A 3D Hybrid Perovskite Ferroelastic with Triclinic‐to‐Cubic Phase Transition Boosts Temperature/Pressure Dual On/Off Switchable Birefringence

Abstract Birefringent crystals have gained enormous attention for decades due to their unique ability to manipulate polarized light. However, achieving the fascinating on/off (active/inactive) switchable birefringence under external stimuli in crystals remains a huge challenge, and the stimuli employed have been constrained predominantly to temperature. Here, through H/F substitution, we designed a 3D hybrid double perovskite ferroelastic [C 3 H 5 FNH 2 ] 2 [(NH 4 )Fe(CN) 6 ] ( 1‐F ), which undergoes a ferroelastic transition at 296 K, with the highest possible orientation states of 24 among ferroelastic crystals. Notably, the ferroelastic phase transition of 1‐F can also be triggered by pressure with a low critical pressure of ∼0.3 GPa. More importantly, 1‐F shows the unprecedented temperature/pressure dual stimuli‐induced on/off switching of birefringence between the birefringence‐active state in the anisotropic triclinic ferroelastic phase and the birefringence‐inactive state in the isotropic cubic paraelastic phase during the ferroelastic transition. To the best of our knowledge, this is the first report of dual stimuli‐induced switchable birefringence in crystals. Our work paves a new way for the switching of birefringence and sheds light on the further exploration of switchable birefringence in ferroelastics.