We used Zn and Ga, two unique dopants having definite valence states and filled 3d levels, to substitute preferentially on the Cu(2) and Cu(1) sites in the ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ superconductor. Small doping of Ga induces an orthorhombic-to-tetragonal structural transition, but the values of ${\mathrm{T}}_{\mathrm{c}}$ are as high as 81 K in the tetragonal phase. Our results indicate that the oxygen-vacancy order, or the linear-chain structure, is not essential. We also find that the Cu(2) sites are much more crucial than the Cu(1) sites in sustaining the high ${\mathrm{T}}_{\mathrm{c}}$.

• Received 2 November 1987

DOI:https://doi.org/10.1103/PhysRevLett.60.1446