A series of new $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$-like materials are reported in which the oxygen content and the average [Cu-O] charge (or Cu valence) are separately varied over a wide range. We find a distinct boundary between samples showing anomalous insulating behavior and those with high $T_c$. These data are interpreted to demonstrate that the [Cu-O] charge in the sheets largely determines $T_c$ and that the variable oxygen content and the chains merely provide an insulating reservoir of charge.

• Received 19 April 1988

DOI:https://doi.org/10.1103/PhysRevB.38.7156