Unusual symmetry breaking in high-temperature enantiomeric ferroelectrics with large spontaneous polarization

Chiral organic-inorganic hybrid perovskites have gained extensive research interest due to their combination of chirality and the excellent optical, electrical and spin properties of perovskite materials, especially in two-dimensional hybrid perovskites. Herein, we report two-dimensional organic-inorganic perovskite enantiomeric ferroelectric [(R)-β-MPA]₂CdCl₄ (1) and [(S)-β-MPA]₂CdCl₄ (2) (MPA⁺ =methylphenethylammonium). Their mirror relationships are verified by both circular dichroism (CD) and crystal structures. At the same time, the two exhibit very similar ferroelectricity and related properties, including high Curie temperature (343 K), large spontaneous polarization (4.65 μC cm^-2), and low coercive force field (13 kV cm^-1). Unusually, at room temperature the crystal phase is monoclinic with the space group C2 and above the phase transition temperature it is triclinic with the space group P1, which means that the symmetry decreases with the increase of temperature. In addition, it exhibits a flexible switchable SHG response, while [(R)-β-MPA]₂CdCl₄ and [(S)-β-MPA]₂CdCl₄ have wide band gaps of 4.21 and 4.26 eV, respectively, mainly contributed by inorganic CdCl₆ octahedra. This discovery opens a new way for the construction of two-dimensional enantiomeric molecular ferroelectrics.