Designing dynamic coordination bonds in polar hybrid crystals for a high-temperature ferroelastic transition

Ferroelastic materials have gained widespread attention as promising candidates for mechanical switches, shape memory, and information processing. Their phase-transition mechanisms usually originate from conventional order-disorder and/or displacive types, while those involving dynamic coordination bonds are still scarce. Herein, based on a strategic molecular design of organic cations, we report three new polar hybrid crystals with a generic formula of AA'RbBiCl₆ (A = A' = Me₃SO⁺ for 1; A = Me₃SO⁺ and A' = Me₄N⁺ for 2; A = A' = Me₃NNH₂⁺ for 3). Their A-site cations link to the [RbBiCl₆]² inorganic framework with lon topology through Rb-O/N coordination bonds, while their significantly different interactions between A'-site cations and inorganic frameworks provide distinct phase-transition behaviour. In detail, the strongly coordinative A'-site Me₃SO⁺ cations prevent 1 from a structural phase transition, while coordinatively free A'-site Me₄N⁺ cations trigger a conventional order-disorder ferroelastic transition at 247 K in 2, accompanied by a latent heat of 0.63 J g^-1 and a usual "high → low" second-harmonic-generation (SHG) switch. Interestingly, the A'-site Me₃NNH₂⁺ cations in 3 reveal unusual dynamic coordination bonds, driving a high-temperature ferroelastic transition at 369 K with a large latent heat of 18.34 J g^-1 and an unusual "low → high" SHG-switching behaviour. This work provides an effective molecular assembly strategy to establish dynamic coordination bonds in a new type of host-guest model and opens an avenue for designing advanced ferroelastic multifunctional materials.