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

Crown Ether Host-Guest Molecular Ferroelectrics

In recent years, molecular ferroelectrics have received great attention due to their low weight, mechanical flexibility, easy preparation and excellent ferroelectric properties. Among them, crown-ether-based molecular ferroelectrics, which are typically composed of the host crown ethers, the guest cations anchored in the crown ethers, and the counterions, are of great interest because of the host-guest structure. Such a structure allows the components to occur order-disorder transition easily, which is beneficial for inducing ferroelectric phase transition. Herein, we summarized the research progress of crown ether host-guest molecular ferroelectrics, focusing on their crystal structure, phase transition, ferroelectric-related properties. In view of the small spontaneous polarization and uniaxial nature, we outlook the chemical design strategies for obtaining high-performance crown-ether-based molecular ferroelectrics. This minireview will be of guiding significance for the future exploration of crown ether host-guest molecular ferroelectrics.

In Situ Observation of Ferroelastic Domain and Phase Transition in a Three-Dimensional Molecular Crystal

Massive efforts have been devoted to designing molecular ferroic materials by molecular modification. For molecular ferroelastic, previous work is focused on the temperature-dependent ferroelastic domains, however, few are related to controlling the ferroelastic domain by the stress. Inspired by the "quasi-spherical theory" and fluorination effect, we designed a more flexible (MedabcoF)²⁺ (MedabcoF=1-fluoro-4-methyl-1,4-diazoniabicyclo[2.2.2]octane) cation by introducing a methyl group and a fluorine atom at the two symmetrical ends of the Dabco (1,4-diazoniabicyclo[2.2.2]octane) and synthesized a hybrid 3D perovskite (MedabcoF)Rb(BF₄ )₃ (1) which displays three reversible phase transitions accompanying dual ferroelastic behavior. Besides, it not only exhibits ferroelastic domains switching by the thermal stimulation, and the sensitive reaction of in situ domains under the stress of it is also realized. This work not only achieves a force-controlled ferroelastic domain but develops a more profound comprehension of the relationship between the thermal motion behavior of guest cations and the intriguing properties of materials.

Centimeter-Sized Single Crystal of a One-Dimensional Lead-Free Mixed-Cation Perovskite Ferroelectric for Highly Polarization Sensitive Photodetection

Linear dichroic anisotropic photonic materials are highly attractive due to their great potentials in many applications, which in combination with the ferroelectric properties could broaden their research and applications. However, to date, the linear dichroism conversion phenomenon has not been observed in one-dimensional (1D) large-size single-crystal materials: in particular, lead-free perovskite ferroelectric crystals. Here, we propose a new ferroelectric design strategy: namely, partial organic cation substitution for precisely designing 1D polarization-sensitive perovskite ferroelectrics. As an example, the 1D mixed-cation perovskite ferroelectric (n-propylammonium)(methylammonium)SbBr₅ was synthesized, which exhibits a fascinating ferroelectricity with a notable reversible polarization of 2.9 μC/cm² and a large ferroelectricity-driven polarization ratio of 6.9. Importantly, the single-crystalline photodetectors also exhibit superior optoelectronic anisotropic performances at the paraelectric phase, having a large photoelectric anisotropy ratio (∼35), an excellent polarization-sensitive dichroism ratio (∼1.31), highly sensitive detectivity up to ∼10⁹ Jones, and a fast response rate (∼45/68 μs). This finding provides a significant and effective pathway for the targeted design of new functional lead-free linear dichroic anisotropic photonic ferroelectrics.

The Role of Fluorine-Substituted Positions on the Phase Transition in Organic-Inorganic Hybrid Perovskite Compounds

Although research on organic-inorganic hybrid perovskites (OIHPs) has grown exponentially in the past two decades, the high phase transition temperature of OIHP materials is still one of the insurmountable difficulties. Herein, a series of A₂BX₄ type OIHP materials [(2,n-DFBA)₂PbCl₄] (n = 3, for 1; n = 4, for 2; n = 5, for 3; n = 6, for 4) have been prepared by reactions of double-substituted difluorobenzylamine (difluorobenzylamine = DFBA) with lead chloride in concentrated HCl aqueous solution. It was found the OIHP compounds 1-3 proceed a switchable phase transition with phase transition temperatures (T_c) at 449 K (1), 462 K (2) and 500 K (3), higher than that of the parent compound [(BA)₂PbCl₄] (BA = benzylammonium) at 438 K, but compound 4 exhibits no phase transition. A crystal structure analysis elucidated that the organic template ligands DFBA lead in the inorganic part in compounds 1-3 to a two-dimensional (2D) perovskite structure, while that in compound 4 leads to a one-dimensional (1D) chain structure. The different double-substituted positions of fluorine atoms on benzylamine have important influences on the phase transition in compounds 1-4.

Optical Control of Polarization Switching in a Single-Component Organic Ferroelectric Crystal

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 Pna2₁, 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.

Dynamics of proton, ion, molecule, and crystal lattice in functional molecular assemblies

Dynamic molecular processes, such as short- or long-range proton (H⁺) and ion (M⁺) motions, and molecular rotations in electrical conducting and magnetic molecular assemblies enable the fabrication of electron-H⁺ (or M⁺) coupling systems, while crystal lattice dynamics and molecular conformation changes in hydrogen-bonded molecular crystals have been utilised in external stimuli responsive reversible gas-induced gate opening and molecular adsorption/desorption behavior. These dynamics of the polar structural units are responsible for the dielectric measurements. The H⁺ dynamics are formed from ferroelectrics and H⁺ conductors, while the dynamic M⁺ motions of Li⁺ and Na⁺ involve ionic conductors and coupling to the conduction electrons. In n-type organic semiconductors, the crystal lattices are modulated by replacing M⁺ cations, with cations such as Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺. The use of polar rotator or inversion structures such as alkyl amides, m-fluoroanilinium cations, and bowl-shaped trithiasumanene π-cores enables the formation of ferroelectric molecular assemblies. The host-guest molecular systems of ESIPT fluorescent chromic molecules showed interesting molecular sensing properties using various bases, where the dynamic transformation of the crystal lattice and the molecular conformational change were coupled to each other.

Giant Spin Splitting in Chiral Perovskites Based on Local Electrical Field Engineering

Chiral hybrid organic-inorganic perovskites (chiral HOIPs) present potential spintronic and spin-optoelectronic applications due to their unique spin-related properties. However, the spin physics in chiral HOIPs has rarely been explored by theoretical studies. Here, with first-principles calculations, we investigate the spin characteristics of the Pb-I based chiral HOIPs and propose an effective approach to significantly increase the spin splitting with a halogen-substituted chiral molecule. Compared to the value of 13 meV without halogen substitution, the spin splitting energy can be significantly enhanced to 73, 90, and 105 meV with F, Cl, and Br substitution, respectively. A k·p model Hamiltonian based on a symmetry argument reveals that the halogen substitution enhances the local electric field, inducing distortion of the PbI₆ octahedron. Further calculation demonstrates that halogen substitution can strongly modify the electrostatic potential surface of the chiral molecules. This work presents an effective molecular engineering approach to modulate spin splitting of chiral HOIPs, shedding light on the design of spintronic materials.

The First High-Temperature Supramolecular Radical Ferroics

Organic radical ferroics such as TEMPO have attracted widespread interest. However, the relatively low Curie temperature of 287 K and melting point of 311 K severely hinder its application potential. Despite extensive interest, high-temperature radical ferroics have not yet been found. Here, taking advantage of chemical design and supramolecular radical chemistry, we designed two high-temperature organic supramolecular radical ferroics [(NH₃ -TEMPO)([18]crown-6)](ReO₄ ) (1) and [(NH₃ -TEMPO)([18]crown-6)](ClO₄ ) (2), which can retain ferroelectricity up to 413 K and 450 K, respectively. To our knowledge, they are both the first supramolecular radical ferroics and unprecedented high-temperature radical ferroics, where the supramolecular component is vital for the stabilization of the radical and extending the working temperature window. Both also have paramagnetism, non-interacting spin moments, and excellent piezoelectric and electrostrictive behaviors comparable to that of LiNbO₃ .

Room-temperature ferroelectric and ferroelastic orders coexisting in a new tetrafluoroborate-based perovskite

The coexistence of multiferroic orders has attracted increasing attention for its potential applications in multiple-state memory, switches, and computing, but it is still challenging to design single-phase crystalline materials hosting multiferroic orders at above room temperature. By utilizing versatile ABX3-type perovskites as a structural model, we judiciously introduced a polar organic cation with easily changeable conformations into a tetrafluoroborate-based perovskite system, and successfully obtained an unprecedented molecular perovskite, (homopiperazine-1,4-diium)[K(BF4)3], hosting both ferroelectricity and ferroelasticity at above room temperature. By using the combined techniques of variable-temperature single-crystal X-ray structural analyses, differential scanning calorimetry, and dielectric, second harmonic generation, and piezoresponse force microscopy measurements, we demonstrated the domain structures for ferroelectric and ferroelastic orders, and furthermore disclosed how the delicate interplay between stepwise changed dynamics of organic cations and cooperative deformation of the inorganic framework induces ferroelectric and ferroelastic phase transitions at 311 K and 455 K, respectively. This instance, together with the underlying mechanism of ferroic transitions, provides important clues for designing advanced multiferroic materials based on organic-inorganic hybrid crystals.

PFM (piezoresponse force microscopy)-aided design for molecular ferroelectrics

With prosperity, decay, and another spring, molecular ferroelectrics have passed a hundred years since Valasek first discovered ferroelectricity in the molecular compound Rochelle salt. Recently, the proposal of ferroelectrochemistry has injected new vigor into this century-old research field. It should be highlighted that piezoresponse force microscopy (PFM) technique, as a non-destructive imaging and manipulation method for ferroelectric domains at the nanoscale, can significantly speed up the design rate of molecular ferroelectrics as well as enhance the ferroelectric and piezoelectric performances relying on domain engineering. Herein, we provide a brief review of the contribution of the PFM technique toward assisting the design and performance optimization of molecular ferroelectrics. Relying on the relationship between ferroelectric domains and crystallography, together with other physical characteristics such as domain switching and piezoelectricity, we believe that the PFM technique can be effectively applied to assist the design of high-performance molecular ferroelectrics equipped with multifunctionality, and thereby facilitate their practical utilization in optics, electronics, magnetics, thermotics, and mechanics among others.

Molecular Disorder Induces an Unusual Phase Transition in a Potential 2D Chiral Ferroelectric Perovskite

Two-dimensional hybrid halide perovskites with single chiral and ferroelectricity together with various structural phase transitions provide the possibility for more diverse functional properties. Here, we present a 2D chiral hybrid halide perovskite ferroelectric, [C₆ H₅ (CH₂ )₄ NH₃ ]₂ CdCl₄ (4PBA-CdCl₄ , 4PBA=4-phenylbutylamine) that experiences two continuous phase transitions from centrosymmetric triclinic P 1 ‾ to polar chiral monoclinic P2 and then to another centrosymmetric tetragonal P4/mmm with increasing temperature, accompanied by symmetry breaking, due to the prominent octahedral distortion and disorder transformation of organic 4PBA cations. In the polar chiral phase, 4PBA-CdCl₄ gives a significant CD signal and has a moderate ferroelectric polarization of 0.35 μC/cm² . In addition, 4PBA-CdCl₄ occupies a wide band gap of 4.376 eV that is chiefly contributed by the inorganic CdCl₆ octahedron. This finding offers an alternative pathway for designing new phase transitions and related physical properties in hybrid halide perovskites and other hybrid crystals.

Bromine-Substitution-Induced High-Tc Two-Dimensional Bilayered Perovskite Photoferroelectric

High-Curie-temperature (T_c) ferroelectrics have exhibited broad applications in optoelectronic devices. Recently, two-dimensional multilayered perovskite ferroelectrics with excellent photoelectric attributes are attracting increasing interest as new systems of photoferroelectrics. However, the effective tuning of the T_c value of a multilayered perovskite photoferroelectric system still remains a huge challenge. Here, by a halogen substitution strategy to introduce bromine atoms on n-propylamine cations, the hybrid perovskite photoferroelectric (3-bromopropylaminium)₂(formamidinium)Pb₂Br₇ (BFPB) with a high T_c value (348.5 K) was obtained. It is notable that BFPB adopts a two-dimensional bilayered inorganic framework, with tight linking to the organic cation by C-Br···Br-Pb halogen···halogen interactions and N-H···Br hydrogen bonds. Intriguingly, in comparison with the prototypical compound (n-propylaminium)₂(formamidinium)Pb₂Br₇, a remarkable augmentation of 85.2 K in the resulting T_c value of BFPB is clearly observed, which further broadens the temperature range of its application. In combination with the remarkable ferroelectric and semiconducting attributes, the reversible bulk photovoltaic effect was realized in single crystals of BFPB. This finding can not only enhance the hybrid perovskite ferroelectric family but also further promote the photoelectric application of ferroelectrics.

Stabilization of Ferroelectric Phase in Highly Oriented Quinuclidinium Perrhenate (HQReO4) Thin Films

The low-temperature processability of molecular ferroelectric (FE) crystals makes them a potential alternative for perovskite oxide-based ferroelectric thin films. Quinuclidinium perrhenate (HQReO₄) is one such molecular FE crystal that exhibits ferroelectricity when crystallized in an intermediate temperature phase (ITP). However, bulk HQReO₄ crystals exhibit ferroelectricity only for a narrow temperature window (22 K), above and below which the polar phase transforms to a non-FE phase. The FE phase or ITP of HQReO₄ should be stabilized in a much wider temperature range for practical applications. Here, to stabilize the FE phase (ITP) in a wider temperature range, highly oriented thin films of HQReO₄ were prepared using a simple solution process. A slow evaporation method was adapted for drying the HQReO₄ thin films to control the morphology and the temperature window. The temperature window of the intermediate temperature FE phase was successfully widened up to 35 K by merely varying the film drying temperature between 333 and 353 K. The strategy of stabilizing the FE phase in a wider temperature range can be adapted to other molecular FE materials to realize flexible electronic devices.

Record Enhancement of Curie Temperature in Host-Guest Inclusion Ferroelectrics

Solid-state molecular rotor-type materials such as host-guest inclusion compounds are very desirable for the construction of molecular ferroelectrics. However, they usually have a low Curie temperature (T_c) and uniaxial nature, severely hindering their practical applications. Herein, by regulating the anion to control "momentum matching" in the crystal structure, we successfully designed a high-temperature multiaxial host-guest inclusion ferroelectric [(MeO-C₆H₄-NH₃)(18-crown-6)][TFSA] (MeO-C₆H₄-NH₃ = 4-methoxyanilinium, TFSA = bis(trifluoromethanesulfonyl)ammonium) with the Aizu notation of mmmFm. Compared to the parent uniaxial ferroelectric [(MeO-C₆H₄-NH₃)(18-crown-6)][BF₄] with a T_c of 127 K, the introduction of larger TFSA anions brings a lower crystal symmetry at room temperature and a higher energy barrier of molecular motions in phase transition, giving [(MeO-C₆H₄-NH₃)(18-crown-6)][TFSA] multiaxial ferroelectricity and a high T_c up to 415 K (above that of BaTiO₃). To our knowledge, such a record temperature enhancement of 288 K makes its T_c the highest among the reported crown-ether-based ferroelectrics, giving a wide working temperature range for applications in data storage, temperature sensing, actuation, and so on. This work will provide guidance and inspiration for designing high-T_c host-guest inclusion ferroelectrics.

Pressure-induced ferroelectric-like transition creates a polar metal in defect antiperovskites Hg3Te2X2 (X = Cl, Br)

Ferroelectricity is typically suppressed under hydrostatic compression because the short-range repulsions, which favor the nonpolar phase, increase more rapidly than the long-range interactions, which prefer the ferroelectric phase. Here, based on single-crystal X-ray diffraction and density-functional theory, we provide evidence of a ferroelectric-like transition from phase I2₁3 to R3 induced by pressure in two isostructural defect antiperovskites Hg₃Te₂Cl₂ (15.5 GPa) and Hg₃Te₂Br₂ (17.5 GPa). First-principles calculations show that this transition is attributed to pressure-induced softening of the infrared phonon mode Γ₄, similar to the archetypal ferroelectric material BaTiO₃ at ambient pressure. Additionally, we observe a gradual band-gap closing from ~2.5 eV to metallic-like state of Hg₃Te₂Br₂ with an unexpectedly stable R3 phase even after semiconductor-to-metal transition. This study demonstrates the possibility of emergence of polar metal under pressure in this class of materials and establishes the possibility of pressure-induced ferroelectric-like transition in perovskite-related systems.

Generally, ferroelectricity in ABO₃ perovskites is suppressed by hydrostatic compression, but the evidence for pressure-induced ferroelectricity remains elusive. Here, the authors find a direct ferroelectric-like structural transition induced by pressure in defect antiperovskites.

Mechanisms for collective inversion-symmetry breaking in dabconium perovskite ferroelectrics

Dabconium hybrid perovskites include a number of recently-discovered ferroelectric phases with large spontaneous polarisations. The origin of ferroelectric response has been rationalised in general terms in the context of hydrogen bonding, covalency, and strain coupling. Here we use a combination of simple theory, Monte Carlo simulations, and density functional theory calculations to assess the ability of these microscopic ingredients-together with the always-present through-space dipolar coupling-to account for the emergence of polarisation in these particular systems whilst not in other hybrid perovskites. Our key result is that the combination of A-site polarity, preferred orientation along 〈111〉 directions, and ferroelastic strain coupling drives precisely the ferroelectric transition observed experimentally. We rationalise the absence of polarisation in many hybrid perovskites, and arrive at a set of design rules for generating FE examples beyond the dabconium family alone.

Multifunctional rare earth molecular ferroelectrics with a piezoelectric response: ((nBu)4N)3[Ce(NO3)4(SCN)2]((CH3CH2CH2CH2)4N = tetrabutylammonium)

A new type of multipolar rare earth molecular ferroelectric: ((nBu)4N)3[Ce(NO3)4(SCN)2] (BuCH3CH2CH2CH2)4N), undergoes a high-temperature ferroelectric phase transition, possesses flexible switchable SHG effect, moderate spontaneous polarization and a narrow band gap.

Diverse physical functionalities of rare-earth hexacyanidometallate frameworks and their molecular analogues

The combination of rare-earth metal complexes and hexacyanidometallates of transition metals is a fruitful pathway for achieving functional materials exhibiting a wide scope of mechanical, magnetic, optical, and electrochemical properties.

Linear Electro-Optic Modulation in Highly Polarizable Organic Perovskites

Electrical-to-optical signal conversion is widely employed in information technology and is implemented using on-chip optical modulators. State-of-the-art modulator technologies are incompatible with silicon manufacturing techniques: inorganic nonlinear crystals such as LiNbO₃ are integrated with silicon photonic chips only using complex approaches, and hybrid silicon-LiNbO₃ optical modulators show either low bandwidth or high operating voltage. Organic perovskites are solution-processed materials readily integrated with silicon photonics; and organic molecules embedded within the perovskite scaffold allow in principle for high polarizability. However, it is found that the large molecules required for high polarizability also require an increase of the size of the perovskite cavity: specifically, using the highly polarizable DR²⁺ (R = H, F, Cl) in the A site necessitates the exploration of new X-site options. Only by introducing BF₄ ^- as the X-site molecule is it possible to synthesize (DCl)(NH₄ )(BF₄ )₃ , a material exhibiting a linear EO coefficient of 20 pm V^-1 , which is 10 times higher than that of metal halide perovskites and is a 1.5 fold enhancement compared to reported organic perovskites. The EO response of the organic perovskite approaches that of LiNbO₃ (r_eff  ≈ 30 pm V^-1 ) and highlights the promise of rationally designed organic perovskites for use in efficient EO modulators.

A high-temperature halide perovskite molecular ferroelastic with evident dielectric switching

Under the quasi-spherical strategy, a hybrid halide perovskite (TMTB)CdCl₃ is designed and synthesized and shows evident high-temperature ferroelastic phase transition and dielectric switching.

A polar oxyhalogen-vanadate compound (C5NH13Cl)2VOCl4 with optical and two-staged dielectric switch behavior

Polar organic-inorganic hybrid materials have been applied in ferroelectricity, as well as in devices based on nonlinear optical and piezoelectricity properties. Here, we used a rectangular pyramid structure of [VOCl4]2- to construct a polar compound (C5NH13Cl)2VOCl4 (1). Compound 1 crystallized in the monoclinic P21 space group. Coexistence of nonlinear optical switching behavior (space-group change from P21 to P21/n) and two-staged thermosensitive dielectric switching properties could be achieved under the stimulus of temperature. Our findings provide an effective approach for construction of polar materials.

Room-temperature dielectric switching in a host–guest crown ether inclusion complex

Using the “momentum matching” theory, we have designed a new host–guest crown ether inclusion complex, which exhibits prominent room temperature bistable dielectric switching.

Halogen regulation triggers NLO and dielectric dual switches in hybrid compounds with green fluorescence

An effective strategy of using halogens to modify organic–inorganic hybrid materials to obtain NLO switching characteristics, which is expected to be used for the directional adjustment of NLO switch activity.

Red/green-light emission in continuous dielectric phase transition materials: [Me3NVinyl]2[MnX4] (X = Cl, Br)

The luminescence of dielectric phase transition materials is one important property for technological applications, such as low-energy electron excitation. The combination of dielectric phase transitions and luminescence within organic–inorganic hybrids would lead to a new type of luminescent dielectric phase transition multifunctional material. Here, we report two novel A₂BX₄ organic–inorganic hybrid complexes [Me₃NVinyl]₂[MnCl₄] 1 and [Me₃NVinyl]₂[MnBr₄] 2, ([Me₃NVinyl] = trimethylvinyl ammonium cation). The complexes 1 and 2 were found to undergo continuous reversible phase transitions as well as switch dielectric phase transitions. Strikingly, intensive red luminescence and green luminescence were obtained under UV excitation respectively to reveal potential application of the two complexes in multi-functional materials along with dielectric switches and so on.

The luminescence of dielectric phase transition materials is one important property for technological applications, such as low-energy electron excitation.

Chiral organic–inorganic lead halide perovskites based on α-alanine

Novel hybrid organic–inorganic perovskites AlaH⁺PbHal₃⁻·H₂O incorporating chiral α-alanine were synthesized.

Multisequential reversible phase transition materials with semiconducting and fluorescence properties: (C8H18BrN)2SnBr6

A novel organic–inorganic hybrid metal halide has synthesized, namely (C8H18BrN)2SnBr6 (1), which have excellent switchable dielectric properties, narrow band gap and broadband yellow fluorescence characteristics.

Above room-temperature dielectric switching and semiconducting properties of a layered organic-inorganic hybrid compound: (C6H12N)2Pb(NO3)4

The well-studied star compound, CH3NH3PbI3, has attracted plenty of attention because of its remarkable optical and electrical properties. Consequently, new switching multifunctional hybrid compounds can be widely used in many fields such as solar cells, light-emitting diodes, optical data storage and so on. Therefore, switching multifunctional hybrid compounds with dielectric and semiconducting properties simultaneously will also find roles in the next generation of optoelectronic coupling materials. In fact, discovering an effective method to synthesize (multi)functional hybrid materials remains a pressing challenge. Thanks to the "quasi-spherical theory" proposed by Xiong et al., we used 7-azabicyclo[2.2.1]heptane as the quasi-spherical cation to construct molecule-based crystalline materials that exhibit responsive properties. Then, we tried to exploit the knowledge of crystal engineering and coordination chemistry to explain (multi)functional molecular materials. A layered organic-inorganic hybrid compound, (C6H12N)2Pb(NO3)4 (1), was grown and its dielectric switching property and semiconducting behaviour were investigated. Insights from differential scanning calorimetry measurements, variable-temperature X-ray structural studies, and dielectric spectroscopy revealed the origin of the phase transition, which is related to the motion of the organic ammonium and inorganic framework in solid-state crystals. Furthermore, 1 is also a wide bandgap semiconductor with an optical bandgap of 3.53 eV. The realization of switching and semiconducting properties simultaneously in layered Pb-based perovskites has a great significance toward research into hybrid compounds and the development of dielectric-optoelectronic integrated materials.

Two-Dimensional Hybrid Perovskite Ferroelectric Induced by Perfluorinated Substitution

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs), which possess the merits of good material stability, structural diversity, and ease of fabrication, are highly desirable for widespread applications of ferroelectrics, solar cells, and electroluminescent devices. Although some molecular design strategies toward ferroelectrics have been proposed, however, it is still a great challenge to precisely induce and optimize the ferroelectricity in 2D HOIPs. Here, for the first time through perfluorinated substitution strategy, we successfully design a high-performance 2D HOIP ferroelectric, (perfluorobenzylammonium)₂PbBr₄, exhibiting more obvious second harmonic generation intensity, larger piezoelectric response, more polar axes, larger spontaneous polarization of 4.2 μC cm^-2, and higher Curie temperature of 440 K than those of parent (benzylammonium)₂PbBr₄. Compared to the selective effect of monofluorinated substitution on different positions of the benzene ring, where (3-fluorobenzylammonium)₂PbBr₄ and (4-fluorobenzylammonium)₂PbBr₄ are not ferroelectrics, the pioneering perfluorinated substitution is more universal and effective for targeted design of aromatic ferroelectrics. This work offers an efficient strategy for precisely designing high-performance 2D HOIP ferroelectrics.

Thermo- and light-triggered reversible interconversion of dysprosium-anthracene complexes and their responsive optical, magnetic and dielectric properties

Artificial smart materials with switchable multifunctionality are of immense interest owing to their wide application in sensors, displays and memory devices. Lanthanide complexes are promising multifunctional materials integrating optical and magnetic characteristics. However, synergistic manipulation of different physical properties in lanthanide systems is still challenging. Herein we designed and synthesized a mononuclear complex [DyIII(SCN)3(depma)2(4-hpy)2] (1), which incorporates 9-diethylphosphonomethylanthracene (depma) as a photo-active component and 4-hydroxypyridine (4-hpy) as a polar component. This compound shows several unusual features: (a) reversible thermo-responsive phase transition associated with the order-disorder transition of 4-hpy and SCN-, which leads to thermochromic behavior and dielectric anomaly; (b) reversible photo-induced dimerization of anthracene groups, which leads to synergistic switching of luminescence, magnetic and dielectric properties. To our knowledge, compound 1 is the first example of lanthanide complexes that show stimuli-triggered synergistic and reversible switching of luminescence, magnetic and dielectric properties.

Methylphosphonium Tin Bromide: A 3D Perovskite Molecular Ferroelectric Semiconductor

3D ABX₃ organic-inorganic halide perovskite (OIHP) semiconductors like [CH₃ NH₃ ]PbI₃ have received great attention because of their various properties for wide applications. However, although a number of low-dimensional lead-based OIHP ferroelectric semiconductors have been documented, obtaining 3D ABX₃ OIHP ferroelectric semiconductors is challenging. Herein, an A-site cation [CH₃ PH₃ ]⁺ (methylphosphonium, MP) is employed to successfully obtain a lead-free 3D ABX₃ OIHP ferroelectric semiconductor MPSnBr₃ , which shows clear above-room-temperature ferroelectricity and a direct bandgap of 2.62 eV. It is emphasized that MPSnBr₃ is a multiaxial molecular ferroelectric with the number of ferroelectric polar axes being as many as 12, which is far more than those of the other OIHP ferroelectric semiconductors and even the classical inorganic perovskite ferroelectric semiconductors BiFeO₃ (4 polar axes) and BaTiO₃ (3 polar axes). MPSnBr₃ is the first MP-based 3D ABX₃ OIHP ferroelectric semiconductor. This finding throws light on the exploration of other excellent 3D ABX₃ OIHP ferroelectric semiconductors with great application prospects.

Organometallic-Based Hybrid Perovskite Piezoelectrics with a Narrow Band Gap

Hybrid organic-inorganic perovskites (HOIPs) with the general formula ABX₃ hold phenomenal research interest for their great scientific and technological potential in photovoltaic, piezoelectric, and electroluminescent devices. It is their considerable structural diversity that offers a good opportunity to build a variety of HOIP structures with various functionalities. However, no organometallic-based HOIP piezoelectrics have yet been found, despite the structural diversity and functional richness of organometallic compounds such as the ferrocene-based family. Here, for the first time, we report an organometallic-based HOIP piezoelectric, [(ferrocenylmethyl)trimethylammonium]PbI₃. Benefitting from the stability of ferrocene-based cations, excellent piezoelectric performance, comparable to that of LiNbO₃, can be obtained and optimized by tuning the anionic framework. The involvement of organometallic cations enables a narrow band gap of 2.37 eV, much lower than those of most HOIPs and some inorganic semiconductors. This work provides a new future direction for the study of perovskites and will inspire intriguing research on organometallic-based HOIP piezoelectrics.

An unusual high-frequency ferroelectric obtained via the post-synthetic modification of a metal-organic framework

Ferroelectrics as crucial functional materials have attracted much interest since ferroelectricity was discovered in 1920. Herein, an unusual high-frequency ferroelectric, (CH3)2NH·HCl@Cd-MOF, was successfully obtained through a dual-step synthetic methodology. A chiral porous Cd-MOF with a channel size of 6.8 × 6.8 Å was synthesized via self-assembly of chiral Schiff-base ligands and Cd2+ ions. Subsequently, polarizable (CH3)2NH·HCl was introduced into the channels of the Cd-MOF and hence the host-guest system (CH3)2NH·HCl@Cd-MOF was formed. The as-synthesized (CH3)2NH·HCl@Cd-MOF displays obvious ferroelectricity at a high frequency of 1 kHz. Such a high-frequency ferroelectric is extremely rare among MOF-based ferroelectric materials, and the high-frequency ferroelectricity means that (CH3)2NH·HCl@Cd-MOF has potential for use in ferroelectric memories. The results again demonstrate that post-synthetic modification is a promising approach for achieving rational and precise design of ferroelectric materials.

First-principles experimental demonstration of ferroelectricity in a thermotropic nematic liquid crystal: Polar domains and striking electro-optics

We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable polarization locally parallel to the director. This polarization density saturates at a low temperature value of ∼6 µC/cm², the largest ever measured for a fluid or glassy material. This polarization is comparable to that of solid state ferroelectrics and is close to the average value obtained by assuming perfect, polar alignment of molecular dipoles in the nematic. We find a host of spectacular optical and hydrodynamic effects driven by ultralow applied field (E ∼ 1 V/cm), produced by the coupling of the large polarization to nematic birefringence and flow. Electrostatic self-interaction of the polarization charge renders the transition from the nematic phase mean field-like and weakly first order and controls the director field structure of the ferroelectric phase. Atomistic molecular dynamics simulation reveals short-range polar molecular interactions that favor ferroelectric ordering, including a tendency for head-to-tail association into polar, chain-like assemblies having polar lateral correlations. These results indicate a significant potential for transformative, new nematic physics, chemistry, and applications based on the enhanced understanding, development, and exploitation of molecular electrostatic interaction.