One-Dimensional Cadmium Thiocyanate Perovskite Ferroelastics Tuned by Halogen Substitution

Organic–inorganic hybrid molecular ferroelastics have gained widespread attention as a promising candidate for data storage, sensor, and mechanical switch applications. However, it remains a great challenge to construct new molecular ferroelastic materials. Based on the “quasi-spherical theory”, we designed and synthesized a new quasi-spherical cation [DMIE]⁺ ([DMIE]⁺ is dimethyl-isopropyl-ethyl-ammonium cation) to expect ferroelastic materials. Unfortunately, [DMIE]­[Cd­(SCN)₃] (1) undergoes a structural phase transition from the space group P6₃ to P6₃/mmc, which is not among the 94 species of ferroelastic phase transitions as suggested by Aizu. Herein, by introducing electronegative halogen (F, Cl, or Br) atoms to the [DMIE]⁺ cation, the symmetry groups of the corresponding cadmium thiocyanate perovskites get efficiently lowered to orthorhombic Pbca to induce ferroelastic phase transitions with an Aizu notation of 6/mmmFmmm. The spontaneous strain value reduces from 0.0778 in a fluorinated product to 0.0428 in a brominated product. Accompanied by the introduction of halogen groups from F to Br, the phase transition temperature increases from 248.6 to 367.8 K. This work demonstrates that the strategy of combining a quasi-spherical molecule with specific electronegative groups provides an efficient way to offer molecular ferroelastic materials.