Reversible Thermal Dielectric Switch Triggered by Blooming-Flower Structural Phase Transition in Ionic Crystal without Metal

Due to having excellent properties of sensitive switchable physical and/or chemical response, simple preparation, and environmentally friendly processing, bistable switches (electric switching between “on” and “off” bistable states) have gradually developed into an ideal class of highly smart materials. However, most of them contain metals, especially heavy metals, which are highly toxic to the environment, and metal-free switch materials are rarely reported. Based on this issue, we successfully designed and synthesized organic ion crystals and realized thermal dielectric switching characteristics. Differential scanning calorimetry and dielectric measurements show that the large-size crystal (F-TEDA)­(BF₄)₂ (1) can be regarded as an sensitive dielectric bistable switching between high (switch on) and low (switch off) dielectric states. Variable-temperature single crystal structure reveals one-half of the BF₄^– anions in the crystal undergoes order–disorder transition around 200 K, similar to the transition between flower buds and blooming flowers. This flower-style transition of BF₍₁₎₄^–/BF_(0.5)8^– triggered the rapid switching performance; those properties establish the basis of their applications in excellent temperature-responsive electrical switches, especially lightweight devices.