Body-centered tetragonal ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ is shown to have its electronic structure and properties dominated by the layered in-plane Cu-3d–O-2p interactions. A strong Fermi surface instability along [110] with ‖q‖=2$k_{\mathrm{F}}$ leads, via a soft-phonon mode, to the observed orthorhombic phase and accounts for its semiconducting properties. The addition of divalent metals, i.e., Ba, Sr, suppresses the instability and stabilizes the tetragonal phase where the same soft-phonon branch apparently contributes to a large electron-phonon interaction and a high $T_c$.

• Received 9 January 1987

DOI:https://doi.org/10.1103/PhysRevLett.58.1035