Microscopic mechanisms behind hyperferroelectricity
Mohamed Khedidji, Danila Amoroso, Hania Djani
Abstract
Hyperferroelectrics are observing a growing interest thanks to their unique property to retain a spontaneous polarization even in the presence of a depolarizing field, corresponding to zero macroscopic displacement field $(\mathbf{D}=0)$ conditions. Hyperferroelectricity is ascribed to the softening of a polar $LO$ mode, but the microscopic mechanisms behind this softening are not totally resolved. Here, by means of phonon calculations and force constants analysis, performed in two classes of hyperferroelectrics, the ${\mathrm{ABO}}_{3}$-LiNbO3-type systems and the hexagonal-ABC systems, we unveil the common features in the dynamical properties of a hyperferroelectric that lead the $LO$ instability: negative or vanishing on-site force constant associated to the cation driving the $LO$ polar mode and a destabilizing cation-anion interaction; both induced by short-range forces. We also predict a possible enhancement of the hyperferroelectric properties under increasing external positive pressures: pressure strengthens the destabilizing short-range interactions, inducing a stronger $LO$ mode instability and the increase of the longitudinal mode effective charges associated to the unstable $LO$ mode. This suggests an eventual enhancement of the $\mathbf{D}=0$ polarization under compressive strain.