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

High-purity (97%) samples of 27-Å phase of Y-Ba-Cu-O have been prepared in superconducting thin films on (100) ${\mathrm{SrTiO}}_3$. The possibility that the observed superconductivity might arise from a residual 3% ${\mathrm{Ba}}_2$ ${\mathrm{YCu}}_3$ ${\mathrm{O}}_7$ 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-Å phase is superconducting with an onset $T_c$ of 82 K and $T_c$(R=0) of 79 K.