Inch-Size Molecular Ferroelectric Crystal with a Large Electromechanical Coupling Factor on Par with Barium Titanate

Molecular ferroelectrics with large piezoelectric responses have long been sought for their advantages of light weight, mechanical flexibility, and easy preparation, in contrast to the widely used inorganic counterparts. Representatively, a molecular ferroelectric crystal [Me₃NCH₂Cl]CdCl₃ (TMCM-CdCl₃) has been found to show a large piezoelectric coefficient d₃₃ of 220 pC/N exceeding that of BaTiO₃ (You et al. Science2017, 357, 306-309). However, although the d₃₃ of molecular ferroelectrics has achieved great progress, their electromechanical coupling factor k₃₃, which is essential for various piezoelectric applications, including ultrasonic transducers and actuators, was rarely explored and is far below the level of inorganic ferroelectrics. The major reason for this situation is the great challenge of growing large-size crystals which is a key limiting factor for measuring k₃₃. Here, we grew inch-size crystals of organic-inorganic perovskite ferroelectric TMCM-CdCl₃ with a high d₃₃ (383 pC/N) for investigating its piezoelectric responses including the k₃₃ (0.483) by the resonance method. Such high k₃₃ (0.483) is much larger than those of other molecular ferroelectrics and competitive with that of BaTiO₃ (0.5). In addition, TMCM-CdCl₃ has a low elastic modulus of 13.03 GPa, an order of magnitude lower than that of BaTiO₃. This finding sheds light on the exploration of large electromechanical coupling factors in molecular ferroelectrics for potential applications in flexible and portable piezoelectric devices.