Evidence of Lone Pair Crafted Emphanisis in the Ruddlesden–Popper Halide Perovskite Cs2PbI2Cl2

Abstract A hallmark of typical structural transformations is an increase in symmetry upon heating due to entropic favourability. However, local symmetry breaking upon warming is recently evidenced in rare crystalline phases. Termed as emphanisis, the phenomenon implores exploration of fascinating thermodynamic nuances that drive unusual structural evolutions. Here, synchrotron X‐ray total scattering measurements are presented on a Ruddlesden–Popper mixed halide perovskite, Cs 2 PbI 2 Cl 2 , which reveal signatures of emphanisis. The genesis of symmetry lowering upon heating is traced to a lone pair‐driven cooperative local structural distortion composed of thermally actuated Pb off‐centring and static Cl displacement. Mapping the thermal evolution of low‐lying phonon modes with inelastic neutron scattering uncovers instances of mode hardening with picosecond lifetime and an intriguing soft mode at the X‐point of the Brillouin zone—features conducive to ultralow thermal transport. Together, these observations highlight the fundamental and functional implications of chemical design in engendering unconventional phenomena in crystalline materials and associated properties.