Ab initio study of the ferroelectric strain dependence and 180$^\circ$ domain walls in the barium metal fluorides BaMgF$_4$ and BaZnF$_4$

TL;DR

This study uses first-principles calculations to explore how strain affects ferroelectric polarization and domain wall structures in barium fluorides, revealing differences from conventional ferroelectrics due to their geometric ferroelectricity.

Contribution

It provides the first detailed analysis of strain effects and domain wall structures in BaMgF$_4$ and BaZnF$_4$, highlighting their unique geometric ferroelectric mechanism.

Findings

Polarization varies with strain, showing distinct behavior from conventional ferroelectrics.

Domain wall energies suggest which configurations are most stable.

Comparison with perovskite oxides offers insights into non-oxide ferroelectrics.

Abstract

We investigate the strain dependence of the ferroelectric polarization and the structure of the ferroelectric domain walls in the layered perovskite-related barium fluorides, BaMF$_4$ (M=Mg, Zn). The unusual "geometric ferroelectricity" in these materials is driven by the softening of a single polar phonon mode consisting of rotations of the MF$_6$ octahedra accompanied by polar displacements of the Ba cations, and in contrast to conventional ferroelectrics involves minimal electronic rehybridization. We therefore anticipate a different strain dependence of the polarization, and alternative domain wall structures compared with those found in conventional ferroelectric materials. Using first-principles calculations based on density functional theory (DFT) within the general gradient approximation (GGA), we calculate the variation of the crystal structure and the ferroelectric polarization under both compressive and tensile strain. We perform structural relaxations of neutral domain walls between oppositely oriented directions of the ferroelectric polarization and calculate their corresponding energies to determine which are most likely to form. We compare our results to literature values for conventional perovskite oxides to provide a source of comparison for understanding the ferroelectric properties of alternative non-oxide materials such as the barium fluorides.