A 3D Hybrid Perovskite Ferroelastic with Triclinic-to-Cubic Phase Transition Boosts Temperature/Pressure Dual On/Off...

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 [C₃H₅FNH₂]₂[(NH₄)Fe(CN)₆] (1-F), which undergoes a $m\overline{3}mF\overline{1}$ 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 $P\overline{1}$ phase and the birefringence-inactive state in the isotropic cubic paraelastic $Fm\overline{3}m$ phase during the $m\overline{3}mF\overline{1}$ 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.