High Saturation Polarization Ferroelectricity in a Pair of α-Phase Enantiomorphic Crystals Based on Calabash-Shaped...

Molecular ferroelectrics have become promising alternatives to traditional inorganic ferroelectrics due to their structural tunability, flexibility, and biocompatibility. The homochiral strategy provides an effective route for constructing ferroelectric materials by increasing the probability of obtaining polar crystal structures. In this work, we designed a pair of unique calabash-shaped chiral organic cations, S/R-430 ((S/R)-hexahydro-1H-pyrrolo[2,1-c][1,4]oxazine), and successfully synthesized two novel enantiomeric ferroelectric materials, [S/R-430 M][FeBr₄] (S/R-430 M = (5R,8aS/5S,8aR)-5-methyloctahydropyrrolo[2,1-c][1,4]oxazin-5-ium). Both compounds exhibit three room-temperature crystalline phases (α, β, γ) with chiral polar point groups C₁, C₂ and C₄, respectively. Among these polymorphs, the α-phase undergoes a reversible $\overline{1}F1$ -type first-order ferroelectric phase transition at approximately 355 K, in which the second-harmonic generation (SHG) signal decreases from a nonzero value to nearly zero. Benefiting from the lowest polar symmetry of the triclinic P1 space group, the compounds display ferroelectric behavior with well-saturated polarization-electric field hysteresis loops along the a-, b-, and c-axes at room temperature. Impressively, the saturation polarization (P_s) of α-[S-430 M][FeBr₄] along the b-axis reaches 31.7 µC/cm². This work not only enriches the family of chiral organic-inorganic hybrid ferroelectrics but also facilitates the development of novel molecular ferroelectric functional materials.