Molecular Disorder Induces an Unusual Phase Transition in a Potential 2D Chiral Ferroelectric Perovskite.

Two-dimensional hybrid halide perovskites with single chiral and ferroelectricity together with various structural phase transitions provide the possibility for more diverse functional properties. Here, we present a 2D chiral hybrid halide perovskite ferroelectric, [C6 H5 (CH2 )4 NH3 ]2 CdCl4 (4PBA-CdCl4 , 4PBA=4-phenylbutylamine) that experiences two continuous phase transitions from centrosymmetric triclinic P 1 ‾ to polar chiral monoclinic P2 and then to another centrosymmetric tetragonal P4/mmm with increasing temperature, accompanied by symmetry breaking, due to the prominent octahedral distortion and disorder transformation of organic 4PBA cations. In the polar chiral phase, 4PBA-CdCl4 gives a significant CD signal and has a moderate ferroelectric polarization of 0.35 µC/cm2 . In addition, 4PBA-CdCl4 occupies a wide band gap of 4.376 eV that is chiefly contributed by the inorganic CdCl6 octahedron. This finding offers an alternative pathway for designing new phase transitions and related physical properties in hybrid halide perovskites and other hybrid crystals.