orthorhombic phase for 6.4≤y<6.8 and (3) a high-Tc (Tc=90 K) orthorhombic phase for 6.8≤y<7.0. Evidence for a distinct critical concentration at y=6.7∼6.8, necessary to sustain the bulk high-Tc (90 K) superconductivity, is presented.