Record Enhancement of Phase Transition Temperature Realized by H/F Substitution

Abstract

A high transition temperature (Tc) is essential for the practical application of ferroelectrics as electronic devices under extreme thermal conditions in the aerospace, automotive, and energy industries. In recent decades, the isotope effect and strain engineering are found to effectively modulate Tc; however, these strategies are limited to certain systems. Developing simple, universal, and practical methods to improve Tc has become an imminent challenge for expanding the applications of ferroelectrics. Here, by adopting a molecular design strategy involving H/F substitution on an organic–inorganic hybrid perovskite (1‐azabicyclo[2.2.1]heptane)CdCl3 at a Tc of 190 K, the successful synthesis of a multiaxial, ferroelectric hybrid perovskite (4‐fluoro‐1‐azabicyclo[2.2.1]heptane)CdCl3 is reported, which demonstrates a large spontaneous polarization of 11.2 µC cm−2 (greater than that of polyvinylidene difluoride) and a Tc of 419 K (greater than that of BaTiO3). This temperature enhancement (229 K) is the largest reported for molecular ferroelectrics, far exceeding the reported enhancements induced by the isotope effect and other techniques. This pioneering technique provides an effective and universal method for improving Tc in ferroelectrics and represents an important step toward the development of high‐performance ferroelectric technology.

Ferroic phase transition molecular crystals

“…39 The molecular This journal is © The Royal Society of Chemistry 2022 origin of the phase transition is attributed to the orderdisorder transition of the N atom of the cation and cooperative atomic distortion, resulting in a relatively large barrier to overcome. Experimentally, it is easy to further increase and tune the T C by atom substitution and doping/ mixing, including deuteration in hydrogen ferroelectrics, 3 replacement of H/N atoms with F/P atoms in organoammonium salts, 40,41 cation/metal/halogen substitution, and alloying in organic/hybrid halides. [42][43][44] Meanwhile, along with the T C , numerous other structurerelated properties can be tuned.…”

H/F substitution for advanced molecular ferroelectrics

“…As most of the molecular ferroelectrics undergo order-disorder-type transition, the potential energy barrier of rotating motions of organic cations may be raised after fluorination and cause the T c enhancement. After a prolonged endeavor, a record enhancement of T c by 229 K has been achieved in the fluorinated [4-FABCH]CdCl 3 (4-FABCH = 4-fluoro-1-azabicyclo[2.2.1]heptane) compared with the parent compound [ABCH]CdCl 3 (ABCH = 1-azabicyclo[2.2.1]heptane), and they both undergo the same 6m2Fmm2-type phase transition [36]. To our knowledge, this shows a record enhancement of T c in molecular ferroelectrics realized by H/F substitution, far beyond that of 142 K in PbDPO 4 achieved by the isotope effect [24].…”

“…Schematic diagram of the chemical design strategy H/F substitution aimed at the targeted design and performance optimization of molecular ferroelectrics. Adapted, with permission, from [36,39,40,42,44,47,48,51,52,81]. Abbreviations: CPL, circularly polarized luminescence; FEPV, ferroelectric photovoltaic.…”

“…OPEN ACCESS There is no doubt that the H/F substitution examples raised in this review will inspire more creative works. Briefly, with H/F substitution, a significant T c enhancement was discovered in [4-FABCH] CdCl 3 [36], (R)-and (S)-3-(fluoropyrrolidinium)MnCl 3 [32], (R)-and (S)-3-(fluoropyrrolidinium) CdCl 3 [35], (R)-and (S)-DMFPI [31], and [4-F-Q]ReO 4 [82], among others. The H/F substitution method is now beyond the H/D isotope effect concerning T c enhancing.…”

The first salicylaldehyde Schiff base organic–inorganic hybrid lead iodide perovskite ferroelectric

“…Xiong and co-authors have discovered numerous advanced perovskite ferroelectrics using important chemical design principles. [21][22][23][24][25][26][27] Moreover, heterostructure engineering of multilayer perovskite ferroelectrics for promising opto-electronic applications has also been widely investigated. [28][29][30][31] Of these materials, several 1D ABX 3 lead halide perovskite ferroelectrics have been documented.…”