Superconducting properties of a 27-Aphase of Ba-Y-Cu-O

High-purity (97%) samples of 27-A{} phase of Y-Ba-Cu-O have been prepared in superconducting thin films on (100) SrTiO₃. The possibility that the observed superconductivity might arise from a residual 3% Ba₂{YCu}₃O₇ impurity in this sample is ruled out using transmission electron microscopy which shows that it is present only as isolated particles. This allows for an unambiguous determination that the 27-A{} phase is superconducting with an onset Tc of 82 K and Tc(R=0) of 79 K.