Regulating Noncovalent Interactions to Construct High-Temperature Multiaxial Lead-Free Metal Halide Hybrid Ferroelectrics

Lead-free molecular ferroelectrics are expected to promote the development of green electronics technology due to their environmental friendliness and functional diversity. Nevertheless, the limited variety, low Curie temperature (<i>T</i>_c), and uniaxial characteristics pose significant obstacles in their application. Herein, on the basis of (PMA)₃MBr₆ (PMA⁺ is benzylammonium, and M is Bi³⁺ or Sb³⁺), we constructed a series of lead-free metal halide hybrids (PMA)₂(A)MBr₆ (A is dimethylammonium (DMA⁺), formamidinium (FA⁺), or guanidinium (GA⁺)) using a cation mixing strategy. The cation mixing strategy introduces cation-π interactions and effectively regulates the hydrogen bonding network in the structure. The different strengths of noncovalent interactions in the structures lead to differences in the phase transition temperature and ferroelectricity of (PMA)₂(A)MBr₆. In (PMA)₂(GA)MBr₆ and (PMA)₂(FA)MBr₆, the excessively strong hydrogen bond network and cation-π interactions result in the system lacking the necessary kinetic degrees of freedom to achieve ferroelectric inversion. It is worth noting that moderate noncovalent interactions can ensure spontaneous polarization below the Curie temperature (<i>T</i>_c) and provide necessary dynamic channels for the flipping of polar units, thus successfully achieving ferroelectricity in (PMA)₂(DMA)MBr₆ (3̅<i>mFm</i>). The results revealed that in the design of molecular ferroelectrics, a delicate balance needs to be struck between the different strengths of noncovalent interactions. This finding provides a significant and effective pathway to design high-<i>T</i>_c and multiaxial environmentally friendly ferroelectrics.