Electronic structure and biaxial strain in RbHgF3 perovskite and hybrid improper ferroelectricity in (Na,Rb) Hg2 F6 and (K,Rb) Hg2 F6 superlattices

Here we study geometry, electronic structure, and effects of biaxial strain on RbHgF3 fluoro-perovskite from first-principles based density-functional theory computations. It has been shown that while an epitaxial strain of ∼±2% is sufficient to produce a significant ferroelectric polarization in the prototypical cubic Pm3m structure, the ground state orthorhombic Pnma structure remains effectively immune to the strain induced ferroelectricity even at biaxial strains as high as ±5%. We further show that RbHgF3 in the Pnma structure can accommodate compressive and tensile strains, respectively, by a-a-b0 tilting (out-of-phase tilts along a and b axes) and a0a0b+ rotations (in-phase rotations along c axis) of HgF2 octahedra. Similar to many perovskite oxides, HgF2 octahedral rotations in RbHgF3 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 RbHgF3/NaHgF3 and RbHgF3/KHgF3 superlattices to produce significant net polarizations of 4.93 μC/cm2 and 1.70 μC/cm2, respectively.