${\mathrm{La}}_{1.85}$${\mathrm{Ba}}_{0.15}$${\mathrm{CuO}}_4$ exhibits high-${\mathrm{T}}_{\mathrm{c}}$ superconductivity with an onset temperature near 33 K as measured by resistivity and magnetic susceptibility. The nonsuperconducting end-member compound, ${\mathrm{La}}_2$${\mathrm{CuO}}_4$, which has an orthorhombic, Cmca structure, exhibits semiconducting behavior. The orthorhombic distortion is proposed to result from a Peierls 2${\mathrm{k}}_{\mathrm{F}}$ instability or a soft zone-boundary phonon mode. The role of Ba is to suppress the instability and stabilize a higher-symmetry tetragonal, I4/mmm structure which is metallic and superconducting.

• Received 9 February 1987

DOI:https://doi.org/10.1103/PhysRevLett.58.1024