Origin of ferroelectricity in the multiferroic barium fluorides BaMF4

TL;DR

This study uses first principles calculations to investigate the origin of ferroelectricity in BaMF4 multiferroics, revealing it arises from a polar phonon mode without charge transfer, driven by size effects and structural constraints.

Contribution

It identifies the mechanism of ferroelectricity in BaMF4 compounds as arising from a polar phonon mode, distinct from charge transfer mechanisms in perovskites.

Findings

Ferroelectricity results from a single unstable polar phonon mode.

No charge transfer occurs during the structural distortion.

Size effects and structural geometry drive the ferroelectric instability.

Abstract

We present a first principles study of the series of multiferroic barium fluorides with the composition BaMF4, where M is Mn, Fe, Co, or Ni. We discuss trends in the structural, electronic, and magnetic properties, and we show that the ferroelectricity in these systems results from the "freezing in" of a single unstable polar phonon mode. In contrast to the case of the standard perovskite ferroelectrics, this structural distortion is not accompanied by charge transfer between cations and anions. Thus, the ferroelectric instability in the multiferroic barium fluorides arises solely due to size effects and the special geometrical constraints of the underlying crystal structure.