Mixed Cation-Ordering-Driven Ferroelastic Phase Transition in Hybrid Lead Halide Enabling Large Switchable...

Solid-state phase transition materials are of crucial significance for regulating fundamental physical properties, particularly optical and electrical behaviors, offering a robust platform for developing stimuli-responsive smart materials. However, achieving phase transitions in hybrid halides containing aromatic cations, a promising category of materials with fascinating functions, still presents a huge challenge due to the rigid and stable frameworks that resist structural flexibility for reversible switching. Herein, we present a mixed cation-ordering strategy to develop a two-dimensional hybrid halide ferroelastic phase transition material (NMPA)(BrEA)PbBr₄ (1, NMPA = N-methyl-p-toluidine, BrEA = 2-bromoethylamine). By incorporating a flexible secondary cation, the lattice is effectively softened and imbued with dynamic disorder, significantly reducing the energy barrier for structural transformation and enabling a ferroelastic phase transition of 4/mmmFmmm at 353 K. Notably, benefiting from the reorientation of organic moieties and the consequent distortion of the inorganic framework, the ferroelastic transition in 1 drives a remarkable and reversible switch in birefringence, demonstrating an excellent phase-transition-driven switchable birefringence response with Δn from 0.028 (on) to 0 (off). This work not only demonstrates promising potential for optical applications but also provides an effective way for the switchable birefringence in organic-inorganic hybrid metal halides.