Structure and Born effective charge determination for planar-zigzag <beta>-poly(vinylidene fluoride) using density-functional theory

TL;DR

This study uses density-functional theory to identify the preferred planar-zigzag structure of b-PVDF and calculates its dynamic Born effective charges, revealing atomic-level origins of its ferroelectricity and charge transfer behavior.

Contribution

First determination of dynamic Born effective charges for b-PVDF's planar-zigzag structure using a Berry-phase approach, clarifying atomic origins of ferroelectricity.

Findings

Planar-zigzag structure is energetically preferred.

Dynamic Born effective charges differ from static Mulliken charges.

Atomic-motion-induced charge transfer explains ferroelectricity.

Abstract

Two structures have been proposed in the literature for the b-phase of the ferroelectric polymer, poly(vinylidene fluoride) (b-PVDF); planar-zigzag and alternatively-deflected forms. Using density-functional theory, we have found the planar-zigzag structure is the preferred form and upon atomic relaxation, the alternatively-deflected structure attains a structure very similar to the planar-zigzag structure. In order to better understand the atomic origin of the ferroelectricity in b-PVDF, we have for the first time determined the dynamic Born effective charges (Z*) for the planar-zigzag structure using a Berry-phase approach. When compared to their nominal ionic values, the Z* show anomalous differences. Using these effective charges, we describe the polarity of the bonds with b-PVDF and show the extent of atomic-motion-induced (or dynamic) charge transfer within this ferroelectric material. In addition, our effective charges are different to previously-determined Mulliken charges, due to the inherent differences between static and dynamic charges.