An anomalous ferroelastic phase transition arising from an unusual cis-/ anti-conformational reversal of polar organic cations

Abstract

Hybrid ferroelastics have attracted increasing attention for their potential application as mechanical switches. The sporadically documented anomalous ferroelastic phase transitions, i.e., ferroelasticity that appears at a high-temperature phase rather than a low-temperature phase, are of particular interest but are not well understood at the molecular level. By judiciously choosing a polar and flexible organic cation (Me₂NH(CH₂)₂Br⁺) with cis-/anti- conformations as an A-site component, we obtained two new polar hybrid ferroelastics, A₂[MBr₆] (M = Te for 1 and Sn for 2). These materials undergo distinct thermal-induced ferroelastic phase transitions. The larger [TeBr₆]^2- anions anchor the adjacent organic cations well and essentially endow 1 with a conventional ferroelastic transition (P2₁ → Pm2₁n) arising from a common order-disorder transition of organic cations without conformational changes. Moreover, the smaller [SnBr₆]^2- anions can interact with the adjacent organic cations in energetically similar sets of intermolecular interactions, enabling 2 to undergo an anomalous ferroelastic phase transition (P2₁2₁2₁ → P2₁) arising from an unusual cis-/anti-conformational reversal of organic cations. These two instances demonstrate the importance of the delicate balance of intermolecular interactions for inducing anomalous ferroelastic phase transitions. The findings here provide important insights for seeking new multifunctional ferroelastic materials.