Toward the Targeted Design of Molecular Ferroelectrics: Modifying Molecular Symmetries and Homochirality

Abstract

Conspectus Although the first ferroelectric discovered in 1920 is Rochelle salt, a typical molecular ferroelectric, the front-runners that have been extensively studied and widely used in diverse applications, such as memory elements, capacitors, sensors, and actuators, are inorganic ferroelectrics with excellent electrical, mechanical, and optical properties. With the increased concerns about the environment, energy, and cost, molecular ferroelectrics are becoming promising supplements for inorganic ferroelectrics. The unique advantages of high structural tunability and homochirality, which are unavailable in their inorganic counterparts, make molecular systems a good platform for manipulating ferroelectricity. Remarkably, based on the Neumann’s principle and the Curie symmetry principle defining the group-to-subgroup relationship, we have found some outstanding high-temperature molecular ferroelectrics, like diisopropylammonium bromide (DIPAB) with a large spontaneous polarization up to 23 μC/cm2 (Science2013339425). However, their application potential is severely limited by the uniaxial nature, leading to major issues in finding proper substrates for thin-film growth and achieving high thin-film performance. Inspired by the commercialized inorganic ferroelectrics like Pb­(Zr, Ti)­O3 (PZT), where the multiaxial nature contributes greatly to the optimized ferroelectric and piezoelectric performance, developing high-temperature multiaxial molecular ferroelectrics is an imminent task. In this Account, we review our recent research progress on the targeted design of multiaxial molecular ferroelectrics. We first propose the “quasi-spherical theory”, a phenomenological theory based on the Curie symmetry principle, to modify the spherical cations to a low-symmetric quasi-spherical geometry for acquiring the highly symmetric paraelectric phase and the polar ferroelectric phase of multiaxial ferroelectrics simultaneously. Besides the sizes and weights of the cation and anion, the intermolecular interactions are particularly crucial for decelerating the molecular rotation at low temperature to reasonably induce ferroelectricity. It means that the momentums of the cation and anion should be matched, so we describe the “momentum matching theory”. In particular, introducing homochirality, a superiority of molecular materials over the inorganic ones, was demonstrated as an effective approach to increase the incidence of ferroelectric crystal structures. Thanks to the striking chemical variability and structure–property flexibility of molecular materials, our research efforts outlined in this Account have led to and will further motivate the richness and the application exploration of high-temperature, high-performance multiaxial molecular ferroelectrics, along with the implementation and perfection of the targeted design strategies.

Extraordinarily Large Electrocaloric Strength of Metal‐Free Perovskites

“…In particular, we expect that the replacement of MDABCO by cations with lower symmetry and larger dipole moment may induce even higher ferroelectric and electrocaloric properties. More detailed discussions on the effects of cation and anion modifications on the properties of metal‐free perovskites can be found in the literature …”

A Three‐Dimensional M3AB‐Type Hybrid Organic–Inorganic Antiperovskite Ferroelectric: [C3H7FN]3[SnCl6]Cl

“…Crystal structure analysis For convenience, the phase below the Curie temperature can be labeled as low-temperature phase (LTP) and the phase above T c can be labeled ash igh-temperature phase (HTP). In the LTP( 293 K), 1 crystallizesi nt he hexagonal polar space group P6 1 ,w ith cell parameters a = 10.9523(2), b = 10.9523(2), c = 30.7971 (8) , V = 3199.27(14) 3 ,a nd Z = 6. The asymmetric unit of 1 contains three [C 3 H 7 FN] + ,o ne hexacoordinated SnCl 6 2À ,a nd one Cl À ion, which coincides with the formula M 3 AB ( Figure 1a).…”

“…[7] For the design of organic-inorganic hybrid ferroelectrics, molecular design which is the applicationofmolecular modification or molecular tailoringtoendow organic-inorganic hybrids with ferroelectricity was recently developed as an effective strategy. [8] When introducing specific chemical groups into organic cations in the organic-inorganic hybrid structure, the changes of the molecular symmetry and molecular dipole moment of the organic cations may lead to apolar crystal structure for realizing ferroelectricity.F or example, by equipping am ethyl group to break a mirrorp lane of 1,4-diazabicyclo[2.2.2]octane (dabco) molecule, the organic-inorganic perovskite [H 2 dabco]NH 4 I 3 without ferroelectricity was modified to the perovskite [MeHdabco]NH 4 I 3 with ferroelectricity. [8] Very recently,b yt he molecular design method of H/F substitution on the [(CH 3 ) 2 (CH 2 CH 3 )NH] + ion, we successfully regulated the non-ferroelectric perovskite [(CH 3 ) 2 (CH 2 CH 3 )NH]CdCl 3 to the ferroelectric antiperovskite [(CH 3 ) 2 (F-CH 2 CH 2 )NH] 3 (CdCl 4 )(CdCl 3 ).…”

Organic‐inorganic Hybrid ([BrCH2CH2N(CH3)3]+2[CdBr4]2−) with Unusual Ferroelectric and Switchable Dielectric Bifunctional Properties over Different Temperature Range

“…For the past few decades, much efforts have been paid to the precise design and synthesis of molecular ferroelectric and switchable dielectric materials, which are driven by the reconstructive, displacive and order‐disorder types of solid‐state structural phase transition in the respective of crystallolography . Although predicting the ferroelectric and/or switchable dielectric behavior is still unreached at present, both the structural modification of three‐dimensional spherical organic ammonium and the size adjustment of metal halide have usually been found to be useful methods to endow organic‐inorganic hybrid metal halides with ferroelectric or switchable dielectric properties, according to “quasi‐spherical theory” and “momentum matching theory” . Inspired by the pioneered work of trimethylchloromethylammonium trichloromanganese(ΙΙ) (TMCM‐MnCl 3 ), which has a high T c and large piezoelectric coefficients, a series of perovskite ferroelectrics TMBM‐MnBr 3, [Me 3 NOH] 2 [KFe(CN) 6 ], [MeHdabco]RbI 3 , TMCM‐CdBr 3, [(CH 3 ) 3 NCH 2 I]PbI 3 , [Mdabco]NH 4 X 3 (X=Br − , I − ) and [Odabco]NH 4 X 3 (X=Cl − , Br − ) (Odabco=N‐hydroxy‐N′‐1,4‐diazoniabicyclo[2.2.2]octonium) and (TMFM) x (TMCM) 1−x CdCl 3 (0≤x≤1) have been successfully constructed and exhibited a series of distinct physical properties such as high Curie temperature, strong piezoelectric coefficient and fast polarization switching behavior.…”