Molecular Symmetry and Geometry Engineering for High-Temperature Ferroelectricity and Low Coercive Field in Hybrid...

Optimizing ferroelectric properties is critical for molecule-based ferroelectrics toward practical applications, including enhanced saturation polarization (P_s), elevated Curie temperature (T_C), and reduced coercive field (E_c). Recent advances in ferroelectrochemistry have provided efficient synthetic strategies to tailor these properties, with a focus on functionalizing organic components. However, the impact of combined molecular symmetry and geometry on ferroelectricity remains less understood. In this work, we construct a series of one-dimensional ferroelectric hybrid metal halides (HMHs) using C_3v-symmetric trigonal pyramidal polar cations to systematically investigate how molecular symmetry and geometry modulate ferroelectric behavior. The model compound (TMS)PbI₃ (TMS = trimethylsulfonium) exhibits ferroelectricity up to its decomposition temperature (530 K), the highest among known HMH ferroelectrics, alongside an exceptionally low E_c (0.25 kV cm^-1 at 298 K). We demonstrate that the unique C_3v symmetry and trigonal pyramidal geometry of the TMS cation facilitate energy-favorable uniaxial rotation about the polar 3-fold axis and 90° polarity flipping during disordering in the ferroelectric-ferroelectric phase transition near 271 K. This partial disorder transition underpins the remarkable high-temperature ferroelectric phase and low E_c. Selenium- and phosphorus-based analogs show similar properties with E_c values of 0.55 and 0.47 kV cm^-1, respectively.