Cohesion energy and structural phase stability inLa2CuO4: The orthorhombic state

Using an atom-atom pair potential, including Coulomb, short-range repulsion, and van der Waals terms, we analyze the zero-temperature cohesive energy ${\mathit{E}}_{\mathit{c}}$ of ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ both in its tetragonal and orthorhombic phases. In the tetragonal structure we emphasize that the largest part of ${\mathit{E}}_{\mathit{c}}$ is due to interplane interactions, with the intraplane ones being as a whole repulsive. A model in which the ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ units are approximated by electrical quadrupoles, with a single component along their long axis, qualitatively accounts for this result. The orthorhombic phase is found to be more stable than the tetragonal one, and the structural parameters are reproduced with an accuracy better than 0.1 Å. An important improvement, yielding an accuracy of about 0.01 Å, is achieved by introducing in the total energy a term representing covalency effects. We show that such a description accounts rather well for the observed values of the compressibility coefficients, as well as for the critical pressure for destabilization of the orthorhombic phase. The microscopic interactions responsible for the orthorhombic phase stability are studied in detail, with use of a phase-transition-like approach. A Landau expansion of the total energy with respect to the order parameters is obtained numerically and the importance of the coupling between the tilt and the movements of the lanthanum atoms to stabilize the distorted phase is emphasized. The shear distortion of the ${\mathrm{CuO}}_4$ basal planes of the units and the readjustments of all the tetragonal structural parameters are shown to be secondary order parameters. The destabilization of the orthorhombic phase under pressure is also analyzed from a microscopic point of view.