Unconventional Inverse Temperature Symmetry Breaking in a Ferroelastic Crystal

Molecular ferroelastics have garnered significant attention due to their potential applications in the fields of shape memory devices, mechanical sensors, and switches. Despite great efforts to elevate Tc, it remains a challenge to design high-temperature ferroelastics to meet the requirements for practical applications. Here, we present an organic salt molecular ferroelastic crystal, [Piperazine][CH3SO3H]2 (CH-P), which undergoes a ferroelastic structural phase transition at 383 K, characterized by an unconventional inverse temperature symmetry breaking (ITSB) mechanism. CH-P retains its ferroelastic properties at a high-temperature range. Crystallographic analysis reveals that the displacement of organic cations and anions drives the ITSB-induced ferroelastic phase transition. Additionally, H/F substitution on the anion moieties leads to a substantial enhancement of the ferroelastic phase transition temperature, reaching up to 512 K for the crystal [Piperazine][CF3SO3H]2 (CF-P). This work highlights the concept of designing above-room-temperature ferroelastic materials through the ITSB phenomenon and the H/F substitution strategy.