[(CH 3 ) 3 PCH 2 CH 2 CH 3 ] 2 ( n -C 4 H 9 NH 3 )Bi 2 Br 9 (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 -C 4 H 9 NH 3 ] 2 [BiBr 5 ]. This provides a new idea for exploring and designing molecular ferroelastic materials with excellent performance.