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-la yer'' compounds CaCuO2, SrCuO2, and BaCuO2. We find an increase of bot h the planar and the axial lattice constant when going from CaCuO2 thr ough SrCuO2 to BaCuO2. The rate of increase of the planar lattice cons tant 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. T he phonon dispersion in the RIM exhibits several unstable branches mai nly related to axial displacements of the oxygens, indicating the tend ency of the crystal to reconstruct in a lower-symmetry structure. The structural stability increases, however, towards BaCuO2; simultaneousl y, the maximum phonon frequency decreases. An A(2u) zone-center mode w ith very large LO-TO-splitting exists in all three compounds (''ferroe lectric'' mode). In a second step charge fluctuations (CF) are taken i nto account at the copper- and oxygen ions, using SrCuO2 as an example . Due to the vanishing of the ''ferroelectric split'' a branch with ve ry steep dispersion forms in the [001] direction in the metallic phase whereas the zone-center A(2u) modes are unchanged in the insulating p hase because of the two-dimensional (2D) electronic structure assumed. Characteristic nonlocal electron-phonon-interaction effects are assoc iated with the Z-point Sr-axial-breathing mode: CF of uniform sign wit hin 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 electroni c structure assumed. Instead, large induced site-potential changes eme rge in this case.