Disentangling the Effects of Structure and Lone-Pair Electrons in the Lattice Dynamics of Halide Perovskites

Metal halide perovskites have shown great performance as solar energymaterials, but their outstanding optoelectronic properties are paired withunusually strong anharmonic effects. It has been proposed that this intriguingcombination of properties derives from the "lone pair" 6s^2 electronconfiguration of the Pb^2+ cations, and associated weak pseudo-Jahn-Tellereffect, but the precise impact of this chemical feature remains unclear. Herewe show that in fact an ns^2 electron configuration is not a prerequisite forthe strong anharmonicity and low-energy lattice dynamics encountered in thisclass of materials. We combine X-ray diffraction, infrared and Ramanspectroscopies, and first-principles molecular dynamics calculations todirectly contrast the lattice dynamics of CsSrBr_3 with those of CsPbBr_3,two compounds which bear close structural similarity but with the formerlacking the propensity to form lone pairs on the 5s^0 octahedral cation. Weexploit low-frequency diffusive Raman scattering, nominally symmetry-forbiddenin the cubic phase, as a fingerprint to detect anharmonicity and reveal thatlow-frequency tilting occurs irrespective of octahedral cation electronconfiguration. This work highlights the key role of structure in perovskitelattice dynamics, providing important design rules for the emerging class ofsoft perovskite semiconductors for optoelectronic and light-harvesting devices.