NONLOCAL CHARGE-TRANSFER EFFECTS IN THE LATTICE-DYNAMICS OF CLASSICALIONIC-CRYSTALS

A formalism recently developed to study nonlocal, long-ranged electron -phonon interaction effects of ionic origin in the high-temperature su perconductors is applied to investigate the question if similar, in al l probability weaker effects, are already present in classical ionic c rystals like MgO. In a first step an ab initio rigid-ion model with no minal ionic charges is set up to calculate the lattice constant and th e phonon dispersion. improvement of the model is achieved by taking io n softening into account in terms of effective ionic charge as calcula ted from a tight-binding analysis of the electronic band structure for MgO in the local-density approximation. On the basis of such a band s tructure the electronic polarizability in the tight-binding representa tion is established and nonlocal screening effects of the charge-fluct uation type are considered. Then, the influence of this type of nonloc al electron-phonon interaction effect on the phonon dispersion of a cl assical ionic crystal, like MgO, is investigated. The result for MgO a nd most likely for all classical ionic crystals is less dramatic as co mpared with the situation found for the high-temperature superconducto rs. 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 an d the renormalization of the LA phonon at the L point.