Ferroelastic-switching-driven large shear strain and piezoelectricity in a hybrid ferroelectric

High-κ monocrystalline dielectrics for low-power two-dimensional electronics

The downscaling of complementary metal-oxide-semiconductor technology has produced breakthroughs in electronics, but more extreme scaling has hit a wall of device performance degradation. One key challenge is the development of insulators with high dielectric constant, wide bandgap and high tunnel masses. Here, we show that two-dimensional monocrystalline gadolinium pentoxide, which is devised through combining particle swarm optimization algorithm and theoretical calculations and synthesized via van der Waals epitaxy, could exhibit a high dielectric constant of ~25.5 and a wide bandgap simultaneously. A desirable equivalent oxide thickness down to 1 nm with an ultralow leakage current of ~10-4 A cm-2 even at 5 MV cm-1 is achieved. The molybdenum disulfide transistors gated by gadolinium pentoxide exhibit high on/off ratios over 108 and near-Boltzmann-limit subthreshold swing at an operation voltage of 0.5 V. We also constructed inverter circuits with high gain and nanowatt power consumption. This reliable approach to integrating ultrathin monocrystalline insulators paves the way to future nanoelectronics.

Organic Ferroelastic with Dual-Channel Manipulation Obtained by H/F Substitution

Ferroelastic materials with high phase transition temperature have broad application prospects in information conversion and storage, shape memory, energy conversion, hyperelasticity, etc. However, most of the current reports focus on inorganic ferroelastic materials. Inorganic ferroelastic materials have the disadvantages of high energy consumption and harmful metals, which limit their application in practical work. In contrast, organic ferroelastic materials have the advantages of structural adjustability, environmental protection, easy processing, low cost, mechanical flexibility, and so on, which have great development potential in new ferroelastic materials. Here, we have successfully designed and synthesized a pair of homochiral enantiomers [(R/S)-4-fluorobenzoic acid-2-amino-2-phenylethanol] (R- and S-F) using the chemical design strategy of H/F substitution. Compared with the non-F substitution [(R/S)-benzoic acid-2-amino-2-phenylethanol] (R- and S-H), they undergo 2F1-type ferroelastic phase transitions at 370 K. Notably, the ferroelastic domains of R/S-F can be controlled through two physical channels that are temperature and stress, showing great potential in dual-channel switches.

Exploring Aqueous Solution-Processed Pseudohalide Rare-Earth Double Perovskite Ferroelectrics toward X-Ray Detection with High Sensitivity

Three-dimensional (3D) pseudohalide rare-earth double perovskites (PREDPs) have garnered significant attention for their versatile physical properties, including ferroelectricity, ferroelasticity, large piezoelectric responses, and circularly polarized luminescence. However, their potential for X-ray detection remains unexplored, and the low Curie temperature (TC) limits the performance window for PREDP ferroelectrics. Here, by applying the chemical regulation strategies involving halogen substitution on the organic cation and Rb/Cs substitution to the PREDP [(R)-M3HQ]2RbEu(NO3)6 [(R)-M3HQ=(R)-N-methyl-3-hydroxylquinuclidinium] with a low TC of 285 K, a novel 3D PREDP ferroelectric [(R)-CM3HQ]2CsEu(NO3)6 [(R)-CM3HQ=(R)-N-chloromethyl-3-hydroxylquinuclidinium] are successfully synthesized, for which the TC reaches 344 K. More importantly, such a strategy endowed [(R)-CM3HQ]2CsEu(NO3)6 with notable X-ray detection capabilities. Centimeter-sized [(R)-CM3HQ]2CsEu(NO3)6 single crystals fabricated from aqueous solutions demonstrated a sensitivity of 1307 μC Gyair -1 cm-2 and a low detectable dose rate of 152 nGyair s-1, the highest sensitivity reported for hybrid double perovskite ferroelectric detectors. This work positions PREDPs as promising candidates for the next generation of eco-friendly optoelectronic materials and also offers substantial insights into the interaction between structure, composition, and functionality in ferroelectric materials.

Achievement of a giant electromechanical conversion coefficient in a molecule-based ferroelectric

Molecule-based ferroelectrics are promising candidates for flexible self-powered power supplies (i.e., piezoelectric generators (PEGs)). Although the large electromechanical conversion coefficients (d 33 × g 33) of piezoelectrics are key to enhancing the performance of PEGs in their nonresonant states, it remains a great challenge to obtain molecule-based piezoelectrics with large d 33 × g 33. Here, we report a molecule-based ferroelectric [(CH3)3NCH2CH2Cl][GaBr4] (1) that exhibits the largest piezoelectric coefficient (∼454 pC N-1) and electromechanical conversion coefficient (4953.1 × 10-12 m2 N-1) among all known free-standing polycrystalline pellets. Notably, the PEG comprising 15 wt% 1 and polydimethylsiloxane (PDMS) achieves a power density of up to 120 μW cm-2, marking the highest reported power density for ferroelectric@PDMS PEGs to date.

Flexible self-powered bioelectronics enables personalized health management from diagnosis to therapy

Flexible self-powered bioelectronics (FSPBs), incorporating flexible electronic features in biomedical applications, have revolutionized the human-machine interface since they hold the potential to offer natural and seamless human interactions while overcoming the limitations of battery-dependent power sources. Furthermore, as biosensors or actuators, FSPBs can dynamically monitor physiological signals to reveal real-time health abnormalities and provide timely and precise treatments. Therefore, FSPBs are increasingly shaping the landscape of health monitoring and disease treatment, weaving a sophisticated and personalized bond between humans and health management. Here, we examine the recent advanced progress of FSPBs in developing working mechanisms, design strategies, and structural configurations toward personalized health management, emphasizing its role in clinical medical scenarios from biophysical/biochemical sensors for sensing diagnosis to robust/biodegradable actuators for intervention therapy. Future perspectives on the challenges and opportunities in emerging multifunctional FSPBs for the next-generation health management systems are also forecasted.

Strong Vertical Piezoelectricity and Broad-pH-Value Photocatalyst in Ferroelastic Y2Se2BrF Monolayer

With the development of miniaturized devices, there is an increasing demand for 2D multifunctional materials. Six ferroelastic semiconductors, Y2Se2XX' (X, X' = I, Br, Cl, or F; X ≠ X') monolayers, are theoretically predicted here. Their in-plane anisotropic band structure, elastic and piezoelectric properties can be switched by ferroelastic strain. Moderate energy barriers can prevent the undesired ferroelastic switching that minor interferences produce. These monolayers exhibit high carrier mobilities (up to 104 cm2 V-1 s-1) with strong in-plane anisotropy. Furthermore, their wide bandgaps and high potential differences make them broad-pH-value and high-performance photocatalysts at pH value of 0-14. Strikingly, Y2Se2BrF possesses outstanding d33 (d33 = -405.97 pm/V), greatly outperforming CuInP2S6 by 4.26 times. Overall, the nano Y2Se2BrF is a hopeful candidate for multifunctional devices to generate a direct current and achieve solar-free photocatalysis. This work provides a new paradigm for the design of multifunctional energy materials.

The past 10 years of molecular ferroelectrics: structures, design, and properties

Ferroelectricity, which has diverse important applications such as memory elements, capacitors, and sensors, was first discovered in a molecular compound, Rochelle salt, in 1920 by Valasek. Owing to their superiorities of lightweight, biocompatibility, structural tunability, mechanical flexibility, etc., the past decade has witnessed the renaissance of molecular ferroelectrics as promising complementary materials to commercial inorganic ferroelectrics. Thus, on the 100th anniversary of ferroelectricity, it is an opportune time to look into the future, specifically into how to push the boundaries of material design in molecular ferroelectric systems and finally overcome the hurdles to their commercialization. Herein, we present a comprehensive and accessible review of the appealing development of molecular ferroelectrics over the past 10 years, with an emphasis on their structural diversity, chemical design, exceptional properties, and potential applications. We believe that it will inspire intense, combined research efforts to enrich the family of high-performance molecular ferroelectrics and attract widespread interest from physicists and chemists to better understand the structure-function relationships governing improved applied functional device engineering.

Oxide-Halide Perovskite Composites for Simultaneous Recycling of Lead Zirconate Titanate Piezoceramics and Methylammonium Lead Iodide Solar Cells

Global concerns over energy availability and the environment impose an urgent requirement for sustainable manufacturing, usage, and disposal of electronic components. Piezoelectric and photovoltaic components are being extensively used. They contain the hazardous element, Pb (e.g., in widely used and researched Pb(Zr,Ti)O3 and halide perovskites), but they are not being properly recycled or reused. This work demonstrates the fabrication of upside-down composite sensor materials using crushed ceramic particles recycled from broken piezoceramics, polycrystalline halide perovskite powder collected from waste dye-sensitized solar cells, and crystal particles of a Cd-based perovskite composition, C6H5N(CH3)3CdBr3 xCl3(1- x ). The piezoceramic and halide perovskite particles are used as filler and binder, respectively, to show a proof of concept for the chemical and microstructural compatibility between the oxide and halide perovskite compounds while being recycled simultaneously. Production of the recycled and reusable materials requires only a marginal energy budget while achieving a very high material densification of >92%, as well as a 40% higher piezoelectric voltage coefficient, i.e., better sensing capability, than the pristine piezoceramics. This work thus offers an energy- and environmentally friendly approach to the recycling of hazardous elements as well as giving a second life to waste piezoelectric and photovoltaic components.

Modulating the ferroelectric phases in cholesteryl-based organic compounds with perfluoroalkyl tail engineering

Here, we synthesized a series of cholesteryl-based compounds, whose phases and their transformation can be modulated by temperature and the chain length of the fluoroalkyl moieties. To our knowledge, this is the first time that the phase transition could be modulated with perfluoroalkyl tail engineering in organic single-component ferroelectric crystals.

Ferroics in Hybrid Organic-Inorganic Perovskites: Fundamentals, Design Strategies, and Implementation

Hybrid organic-inorganic perovskites (HOIPs) afford highly versatile structure design and lattice dimensionalities; thus, they are actively researched as material platforms for the tailoring of ferroic behaviors. Unlike single-phase organic or inorganic materials, the interlayer coupling between organic and inorganic components in HOIPs allows the modification of strain and symmetry by chirality transfer or lattice distortion, thereby enabling the coexistence of ferroic orders. This review focuses on the principles for engineering one or multiple ferroic orders in HOIPs, and the conditions for achieving multiferroicity and magnetoelectric properties. The prospects of multilevel ferroic modulation, chiral spin textures, and spin orbitronics in HOIPs are also presented.

Organic-Inorganic Rare-Earth Double Perovskite Ferroelectric with Large Piezoelectric Response and Ferroelasticity for Flexible Composite Energy Harvesters

Hybrid organic-inorganic perovskite (HOIP) ferroelectric materials have great potential for developing self-powered electronic transducers owing to their impressive piezoelectric performance, structural tunability and low processing temperatures. Nevertheless, their inherent brittle and low elastic moduli limit their application in electromechanical conversion. Integration of HOIP ferroelectrics and soft polymers is a promising solution. In this work, a hybrid organic-inorganic rare-earth double perovskite ferroelectric, [RM3HQ]2RbPr(NO3)6 (RM3HQ = (R)-N-methyl-3-hydroxylquinuclidinium) is presented, which possesses multiaxial nature, ferroelasticity and satisfactory piezoelectric properties, including piezoelectric charge coefficient (d33) of 102.3 pC N-1 and piezoelectric voltage coefficient (g33) of 680 × 10-3 V m N-1. The piezoelectric generators (PEG) based on composite films of [RM3HQ]2RbPr(NO3)6@polyurethane (PU) can generate an open-circuit voltage (Voc) of 30 V and short-circuit current (Isc) of 18 µA, representing one of the state-of-the-art PEGs to date. This work has promoted the exploration of new HOIP ferroelectrics and their development of applications in electromechanical conversion devices.

Designing dynamic coordination bonds in polar hybrid crystals for a high-temperature ferroelastic transition

Ferroelastic materials have gained widespread attention as promising candidates for mechanical switches, shape memory, and information processing. Their phase-transition mechanisms usually originate from conventional order-disorder and/or displacive types, while those involving dynamic coordination bonds are still scarce. Herein, based on a strategic molecular design of organic cations, we report three new polar hybrid crystals with a generic formula of AA'RbBiCl6 (A = A' = Me3SO+ for 1; A = Me3SO+ and A' = Me4N+ for 2; A = A' = Me3NNH2+ for 3). Their A-site cations link to the [RbBiCl6]n2n- inorganic framework with lon topology through Rb-O/N coordination bonds, while their significantly different interactions between A'-site cations and inorganic frameworks provide distinct phase-transition behaviour. In detail, the strongly coordinative A'-site Me3SO+ cations prevent 1 from a structural phase transition, while coordinatively free A'-site Me4N+ cations trigger a conventional order-disorder ferroelastic transition at 247 K in 2, accompanied by a latent heat of 0.63 J g-1 and a usual "high → low" second-harmonic-generation (SHG) switch. Interestingly, the A'-site Me3NNH2+ cations in 3 reveal unusual dynamic coordination bonds, driving a high-temperature ferroelastic transition at 369 K with a large latent heat of 18.34 J g-1 and an unusual "low → high" SHG-switching behaviour. This work provides an effective molecular assembly strategy to establish dynamic coordination bonds in a new type of host-guest model and opens an avenue for designing advanced ferroelastic multifunctional materials.

Large Enhancement of Polarization in a Layered Hybrid Perovskite Ferroelectric Semiconductor via Molecular Engineering

Switchable spontaneous polarization is the vital property of ferroelectrics, which leads to other key physical properties such as piezoelectricity, pyroelectricity, and nonlinear optical effects, etc. Recently, organic-inorganic hybrid perovskites with 2D layered structure have become an emerging branch of ferroelectric materials. However, most of the 2D hybrid ferroelectrics own relatively low polarizations (<15 µC cm-2 ). Here, a strategy to enhance the polarization of these hybrid perovskites by using ortho-, meta-, para-halogen substitution is developed. Based on (benzylammonium)2 PbCl4 (BZACL), the para-chlorine substituted (4-chlorobenzylammonium)2 PbCl4 (4-CBZACL) ferroelectric semiconductor shows a large spontaneous polarization (23.3 µC cm-2 ), which is 79% larger than the polarization of BZACL. This large enhancement of polarization is successfully explained via ab initio calculations. The study provides a convenient and efficient strategy to promote the ferroelectric property in the hybrid perovskite family.

A High Working Temperature Multiferroic Induced by Inverse Temperature Symmetry Breaking

Molecular-based multiferroic materials that possess ferroelectric and ferroelastic orders simultaneously have attracted tremendous attention for their potential applications in multiple-state memory devices, molecular switches, and information storage systems. However, it is still a great challenge to effectively construct novel molecular-based multiferroic materials with multifunctionalities. Generally, the structure of these materials possess high symmetry at high temperatures, while processing an obvious order-disorder or displacement-type ferroelastic or ferroelectric phase transition triggered by symmetry breaking during the cooling processes. Therefore, these materials can only function below the Curie temperature (Tc), the low of which is a severe impediment to their practical application. Despite great efforts to elevate Tc, designing single-phase crystalline materials that exhibit multiferroic orders above room temperature remains a challenge. Here, an inverse temperature symmetry-breaking phenomenon was achieved in [FPM][Fe3(μ3-O)(μ-O2CH)8] (FPM stands for 3-(3-formylamino-propyl)-3,4,5,6-tetrahydropyrimidin-1-ium, which acts as the counterions and the rotor component in the network), enabling a ferroelastoelectric phase at a temperature higher than Tc (365 K). Upon heating from room temperature, two-step distinct symmetry breaking with the mm2Fm species leads to the coexistence of ferroelasticity and ferroelectricity in the temperature interval of 365-426 K. In the first step, the FPM cations undergo a conformational flip-induced inverse temperature symmetry breaking; in the second step, a typical ordered-disordered motion-induced symmetry breaking phase transition can be observed, and the abnormal inverse temperature symmetry breaking is unprecedented. Except for the multistep ferroelectric and ferroelastic switching, this complex also exhibits fascinating nonlinear optical switching properties. These discoveries not only signify an important step in designing novel molecular-based multiferroic materials with high working temperatures, but also inspire their multifunctional applications such as multistep switches.

Halogen-Regulated Tc and X-ray Radiation Detection in 2D Hybrid Perovskite Ferroelastic Semiconductor

Organic-inorganic hybrid perovskites (OIHPs) have received particular attention due to their characteristic structural tunability and flexibility. These features make OIHPs behave with excellent modifications on macroscopic properties, such as ferroicity or semiconductor performances, etc. Herein, we report two 2D hybrid stibium-based halide perovskite (C3H7N)3Sb2X9 (X = Br, 1; Cl, 2) ferroelastic semiconductor possessing dual switching properties of dielectric and second harmonic generation (SHG). Notably, these two hybrids exhibit halogen-regulated ferroelasticity and semiconductor properties. There is a significant difference in Curie temperature (Tc) and X-ray radiation detection sensitivity (S), i.e., the ΔTc and ΔS are 38 K and 87 μC Gyair-1 cm-2, respectively. Meanwhile, crystals 1 and 2 do not show dark current drift in cyclic measurements of different radiation doses with stable switching ratios of 30 and 10, separately. Meanwhile, these results were proven by scientific experimental results and density functional theory (DFT) calculations. Our work presents a facile and practical method to regulate macroproperties on the molecular level, providing a new vision to develop hybrid perovskite ferroic-photoelectric materials.

Tuning ferroelectric phase transition temperature by enantiomer fraction

Tuning phase transition temperature is one of the central issues in phase transition materials. Herein, we report a case study of using enantiomer fraction engineering as a promising strategy to tune the Curie temperature (TC) and related properties of ferroelectrics. A series of metal-halide perovskite ferroelectrics (S-3AMP)x(R-3AMP)1-xPbBr4 was synthesized where 3AMP is the 3-(aminomethyl)piperidine divalent cation and enantiomer fraction x varies between 0 and 1 (0 and 1 = enantiomers; 0.5 = racemate). With the change of the enantiomer fraction, the TC, second-harmonic generation intensity, degree of circular polarization of photoluminescence, and photoluminescence intensity of the materials have been tuned. Particularly, when x = 0.70 - 1, a continuously linear tuning of the TC is achieved, showing a tunable temperature range of about 73 K. This strategy provides an effective means and insights for regulating the phase transition temperature and chiroptical properties of functional materials.

Halogen Substitution Regulates High Temperature Dielectric Switch in Lead-Free Chiral Hybrid Perovskites

Hybrid metal halides (HMHs) based phase transition materials have received widespread attention due to their excellent performance and potential applications in energy harvesting, optoelectronics, ferroics, and actuators. Nevertheless, effectively regulating the properties of phase transitions is still a thorny problem. In this work, two chiral lead-free HMHs (R-3FP)2 SbCl5 (1; 3FP=3-fluoropyrrolidinium) and (R-3FP)2 SbBr5 (2) were synthesized. By replacing the halide ions in the inorganic skeleton, the phase transition temperature of 2 changes with an increase of about 20 K, compared with 1. Meanwhile, both compounds display reversible dielectric switching properties. Through crystal structure analysis and Hirshfeld surface analysis, their phase transitions are ascribed to the disorder of the cations and deformation of the inorganic chains.

Resolving electron and hole transport properties in semiconductor materials by constant light-induced magneto transport

The knowledge of minority and majority charge carrier properties enables controlling the performance of solar cells, transistors, detectors, sensors, and LEDs. Here, we developed the constant light induced magneto transport method which resolves electron and hole mobility, lifetime, diffusion coefficient and length, and quasi-Fermi level splitting. We demonstrate the implication of the constant light induced magneto transport for silicon and metal halide perovskite films. We resolve the transport properties of electrons and holes predicting the material's effectiveness for solar cell application without making the full device. The accessibility of fourteen material parameters paves the way for in-depth exploration of causal mechanisms limiting the efficiency and functionality of material structures. To demonstrate broad applicability, we further characterized twelve materials with drift mobilities spanning from 10-3 to 103 cm2V-1s-1 and lifetimes varying between 10-9 and 10-3 seconds. The universality of our method its potential to advance optoelectronic devices in various technological fields.

Zero-Dimensional Sn-Based Enantiomeric Phase-Transition Materials with High-Tc and Dielectric Switching

The combination of chirality and phase-transition materials has broad application prospects. Therefore, based on the quasi-spherical theory and the thought strategy of introducing chirality, we have successfully synthesized a pair of chiral enantiomeric ligands (R/S)-triethyl-(2-hydroxypropyl)ammonium iodide, which can be combined with a tin hexachloride anion to obtain a pair of new organic-inorganic hybrid enantiomeric high-temperature plastic phase-transition materials: (R/S)-[CH3 CH(OH)CH2 N(CH2 CH3 )3 ]2 SnCl6 (1-R/1-S), which have a high temperature phase transition of Tc =384 K, crystallize in the P21 chiral space group at room temperature, and have obvious CD signals. In addition, compounds 1-R and 1-S have a good low-loss dielectric switch and broadband gap. This work is conducive to the research into chiral high-temperature reversible plastic phase-transition materials, and promotes the development of multi-functional phase-transition materials.

A Two-Dimensional Hybrid Lead Bromide Ferroelectric Semiconductor with an Out-of-Plane Polarization

A two-dimensional (2D) organic-inorganic hybrid perovskite (OIHP) material with out-of-plane ferroelectricity is the key to the miniaturization of vertical-sandwich-type ferroelectric optoelectronic devices. However, 2D OIHP ferroelectrics with out-of-plane polarization are still scarce, and effective design strategies are lacking. Herein, we report a novel 2D Dion-Jacobson perovskite ferroelectric semiconductor synthesized by a rigid-to-flexible cationic tailoring strategy, achieving an out-of-plane polarization of 1.7 μC/cm2 and high photoresponse. Integrating out-of-plane ferroelectricity with excellent photoelectric properties affords a promising platform to investigate ferroelectricity-related effects in vertical optoelectronic devices.

Solid-Liquid Crystal Biphasic Ferroelectrics with Tunable Biferroelectricity

Ferroelectricity has been separately found in numerous solid and liquid crystal materials since its first discovery in 1920. However, a single material with biferroelectricity existing in both solid and liquid crystal phases is very rare, and the regulation of biferroelectricity has never been studied. Here, solid-liquid crystal biphasic ferroelectrics, cholestanyl 4-X-benzoate (4X-CB, X = Cl, Br, and I), which exhibits biferroelectricity in both the solid and liquid crystal phases, is presented. It is noted that the ferroelectric liquid crystal phase of 4X-CB is a cholesteric one, distinct from the ordinary chiral smectic ferroelectric liquid crystal phase. Moreover, 4X-CB shows solid-solid and solid-liquid crystal phase transitions, of which the transition temperatures gradually increase from Cl to Br to I substitution. The spontaneous polarization (Ps ) of 4X-CB in both solid and liquid crystal phases can also be regulated by different halogen substitutions, where the 4Br-CB has the optimal Ps because of the larger molecular dipole moment. To the authors' knowledge, 4X-CB is the first ferroelectric with tunable biferroelectricity, which offers a feasible case for the performance optimization of solid-liquid crystal biphasic ferroelectrics.

Facile Control of Ferroelectricity Driven by Ingenious Interaction Engineering

Construction of ferroelectric and optimization of macroscopic polarization has attracted tremendous attention for next generation light weight and flexible devices, which brings fundamental vitality for molecular ferroelectrics. However, effective molecular tailoring toward cations makes ferroelectric synthesis and modification relatively elaborate. Here, the study proposes a facile method to realize triggering and optimization of ferroelectricity. The experimental and theoretical investigation reveals that orientation and alignment of polar cations, dominated factors in molecular ferroelectrics, can be controlled by easily processed anionic modification. In one respect, ferroelectricity is induced by strengthened intermolecular interaction. Moreover, ≈50% of microscopic polarization enhancement (from 8.07 to 11.68 µC cm-2 ) and doubling of equivalent polarization direction (from 4 to 8) are realized in resultant ferroelectric FEtQ2ZnBrI3 (FEQZBI, FEtQ = N-fluoroethyl-quinuclidine). The work offers a totally novel platform for control of ferroelectricity in organic-inorganic hybrid ferroelectrics and a deep insight of structure-property correlations.

The First Ring Enlargement Induced Large Piezoelectric Response in a Polycrystalline Molecular Ferroelectric

Inorganic ferroelectrics have long dominated research and applications, taking advantage of high piezoelectric performance in bulk polycrystalline ceramic forms. Molecular ferroelectrics have attracted growing interest because of their environmental friendliness, easy processing, lightweight, and good biocompatibility, while realizing the considerable piezoelectricity in their bulk polycrystalline forms remains a great challenge. Herein, for the first time, through ring enlargement, a molecular ferroelectric 1-azabicyclo[3.2.1]octonium perrhenate ([3.2.1-abco]ReO4 ) with a large piezoelectric coefficient d33 up to 118 pC/N in the polycrystalline pellet form is designed, which is higher than that of the parent 1-azabicyclo[2.2.1]heptanium perrhenate ([2.2.1-abch]ReO4 , 90 pC/N) and those of most molecular ferroelectrics in polycrystalline or even single crystal forms. The ring enlargement reduces the molecular strain for easier molecular deformation, which contributes to the higher piezoelectric response in [3.2.1-abco]ReO4 . This work opens up a new avenue for exploring high piezoelectric polycrystalline molecular ferroelectrics with great potential in piezoelectric applications.

Near-90° Switch in the Polar Axis of Dion-Jacobson Perovskites by Halide Substitution

Ferroelectricity in two-dimensional hybrid (2D) organic-inorganic perovskites (HOIPs) can be engineered by tuning the chemical composition of the organic or inorganic components to lower the structural symmetry and order-disorder phase change. Less efforts are made toward understanding how the direction of the polar axis is affected by the chemical structure, which directly impacts the anisotropic charge order and nonlinear optical response. To date, the reported ferroelectric 2D Dion-Jacobson (DJ) [PbI₄]^2- perovskites exhibit exclusively out-of-plane polarization. Here, we discover that the polar axis in ferroelectric 2D Dion-Jacobson (DJ) perovskites can be tuned from the out-of-plane (OOP) to the in-plane (IP) direction by substituting the iodide with bromide in the lead halide layer. The spatial symmetry of the nonlinear optical response in bromide and iodide DJ perovskites was probed by polarized second harmonic generation (SHG). Density functional theory calculations revealed that the switching of the polar axis, synonymous with the change in the orientation of the sum of the dipole moments (DMs) of organic cations, is caused by the conformation change of organic cations induced by halide substitution.

Superior ferroelectricity and nonlinear optical response in a hybrid germanium iodide hexagonal perovskite

Abundant chemical diversity and structural tunability make organic-inorganic hybrid perovskites (OIHPs) a rich ore for ferroelectrics. However, compared with their inorganic counterparts such as BaTiO₃, their ferroelectric key properties, including large spontaneous polarization (P_s), low coercive field (E_c), and strong second harmonic generation (SHG) response, have long been great challenges, which hinder their commercial applications. Here, a quasi-one-dimensional OIHP DMAGeI₃ (DMA = Dimethylamine) is reported, with notable ferroelectric attributes at room temperature: a large P_s of 24.14 μC/cm² (on a par with BaTiO₃), a low E_c below 2.2 kV/cm, and the strongest SHG intensity in OIHP family (about 12 times of KH₂PO₄ (KDP)). Revealed by the first-principles calculations, its large P_s originates from the synergistic effects of the stereochemically active 4s² lone pair of Ge²⁺ and the ordering of organic cations, and its low kinetic energy barrier of small DMA cations results in a low E_c. Our work brings the comprehensive ferroelectric performances of OIHPs to a comparable level with commercial inorganic ferroelectric perovskites.

Pressure-Induced Ferroelastic Transition Drives a Large Shape Change in a Ni(II) Complex Single Crystal

Crystals with significant length reduction at an accessible low pressure are highly desirable for piezo-responsive devices. Here, we show a molecular crystal [Ni(en)₃](ox) (en = ethylenediamine and ox = oxalate anion) that exhibits an abrupt shape change with a contraction rate of ∼4.7% along its c axis near the phase transition pressure of ∼0.2 GPa. High-pressure single-crystal X-ray diffraction and Raman spectroscopy measurements reveal that this material undergoes a first-order ferroelastic transition from high-symmetry trigonal P3̅1c to low-symmetry monoclinic P2₁/n at ∼0.2 GPa. The oxalate anions serve as unique components, and their disorder-order transformation and rotation of 90° through cooperative intermolecular hydrogen bonding triggered unconventional anisotropic microsize contraction under compression, which can be appreciated visually. Such a prominent directional deformation at a low pressure driven by molecular motors of oxalate anions provides insights for the design of novel molecular crystal-based piezo-responsive switches and actuators in deep-sea environments.

Engineering Second-Order Corner States in 2D Multiferroics

The understanding and manipulate of the second-order corner states are central to both fundamental physics and future topotronics applications. Despite the fact that numerous second-order topological insulators (SOTIs) are achieved, the efficient engineering in a given material remains elusive. Here, the emergence of 2D multiferroics SOTIs in SbAs and BP₅ monolayers is theoretically demonstrated, and an efficient and straightforward way for engineering the nontrivial corner states by ferroelasticity and ferroelectricity is remarkably proposed. With ferroelectric polarization of SbAs and BP₅ monolayers, the nontrivial corner states emerge in the mirror symmetric corners and are perpendicular to orientations of the in-plane spontaneous polarization. And remarkably the spatial distribution of the corner states can be effectively tuned by a ferroelastic switching. At the intermediate states of both ferroelectric and ferroelastic switchings, the corner states disappear. These finding not only combines exotic SOTIs with multiferroics but also pave the way for experimental discovery of 2D tunable SOTIs.

Inversion of Molecular Chirality Associated with Ferroelectric Switching in a High-Temperature Two-Dimensional Perovskite Ferroelectric

Controlling molecular chirality by external stimuli is of great significance in both fundamental research and technological applications. Herein, we report a high-temperature (384 K) molecular ferroelectric of a Cu(II) complex whose spontaneous polarization can be switched associated with flipping of molecular chirality. In this two-dimensional perovskite structure, the inorganic layer is separated by (NH₃(CH₂)₂SS(CH₂)₂NH₃)²⁺ organic cations skewed in a chiral conformation (P- or M-helicity in an individual crystal). As the stereodynamic disulfide bridge determines the molecular dipole moment along the polar axis, the chiral organic cation can be converted to its enantiomer as a consequence of an electric field-induced shift of the S-S moiety relative to its screw axis during the ferroelectric switching. The variation of the molecular chirality is examined with single-crystal X-ray diffraction and circular dichroism spectra. The simultaneous switching of molecular chirality and spontaneous polarization in this perovskite ferroelectric may lead to novel chiral electronic phenomena.

Large Piezoelectric Response in a Metal-Free Three-Dimensional Perovskite Ferroelectric

Metal-free perovskites with light weight and eco-friendly processability have received great interest in recent years due to their superior physical features in ferroelectrics, X-ray detection, and optoelectronics. The famous metal-free perovskite ferroelectric MDABCO-NH₄-I₃ (MDABCO = N-methyl-N'-diazabicyclo[2.2.2]octonium) has been demonstrated to exhibit excellent ferroelectricity comparable to that of inorganic ceramic ferroelectric BaTiO₃, such as large spontaneous polarization and high Curie temperature (Ye et al. Science 2018, 361, 151). However, piezoelectricity as a vitally important index is far from enough in the metal-free perovskite family. Here, we report the discovery of large piezoelectric response in a new metal-free three-dimensional perovskite ferroelectric NDABCO-NH₄-Br₃ (NDABCO = N-amino-N'-diazabicyclo[2.2.2]octonium) by replacing the methyl group of MDABCO with the amino group. Besides the evident ferroelectricity, strikingly, NDABCO-NH₄-Br₃ shows a large d₃₃ of 63 pC/N more than 4 times that of MDABCO-NH₄-I₃ (14 pC/N). The d₃₃ value is also strongly supported by the computational study. To the best of our knowledge, such a large d₃₃ value ranks the highest among the documented organic ferroelectric crystals to date and represents a major breakthrough in metal-free perovskite ferroelectrics. Combined with decent mechanical properties, NDABCO-NH₄-Br₃ is expected to be a competitive candidate for medical, biomechanical, wearable, and body-compatible ferroelectric devices.

High-T c Single-Component Organosilicon Ferroelectric Crystal Obtained by H/F Substitution

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/mmmFmm2-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.

A nickel(ii)-based one-dimensional organic-inorganic halide perovskite ferroelectric with the highest Curie temperature

Organic-inorganic halide perovskites (OIHPs) are very eye-catching due to their chemical tunability and rich physical properties such as ferroelectricity, magnetism, photovoltaic properties and photoluminescence. However, no nickel-based OIHP ferroelectrics have been reported so far. Here, we designed an ABX3 OIHP ferroelectric (3-pyrrolinium)NiCl3, where the 3-pyrrolinium cations are located on the voids surrounded by one-dimensional chains composed of NiCl6-face-sharing octahedra via hydrogen bonding interactions. Such a unique structure enables the (3-pyrrolinium)NiCl3 with a high spontaneous polarization (P s) of 5.8 μC cm-2 and a high Curie temperature (T c) of 428 K, realizing dramatic enhancement of 112 and 52 K compared to its isostructural (3-pyrrolinium)MCl3 (M = Cd, Mn). To our knowledge, remarkably, (3-pyrrolinium)NiCl3 should be the first case of nickel(ii)-based OIHP ferroelectric to date, and its T c of 428 K (35 K above that of BaTiO3) is the highest among all reported one-dimensional OIHP ferroelectrics. This work offers a new structural building block for enriching the family of OIHP structures and will inspire the further exploration of new nickel(ii)-based OIHP ferroelectrics.

A Poling-Free Supramolecular Crown Ether Compound with Large Piezoelectricity

Supramolecular host-guest ferroelectrics based on solution processing are highly desirable because they are generally created with intrinsic piezoelectricity/ferroelectricity and do not need further poling. Poly(vinylidene fluoride) (PVDF) in the electric-active beta phase after stretching/annealing still shows no piezoelectric response unless poled. Although many supramolecular host-guest ferroelectrics have been discovered, their piezoelectricity is relatively small. Based on H/F substitution, we reported a supramolecular host-guest compound [(CF₃-C₆H₄-NH₃)(18-crown-6)][TFSA] (CF₃-C₆H₄-NH₃ = 4-trifluoromethylanilinium, TFSA = bis(trifluoromethanesulfonyl)ammonium) with a remarkable piezoelectric response of 42 pC/N under no poling condition. The introduction of F atoms increases phase transition temperature, polar axes, second harmonic generation (SHG) intensity, and piezoelectric coefficient d₃₃. To our knowledge, such a large piezoelectric performance of [(CF₃-C₆H₄-NH₃)(18-crown-6)][TFSA] makes its d₃₃, piezoelectric voltage coefficient g₃₃, and mechanical quality factor Q_m the highest among the reported supramolecular host-guest ferroelectric compounds and even larger than the values of PVDF. This work provides inspiration for optimizing piezoelectricity on molecular materials.

Remarkable Enhancement of Piezoelectric Performance by Heavy Halogen Substitution in Hybrid Perovskite Ferroelectrics

Piezoelectric materials that enable electromechanical conversion have great application value in actuators, transducers, sensors, and energy harvesters. Large piezoelectric (d₃₃) and piezoelectric voltage (g₃₃) coefficients are highly desired and critical to their practical applications. However, obtaining a material with simultaneously large d₃₃ and g₃₃ has long been a huge challenge. Here, we reported a hybrid perovskite ferroelectric [Me₃NCH₂Cl]CdBrCl₂ to mitigate and roughly address this issue by heavy halogen substitution. The introduction of a large-size halide element softens the metal-halide bonds and reduces the polarization switching barrier, resulting in excellent piezoelectric response with a large d₃₃ (∼440 pC/N), which realizes a significant optimization compared with that of previously reported [Me₃NCH₂Cl]CdCl₃ (You et al. Science2017, 357, 306-309). More strikingly, [Me₃NCH₂Cl]CdBrCl₂ simultaneously shows a giant g₃₃ of 6215 × 10^-3 V m/N, far exceeding those of polymers and conventional piezoelectric ceramics. Combined with simple solution preparation, easy processing of thin films, and a high Curie temperature of 373 K, these attributes make [Me₃NCH₂Cl]CdBrCl₂ promising for high-performance piezoelectric sensors in flexible, wearable, and biomechanical devices.

Competition between Ferroelectric and Ferroelastic Domain Wall Dynamics during Local Switching in Rhombohedral PMN-PT Single Crystals

The possibility to control the charge, type, and density of domain walls allows properties of ferroelectric materials to be selectively enhanced or reduced. In ferroelectric-ferroelastic materials, two types of domain walls are possible: pure ferroelectric and ferroelastic-ferroelectric. In this paper, we demonstrated a strategy to control the selective ferroelectric or ferroelastic domain wall formation in the (111) single-domain rhombohedral PMN-PT single crystals at the nanoscale by varying the relative humidity level in a scanning probe microscopy chamber. The solution of the corresponding coupled electro-mechanical boundary problem allows explaining observed competition between ferroelastic and ferroelectric domain growth. The reduction in the ferroelastic domain density during local switching at elevated humidity has been attributed to changes in the electric field spatial distribution and screening effectiveness. The established mechanism is important because it reveals a kinetic nature of the final domain patterns in multiaxial materials and thus provides a general pathway to create desirable domain structure in ferroelectric materials for applications in piezoelectric and optical devices.

Ferroelectric hybrid organic-inorganic perovskites and their structural and functional diversity

Molecular ferroelectrics have gradually aroused great interest in both fundamental scientific research and technological applications because of their easy processing, light weight and mechanical flexibility. Hybrid organic-inorganic perovskite ferroelectrics (HOIPFs), as a class of molecule-based ferroelectrics, have diverse functionalities owing to their unique structure and have become a hot spot in molecular ferroelectrics research. Therefore, they are extremely attractive in the field of ferroelectrics. However, there seems to be a lack of systematic review of their design, performance and potential applications. Herein, we review the recent development of HOIPFs from lead-based, lead-free and metal-free perovskites, and outline the versatility of these ferroelectrics, including piezoelectricity for mechanical energy-harvesting and optoelectronic properties for photovoltaics and light detection. Furthermore, a perspective view of the challenges and future directions of HOIPFs is also highlighted.

Large-Area Flexible Memory Arrays of Oriented Molecular Ferroelectric Single Crystals with Nearly Saturated Polarization

Molecular ferroelectrics (MFs) have been proven to demonstrate excellent properties even comparable to those of inorganic counterparts usually with heavy metals. However, the validation of their device applications is still at the infant stage. The polycrystalline feature of conventionally obtained MF films, the patterning challenges for microelectronics and the brittleness of crystalline films significantly hinder their development for organic integrated circuits, as well as emerging flexible electronics. Here, a large-area flexible memory array is demonstrated of oriented molecular ferroelectric single crystals (MFSCs) with nearly saturated polarization. Highly-uniform MFSC arrays are  prepared on large-scale substrates including Si wafers and flexible substrates using an asymmetric-wetting and microgroove-assisted coating (AWMAC) strategy. Resultant flexible memory arrays exhibit excellent nonvolatile memory properties with a low-operating voltage of <5 V, i.e., nearly saturated ferroelectric polarization (6.5 µC cm^-2 ), and long bending endurance (>10³ ) under various bending radii. These results may open an avenue for scalable flexible MF electronics with high performance.

Negative Longitudinal Piezoelectricity Coexisting with both Negative and Positive Transverse Piezoelectricity in a Hybrid Formate Perovskite

Negative longitudinal piezoelectric response is a rare property, which has been found mostly in inorganic materials. We use first-principles density functional theory simulations to predict such an unusual response in [NH₂NH₃]Co(HCOO)₃ ─a representative of a large family of hybrid organic-inorganic formate perovskites. A feature that sets aside [NH₂NH₃]Co(HCOO)₃ from inorganic compounds with a negative longitudinal piezoelectric response is that this rare property coexists with both negative and positive transverse piezoelectric responses, which is highly desirable for tunable applications. Atomistic analysis reveals that this unusual electromechanical coupling originates from the high anisotropy of materials response to uniaxial stress. Such a deformation produces oxygen octahedral tilts in the framework, whose magnitude depends strongly on the direction of the applied strain. For hard directions, the tilts make the dominant contribution to the deformation-induced change in polarization, while for the softer direction, it is the tilts of the NH₂NH₃⁺ cation that dominate the polarization response. The latter occur as the complex hydrogen bond network responds to the octahedral tilts. As high anisotropy of mechanical properties is a common feature across the formate perovskites, we expect our findings to stimulate more discoveries of unusual electromechanical couplings in this family.

Biferroelectricity of a homochiral organic molecule in both solid crystal and liquid crystal phases

Ferroelectricity, existing in either solid crystals or liquid crystals, gained widespread attention from science and industry for over a century. However, ferroelectricity has never been observed in both solid and liquid crystal phases of a material simultaneously. Inorganic ferroelectrics that dominate the market do not have liquid crystal phases because of their completely rigid structure caused by intrinsic chemical bonds. We report a ferroelectric homochiral cholesterol derivative, β-sitosteryl 4-iodocinnamate, where both solid and liquid crystal phases can exhibit the behavior of polarization switching as determined by polarization–voltage hysteresis loops and piezoresponse force microscopy measurements. The unique long molecular chain, sterol structure, and homochirality of β-sitosteryl 4-iodocinnamate molecules enable the formation of polar crystal structures with point group 2 in solid crystal phases, and promote the layered and helical structure in the liquid crystal phase with vertical polarization. Our findings demonstrate a compound that can show the biferroelectricity in both solid and liquid crystal phases, which would inspire further exploration of the interplay between solid and liquid crystal ferroelectric phases.

Ferroelectricity normally exists in either solid crystals or liquid crystals. Here, the authors report a homochiral organic compound which shows ferroelectricity in both solid crystal and liquid crystal phases.

Axial-Chiral BINOL Multiferroic Crystals with Coexistence of Ferroelectricity and Ferroelasticity

Chirality exists everywhere from natural amino acids to particle physics. The introduction of point chirality has recently been shown to be an efficient strategy for the construction of molecular ferroelectrics. In contrast to point chirality, however, axial chirality is rarely used to design ferroelectrics so far. Here, based on optically active 1,1'-bi-2-naphthol (BINOL), which has been applied extensively as a versatile chiral reagent in asymmetric catalysis, chiral recognition, and optics, we successfully design a pair of axial-chiral BINOL multiferroics, (R)-BINOL-DIPASi and (S)-BINOL-DIPASi. They experience a 2F1-type full ferroelectric/ferroelastic phase transition at a high temperature of 362 and 363 K, respectively. Piezoelectric force microscopy and polarization-voltage hysteresis loops demonstrate their ferroelectric domains and domain switching, and polarized light microscopy visualizes the evolution of stripe-shaped ferroelastic domains. The axial-chiral BINOL building block promotes the generation of the polar structure and ferroelectricity, and the organosilicon component increases the rotational energy barrier and thus the phase transition temperature. This work presents the first axial-chiral high-temperature multiferroic crystals, offering an efficient path for designing molecular multiferroics through the introduction of axial chirality.

Bond engineering of molecular ferroelectrics renders soft and high-performance piezoelectric energy harvesting materials

Piezoelectric materials convert mechanical stress to electrical energy and thus are widely used in energy harvesting and wearable devices. However, in the piezoelectric family, there are two pairs of properties that improving one of them will generally compromises the other, which limits their applications. The first pair is piezoelectric strain and voltage constant, and the second is piezoelectric performance and mechanical softness. Here, we report a molecular bond weakening strategy to mitigate these issues in organic-inorganic hybrid piezoelectrics. By introduction of large-size halide elements, the metal-halide bonds can be effectively weakened, leading to a softening effect on bond strength and reduction in polarization switching barrier. The obtained solid solution C₆H₅N(CH₃)₃CdBr₂Cl_0.75I_0.25 exhibits excellent piezoelectric constants (d₃₃ = 367 pm/V, g₃₃ = 3595 × 10⁻³ Vm/N), energy harvesting property (power density is 11 W/m²), and superior mechanical softness (0.8 GPa), promising this hybrid as high-performance soft piezoelectrics.

Improving piezoelectric strain and voltage constant generally compromises piezoelectric performance and mechanical softness. Here, the authors report a bond weakening strategy for organic-inorganic hybrid piezoelectrics and mitigated these issues.

An Effective Strategy of Introducing Chirality to Achieve Multifunctionality in Rare-Earth Double Perovskite Ferroelectrics

The hybrid rare-earth double perovskite (HREDP) system provides great convenience for the construction of multifunctional materials. However, suffering from the high symmetry of their intrinsic structure, HREDPs face the challenges in the realization and optimization of ferroelectric and piezoelectric properties. For the first time, after a systematic investigation of the chirality transformation principle, it is found that the introduction of chirality is an efficient strategy for the targeted construction of multifunctionality, which simultaneously increases the possibility of obtaining multiaxial ferroelectricity and ferroelasticity, and effectively realizes a large piezoelectric response. Moreover, chirality induced ferroelasticity will also achieve excellent magnetic or optical response driven by pressure-sensitive. To verify the feasibility of the above ideas, by using rare-earth ions (Ce³⁺ ) and suitable chiral organic cations, a new HREDP, (R-N-methyl-3-hydroxylquinuclidinium)₂ RbCe(NO₃ )₆ (R1) is successfully designed, in which ferroelasticity, multiaxial ferroelectricity, satisfactory piezoelectric response, and the pressure-driven single-ion magnetics switch are simultaneously achieved for the first time. This work shows that the induction of chirality and the HREDP system provide an effective strategy and ideal platform for the expansion and optimization of the functions in perovskite ferroelectrics.

Achieving circularly polarized luminescence and large piezoelectric response in hybrid rare-earth double perovskite by a chirality induction strategy

Chirality, an intrinsic property of nature, has received increased attention in chemistry, biology, and materials science because it can induce optical rotation, ferroelectricity, nonlinear optical response, and other unique properties. Here, by introducing chirality into hybrid rare-earth double perovskites (HREDPs), we successfully designed and synthesized a pair of enantiomeric three-dimensional (3D) HREDPs, [(R)-N-methyl-3-hydroxylquinuclidinium]₂RbEu(NO₃)₆ (R1) and [(S)-N-methyl-3-hydroxylquinuclidinium]₂RbEu(NO₃)₆ (S1), which possess ferroelasticity, multiaxial ferroelectricity, high quantum yields (84.71% and 83.55%, respectively), and long fluorescence lifetimes (5.404 and 5.256 ms, respectively). Notably, the introduction of chirality induces the coupling of multiaxial ferroelectricity and ferroelasticity, which brings about a satisfactory large piezoelectric response (103 and 101 pC N^-1 for R1 and S1, respectively). Moreover, in combination with the chirality and outstanding photoluminescence properties, circularly polarized luminescence (CPL) was first realized in HREDPs. This work sheds light on the design strategy of molecule-based materials with a large piezoelectric response and excellent CPL activity, and will inspire researchers to further explore the role of chirality in the construction of novel multifunctional materials.

Hydrogen Bonding Propagated Phase Separation in Quasi-Epitaxial Single Crystals: A Pd-Br Molecular Insulator

In condensed matter, phase separation is strongly related to ferroelasticity, ferroelectricity, ferromagnetism, electron correlation, and crystallography. These ferroics are important for nano-electronic devices such as non-volatile memory. However, the quantitative information regarding the lattice (atomic) structure at the border of phase separation is unclear in many cases. Thus, to design electronic devices at the molecular level, a quantitative electron-lattice relationship must be established. Herein, we elucidated a Pd^II-Pd^IV/Pd^III-Pd^III phase transition and phase separation mechanism for [Pd(cptn)₂Br]Br₂ (cptn = 1R,2R-diaminocyclopentane), propagated through a hydrogen-bonding network. Although the Pd···Pd distance was used to determine the electronic state, the differences in the Pd···Pd distance and the optical gap between Mott-Hubbard (MH) and charge-density-wave (CDW) states were only 0.012 Å and 0.17 eV, respectively. The N-H···Br···H-N hydrogen-bonding network functioned as a jack, adjusting the structural difference dynamically, and allowing visible ferroelastic phase transition/separation in a fluctuating N₂ gas flow. Additionally, the effect of the phase separation on the spin susceptibility and electrical conductivity were clarified to represent the quasi-epitaxial crystals among CDW-MH states. These results indicate that the phase transitions and separations could be controlled via atomic and molecular level modifications, such as the addition of hydrogen bonding.

Atomic and Electronic Manipulation of Robust Ferroelectric Polymorphs

Polymorphism allows the symmetry of the lattice and spatial charge distributions of atomically thin materials to be designed. While various polymorphs for superconducting, magnetic, and topological states have been extensively studied, polymorphic control is a challenge for robust ferroelectricity in atomically thin geometries. Here, the atomic and electric manipulation of ferroelectric polymorphs in Mo_{1- x} W_x Te₂ is reported. Atomic manipulation for polymorphic control via chemical pressure (substituting tungsten for molybdenum atoms) and charge density modulation can realize tunable polar lattice structures and robust ferroelectricity up to T = 400 K with a constant coercive field in an atomically thin material. Owing to the effective inversion symmetry breaking, the ferroelectric switching withstands a charge carrier density of up to 1.1 × 10¹³ cm^-2 , developing an original diagram for ferroelectric switching in atomically thin materials.

Ferroelasticity in Organic-Inorganic Hybrid Perovskites

Molecular ferroelastics have received particular attention for potential applications in mechanical switches, shape memory, energy conversion, information processing, and solar cells, by taking advantages of their low-cost, light-weight, easy preparation, and mechanical flexibility. The unique structures of organic-inorganic hybrid perovskites have been considered to be a design platform for symmetry-breaking-associated order-disorder in lattice, thereby possessing great potential for ferroelastic phase transition. Herein, we review the research progress of organic-inorganic hybrid perovskite ferroelastics in recent years, focusing on the crystal structures, dimensions, phase transitions and ferroelastic properties. In view of the few reports on molecular-based hybrid ferroelastics, we look forward to the structural design strategies of molecular ferroelastic materials, as well as the opportunities and challenges faced by molecular-based hybrid ferroelastic materials in the future. This review will have positive guiding significance for the synthesis and future exploration of organic-inorganic hybrid molecular ferroelastics.

Van der Waals Exfoliation Processed Biopiezoelectric Submucosa Ultrathin Films

Piezoelectric biomaterials have attracted significant attention due to the potential effect of piezoelectricity on biological tissues and their versatile applications. However, the high cost and complexity of assembling and domain aligning biomolecules at a large scale, and the disordered arrangement of piezoelectric domains as well as the lack of ferroelectricity in natural biological tissues remain a roadblock toward practical applications. Here, utilizing the weak van der Waals interaction in the layered structure of small intestinal submucosa (SIS), a van der Waals exfoliation (vdWE) process is reported to fabricate ultrathin films down to the thickness of the effective piezoelectric domain. Based on that, the piezoelectric property is revealed of SIS stemming from the collagen fibril, with piezoelectric coefficients up to 4.1 pm V^-1 and in-plane polarization orientation parallel to the fibril axis. Furthermore, a biosensor based on the vdWE-processed SIS film with an in-plane electrode is demonstrated that produces open-circuit voltages of ≈250 mV under the cantilever vibration condition. The vdWE method shows great potential in facilely fabricating ultrathin films of soft tissues and biosensors.

Intercalation-driven ferroelectric-to-ferroelastic conversion in a layered hybrid perovskite crystal

Two-dimensional (2D) organic-inorganic hybrid perovskites have attracted intense interests due to their quantum well structure and tunable excitonic properties. As an alternative to the well-studied divalent metal hybrid perovskite based on Pb²⁺, Sn²⁺ and Cu²⁺, the trivalent metal-based (eg. Sb³⁺ with ns2 outer-shell electronic configuration) hybrid perovskite with the A₃M₂X₉ formula (A = monovalent cations, M = trivalent metal, X = halide) offer intriguing possibilities for engineering ferroic properties. Here, we synthesized 2D ferroelectric hybrid perovskite (TMA)₃Sb₂Cl₉ with measurable in-plane and out-of-plane polarization. Interestingly, (TMA)₃Sb₂Cl₉ can be intercalated with FeCl₄ ions to form a ferroelastic and piezoelectric single crystal, (TMA)₄-Fe(iii)Cl₄-Sb₂Cl₉. Density functional theory calculations were carried out to investigate the unusual mechanism of ferroelectric-ferroelastic crossover in these crystals.

Unusual features of lattice dynamics in lawsonite near its phase transitions

Lattice dynamics of a single crystal of lawsonite were studied over a broad range of frequencies (1 Hz to 20 THz) using impedance, THz time-domain and infrared spectroscopies. Based on polarized spectra of complex permittivity \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hat{\varepsilon }$$\end{document}ε^ measured as a function of temperature between 10 K and 500 K, we analyzed the properties of the two known phase transitions—an antiferrodistortive one near \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}1}=270\,\mathrm{K}$$\end{document}Tc1=270K and a ferroelectric one, occurring at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}2}=124\,\mathrm{K}$$\end{document}Tc2=124K. The former one is accompanied by a flat maximum in the THz-range permittivity \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hat{\varepsilon }_{\mathrm{c}}$$\end{document}ε^c near \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}1}$$\end{document}Tc1, which is due to an overdamped polar excitation in the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathbf {E} \parallel c$$\end{document}E‖c spectra reflecting the dynamics of water and hydroxyl groups. The strength of this mode decreases on cooling below \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}1}$$\end{document}Tc1, and the mode vanishes below \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}2}$$\end{document}Tc2 due to hydrogen ordering. At the pseudoproper ferroelectric phase transition, two independent anomalies in permittivity were observed. First, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hat{\varepsilon }_a$$\end{document}ε^a exhibits a peak at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}2}=124\,\mathrm{K}$$\end{document}Tc2=124K due to critical slowing down of a relaxation in the GHz range. Second, infrared and THz spectra revealed an optical phonon softening towards \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}2}$$\end{document}Tc2 which causes a smaller but pronounced maximum in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hat{\varepsilon }_b$$\end{document}ε^b. Such anomaly, consisting in a soft mode polarized perpendicularly to the ferroelectric axis, is unusual in ferroelectrics.