Characterization of the High T_c Superconductor (Ba_0.7Y_0.3)CuO_3-δ

By establishing a partial phase diagram of the Ba-Y-Cu-O system, we synthesized a single-phased sample with T c above 90 K. The crystal structure was determined to be an orthorhombic derivative of the perovskite. It was also found that there exist several other phases competing with the high T c phase. Among them a tetragonal phase and another orthorhombic phase were obtained in a single-phased state, the former being non-superconducting above 1.3 K, while the latter superconducting below 60 K. The effects of composition and sintering conditions on the physical properties of these phases are briefly reported.