Dipole polarization and charge transfer effects in the lattice dynamics and dielectric properties of ionic crystals

Abstract

We extend our formalism of an ab initio description of electronic screening in solids with a strong ionic component of bonding by introducing electronic density polarization effects of dipole type in addition to charge transfer polarization processes. The latter have been found earlier to be very important for the lattice dynamics and the electron-phonon interaction in the high-temperature superconductors. The dipole deformations are calculated with the Sternheimer method and the influence on the polarizability induced by the crystalline environment and by self-interaction corrections is studied. We further derive detailed expressions for the various coupling coefficients appearing in the dynamical matrix in our formalism and apply this theory in a first step in view of the ultimate goal, namely, the high-temperature superconductors, to the calculation of complete dispersion curves for the alkali halide NaCl and the alkaline-earth oxide MgO taking into account for the first time dipole polarization and charge transfer effects together. The agreement of our calculated results with the experiments is good. As a main effect we find a renormalization of the LO modes and a reduction of the LO-TO splitting by both types of polarization processes. The dipole polarization, however, dominates the electronic screening by far. Finally, we investigate the macroscopic dielectric constant and the transverse effective charges and discuss the results in context with the Clausius-Mossotti relation.