Inorganic perovskite ferroelectrics are widely used in nonvolatile memory elements, capacitors, and sensors because of their excellent ferroelectric and other properties. Organic ferroelectrics are desirable for their mechanical flexibility, low weight, environmentally friendly processing, and low processing temperatures. Although almost a century has passed since the first ferroelectric, Rochelle salt, was discovered, examples of highly desirable organic perovskite ferroelectrics are lacking. We found a family of metal-free organic perovskite ferroelectrics with the characteristic three-dimensional structure, among which MDABCO (-methyl--diazabicyclo[2.2.2]octonium)-ammonium triiodide has a spontaneous polarization of 22 microcoulombs per square centimeter [close to that of barium titanate (BTO)], a high phase transition temperature of 448 kelvins (above that of BTO), and eight possible polarization directions. These attributes make it attractive for use in flexible devices, soft robotics, biomedical devices, and other applications.

To predict or identify ferroelectricity is essential for extending the family of molecular ferroelectrics and thereby promoting their practical applications in nonvolatile memories, capacitors, piezoelectric sensors and nonlinear optical devices. In this respect, symmetry breaking is of particular importance, since the paraelectric phase adopting any of the 32 crystallographic point groups is always broken into one of the 10 ferroelectric point groups, i.e. C1, C2, C1h, C2v, C4, C4v, C3, C3v, C6 and C6v.1 It is the Curie symmetry principle that determines the group-subgroup relationship between paraelectric and ferroelectric phases, and thus 88 species of potential ferroelectric phase transitions are deduced. However, in some cases such as croconic acid and triglycine sulfate (TGS), the existence of pseudo center of symmetry makes it difficult to accurately recognize the ferroelectric phase. Then inspired by the Neumann's principle, which states that the symmetry of any physical property of a crystal must include the symmetry elements of the point group of the crystal, the temperature-dependent SHG effect and dielectric property become useful for detecting symmetry breaking and ferroelectricity. Consequently, in the light of the Curie symmetry principle and Neumann's principle, ferroelectrics can be effectively distinguished from innumerable compounds with various crystal structures collected in the Cambridge Structural Database. Taking advantage of such strategy and combining with the measurements of ferroelectric hysteresis loops and ferroelectric domains, we have successfully discovered a series of low-temperature and high-temperature molecular ferroelectrics with high performance.2-6 This study does help to avoid blindly searching for molecular ferroelectrics.

Piezoelectric materials produce electricity when strained, making them ideal for different types of sensing applications. The most effective piezoelectric materials are ceramic solid solutions in which the piezoelectric effect is optimized at what are termed morphotropic phase boundaries (MPBs). Ceramics are not ideal for a variety of applications owing to some of their mechanical properties. We synthesized piezoelectric materials from a molecular perovskite (TMFM)x(TMCM)1–xCdCl3 solid solution (TMFM, trimethylfluoromethyl ammonium; TMCM, trimethylchloromethyl ammonium, 0 ≤ x ≤ 1), in which the MPB exists between monoclinic and hexagonal phases. We found a composition for which the piezoelectric coefficient d33 is ~1540 picocoulombs per newton, comparable to high-performance piezoelectric ceramics. The material has potential applications for wearable piezoelectric devices.

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.

Though dominating most of the practical applications, inorganic ferroelectric thin films usually suffer from the high processing temperatures, the substrate limitation, and the complicated fabrication techniques that are high-cost, energy-intensive, and time-consuming. By contrast, molecular ferroelectrics offer more opportunities for the next-generation flexible and wearable devices due to their inherent flexibility, tunability, environmental-friendliness, and easy processability. However, most of the discovered molecular ferroelectrics are uniaxial, one major obstacle for improving the thin-film performance and expanding the application potential. In this Perspective, we overview the recent advances on multiaxial molecular ferroelectric thin films, which is a solution to this issue. We describe the strategies for screening multiaxial molecular ferroelectrics and characterizations of the thin films, and highlight their advantages and future applications. Upon rational and precise design as well as optimizing ferroelectric performance, the family of multiaxial molecular ferroelectric thin films surely will get booming in the near future and inject vigor into the century-old ferroelectric field.

Organic-inorganic hybrid perovskite, [CHNH]PbI, holds a great potential for next-generation solar devices. However, whether the ferroelectricity exists in [CHNH]PbI and results in the ultrahigh performance is not at present clear. Beyond that, no hybrid lead iodide perovskite ferroelectric has yet been found. Here, using precise molecular modifications, we successfully designed a room-temperature hybrid perovskite ferroelectric, [(CH)NCHI]PbI. Because of the high-symmetry and nearly spherical shape of [(CH)N] cation, [(CH)N]PbI crystallizes in a centrosymmetric space group P6/ m at room temperature and undergoes a structural phase transition at 184 K. Accompanied by the introduction of halogen atoms on the cation from F to I, the phase transition temperature gradually increases to 312 K and the space group transforms into a polar C2 at room temperature. The strongest halogen bond energy of [(CH)NCHI]-I and the largest volume of [(CH)NCHI] among these compounds might be possible reasons for the stabilization of ordered [(CH)NCHI] cation array and further reservation of its ferroelectricity at relatively high temperature. This work provides an efficient molecular design strategy toward the targeted harvest of room-temperature organic-inorganic perovskite ferroelectrics, and should inspire further exploration of the interplay between structure and ferroelectricity. The discovery of lead iodide perovskite ferroelectric also offers a foothold to the possibility for the existence of ferroelectricity in [CHNH]PbI.

The past decade has witnessed much progress in designing molecular ferroelectrics, whose intrinsic mechanical flexibility, structural tunability, and easy processability are desirable for next-generation flexible and wearable electronic devices. However, an obstacle in expanding their promising applications in nonvolatile memory elements, capacitors, and sensors is effectively modulating the Curie temperature (T c ).Here, taking advantage of fluorine substitution on the reported molecular ferroelectric, (pyrrolidinium)MnCl 3 , we present enantiomeric perovskite ferroelectrics, namely, (R)and (S)-3-(fluoropyrrolidinium)MnCl 3 . The close van der Waal's radii and the similar steric parameters between H and F atoms ensure the minimum disruption of the crystal structure, while their different electronegativity and polarizability can trigger significant changes in the physical and chemical properties. As expected, the T c gets successfully increased from 295 K in (pyrrolidinium)MnCl 3 to 333 K in these two homochiral compounds. Such a dramatic enhancement of 38 K signifies an important step toward designing high-T c molecular ferroelectrics. In the light of the conceptually new idea of fluorine substitution, one could look forward to a continuous succession of new molecular ferroelectric materials and technology developments.

Two-dimensional (2D) organic–inorganic perovskites (OIPs), with improved material stability over their 3D counterparts, are highly desirable for device applications. It is their considerable structural diversity that offers an unprecedented opportunity to engineer materials with fine-tuning functionalities. The isosteric substitution of hydrogen by an electronegative fluorine atom has been proposed as a useful route to improve the photovoltaic performance of 2D OIPs, whereas its valuable role in developing ferroelectricity is still waiting for further exploration. Herein, for the first time we applied fluorinated aromatic cations in extending the family of 2D OIP ferroelectrics, and successfully obtained [2-fluorobenzylammonium]2PbCl4 as a high-performance ferroelectric semiconductor. The failures in the nonferroelectric [4-fluorobenzylammonium]2PbCl4 and [3-fluorobenzylammonium]2PbCl4 demonstrate that the selective introduction of fluorine in correct structural positions is particularly essential. This work represents an unprecedented proof-of-concept in the use of fluorinated aromatic cations for the targeted design of excellent 2D OIP ferroelectrics, and is believed to inspire the future development of low-cost, high-efficiency, and stable device applications.