  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.
