Giant hyperferroelectricity in LiZnSb and its origin

Abstract

Hyperferroelectricity (HyFE) is an intriguing phenomenon in which spontaneous polarization persists under an open-circuit boundary condition (OCBC) despite the existence of a depolarization field. However, small polarization and shallow well depth have been found so far in most hyperferroelectric materials, which severely limits their applications. Using first-principles density functional theory, we report the discovery of a giant hyperferroelectricity in LiZbSb. The HyFE spontaneous polarization is shown to be remarkably large with $\mathrm{P}=0.282\phantom{\rule{0.16em}{0ex}}\mathrm{C}/{\mathrm{m}}^{2}$ under OCBC, which is one order of magnitude greater than the HyFE polarization in ${\mathrm{LiNbO}}_{3}$. Furthermore, HyFE in LiZnSb is found to be exceptionally stable with a well depth of electric free energy $\mathrm{\ensuremath{\Delta}}F=\ensuremath{-}332\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, which makes LiZnSb a possible hyperferroelectric solid at room temperature. The origin of the giant hyperferroelectricity in LiZnSb is identified and is attributed to the large mode effective charge of the soft longitudinal-optic phonon and the large high-frequency dielectric constant.