Use and Limitations of the Shell Model in Calculations on Perovskites

摘要

A conventional shell model (SM) fitted to static lattice properties and just one phonon frequency, namely that of the transverse Raman frequency at the R point, has been used to calculate the phonon dispersion in cubic strontium titanate at 298 K along principal high-symmetry directions in the Brillouin zone. The agreement between these calculated data and experimental results from neutron scattering is fully comparable with that of SMs actually fitted to experimental neutron data. The usual SM cannot account for the strong temperature dependence of the soft-mode frequency and the static permittivity, but it is shown that this temperature dependence can be accounted for by small continuous variations in just two of the SM parameters. While purely phenomenological at present, these variations point to the strong dependence of metal-oxygen bonding on lattice constant.