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.

Microscopic modeling of phonon dynamics and charge response in metallicBaBiO3

“…They are given in Ref. 15. In this context we remark that the coupling matrix C κκ ′ (q ) of the EDF-EDF interaction, whose inverse appears in Eq.…”

“…1 and in particular in Ref. 15 where the calculation of the microscopic coupling parameters is given. The local, rigid part of the electronic charge response and the EPI is approximated by an ab initio RIM taking into account ion-softening in terms of (static) effective ionic charges and scaling of the short-ranged part of certain pair potentials between the ions to simulate covalence effects in the model.…”

“…Especially in the metallic state the CF's dominate the nonlocal contribution of the charge response and the EPI. In addition DF's are admitted in our approach 5,15 . Thus, the basic variable of our model is the ionic density which is given in the perturbed state by…”

“…(1). W αβ (R, ζ) represents the short-ranged Coulomb contribution of the interaction of the density ρ α with the density ρ β and G αβ (R, ζ) is the sum of the kinetic one-particle-and the exchangecorrelation (XC) contribution of the interaction between the two ions 15 . The short-ranged part of the potentials and the various coupling coefficients are calculated numerically for a set of distances R between the ions.…”

“…In the past, this theoretical approach, which is well suited for systems with a strong component of ionic binding, has been applied successfully to the cuprate-HTSC's 1,2,3,4,5,6 , i. e., "ionic" metals in which the band crossing the Fermi level has admixture of both anion and cation orbitals, simple ionic crystals 15 and the perovskite oxides (SrTiO 3 , BaTiO 3 ) 16 . From a general point of view our treatment of the electronic density response and lattice dynamics in terms of dipole fluctuations (DF's) and CF's can be considered as a microscopic (semi-ab initio) implementation of the phenomenological dipole-shell model or the charge-fluctuation models, respectively.…”