The physics of dynamical atomic charges: the case of ABO3 compounds

TL;DR

This paper investigates the nature of dynamical atomic charges in ABO3 compounds, especially BaTiO3, contrasting static and dynamical charge concepts, and analyzing their origins and implications for ferroelectric properties.

Contribution

It provides a unified approach to understanding dynamical charges in periodic solids and large clusters, clarifying their physical origin and relation to atomic polarizability and charge transfer.

Findings

Static and dynamical charges are driven by different parameters.

Ab initio calculations support charge transfer as the origin of anomalous effective charges.

Estimated spontaneous polarization in barium titanate phases.

Abstract

Based on recent first-principles computations in perovskite compounds, especially BaTiO3, we examine the significance of the Born effective charge concept and contrast it with other atomic charge definitions, either static (Mulliken, Bader...) or dynamical (Callen, Szigeti...). It is shown that static and dynamical charges are not driven by the same underlying parameters. A unified treatment of dynamical charges in periodic solids and large clusters is proposed. The origin of the difference between static and dynamical charges is discussed in terms of local polarizability and delocalized transfers of charge: local models succeed in reproducing anomalous effective charges thanks to large atomic polarizabilities but, in ABO3 compounds, ab initio calculations favor the physical picture based upon transfer of charges. Various results concerning barium and strontium titanates are presented. The origin of anomalous Born effective charges is discussed thanks to a band-by-band decomposition which allows to identify the displacement of the Wannier center of separated bands induced by an atomic displacement. The sensitivity of the Born effective charges to microscopic and macroscopic strains is examined. Finally, we estimate the spontaneous polarization in the four phases of barium titanate.