Fluoridation Achieved Antiperovskite Molecular Ferroelectric\nin [(CH₃)₂(F-CH₂CH₂)NH]₃(CdCl₃)(CdCl₄)

Abstract

Antiperovskites\nhave developed to be one kind of important functional\nmaterial over the past few decades, showing abundant physical properties\nsuch as negative thermal expansion and superconductivity, <i>etc</i>. However, antiperovskite ferroelectrics have scarcely\nbeen discovered in inorganic ceramics. In this article, we report\na new organic–inorganic hybrid antiperovskite ferroelectric\n[(CH₃)₂(F-CH₂CH₂)­NH]₃(CdCl₃)­(CdCl₄) based on the strategy\nof molecular design. The replacement of one methyl in [(CH₃)₃NH]­CdCl₃ with ethyl produces the lower symmetric\n[(CH₃)₂(CH₂CH₃)­NH]­CdCl₃ with nonpolar perovskite structure, while the polar hexagonal\nantiperovskite structure with the formula of X₃BA (where\nX = [(CH₃)₂(F-CH₂CH₂)­NH]⁺, B = [CdCl₃]⁻, and A = [CdCl₄]^2–) was received after further fluoridation\nof the ethyl group. Therefore, fluoridation successfully achieves\nthe structural transformation from perovskite to antiperovskite, as\nwell as the significant changes in physical properties from nonferroelectric\nto ferroelectric. The antiperovskite [(CH₃)₂(F-CH₂CH₂)­NH]₃(CdCl₃)­(CdCl₄) exhibits typical ferroelectric phase transition above room\ntemperature (<i>T</i>_c = 333 K) including thermal\nanomalies, dielectric transitions, and second harmonic generation\n(SHG) responses. Moreover, lower coercive fields and easy polarization\nswitching are observed by the measurements of hysteresis loops and\nferroelectric domains. The saturated polarization (<i>P</i>_s) of 4.0 μC/cm² is almost 10 times as\nlarge as those recently discovered antiperovskite molecular ferroelectrics.\nThis finding provides a novel strategy to design and explore more\nantiperovskite organic–inorganic hybrid ferroelectric materials.