Photochromic ferroelasticity observed in hybrid lead halide through effective structural regulation

Ferroelastics, the mechanism analogous to ferroelectrics and ferromagnets, have attracted comprehensive attention owing to the prominent spontaneous strain characteristics. However, the contingency of symmetry changes during the phase transition brings a great challenge to the construction of ferroelastic materials. Currently, most of the ferroelastics are accompanied by various physical properties, such as, photoluminescence and thermochromism. There are few reports of ferroelastics with a photochromic effect. Here, we have successfully designed two hybrid metal halide materials, (pyrrolidinium)PbBr 3 (M-1) and (3-hydroxypyrrolidinium) 4 Pb 3 Br 10 (M-2), both of which possess a two-step dielectric response. The organic-inorganic hybrid materials with structural adjustability provide a good design platform and great possibilities. Meanwhile, the strategy of introducing intermolecular H-bonds increases the phase transition temperature by about 90 K, and also achieves dimensional regulation (3D→2D). Exhilaratingly, the second phase transition of M-2 belongs to the 2/mF1̅ -type ferroelastic phase transition according to the Aizu rules, and the ferroelastic domains are observed by a polarizing microscope, which strongly proves the ferroelasticity. Furthermore, we find that these compounds possess a photochromic effect. Especially, the photochromic phenomenon (from milky white to beige) of M-2 is more fascinating, making it a potential candidate for sensors and display devices. The key properties are well-verified through theoretical calculations with high consistency. In short, this photochromic ferroelastic might be a potential candidate in the fields of mechanical switches, shape memory and energy conversion.