Electronic structure and biaxial strain inRbHgF3perovskite and hybrid improper ferroelectricity in(Na,Rb)Hg2F6and(K,Rb)Hg2F6superlattices

Abstract

Here we study geometry, electronic structure, and effects of biaxial strain on ${\mathrm{RbHgF}}_{3}$ fluoro-perovskite from first-principles based density-functional theory computations. It has been shown that while an epitaxial strain of $\ensuremath{\sim}\ifmmode\pm\else\textpm\fi{}2%$ is sufficient to produce a significant ferroelectric polarization in the prototypical cubic $Pm\overline{3}m$ structure, the ground state orthorhombic $Pnma$ structure remains effectively immune to the strain induced ferroelectricity even at biaxial strains as high as $\ifmmode\pm\else\textpm\fi{}5%$. We further show that ${\mathrm{RbHgF}}_{3}$ in the $Pnma$ structure can accommodate compressive and tensile strains, respectively, by ${a}^{\ensuremath{-}}{a}^{\ensuremath{-}}{b}^{0}$ tilting (out-of-phase tilts along $a$ and $b$ axes) and ${a}^{0}{a}^{0}{b}^{+}$ rotations (in-phase rotations along $c$ axis) of ${\mathrm{HgF}}_{2}$ octahedra. Similar to many perovskite oxides, ${\mathrm{HgF}}_{2}$ octahedral rotations in ${\mathrm{RbHgF}}_{3}$ are found to be accompanied by large Rb-site antipolar displacements along the [001] direction. We demonstrate that this coupling between the octahedral rotations and Rb-site antipolar modes can be harnessed in ${\mathrm{RbHgF}}_{3}/{\mathrm{NaHgF}}_{3}$ and ${\mathrm{RbHgF}}_{3}/{\mathrm{KHgF}}_{3}$ superlattices to produce significant net polarizations of 4.93 $\ensuremath{\mu}\text{C}/{\mathrm{cm}}^{2}$ and 1.70 $\ensuremath{\mu}\text{C}/{\mathrm{cm}}^{2}$, respectively.