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Molecular piezoelectrics are attracting tremendous interest because of their easy processing, light weight, low acoustical impedance, and mechanical flexibility. However, reports of molecular piezoelectrics with a piezoelectric coefficient d33 comparable to piezoceramics such as barium titanate (BTO, 90-190 pC/N) have been scarce. Here, we present a uniaxial molecular ferroelectric, trimethylchloromethylammonium tribromocadmium(II) (TMCM-CdBr3), in which the halogen bonding might be a possible critical point for the stabilization of one-dimensional (1D) {CdBr3}- chain and further reservation of its ferroelectricity in such organic- inorganic hybrid crystalline systems. It has a large d33 of 139 pC/N, 1 order of magnitude higher than those of most classically uniaxial ferroelectrics such as LiNbO3 (6-16 pC/N) and Rochelle salt (~7 pC/N), and comparable with those of multiaxial ferroelectrics such as BTO and trimethylbromomethylammonium tribromomanganese(II) (112 pC/N). Moreover, the simple single-crystal growth and easyto- find polar axis enable it to hold a great potential for applying in the single-crystal form. In light of the strong, specific, and directional halogen-bonding interactions, this work provides possibilities to explore new classes of molecular piezoelectrics and contribute to further developments. [ABSTRACT FROM AUTHOR]

To date, the field of ferroelectric random access memories (FeRAMs) is mainly dominated by inorganic ferroelectric thin films like Pb(Zr,Ti)O3, which suffer from the issues of environmental harmfulness, high processing temperatures, and high fabrication costs. In these respects, molecular ferroelectric thin films are particularly advantageous and thus become promising alternatives to the conventional inorganic ones. For the prospect of FeRAMs applications, they should fulfill the requirements of effective polarization switching and low-voltage, high-speed operation. Despite recent advancements, molecular ferroelectric thin films with such high performance still remain a huge blank. Herein we present the first example of a large-area continuous biaxial molecular ferroelectric thin film that gets very close to the goal of application in FeRAMs: [Hdabco]BF4 (dabco = diazabicyclo[2.2.2]octane). In addition to excellent film performance, it is the coexistence of a low coercive voltage of ~12 V and ultrafast polarization switching at a significantly high frequency of 20 kHz that affords [Hdabco]BF4 considerable potential for memory devices. Particularly, piezoresponse force microscopy (PFM) clearly demonstrates the four polarization directions and polarization switching at a low voltage down to ~4.2 V (with an ~150 nm thick film). This innovative work on high-performance molecular ferroelectric thin films, which can be compatible with wearable devices, will inject new vitality to the low-power information field. [ABSTRACT FROM AUTHOR]

Organic-inorganic hybrid perovskites (OIHPs) have been emerging as a hot research topic due to their potential applications in energy storage, semiconductors, and electronic devices. Herein, we systematically investigated the synthesis and phase transition behaviors of the enantiomeric OIHPs, ( R ) and ( S )- N , N -dimethyl-3-fluoropyrrolidinium cadmium bromide ([DMFP][CdBr 3 ]), and the hybrid trigonal structure [DMFP] 3 (CdBr 3 )(CdBr 4 ). The enantiomers have a mirror-symmetric structure and enhanced solid-state phase transition points of 417 and 443 K, in contrast to the nonfluorinated parent compound, N , N -dimethyl-pyrrolidinium cadmium bromide ([DMP][CdBr 3 ], 385 K). Moreover, racemic H/F substitution on the pyrrolidinium cations leads to the formation of a trigonal compound, showing above-room-temperature structural phase transition and dominant ferroelasticity. This work discovers chiral enantiomeric OIHPs through H/F substitution, demonstrating a useful chemical synthesis strategy for exploring novel phase transition materials.

Organic single-component ferroelectrics are highly desirable for their low molecular mass, light weight, low processing temperature, and excellent film-forming properties. Organosilicon materials with a strong film-forming ability, weather resistance, nontoxicity, odorlessness, and physiological inertia are very suitable for device applications related to the human body. However, the discovery of high- T c organic single-component ferroelectrics has been very scarce, and the organosilicon ones even less so. Here, we used a chemical design strategy of H/F substitution to successfully synthesize a single-component organosilicon ferroelectric tetrakis(4-fluorophenylethynyl)silane (TFPES). Systematic characterizations and theory calculations revealed that, compared with the parent nonferroelectric tetrakis(phenylethynyl)silane, fluorination caused slight modifications of the lattice environment and intermolecular interactions, inducing a 4/ mmm F mm 2-type ferroelectric phase transition at a high T c of 475 K in TFPES. To our knowledge, this T c should be the highest among the reported organic single-component ferroelectrics, providing a wide operating temperature range for ferroelectrics. Moreover, fluorination also brought about a significant improvement in the piezoelectric performance. Combined with excellent film properties, the discovery of TFPES provides an efficient path for designing ferroelectrics suitable for biomedical and flexible electronic devices., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

The optical control of polarization switching is attracting tremendous interest because photoirradiation stands out as a nondestructive, noncontact, and remote-control means beyond an electric or strain field. The current research mainly uses various photoexcited electronic effects to achieve the photocontrol polarization, such as a light-driven flexoelectric effect and a photovoltaic effect. However, since photochromism was discovered in 1867, the structural phase transition caused by photoisomerization has never been associated with ferroelectricity. Here, we successfully synthesized an organic photochromic ferroelectric with polar space group Pna 2 1 , 3,4,5-trifluoro- N -(3,5-di- tert -butylsalicylidene)aniline, whose color can change between yellow and orange via laser illumination. Its dielectric permittivity and spontaneous polarization can be switched reversibly with a photoinduced phase transition triggered by structural photoisomerization between the enol form and the trans -keto form. To our knowledge, this is the first photoswitchable ferroelectric crystal to achieve polarization switching through a structural phase transition triggered by photoisomerization. This finding paves the way toward photocontrol of smart materials and biomechanical applications in the future.

Topological defects such as vortices in ferroelectric materials are attracting tremendous interest because of their splendid possibilities for unique physical phenomena and potential applications in nanoelectronic devices. However, reports of the vortex structure have been scarce in organic ferroelectrics, which are highly desirable for their mechanical flexibility, easy and environment-friendly processing, and low acoustical impedance. Here, we successfully observed the robust triangular domains in a single-component organic ferroelectric, 2-(hydroxymethyl)-2-nitro-1,3-propanediol ( 1 ), six of which can form a 6-fold vertex domain structure. To our knowledge, it is the first time that such an intriguing topological vortex gets experimentally confirmed in ferroelectrics. Moreover, the symmetry change of 1 with an Aizu notation of m 3 m F1 leads to the most 48 crystallographically equivalent polarization directions among all ferroelectrics. With those benefits and excellent piezoelectric properties, compound 1 shows great potential as a reconfigurable electronic element or a mechanical sensor for soft robotics, flexible and wearable devices, and biomachines.

Organic-inorganic ABX 3 (A, B = cations, X = anion) hybrids with perovskite structure have recently attracted tremendous interest due to their structural tunability and rich functional properties, such as ferroelectricity. However, ABX 3 hybrid ferroelectrics with other structures have rarely been reported. Here, we successfully designed an ABX 3 hybrid ferroelectric [(CH 3 ) 3 NCH 2 F]ZnCl 3 with a spontaneous polarization of 4.8 μC/cm 2 by the molecular modification of [(CH 3 ) 4 N]ZnCl 3 through hydrogen/halogen substitution. It is the first zinc halide ABX 3 ferroelectric, which contains one-dimensional [ZnCl 3 ] - n chains of corner-sharing ZnCl 4 tetrahedra, distinct from the anionic framework of corner-sharing or face-sharing BX 6 octahedra in the ABX 3 perovskites. From zero dimension to one dimension, the high symmetry of ZnCl 4 tetrahedra is broken, and all of them align along one direction to form a polar [ZnCl 3 ] - n chain, beneficial to the generation of ferroelectricity. This finding provides an efficient polar anionic framework for enriching the family of hybrid ferroelectrics by assembling with various cations and should inspire further exploration of new classes of organic-inorganic ABX 3 ferroelectrics.

Topological defects, such as vortices and skyrmions, provide a wealth of splendid possibilities for new nanoscale devices because of their marvelous electronic, magnetic, and mechanical behaviors. Recently, great advances have been made in the study of the ferroelectric vortex in conventional perovskite oxides, such as BaTiO 3 and BiFeO 3 . Despite extensive interest, however, no intriguing ferroelectric vortex structures have yet been found in organic-inorganic hybrid perovskites (OIHPs), which are desirable for their mechanical flexibility, ease of fabrication, and low acoustical impedance. We observed the robust vortex-antivortex topological configurations in a two-dimensional (2D) layered OIHP ferroelectric (4,4-DFPD) 2 PbI 4 (4,4-DFPD is 4,4-difluoropiperidinium). This provides future directions for the study of perovskites and makes it a promising alternative for nanoscale ferroelectric devices in medical, micromechanical, and biomechanical applications.

Molecular ferroelectrics are attracting tremendous interest because of their easy and environmentally friendly processing, light weight, low acoustical impedance, and mechanical flexibility, which are viable alternatives or supplements to conventional ceramic ferroelectrics. However, reports of ceramic-like molecular ferroelectrics that can be applied in the polycrystalline form have been scarce. Here, according to the "quasi-spherical theory", we successfully synthesized a ceramic-like molecular ferroelectric with an m 3 m F mm 2 type phase transition at 357 K, 1,5-diazabicyclo[3.2.1]octonium tetrafluoroborate ([3.2.1-dabco]BF 4 ), which can show excellent ferroelectric performance in the polycrystalline thin-film form at room temperature. On the basis of the reported molecular ferroelectric [2.2.2-dabco]BF 4 (2.2.2-dabco = 1,4-diazabicyclo[2.2.2]octonium) with an Aizu notation of 4/ mmm F mm 2 and two polar axes, we changed the [2.2.2-dabco] + cation to the [3.2.1-dabco] + cation to reduce the molecular symmetry and keep the quasi-spherical shape simultaneously, making the number of polar axes up to six. Moreover, the spontaneous polarization P s gets successfully increased from 4.9 μC cm -2 in [2.2.2-dabco]BF 4 to 5.5 μC cm -2 in [3.2.1-dabco]BF 4 . This precise molecular design strategy offers an efficient pathway to design ceramic-like molecular ferroelectrics.

Piezoelectric sensors that can work under various conditions with superior performance are highly desirable with the arrival of the Internet of Things. For practical applications, a large piezoelectric voltage coefficient g and a high Curie temperature T c are critical to the performance of piezoelectric sensors. Here, we report a two-dimensional perovskite ferroelectric (4-aminotetrahydropyran) 2 PbBr 4 [(ATHP) 2 PbBr 4 ] with a saturated polarization of 5.6 μC cm -2 , high T c of 503 K [above that of BaTiO 3 (BTO, 393 K)], and extremely large g 33 of 660.3 × 10 -3 V m N -1 [much beyond that of Pb(Zr,Ti)O 3 (PZT) ceramics (20 to 40 × 10 -3 V m N -1 ), more than 2 times higher than that of poly(vinylidene fluoride) (PVDF, about 286.7 × 10 -3 V m N -1 )]. Combined with the advantages of molecular ferroelectrics, such as light weight, easy and environmentally friendly processing, and mechanical flexibility, (ATHP) 2 PbBr 4 would be a competitive candidate for next-generation smart piezoelectric sensors in flexible devices, soft robotics, and biomedical devices.

Molecular ferroelectrics as alternatives to the conventional inorganic ferroelectrics have been greatly developed in past decades; many of these have been discovered and designed through various chemical means due to their structural adjustability. However, it is still a huge challenge to obtain high (above room temperature) Curie temperature ( T c ) molecular ferroelectrics to meet the application requirements. Here, we present a new organic-inorganic hybrid molecular ferroelectric, [cyclopentylammonium] 2 CdBr 4 ( 1 ), showing a moderate above-room-temperature T c of 340.3 K. The mechanism of the ferroelectric phase transition from Pnam to Pna 2 1 in 1 is ascribed to the order-disorder transition of both the organic cations and inorganic anions, affording a spontaneous polarization of 0.57 μC/cm 2 for the ferroelectric phase. Using piezoresponse force microscopy (PFM), we clearly observed the antiparallel 180° stripe domains and realized the polarization switching, unambiguously establishing the existence of room-temperature ferroelectricity in the thin film. These attributes make it attractive for use in flexible devices, soft robotics, biomedical devices, and other applications.

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.

Antiperovskites have developed to be one kind of important functional material over the past few decades, showing abundant physical properties such as negative thermal expansion and superconductivity, etc. However, antiperovskite ferroelectrics have scarcely been discovered in inorganic ceramics. In this article, we report a new organic-inorganic hybrid antiperovskite ferroelectric [(CH 3 ) 2 (F-CH 2 CH 2 )NH] 3 (CdCl 3 )(CdCl 4 ) based on the strategy of molecular design. The replacement of one methyl in [(CH 3 ) 3 NH]CdCl 3 with ethyl produces the lower symmetric [(CH 3 ) 2 (CH 2 CH 3 )NH]CdCl 3 with nonpolar perovskite structure, while the polar hexagonal antiperovskite structure with the formula of X 3 BA (where X = [(CH 3 ) 2 (F-CH 2 CH 2 )NH] + , B = [CdCl 3 ] - , and A = [CdCl 4 ] 2- ) was received after further fluoridation of the ethyl group. Therefore, fluoridation successfully achieves the structural transformation from perovskite to antiperovskite, as well as the significant changes in physical properties from nonferroelectric to ferroelectric. The antiperovskite [(CH 3 ) 2 (F-CH 2 CH 2 )NH] 3 (CdCl 3 )(CdCl 4 ) exhibits typical ferroelectric phase transition above room temperature ( T c = 333 K) including thermal anomalies, dielectric transitions, and second harmonic generation (SHG) responses. Moreover, lower coercive fields and easy polarization switching are observed by the measurements of hysteresis loops and ferroelectric domains. The saturated polarization ( P s ) of 4.0 μC/cm 2 is almost 10 times as large as those recently discovered antiperovskite molecular ferroelectrics. This finding provides a novel strategy to design and explore more antiperovskite organic-inorganic hybrid ferroelectric materials.

It is known that perovskites with the general chemical formula of ABX 3 (A, B = cations, X = anion) have been intensively studied over the last half century because of their diverse functional properties, such as ferroelectricity in BaTiO 3 , piezoelectricity in PZT (lead zirconate titanate), and recently developed photovoltaic properties in CH 3 NH 3 PbI 3 . However, rather less attention has been paid to their "inverse" analogs, antiperovskites, which have a chemical formula X 3 BA, where A and B are anions and X is a cation. Although most of important ferroelectrics are perovskites, no antiperovskite ferroelectrics have been found since the discovery of antiperovskites in 1930. Here, for the first time, we report a X 3 BA antiperovskite ferroelectric [(CH 3 ) 3 NH] 3 (MnBr 3 )(MnBr 4 ) (where (CH 3 ) 3 NH is X, MnBr 3 is B, and MnBr 4 is A), which shows outstanding ferroelectricity with a significantly high phase transition temperature of 458 K as well as fascinating photoluminescence properties with two intense emissions. This finding opens a new avenue to explore the golden area of antiperovskites for high-performance functional materials.

Piezoelectric materials have been widely used in various applications, such as high-voltage sources, actuators, sensors, motors, frequency standard, vibration reducer, and so on. In the past decades, lead zirconate titanate (PZT) binary ferroelectric ceramics have dominated the commercial piezoelectric market due to their excellent properties near the morphotropic phase boundary (MPB), although they contain more than 60% toxic lead element. Here, we report a lead-free and one-composition molecular ferroelectric trimethylbromomethylammonium tribromomanganese(II) (TMBM-MnBr 3 ) with a large piezoelectric coefficient d 33 of 112 pC/N along polar axis, comparable with those of typically one-composition piezoceramics such as BaTiO 3 along polar axis [001] (∼90 pC/N) and much greater than those of most known molecular ferroelectrics (almost below 40 pC/N). More significantly, the effective local piezoelectric coefficient of TMBM-MnBr 3 films is comparable to that of its bulk crystals. In terms of ferroelectric performance, it is the low coercive voltages, combined with the multiaxial characteristic, that ensure the feasibility of piezo film applications. Based on these, along with the common superiorities of molecular ferroelectrics like light weight, flexibility, low acoustical impedance, easy and environmentally friendly processing, it will open a new avenue for the exploration of next-generation piezoelectric devices in industrial and medical applications.

Multifunctional properties of chiral molecules arise from the coexistence of mirror-symmetry-induced stereoisomers and optical rotation characteristics in one material. One of these complex phenomena in these molecules is chiral ferroelectricity, providing the coupling between polarized light and the spatial asymmetry induced dipole moment. Herein we describe the chiral polarization and electroresistance in molecular ferroelectric (R)-(-)-3-hydroxyquinuclidinium chloride thin films with a Curie temperature of 340 K. The high transmittance of chiral ferroelectrics is coupled with polarized light for a linear electro-optic effect, which exhibits angle-dependent optical behaviors. The polarization-controlled conductance imposes a large on/off ratio (∼26.6) of electroresistive switching in molecular ferroelectrics with superior antifatigue endurance.

The classical organic ferroelectric, poly(vinylidene fluoride) (PVDF), has attracted much attention as a promising candidate for data storage applications compatible with all-organic electronics. However, it is the low crystallinity, the large coercive field, and the limited thermal stability of remanent polarization that severely hinder large-scale integration. In light of that, we show a molecular ferroelectric thin film of [Hdabco][ReO 4 ] (dabco = 1,4-diazabicyclo[2.2.2]octane) (1), belonging to another class of typical organic ferroelectrics. Remarkably, it displays not only the highest Curie temperature of 499.6 K but also the fastest polarization switching of 100k Hz among all reported molecular ferroelectrics. Combined with the large remanent polarization values (∼9 μC/cm 2 ), the low coercive voltages (∼10 V), and the unique multiaxial ferroelectric nature, 1 becomes a promising and viable alternative to PVDF for data storage applications in next-generation flexible devices, wearable devices, and bionics.

With the flourishing development of (CH 3 NH 3 )PbI 3 , three-dimensional (3D) organic-inorganic perovskites with unique structure-property flexibility have become a worldwide focus. However, they still face great challenges in effectively inducing ferroelectricity. Despite the typical 3D perovskite structure and the ability of dabco (1,4-diazabicyclo[2.2.2]octane) to trigger phase transition, unfortunately [H 2 dabco]RbCl 3 adopts a nonpolar crystal structure without ferroelectricity. Within the larger RbI 3 framework, we assemble N-methyl-1,4-diazoniabicyclo[2.2.2]octane (MeHdabco) obtained by reducing the molecular symmetry of dabco into a new 3D organic-inorganic perovskite. As expected, MeHdabco bearing a molecular dipole moment turns out to be vital in the generation of polar crystal structure and ferroelectric phase transition occurring at 430 K. It is the first time that the dabco component has been successfully wrapped into a 3D cage to achieve ferroelectricity even through there is intensive research on dabco. This precise molecular design strategy based on the modification of molecular symmetry provides an efficient route to enrich the family of 3D organic-inorganic perovskite ferroelectrics. Intriguingly, the iodine-doped crystal can exhibit intense saffron yellow luminescence with a high quantum yield of 17.17% under UV excitation, extending its application in the field of ferroelectric luminescence and/or multifunctional devices.

As a promising candidate for energy storage capacitors, antiferroelectric (AFE) materials have attracted great concern due to their congenital advantages of large energy storage ability from double polarization versus electric field (P-E) hysteresis characteristics in contrast to ferroelectrics and linear dielectrics. However, antiferroelectricity has only been discovered in inorganic oxides and some hydrogen-bonded molecular systems. In view of the structural diversity and unique physical properties of organic-inorganic hybrid system, it remains a great opportunity to introduce antiferroelectricity into organic-inorganic hybrid perovskites. Here, we report that polarizable antiparallel dipole arrays can be realized in an organic-inorganic hybrid perovskite, (3-pyrrolinium)CdBr 3 , which not only exhibits an excellent ferroelectric property (with a high spontaneous polarization of 7.0 μC/cm 2 ), but also presents a striking AFE characteristic revealed by clear double P-E hysteresis loops. To the best of our knowledge, it is the first time that such successive ferroelectric-antiferroelectric-paraelectric phase transitions have been discovered in organic-inorganic perovskites. Besides, a giant dielectric constant of 1600 even at high frequency of 1000 kHz and a bulk electrocaloric effect with entropy change of 1.18 J K -1 kg -1 under 7.41 kV/cm are also observed during the phase transition. Apparently, the combined striking AFE characteristic and giant dielectric constant make (3-pyrrolinium)CdBr 3 a promising candidate for next generation high-energy-storage capacitors.

The underlying phase transitions of ferroelectric mechanisms in molecular crystals are mainly limited to order-disorder and displacive types that are not involved in breaking of the chemical bonds. Here, we show that the bond-switching transition under ambient pressure is designable in molecular crystals, and demonstrate how to utilize the weaker and switchable coordination bonds in a novel molecular perovskite, [(CH 3 ) 3 NOH] 2 [KFe(CN) 6 ] (TMC-1), to afford a scarce multiaxial ferroelectrics with a high Curie temperature of 402 K and 24 equivalent ferroelectric directions (more than BaTiO 3 ). The high-quality thin films of TMC-1 can be easily fabricated by a simple solution process, and to reveal perfect ferroelectric properties at both macroscopic and microscopic scales, suggesting TMC-1 as a promising candidate for applications in next-generation flexible electronics. The presented molecular assembly strategy, together with the achieved bond-switching ferroelectric mechanism, opens a new avenue for designing advanced ferroelectric materials.

It is known that CH 3 NH 3 PbI 3 is particularly promising for next-generation solar devices; therefore, molecular perovskite structures have recently received extraordinary attention from the academic community because of their potential in producing unique physical properties. However, although great efforts have been made, molecular ferroelectrics with three-dimensional (3D) perovskite structures are still rare. So far, reported perovskite-like molecular ferroelectrics are basically one- or two-dimensional, significantly deviating from the inorganic perovskite ferroelectrics. Thus, their ferroelectric properties have to be greatly improved to meet the requirements of practical applications. Here, we report a 3D molecular perovskite ferroelectric: (3-ammoniopyrrolidinium)RbBr 3 [(AP)RbBr 3 ], with a high Curie temperature (T c = 440 K) beyond that of BaTiO 3 . To the best of our knowledge, such above-room-temperature ferroelectricity in the 3D molecular perovskite compound is unprecedented. Furthermore, (AP)RbBr 3 has great potential for applications due to its high thermal stability, ultrafast polarization reversal (greater than 20 kHz), and fascinating multiaxial characteristic. This finding opens a new avenue to the design and controllable synthesis of molecular ferroelectric perovskites, where the metal ion, halogen ion, and organic cation can be easily tuned.

To date, the field of ferroelectric random access memories (FeRAMs) is mainly dominated by inorganic ferroelectric thin films like Pb(Zr,Ti)O 3 , which suffer from the issues of environmental harmfulness, high processing temperatures, and high fabrication costs. In these respects, molecular ferroelectric thin films are particularly advantageous and thus become promising alternatives to the conventional inorganic ones. For the prospect of FeRAMs applications, they should fulfill the requirements of effective polarization switching and low-voltage, high-speed operation. Despite recent advancements, molecular ferroelectric thin films with such high performance still remain a huge blank. Herein we present the first example of a large-area continuous biaxial molecular ferroelectric thin film that gets very close to the goal of application in FeRAMs: [Hdabco]BF 4 (dabco = diazabicyclo[2.2.2]octane). In addition to excellent film performance, it is the coexistence of a low coercive voltage of ∼12 V and ultrafast polarization switching at a significantly high frequency of 20 kHz that affords [Hdabco]BF 4 considerable potential for memory devices. Particularly, piezoresponse force microscopy (PFM) clearly demonstrates the four polarization directions and polarization switching at a low voltage down to ∼4.2 V (with an ∼150 nm thick film). This innovative work on high-performance molecular ferroelectric thin films, which can be compatible with wearable devices, will inject new vitality to the low-power information field.

Molecular ferroelectrics are attracting much attention as valuable complements to conventional ceramic ferroelectrics owing to their solution processability and nontoxicity. Encouragingly, the recent discovery of a multiaxial molecular ferroelectric, tetraethylammonium perchlorate, is expected to be able to solve the problem that in the technologically relevant thin-film form uniaxial molecular ferroelectrics have been found to perform considerably more poorly than in bulk. However, it can show good polarization-electric field (P-E) hysteresis loops only at very low frequency, severely hampering practical applications such as ferroelectric random access memory. Here, we present a biaxial molecular ferroelectric thin film of [Hdabco]ClO 4 (dabco = 1,4-diazabicyclo[2.2.2]octane) (1), where a perfect ferroelectric hysteresis loop can be observed even at 10 kHz. It is the first example of a molecular ferroelectric thin film whose polarization can be switched at such a high frequency. Moreover, using piezoresponse force microscopy, we clearly observed the coexistence of 180° and non-180° ferroelectric domains and provided direct experimental proof that 180° ferroelectric switching and non-180° ferroelastic switching are both realized; that is, a flexible alteration of the polarization axis direction can occur in the thin film by applying an electric field. These results open a new avenue for applications of molecular ferroelectrics and will inspire further exploration of high-performance multiaxial molecular ferroelectric thin films.

Besides the single crystals, ferroelectric materials are actually widely used in the forms of the polycrystals like ceramics. Multiaxial ferroelectrics with multiple equivalent polarization directions are preferable for such applications, because more equivalent ferroelectric axes allow random spontaneous polarization vectors to be oriented along the electric field to achieve a larger polarization after poling. Most of ceramic ferroelectrics like BaTiO 3 have equivalent ferroelectric axes no more than three. We herein describe a molecular-ionic ferroelectric with 12 equivalent ferroelectric axes: tetraethylammonium perchlorate, whose number of axes is the most in the known ferroelectrics. Appearance of so many equivalent ferroelectric axes benefits from the plastic phase transition, because the plastic phase usually crystallizes in a highly symmetric cubic system. A perfect macroscopic ferroelectricity can be obtained on the polycrystalline film of this material. This finding opened an avenue constructing multiaxial ferroelectrics for applications as polycrystalline materials.