High-temperature superconductivity was observed in the system (${\mathit{M}}_{\mathit{x}}$${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$)${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.5+\mathrm{\delta }}$) where M = Ga, Bi, or Fe. Ga and Fe substituted for Y reduce the orthorhombic distortion in the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.5+\mathrm{\delta }}$ tetragonal lattice. The changes in transition temperature at zero resistance are not related to gradual changes in the orthorhombic distortion. High $T_c$ superconductivity ($T_c^{R=0\mathrm{}}$=86 K) can be maintained in an undistorted tetragonal lattice. Bi substituted for Y does not substantially change the lattice parameters of the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.5\mathrm{-}\mathrm{\delta }}$ compound.

• Received 8 October 1987

DOI:https://doi.org/10.1103/PhysRevB.37.7510