Molecular Ferroelectric Crystals Research Articles

Discovery of molecular ferroelectric catalytic annulation for quinolines

Ferroelectrics as emerging and attractive catalysts have shown tremendous potential for applications including wastewater treatment, hydrogen production, nitrogen fixation, and organic synthesis, etc. In this study, we demonstrate that molecular ferroelectric crystal TMCM-CdCl3 (TMCM = trimethylchloromethylammonium) with multiaxial ferroelectricity and superior piezoelectricity has an effective catalytic activity on the direct construction of the pharmacologically important substituted quinoline derivatives via one-pot [3 + 2 + 1] annulation of anilines and terminal alkynes by using N,N-dimethylformamide (DMF) as the carbon source. The recrystallized TMCM-CdCl3 crystals from DMF remain well ferroelectricity and piezoelectricity. Upon ultrasonic condition, periodic changes in polarization contribute to the release of free charges from the surface of the ferroelectric domains in nano size, which then quickly interacts with the substrates in the solution to trigger the pivotal redox process. Our work advances the molecular ferroelectric crystal as a catalytic route to organic synthesis, not only providing valuable direction for molecular ferroelectrics but also further enriching the executable range of ferroelectric catalysis.

The First Discovery of Spherical Carborane Molecular Ferroelectric Crystals

AbstractCarborane compounds, known for their exceptional thermal stability and non‐toxic attributes, have garnered widespread utility in medicine, supramolecular design, coordination/organometallic chemistry, and others. Although there is considerable interest among chemists, the integration of suitable carborane molecules into ferroelectric materials remains a formidable challenge. In this study, we employ the quasi‐spherical design strategy to introduce functional groups at the boron vertices of the o‐carborane cage, aiming to reduce molecular symmetry. This approach led to the successful synthesis of the pioneering ferroelectric crystals composed of cage‐like carboranes: 9‐OH‐o‐carborane (1) and 9‐SH‐o‐carborane (2), which undergo above‐room ferroelectric phase transitions (Tc) at approximately 367 K and 347 K. Interestingly, 1 and 2 represent uniaxial and multiaxial ferroelectrics respectively, with 2 exhibiting six polar axes and as many as twelve equivalent polarization directions. As the pioneering instance of carborane ferroelectric crystals, this study introduces a novel structural archetype for molecular ferroelectrics, thereby providing fresh insights into the exploration of molecular ferroelectric crystals with promising applications.

Halogenated carborane molecular ferroelectric crystals with high-temperature phase transition

Five cage-like halogenated o-carborane molecular ferroelectric crystals were successfully obtained with the highest Tc reaching 437 K. This provides a novel avenue for the further design of high Tc carborane ferroelectric crystals.

Molecular Ferroelectric Crystals with Superior Pyroelectricity, Plasticity, and Recyclability.

The pyroelectric effect is used in a wide range of applications such as infrared (IR) detection and thermal energy harvesting, which require the pyroelectric materials to simultaneously have a high pyroelectric coefficient and a low dielectric constant for high figures of merit. However, in conventional proper ferroelectrics, the positive correlation between the pyroelectric coefficient and the dielectric constant imposes an insurmountable challenge in upgrading the figures of merit. Here, we explored superior pyroelectricity in [(CH3)4N][FeCl4] (TMA-FC) and [(CH3)4N][FeCl3Br] (TMA-FCB) molecular ferroelectric plastic crystals, which could decouple this positive correlation due to the nature of improper polarization behavior. Therefore, TMA-FC and TMA-FCB derive a high pyroelectric coefficient and a low dielectric constant simultaneously, yielding record-high figures of merit around room temperature. Furthermore, the favorable plasticity enables ferroelectric crystals to attach surfaces with different shapes for device design and integration. More interestingly, the molecular ferroelectrics could be softened and reshaped at elevated temperatures without decay in pyroelectricity, making them recyclable for cost savings and e-waste reduction. Combined with the facile fabrication process, the findings of this work would open avenues for employing molecular ferroelectric plastic crystals in the manufacture of high-performance pyroelectric devices.

The First Enantiomeric Stereogenic Sulfur-Chiral Organic Ferroelectric Crystals.

Chiral ferroelectric crystals with intriguing features have attracted great interest and many with point or axial chirality based on the stereocarbon have been successively developed in recent years. However, ferroelectric crystals with stereogenic heteroatomic chirality have never been documented so far. Here, we discover and report a pair of enantiomeric stereogenic sulfur-chiral single-component organic ferroelectric crystals, Rs-tert-butanesulfinamide (Rs-tBuSA) and Ss-tert-butanesulfinamide (Ss-tBuSA) through the deep understanding of the chemical design of molecular ferroelectric crystals. Both enantiomers adopt chiral-polar point group 2 (C2) and exhibit mirror-image relationships. They undergo high-temperature 432F2-type plastic ferroelectric phase transition around 348 K. The ferroelectricity has been well confirmed by ferroelectric hysteresis loops and domains. Polarized light microscopy records the evolution of the ferroelastic domains, according with the fact that the 432F2-type phase transition is both ferroelectric and ferroelastic. The very soft characteristics with low elastic modulus and hardness reveals their excellent mechanical flexibility. This finding indicates the first stereosulfur chiral molecular ferroelectric crystals, opening up new fertile ground for exploring molecular ferroelectric crystals with great application prospects.

The First Enantiomeric Stereogenic Sulfur‐Chiral Organic Ferroelectric Crystals

AbstractChiral ferroelectric crystals with intriguing features have attracted great interest and many with point or axial chirality based on the stereocarbon have been successively developed in recent years. However, ferroelectric crystals with stereogenic heteroatomic chirality have never been documented so far. Here, we discover and report a pair of enantiomeric stereogenic sulfur‐chiral single‐component organic ferroelectric crystals, Rs‐tert‐butanesulfinamide (Rs‐tBuSA) and Ss‐tert‐butanesulfinamide (Ss‐tBuSA) through the deep understanding of the chemical design of molecular ferroelectric crystals. Both enantiomers adopt chiral‐polar point group 2 (C2) and exhibit mirror‐image relationships. They undergo high‐temperature 432F2‐type plastic ferroelectric phase transition around 348 K. The ferroelectricity has been well confirmed by ferroelectric hysteresis loops and domains. Polarized light microscopy records the evolution of the ferroelastic domains, according with the fact that the 432F2‐type phase transition is both ferroelectric and ferroelastic. The very soft characteristics with low elastic modulus and hardness reveals their excellent mechanical flexibility. This finding indicates the first stereosulfur chiral molecular ferroelectric crystals, opening up new fertile ground for exploring molecular ferroelectric crystals with great application prospects.

Ferroelectric Ionic Molecular Crystals with Significant Plasticity and a Low Melting Point: High Performance in Hot‐Pressed Polycrystalline Plates and Melt‐Grown Crystalline Sheets

AbstractAmong ferroelectric crystals based on small molecules, plastic/ferroelectric crystals are currently receiving particular attention because they can be used as bulk polycrystals. Herein, we show that an ionic molecular ferroelectric crystal, guanidinium tetrafluoroborate, exhibits significant malleability and multiaxial ferroelectricity despite the absence of a plastic crystal phase. Powder samples of this crystal can be processed into transparent bulk crystalline plates either by press‐forming or by melt‐growing. The plates show high ferroelectric performance and related properties, demonstrating the largest hitherto reported spontaneous polarization for bulk polycrystals of small‐molecule‐based ferroelectrics. Owing to the ready availability of large‐scale materials and processability into various bulk crystalline forms, this ferroelectric crystal represents a highly promising functional material that will boost research on diverse applications as bulk crystals.

Ferroelectric Ionic Molecular Crystals with Significant Plasticity and a Low Melting Point: High Performance in Hot-Pressed Polycrystalline Plates and Melt-Grown Crystalline Sheets.

Among ferroelectric crystals based on small molecules, plastic/ferroelectric crystals are currently receiving particular attention because they can be used as bulk polycrystals. Herein, we show that an ionic molecular ferroelectric crystal, guanidinium tetrafluoroborate, exhibits significant malleability and multiaxial ferroelectricity despite the absence of a plastic crystal phase. Powder samples of this crystal can be processed into transparent bulk crystalline plates either by press-forming or by melt-growing. The plates show high ferroelectric performance and related properties, demonstrating the largest hitherto reported spontaneous polarization for bulk polycrystals of small-molecule-based ferroelectrics. Owing to the ready availability of large-scale materials and processability into various bulk crystalline forms, this ferroelectric crystal represents a highly promising functional material that will boost research on diverse applications as bulk crystals.

Chemically driven energetic molecular ferroelectrics

Chemically driven thermal wave triggers high energy release rate in covalently-bonded molecular energetic materials. Molecular ferroelectrics bridge thermal wave and electrical energy by pyroelectric associated with heating frequency, thermal mass and heat transfer. Herein we design energetic molecular ferroelectrics consisting of imidazolium cations (energetic ion) and perchlorate anions (oxidizer), and describe its thermal wave energy conversion with a specific power of 1.8 kW kg−1. Such a molecular ferroelectric crystal shows an estimated detonation velocity of 7.20 ± 0.27 km s−1 comparable to trinitrotoluene and hexanitrostilbene. A polarization-dependent heat transfer and specific power suggests the role of electron-phonon interaction in tuning energy density of energetic molecular ferroelectrics. These findings represent a class of molecular ferroelectric energetic compounds for emerging energy applications demanding high power density.

Impact of synchrotron X-ray radiation damage on a molecular ferroelectric crystal.

The small-molecule crystallographic community has recently begun to assess the radiation damage impact of high intensity beams on samples. Bogdanov et al. [Acta Cryst. (2021), B77, 365–370] investigate the radiation damage impact on molecular ferroelectric crystal glycinium phosphite.

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.

Theoretical and Experimental Overview of Structural, Dielectric, Crystallographic, Electronic, Optical, and Physical Tensors of α-DIPAB and Iodine-Doped α-DIPAB Molecular Ferroelectric Crystals

Self-consistent ab initio calculations of the structural, crystallographic, electronic, optical, and physical properties as well as spontaneous polarization of P21 (α)-ferroelectric phase of diisopropylammonium bromide (α-DIPAB) molecular crystal are carried out. Examination of the total density of states as obtained using the generalized gradient approximation and hybrid exchange–correlation functional (HSE06) methods yields an optical bandgap of ∼ 5 eV and 6 eV, respectively. Furthermore, by using the Berry phase approach, we found that polar DIPAB exhibits a spontaneous polarization of 22.64 μC/cm2, indicating that this remarkable ferroelectric molecule could be used as an alternative to replace several known ferroelectrics for piezoelectric and optoelectronic applications such as capacitors, sensors, and data storage as well as microelectromechanical system (MEMS) devices. Using the vdW + DF2 approximation, we identify the majority of the vibrational modes in the Raman spectra and analyze the modes generated by the bromine (Br) deficiency. We find that the Br deficiency strongly affects the electric and elastic properties of α-DIPAB. Furthermore, the elastic, dielectric, and piezoelectric tensors of α-DIPAB are calculated and interpreted. α-DIPAB is found to exhibit a static dielectric tensor of ∼ 2.5, i.e., slightly smaller than that of typical perovskite-based ferroelectrics. In addition, the clamped-ion piezoelectric tensor is calculated. We found that the piezoelectric coefficient e21 corresponding to applying uniaxial strain in the direction perpendicular to the N–C bonds is smaller than the e22 principle coefficient. Moreover, the components e15 = 0.220 C/m2 and e35 = −0.2032 C/m2 correspond to applying shear strain η31, indicating a reasonable piezoelectric response of this polar crystal and making it a cheap attractive candidate for piezoelectric applications. The components of the elastic moduli tensor are calculated and interpreted. The α-DIPAB exhibits Young’s modulus of up to 50 GPa along specific crystalline directions despite its relatively low hardness. We attribute such a large value to the presence of a dense H–Br bond network between the Br and DIPAB molecule. The Poisson ratio is also strongly anisotropic, with values ranging from 0.5 to 0.2. The DIPAB systems are brittle based on the ratio between the bulk and shear elastic constants, suggesting that this material has potential for use in flexible electronic applications. A real-part dielectric anomaly at T = 140 K induced by iodine doping of α-DIPAB exhibits switchable behavior. This abnormality can be attributed to suppression of ferroelectricity induced by trapping of vortices at dislocation defects generated by iodine. Another dielectric anomaly found in the vicinity of the Curie temperature of α-DIPAB can be explained in terms of a structural phase-induced ferroelectric-to-paraelectric transition.

First-Principles Calculation of Physical Tensors of α-Diisopropylammonium Bromide (α-DIPAB) Molecular Ferroelectric Crystal

We report accurate calculations of tonsorial elements of α-Diisopropylammonium bromide (α-DIPAB) molecular ferroelectric crystal. In particular, elastic, piezoelectric and dielectric tensors were calculated using Vienna ab initio simulation package (VASP) within the framework of density functional (DFT) theory. The determination of above parameters allows an accurate description of the energy landscape for modeling of realistic devices at finite temperatures. We determine the major physical tensors in energy expansion of total energy per volume of un-deformed crystal to provide experimentalists with valuable information for designing and fabrication of pyroelectric detectors, capacitors, piezoelectric ferroelectric memories, display devices, electro-optic devices, and ultrasound transducers based on α-DIPAB. The spontaneous polarization P_s was calculated using Berry phase approach and found to be 22.64 μC⁄〖cm〗^2 in agreement with reported theoretical value. The dynamical Born effective charge tensor is reported for all atoms in the unit cell of α-DIPAB to get a deeper insight into the bonding network. The neighboring layers of DIPA molecules were found to be strongly crenelated due to the strong short-ranged electrostatic repulsion between Br sites in the DIPAB crystal structure. The organization of species in DIPA molecular layer as well as in the bromine “stitching” layer is essential for accurate calculation of DIPAB elastic properties. Having understood the actual network bonding in α-DIPAB, we calculated the components of the elastic moduli tensor. Our results indicate that a Young’s modulus of 10-180 GPa and a shear modulus of 4-26 GPa were found. Thus, α-DIPAB phase has a great potential to be a terrific candidate for flexible electronic device applications. The value of the principle component of electronic contribution to the static dielectric tensor of α-DIPAB is found to be ≈ 2.5, i.e. 50% smaller than that in typical perovskite-based ferroelectrics. Therefore, α-DIPAB is anticipated to exhibit creative materials' innovations. It offers highly customized features for specific applications, including insulation on polymer thick film conductive inks in the manufacture of membrane switches and flex circuits. Furthermore, clamped-ion piezoelectric tensor is calculated. Our results indicate a reasonable piezoelectric response of this polar crystal making it a low cost attractive candidate for piezoelectric applications.

Plastic/Ferroelectric Crystals with Easily Switchable Polarization: Low-Voltage Operation, Unprecedentedly High Pyroelectric Performance, and Large Piezoelectric Effect in Polycrystalline Forms.

Molecular ferroelectric crystals have attracted growing interest as potential alternatives to conventional lead-based ceramic ferroelectrics. We have recently discovered that a class of compounds known as plastic crystals can show multiaxial ferroelectricity, which allows ferroelectric performance even in polycrystalline forms. Here, we report new plastic/ferroelectric ionic molecular crystals that exhibit remarkably small coercive electric fields at room temperature. The easily switchable ferroelectric polarization enables low-voltage switching operations and high-frequency performance. Such ferroelectric crystals can be readily processed into bulk polycrystalline forms with desired shapes that are characterized by unprecedentedly high pyroelectric figures of merit and large piezoelectricity. These multifunctional molecular crystals represent highly attractive prospects for device elements with a diverse range of applications, which will significantly boost the development of molecular ferroelectric crystals.

Ferroelectricity and Piezoelectricity in Free-Standing Polycrystalline Films of Plastic Crystals.

Plastic crystals represent a unique compound class that is often encountered in molecules with globular structures. The highly symmetric cubic crystal structure of plastic crystals endows these materials with multiaxial ferroelectricity that allows a three-dimensional realignment of the polarization axes of the crystals, which cannot be achieved using conventional molecular ferroelectric crystals with low crystal symmetry. In this work, we focused our attention on malleability as another characteristic feature of plastic crystals. We have synthesized the new plastic/ferroelectric ionic crystals tetramethylammonium tetrachloroferrate(III) and tetramethylammonium bromotrichloroferrate(III), and discovered that free-standing translucent films can be easily prepared by pressing powdered samples of these compounds. The thus obtained polycrystalline films exhibit ferroelectric polarization switching and a relatively large piezoelectric response at room temperature. The ready availability of functional films demonstrates the practical utility of such plastic/ferroelectric crystals, and considering the vast variety of possible constituent cations and anions, a wide range of applications should be expected for these unique and attractive functional materials.

Effect of bromine deficiency on the lattice dynamics and dielectric properties of alpha-phase diisopropylammonium bromide molecular crystals

Diisopropylammonium bromide (DIPAB) molecular ferroelectric crystals were synthesized and examined to exhibit a large electric polarization (∼23μC/cm2), a large dielectric constant in the α-phase. Although the PXRD pattern indicates that the α-DIPAB sample has an overall excellent crystallinity, our analysis of its FT-IR and Raman vibrational spectra suggests the presence of disorder in the synthesized crystals as indicated by the presence of broad features in the Raman spectrum. Using vdW+DF2 calculations, we identified the majority of vibrational modes in the experimental spectra and analyzed the ones due to Br-disorder. We found that the bromine (Br) deficiency strongly affects the electric properties of α-DIPAB. Particularly, the experimentally measured dielectric constant of α-DIPAB is large (∼20), whereas the DFT-based calculations of the ideal DIPAB give much smaller values (∼2-3). We find that Br-deficiency is responsible for large dielectric constant of the DIPAB crystal with calculated value of ∼15-20. Furthermore, we showed that the van der Waals forces have a slight effect on the structural parameters, only causing a small shift in the vibrational frequencies. The main vibrational features of the DIPAB crystal in the Raman spectrum were shown to be driven by covalent bonding in the DIPA molecules and hydrogen bonds between the molecules with Br.

Directionally tunable and mechanically deformable ferroelectric crystals from rotating polar globular ionic molecules.

Ferroelectrics are used in a wide range of applications, including memory elements, capacitors and sensors. Recently, molecular ferroelectric crystals have attracted interest as viable alternatives to conventional ceramic ferroelectrics because of their solution processability and lack of toxicity. Here we show that a class of molecular compounds-known as plastic crystals-can exhibit ferroelectricity if the constituents are judiciously chosen from polar ionic molecules. The intrinsic features of plastic crystals, for example, the rotational motion of molecules and phase transitions with lattice-symmetry changes, provide the crystals with unique ferroelectric properties relative to those of conventional molecular crystals. This allows a flexible alteration of the polarization axis direction in a grown crystal by applying an electric field. Owing to the tunable nature of the crystal orientation, together with mechanical deformability, this type of molecular crystal represents an attractive functional material that could find use in a diverse range of applications.

A series of high-temperature molecular ferroelectric crystals: chlorine doped diisopropylammonium bromide

A series of chlorine doped diisopropylammonium bromide (DIPAB-C) single crystals were grown successfully.

Terahertz radiation by optical rectification in a hydrogen-bonded organic molecular ferroelectric crystal, 2-phenylmalondialdehyde

Terahertz radiation by optical rectification has been observed at room temperature in a hydrogen-bonded organic molecular ferroelectric crystal, 2-phenyl malondialdehyde (PhMDA). The radiated electromagnetic wave consisted of a single-cycle terahertz pulse with a temporal width of ∼0.5 ps. The terahertz radiation amplitude divided by the sample thickness in PhMDA was nearly equivalent to that in a typical terahertz wave emitter ZnTe. This is attributable to a long coherence length in the range of 130–800 µm for the terahertz radiation from PhMDA. We also discussed the possibility of PhMDA as a terahertz wave emitter in terms of the phase-matching condition.

Can molecular ferroelectrics challenge pure inorganic ones?

Molecular materials have attracted much attention due to their advantages of easy modification and fabrication, environmentally friendly and less energy-cost processing, transparent, light weight, mechanical flexibility and possible multi-functionalities, compared to their counter parts of pure inorganic or metallic ones, and many functionalities and properties have been discovered and realized inmolecule-basedmaterials [1–4]. Ferroelectrics, as an important class of multifunctional electroactive materials possessing electrically or mechanically switchable, temperature-dependent polarization and/or tunable dielectric responses and nonlinear electro-optic effects, have many applications in temperature sensing, data storage, mechanical actuation, energy harvesting, electromagnetic wave manipulation, and so on [5–8]. The first ferroelectrics, Rochelle salt, which was discovered in 1920–1, is in fact a molecular material, and later a few other molecular systems, such as the well-known potassium dihydrogen phosphate and tri-glycine sulfate, were discovered and developed. However, the rapid development of ferroelectrics took place only after the discovery of pure inorganic ferroelectrics, perovskite barium titanate (BTO) and lead zirconate titanate, because of their large spontaneous polarization (PS), high Curie temperature (TC), large dielectric constant (e′), and low dielectric loss (e′′ or tanδ), which are usually unavailable for molecular ferroelectrics [5,6]. These shortcomings for molecular ferroelectrics have been gradually overcome [9–13]; for example, an organic ferroelectrics, croconic acid, has been reported to possess aPS of 21μCcm−2 at room temperature, comparable to BTO [9]. Very recently, a research team led by Professor Ren-Gen Xiong in Southeast University, Nanjing, China, has reported [14] that a molecular ferroelectric crystal of diisopropylammonium bromide salt (DIPAB, Figs 1 and 2) showed a PS of 23 μC cm−2, high TC of 426 K, large e′ up to 103, and low tanδ of ∼0.4%, all close to or beyond the pure inorganic BTO, representing a breakthrough in the research of molecular ferroelectrics.