Nonlocal charge-transfer effects in the lattice dynamics of classical ionic crystals

A formalism recently developed to study nonlocal, long-ranged electron-phonon interaction effects of ionic origin in the high-temperature superconductors is applied to investigate the question if similar, in all probability weaker effects, are already present in classical ionic crystals like MgO. In a first step an ab initio rigid-ion model with nominal ionic charges is set up to calculate the lattice constant and the phonon dispersion. improvement of the model is achieved by taking ion softening into account in terms of effective ionic charge as calculated from a tight-binding analysis of the electronic band structure for MgO in the local-density approximation. On the basis of such a band structure the electronic polarizability in the tight-binding representation is established and nonlocal screening effects of the charge-fluctuation type are considered. Then, the influence of this type of nonlocal electron-phonon interaction effect on the phonon dispersion of a classical ionic crystal, like MgO, is investigated. The result for MgO and most likely for all classical ionic crystals is less dramatic as compared with the situation found for the high-temperature superconductors. Most important is the renormalization of the LO modes resulting in a reduction of the LO-TO splitting of about 15% at the Gamma point and the renormalization of the LA phonon at the L point.