Methyl regulation triggers high-temperature ferroelastic phase transition

The multifunctional tuning of solid-state dielectric switches constructed from organic-inorganic hybrid materials (OIHMs) has received great attention. In particular, molecular ferroelastics with dielectric phase transitions have considerable potential in the optical and electrical fields owing to their adjustable structures and physical features. However, it remains a challenge to effectively design ferroelastics with high phase transition temperature (T_c). We used [TTMA]₂CdI₄ (TTMA = tetramethylammonium, 1) as a template to continuously increase the molecular weight and change the structure of the hybrid material by modifying and extending the alkane chain in the cation. Therefore, a series of OIHMs were eventually developed: [TMEA]₂CdI₄ (TMEA = trimethylethylammonium, 2), [TMPA]₂CdI₄ (TMPA = trimethylpropylammonium, 3), and [TMIPA]₂CdI₄ (TMIPA = trimethyliso-propylammonium, 4). Among them, the T_c of ferroelastic 3 increased up to 387 K. DSC and temperature-related dielectric constant tests prove the occurrence of the phase transition for 1, 2, and 3. The structures further indicate that the phase transition is caused by the order-disorder cation motion. The extension of the alkyl chain greatly increases T_c and endows 3 with ferroelasticity at room temperature.