Calculated phonon dispersion of infinite-layer compounds and the effects of charge fluctuations

In a first step we use an ab initio rigid-ion model (RIM) to calculate the lattice parameters and the phonon dispersion of the infinite-layer compounds CaCuO₂, SrCuO₂, and BaCuO₂. We find an increase of both the planar and the axial lattice constant when going from CaCuO₂ through SrCuO₂ to BaCuO₂. The rate of increase of the planar lattice constant with respect to the alkaline-earth ionic radius is calculated to be smaller for the replacement of Sr by Ba than for the replacement of Ca by Sr. Both results are in accordance with experimental studies. The phonon dispersion in the RIM exhibits several unstable branches mainly related to axial displacements of the oxygens, indicating the tendency of the crystal to reconstruct in a lower-symmetry structure. The structural stability increases, however, towards BaCuO₂; simultaneously, the maximum phonon frequency decreases. AnA_2uzone-center mode with very large LO-TO-splitting exists in all three compounds (ferroelectric mode). In a second step charge fluctuations (CF) are taken into account at the copper- and oxygen ions, using SrCuO₂ as an example. Due to the vanishing of the ferroelectric split a branch with very steep dispersion forms in the [001] direction in the metallic phase whereas the zone-centerA_2umodes are unchanged in the insulating phase because of the two-dimensional (2D) electronic structure assumed. Characteristic nonlocal electron-phonon-interaction effects are associated with theZ-point Sr-axial-breathing mode: CF of uniform sign within the CuO planes but alternating sign in consecutive planes do occur in the metalic phase. This interplane charge transfer is, on the other hand, suppressed in the insulating phase due to the 2D electronic structure assumed. Instead, large induced site-potential changes emerge in this case.