Lone-Pair Induced Ferroelectric Distortion and Rashba Effect in the Halide Perovskites

The lone-pair s states of germanium, tin, and lead endow the inorganic metal halide perovskites with many unconventional properties. Previously, only the germanium perovskites were thought to exhibit crystallographic expression of their lone pairs, but dynamic stereochemical expression of the lone pairs is well established for perovskites based on all three metals. Here we use first-principles calculations, namely hybrid density functional theory together with spin–orbit coupling, to study the effects of lone pairs on the electronic and crystal structures of halide perovskites. Using these calculations, we predict stable monoclinic polar phases for both CsSnI₃ and CsSnBr₃, which exhibit ferroelectric distortions that are driven by stereochemical expression of tin lone pairs [1]. Similar metastable ferroelectric phases are predicted to occur for CsPbI₃ and CsPbBr₃. Together with ferroelectricity, these phases also exhibit the Rashba effect, which has recently been shown to be crucial for controlling exciton fine structure in nanostructured halide perovskites. Spin splitting occurs in both the conduction and valence bands for the polar-ferroelectric phases, suggesting that nanostructures based on these compounds could host bright ground-state excitons if the correct phase is achieved. Finally, we how electric fields and tensile strain might aid in the experimental realization of these phases.<br/><br/>[1] Michael W. Swift and John L. Lyons, Chem. Mater. 35, 9370 (2023).<br/><br/><br/>This work was supported by the ONR/NRL 6.1 Basic Research Program.