On the microscopic mechanisms behind hyperferroelectricity

TL;DR

This paper uncovers the microscopic mechanisms behind hyperferroelectricity, highlighting the role of short-range forces and cation interactions, and suggests pressure can enhance hyperferroelectric properties.

Contribution

It reveals common dynamical features driving hyperferroelectricity and predicts how external pressure can improve their polarization properties.

Findings

Negative or vanishing on-site force constants are linked to hyperferroelectricity.

Destabilizing short-range cation-anion interactions are key to LO mode softening.

External pressure can enhance hyperferroelectric polarization.

Abstract

Hyperferroelectrics are receiving a growing interest thanks to their unique property to retain a spontaneous polarization even in presence of a depolarizing field. Nevertheless, general microscopic mechanisms driving hyperferroelectricity, which is ascribed to the softening of a polar $LO$ mode, are still missing. Here, by means of phonons calculations and force constants analysis in two class of hyperferroelectrics, the ABO$_3$-LiNbO3-type systems and the prototypical hexagonal-ABC systems, we unveiled common features in the dynamical properties of a hyperferroelectric behind such $LO$ instability: negative or vanishing on-site force constant associated to the cation driving the $LO$ polar distortion, and destabilizing cation-anion interactions, both induced by short-range forces. We also predict possible enhancement of the hyperferroelectric properties by applying an external positive pressure; pressure strengthens the destabilizing short-range interactions. Particularly, the increase in the mode effective charges associated to the unstable $LO$ mode under pressure suggests an eventual enhancement of the $D$=0 polarization under compressive strain.