Wannier Orbital Overlap Population (WOOP), Wannier Orbital Position Population (WOPP) and the Origin of Anomalous Dynamical Charges

TL;DR

This paper introduces WOOP and WOPP methods based on Wannier functions to analyze the chemical origins of large Born dynamical charges in transition metal oxides, revealing the key role of oxygen p orbitals in ferroelectricity.

Contribution

It develops new Wannier-based tools (WOOP and WOPP) to dissect atomic orbital contributions to dynamical charges in perovskite oxides, linking electronic structure to ferroelectric behavior.

Findings

Oxygen p orbitals perpendicular to TM-O chains dominate anomalous charges.

Transfer of tiny electronic charge between TM atoms explains large dynamical charges.

Corner-shared TMO_6 octahedra structure favors high dynamical charges and ferroelectricity.

Abstract

Most d^0 transition metal (TM) oxides exhibit anomalously large Born dynamical charges associated with off-centering or motion of atoms along the TM-O chains. To understand their chemical origin, we introduce "Wannier Orbital Overlap Population" (WOOP) and "Wannier ond Orbital Position Population" (WOPP) based on the Wannier function based description of electronic structure obtained within first-principles density functional theory. We apply these concepts in a precise analysis of anomalous dynamical charges in PbTiO_3, BaTiO_3 and BaZrO_3 in the cubic perovskite structure. Determining contributions of different atomic orbitals to the dynamical charge and their break-up into local polarizability, charge transfer and covalency, we find that p orbitals of oxygen perpendicular to the -TM-O- chain contribute most prominently to the anomalous charge, by facilitating a transfer of tiny electronic charge through one unit cell from one TM atom to the next. Our results explain why the corner-shared linkage of TMO_6 octahedra, as in the perovskite structure, is ideal for large dynamical charges and hence for ferroelectricity.