H/F-Substitution-Induced Homochirality for Designing High-Tc Molecular Perovskite Ferroelectrics.

A ferroelectric with a high phase-transition temperature (Tc ) is an indispensable condition for practical applications. Over the past decades, both strain engineering and the isotope effect have been found to effectively improve the Tc within ferroelectric material systems. However, the former strategy seems to prefer working in inorganic ferroelectric thin films, while the latter is also limited to some certain systems, such as hydrogen-bonded ferroelectrics. It is noted that a mono-fluorinated molecule is geometrically very similar to its parent molecule and the substitution of H by an F atom can introduce a chiral center on the molecule to template or stabilize polar structures. Significantly, the barrier of rotation of the fluorinated organic molecules is raised, resulting in a remarkable increase in Tc . Herein, by applying the molecular design strategy of H/F substitution to the organic-inorganic perovskite ferroelectric (pyrrolidinium)CdCl3 with a low Tc of 240 K, two high-Tc chiral perovskite ferroelectrics, (R)- and (S)-3-F-(pyrrolidinium)CdCl3 are successfully synthesized, for which the Tc reaches 303 K. The significant enhancement of 63 K in Tc extends the ferroelectric working temperature range to room temperature. This finding provides a new effective way to regulate the Tc in ferroelectrics and to design high-Tc molecular ferroelectrics.