Designing and Constructing a High-Temperature Molecular Ferroelectric by Anion and Cation Replacement in a Simple Crown Ether Clathrate.

ABSTRACT

Molecular ferroelectrics have displayed a promising future since they are light-weight, flexible, environmentally friendly and easily synthesized, compared to traditional inorganic ferroelectrics. However, how to precisely design a molecular ferroelectric from a non-ferroelectric phase transition molecular system is still a great challenge. Here we designed and constructed a molecular ferroelectric by double regulation of the anion and cation in a simple crown ether clathrate, 4, [K(18-crown-6)]+ [PF6 ]- . By replacing K+ and PF6 - with H3 O+ and [FeCl4 ]- respectively, we obtained a new molecular ferroelectric [H3 O(18-crown-6)]+ [FeCl4 ]- , 1. Compound 1 undergoes a para-ferroelectric phase transition near 350 K with symmetry change from P21/n to the Pmc21 space group. X-ray single-crystal diffraction analysis suggests that the phase transition was mainly triggered by the displacement motion of H3 O+ and [FeCl4 ]- ions and twist motion of 18-crown-6 molecule. Strikingly, compound 1 shows high a Curie temperature (350 K), ultra-strong second harmonic generation signals (nearly 8 times of KDP), remarkable dielectric switching effect and large spontaneous polarization. We believe that this research will pave the way to design and build high-quality molecular ferroelectrics as well as their application in smart materials.