Partial density approach to lattice dynamics of metals

A recently proposed theory of quasi-ions is demonstrated using sodium as an example. In this theory the total electronic charge density is decomposed in 'partial densities', which can unambiguously be assigned to individual atoms or sublattices. The partial densities together with the ion cores constitute the new building blocks (quasi-ions) into which the crystal can be decomposed. For the study of the dynamics of the crystal lattice the phonon-induced charge density variation is split up in one part that rigidly followed the motion of the ions and a second part which is due to distortions during that motion. The effects of both contributions on the phonon dispersion are investigated using special models for the distortions. Introducing small inhomegeneities to the spherical Na quasi-ions the authors further analyse the influence of such charge redistributions on the phonon dispersion. They show that the distortion part of the charge density variation leads to characteristic anomalous structures in the dispersion curves, which are rather similar to those occurring in d-electron metals.