Exceptional dielectric performance induced by the stepwise reversible phase transitions of an organic crystal: betainium chlorodifluoroacetate†
Solid-state phase transition materials with electric ordering are of great interest owing to their technological importance. In the present work, a new molecular co-crystal material, betainium chlorodifluoroacetate (1), was synthesized, which exhibits exceptional dielectric performance, originating from the distinct structural changes and phase transitions induced by the stepwise ordering of chlorodifluoroacetate anions. Measurements of its temperature-dependent dielectric constant revealed pronounced and successive anomalies at 131 (Tc), 178 (T2) and 220 K (T3), respectively. Such dielectric orderings are coincident with a series of reversible thermal peaks in DSC and specific heat curves. Moreover, a large heat hysteresis of ∼15 K at Tc indicates that 1 undergoes a first-order reversible phase transition with a symmetry transformation from P21/m to P21/c, which was further confirmed by structure analyses. For the origin of its pronounced dielectric response, the totally frozen ordering and reorientation of anionic parts were found to dominate its structural change at Tc (i.e. the order–disorder transformation), while the other two dielectric anomalies at 178 and 220 K were merely involved with the partial ordering of CF2ClCOO− moieties. This investigation will afford broader design flexibility and may facilitate the development of molecular functional materials.