  The lone-pair s states of germanium, tin, and lead underlie many of the
unconventional properties of the inorganic metal halide perovskites. Dynamic
stereochemical expression of the lone pairs is well established for perovskites
based on all three metals, but previously only the germanium perovskites were
thought to express the lone pair crystallographically. In this work, we use
advanced first-principles calculations with a hybrid functional and spin-orbit
coupling to predict stable monoclinic polar phases of $\mathrm{CsSnI}_3$ and
$\mathrm{CsSnBr}_3$, which exhibit a ferroelectric distortion driven by
stereochemical expression of the tin lone pair. We also predict similar
metastable ferroelectric phases of $\mathrm{CsPbI}_3$ and $\mathrm{CsPbBr}_3$.
In addition to ferroelectricity, these phases exhibit the Rashba effect. Spin
splitting in both the conduction and valence bands suggests that nanostructures
based on these phases could host bright ground-state excitons. Finally, we
discuss paths toward experimental realization of these phases via electric
fields and tensile strain.
