The orthorhombic-tetragonal structural phase transition of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ is quantitatively analyzed as a function of composition x within an anharmonic electron-phonon interaction model. The correct temperature dependence of the soft mode and the elastic constant ${\mathit{c}}_{66}$ is obtained. The double-well potential in the electron-phonon interaction is derived self-consistently and found to vary strongly with x. In the vicinity of the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ electron–two-phonon interactions dominate the harmonic ones which may explain the high ${\mathit{T}}_{\mathit{c}}$’s observed.

• Received 22 October 1990

DOI:https://doi.org/10.1103/PhysRevLett.67.512