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 T_c, 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.