Increasing the ferroelastic phase transition temperature of hybrid perovskites through a mixed phosphonium and ammonium cation strategy

[(CH3)(3)PCH2CH2CH3](2)(n-C4H9NH3)Bi2Br9 (1) was obtained by a mixed organic cation strategy. The introduction of phosphonium cations increases the potential energy barrier for the motion of cations, which raises the ferroelastic phase transition temperature of 1 above room temperature, unlike the low-temperature ferroelasticity of [n-C4H9NH3](2)[BiBr5]. This provides a new idea for exploring and designing molecular ferroelastic materials with excellent performance.