  Polar and chiral crystal symmetries confer a variety of potentially useful
functionalities upon solids by coupling otherwise noninteracting mechanical,
electronic, optical, and magnetic degrees of freedom. We describe two unstudied
phases of the 3D perovskite, CsSnBr$_3$, which emerge below 85 K due to the
formation of Sn(II) lone pairs and their interaction with extant octahedral
tilts. Phase II (77 K<$T$<85 K, space group $P2_1/m$) exhibits ferroaxial order
driven by a noncollinear pattern of lone pair-driven distortions within the
plane normal to the unique octahedral tilt axis, preserving the inversion
symmetry observed at higher temperatures. Phase I ($T$<77 K, space group
$P2_1$) additionally exhibits ferroelectric order due to distortions along the
unique tilt axis, breaking both inversion and mirror symmetries. This polar and
chiral phase exhibits second harmonic generation from the bulk and a large,
intrinsic polarization$-$electrostriction coefficient along the polar axis
($Q_{22}\approx$1.1 m$^4$ C$^{-2}$), resulting in acute negative thermal
expansion ($\alpha_V=-9\times10^{-5}$ K$^{-1}$) through the onset of
spontaneous polarization. The unprecedented structures of phases I and II were
predicted by recursively following harmonic phonon instabilities to generate a
tree of candidate structures and subsequently corroborated by synchrotron X-ray
powder diffraction and polarized Raman and $^{81}$Br nuclear quadrupole
resonance spectroscopies. Relativistic electronic structure scenarios
compatible with reported photoluminescence measurements are discussed.
Together, the polar symmetry, small bandgap, large spin-orbit splitting of Sn
5$p$ orbitals, and predicted strain sensitivity of the symmetry-breaking
distortions suggest bulk samples and epitaxial films of CsSnBr$_3$ or its
neighboring solid solutions as strong candidates for bulk Rashba effects.
