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

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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>m</mml:mi> <mml:mover><mml:mn>3</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mi>m</mml:mi> <mml:mi>F</mml:mi> <mml:mover><mml:mn>1</mml:mn> <mml:mo>¯</mml:mo></mml:mover> </mml:mrow> <mml:annotation>$m {\mathrm{\bar{3}}}m{\mathrm{F}}{\mathrm{\bar{1}}}$</mml:annotation></mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>P</mml:mi> <mml:mover><mml:mn>1</mml:mn> <mml:mo>¯</mml:mo></mml:mover> </mml:mrow> <mml:annotation>$P{\mathrm{\bar{1}}}$</mml:annotation></mml:semantics> </mml:math> phase and the birefringence-inactive state in the isotropic cubic paraelastic <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>F</mml:mi> <mml:mi>m</mml:mi> <mml:mover><mml:mn>3</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mi>m</mml:mi></mml:mrow> <mml:annotation>$Fm{\mathrm{\bar{3}}}m$</mml:annotation></mml:semantics> </mml:math> phase during the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>m</mml:mi> <mml:mover><mml:mn>3</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mi>m</mml:mi> <mml:mi>F</mml:mi> <mml:mover><mml:mn>1</mml:mn> <mml:mo>¯</mml:mo></mml:mover> </mml:mrow> <mml:annotation>$m{\mathrm{\bar{3}}}m{\mathrm{F}}{\mathrm{\bar{1}}}$</mml:annotation></mml:semantics> </mml:math> 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.