Noncollinear electric dipoles in a polar, chiral phase of CsSnBr${}_3$ perovskite

Abstract

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 $P{2}_1/m$) 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 $P{2}_1$) 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 (${\alpha }_V=-9\times 10^{-5}$ 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.

References in corpus (11)

• Restoring the density-gradient expansion for exchange in solids and surfaces
• Generalized gradient approximation for solids and their surfaces
• Dissipationless Quantum Spin Current at Room Temperature
• Implementation strategies in phonopy and phono3py
• Enhanced Born Charge and Proximity to Ferroelectricity in Thallium Halides
• Defining "Giant" Electrostriction
• The Rashba Scale: Emergence of Band Anti-Crossing as a Design Principle for Materials with Large Rashba coefficient
• Lone-Pair Stereochemistry Induces Ferroelectric Distortion and the Rashba Effect in Inorganic Halide Perovskites
• Origin of Rashba spin-splitting and strain tunability in ferroelectric bulk CsPbF${}_3$
• Multiple lattice instabilities and complex ground state in Cs${}_2$AgBiBr${}_6$
• Coherent Hopping Transport and Giant Negative Magnetoresistance in Epitaxial CsSnBr${}_3$

Cited by in corpus (1)

• Persistent Spin Textures in Nonpolar Chiral Systems