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

Optimizing ferroelectric properties is critical for molecule-based ferroelectrics toward practical applications, including enhanced saturation polarization (Ps), elevated Curie temperature (TC), and reduced coercive field (Ec). 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 C3v-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 Ec (0.25 kV cm-1 at 298 K). We demonstrate that the unique C3v 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 Ec. Selenium- and phosphorus-based analogs show similar properties with Ec values of 0.55 and 0.47 kV cm-1, respectively.