Fluoridation Achieved Antiperovskite Molecular Ferroelectric in [(CH 3 ) 2 (F-CH 2 CH 2 )NH] 3 (CdCl 3 )(CdCl 4 ).

Antiperovskites have developed to be one kind of important functional material over the past few decades, showing abundant physical properties such as negative thermal expansion and superconductivity, etc. However, antiperovskite ferroelectrics have scarcely been discovered in inorganic ceramics. In this article, we report a new organic-inorganic hybrid antiperovskite ferroelectric [(CH ₃ ) ₂ (F-CH ₂ CH ₂ )NH] ₃ (CdCl ₃ )(CdCl ₄ ) based on the strategy of molecular design. The replacement of one methyl in [(CH ₃ ) ₃ NH]CdCl ₃ with ethyl produces the lower symmetric [(CH ₃ ) ₂ (CH ₂ CH ₃ )NH]CdCl ₃ with nonpolar perovskite structure, while the polar hexagonal antiperovskite structure with the formula of X ₃ BA (where X = [(CH ₃ ) ₂ (F-CH ₂ CH ₂ )NH] ⁺ , B = [CdCl ₃ ] ^- , and A = [CdCl ₄ ] ^2- ) was received after further fluoridation of the ethyl group. Therefore, fluoridation successfully achieves the structural transformation from perovskite to antiperovskite, as well as the significant changes in physical properties from nonferroelectric to ferroelectric. The antiperovskite [(CH ₃ ) ₂ (F-CH ₂ CH ₂ )NH] ₃ (CdCl ₃ )(CdCl ₄ ) exhibits typical ferroelectric phase transition above room temperature ( T _c = 333 K) including thermal anomalies, dielectric transitions, and second harmonic generation (SHG) responses. Moreover, lower coercive fields and easy polarization switching are observed by the measurements of hysteresis loops and ferroelectric domains. The saturated polarization ( P _s ) of 4.0 μC/cm ² is almost 10 times as large as those recently discovered antiperovskite molecular ferroelectrics. This finding provides a novel strategy to design and explore more antiperovskite organic-inorganic hybrid ferroelectric materials.