Abstract
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% Ba2YCu3O7 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. © 1988 The American Physical Society.