Metal-free three-dimensional perovskite ferroelectrics

Record Enhancement of Curie Temperature in Host-Guest Inclusion Ferroelectrics

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

2D Perovskite Nanosheets with Intrinsic Chirality

Chiral perovskite materials have been intensively studied because of their unique properties and wide range of potential applications; however, the synthesis of perovskite nanocrystals with improved chirality has been scarcely investigated. In this Letter, two-dimensional perovskite nanosheets with intrinsic chirality are demonstrated. Inserting chiral amines into the perovskite framework leads to the chirality transfer from amine molecules to perovskite structure. The protecting agent, specifically, achiral octylamine, is found to influence the chiral optical signal or dissymmetric factor of nanosheets significantly. By controlling the amount of octylamine, we have synthesized perovskite nanosheets with the highest g-factor ever reported. We expect our primary demonstration could attract more attention toward the synthesis of intrinsic chiral perovskite nanocrystals and the development of nanocrystal-based chiral-optical devices with improved functions.

Target Designing Phase Transition Materials through Halogen Substitution

Crystalline materials have received extensive attention due to their extraordinary physical and chemical properties. Among them, phase transition materials have attracted great attention in the fields of photovoltaic, switchable dielectric devices, and ferroelectric memories, etc. However, many of them suffer from low phase transition temperatures, which limits their practical application. In this work, we systematically designed crystalline materials, (TMXM)₂ PtCl₆ (X=F, Cl, Br, I) through halogen substitution on the cations, aiming to improving phase transition temperature. The resulting phase transition of (TMXM)₂ PtCl₆ (X=F, Cl, Br, I) get a significant enhancement, compared to the parent compound [(CH₃ )₄ N]₂ PtCl₆ ((TM)₂ PtCl₆ ). Such phase transition temperature enhancement can be attributed to the introduction of halogen atoms that increase the potential energy barrier of the cation rotation. In addition, (TMBM)₂ PtCl₆ and (TMIM)₂ PtCl₆ have a low symmetry and crystallize in the space group C₂ /c and P2₁ 2₁ 2₁ , respectively. This work highlights the halogen substitution in designing crystal materials with high phase transition temperature.

Toroidal polar topology in strained ferroelectric polymer

Polar topological texture has become an emerging research field for exotic phenomena and potential applications in reconfigurable electronic devices. We report toroidal topological texture self-organized in a ferroelectric polymer, poly(vinylidene fluoride-ran-trifluoroethylene) [P(VDF-TrFE)], that exhibits concentric topology with anticoupled chiral domains. The interplay among the elastic, electric, and gradient energies results in continuous rotation and toroidal assembly of the polarization perpendicular to polymer chains, whereas relaxor behavior is induced along polymer chains. Such toroidal polar topology gives rise to periodic absorption of polarized far-infrared (FIR) waves, enabling the manipulation of the terahertz wave on a mesoscopic scale. Our observations should inform design principles for flexible ferroic materials toward complex topologies and provide opportunities for multistimuli conversions in flexible electronics.

Mechanisms for collective inversion-symmetry breaking in dabconium perovskite ferroelectrics

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

Hybrid Perovskites, Metal-Organic Frameworks, and Beyond: Unconventional Degrees of Freedom in Molecular Frameworks

ConspectusThe structural degrees of freedom of a solid material are the various distortions most straightforwardly activated by external stimuli such as temperature, pressure, or adsorption. One of the most successful design strategies in materials chemistry involves controlling these individual distortions to produce useful collective functional responses. In a ferroelectric such as lead titanate, for example, the key degree of freedom involves asymmetric displacements of Pb2+ and Ti4+ cations; it is by coupling these together that the system as a whole interacts with external electric fields. Collective rotations of the polyhedral units in oxide ceramics are another commonly exploited distortion, driving anomalous behavior such as negative thermal expansion-the counterintuitive phenomenon of volume contraction on heating. An exciting development in the field has been to take advantage of the interplay between different distortion types: generating polarization by combining two different polyhedral rotations, for example. In this way, degrees of freedom act as geometric "elements" that can themselves be combined to engineer materials with new and interesting properties. Just as the discovery of new chemical elements quite obviously diversified chemical space, we might expect that identifying new and different types of structural degrees of freedom to be an important strategy for developing new kinds of functional materials. In this context, the broad family of molecular frameworks is emerging as an extraordinarily fertile source of new and unanticipated distortion types, the vast majority of which have no parallel in the established families of conventional solid-state chemistry.Framework materials are solids whose structures are assembled from two fundamental components: nodes and linkers. Quite simply, linkers join the nodes together to form scaffolding-like networks that extend from the atomic to the macroscopic scale. These structures usually contain cavities, which can also accommodate additional ions for charge balance. In the well-established systems-such as lead titanate-node, linker, and extra-framework ions are all individual atoms (Ti, O, and Pb, respectively). But in molecular frameworks, at least one of these components is a molecule.In this Account, we survey the unconventional degrees of freedom introduced through the simple act of replacing atoms by molecules. Our motivation is to understand the role these new distortions play (or might be expected to play) in different materials properties. The various degrees of freedom themselves-unconventional rotational, translational, orientational, and conformational states-are summarized and described in the context of relevant experimental examples. The much-improved prospect for generating emergent functionalities by combining these new distortion types is then discussed. We highlight a number of directions for future research-including the design and application of hierarchically structured phases of matter intermediate to solids and liquid crystals-which serve to highlight the extraordinary possibilities for this nascent field.

Chiral Switchable Low-Dimensional Perovskite Ferroelectrics

Low-dimensional hybrid organic-inorganic perovskites (HOIPs) possess more localized electronic states and narrower conduction and valence bands to promote self-trapping of excitons and stronger exciton emission; therefore, they are widely used as building blocks for various applications in the fields of optoelectronics, photovoltaics, light-emitting diodes, luminescence, fluorescence, and so forth. Despite the past decades of intensive study, the discovered low-dimensional chiral HOIPs are rare as of the 1D chiral HOIP single crystals reported in 2003, as well as the low-dimensional chiral HOIP ferroelectrics are particularly scarce since the first chiral two-dimensional (2D) and/or one-dimensional (1D) HOIP ferroelectrics reported. Herein, two new low-dimensional HOIPs with the same conformational formula [R-MPA]₂CdCl₄ (R-MPA⁺ = (R)-(-)-1-methyl-3-phenylpropylamine) were successfully synthetized by means of regulating the stoichiometric proportion of R-MPA and CdCl₂ in two ways of 1:1 (1) and 2:1 (2). By combining single-crystal X-ray diffraction, circular dichroism (CD) spectroscopy, differential scanning calorimetry, temperature-dependent dielectric constant, temperature-dependent second-harmonic generation (SHG) effect, polarization-dependent SHG response, and P-E hysteresis loop, we reveal that 1 is a 1D nonchiral molecular ferroelectric and 2 is the first zero-dimensional (0D) chiral ferroelectric with distinct CD signals; meanwhile, 2 exhibits increased properties of high-T_c, large dielectric constant, SHG isotropy, and ferroelectricity than that of 1. These results not only shed light on the high tunability of the low-dimensional HOIP ferroelectrics but also open up an avenue to explore multifunctional chiral ferroelectrics.

Spontaneous self-formation of molecular ferroelectric heterostructures

A new phase of diisopropylammonium perchlorate (DIPAP) forms during freeze-drying or heat treatment, which generates the heterostructure with its original ferroelectric phase. There is no composition fluctuation in the DIPAP molecular ferroelectric heterostructures, but there is an interface between the two phases of DIPAP. The formation of the new phase resembles that of martensite in alloys. A large internal bias field that is almost 2.5 times of the coercive field was found in the molecular ferroelectric heterostructures, which is comparable to that of doped triglycine sulfate. The large internal bias field will promote the ability of the DIPAP heterostructure to adsorb PM2.5 under light. The spontaneous self-formation of molecular ferroelectric heterostructures may help improve the performance of molecular ferroelectric devices.

Linear Electro-Optic Modulation in Highly Polarizable Organic Perovskites

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

Para–ferroelectric phase transition induces an excellent second harmonic generation response and a prominent switchable dielectric constant change based on a metal-free ionic crystal

A novel metal-free compound, [H₂(bpyp)][ClO₄]₂, undergoes a ferroelectric to paraelectric reversible phase transition at T_c, with excellent NLO response, prominent dielectric constant change, moderate ferroelectric polarization, and wide bandgap.

A hybrid hydrochromic molecular crystal applicable to invisible ink with high reversibility

Highly reversible hydrochromic behavior is realized in a novel hybrid molecular crystal by controlling the gain and loss of coordinated water.

Improper molecular ferroelectrics with simultaneous ultrahigh pyroelectricity and figures of merit

Although ferroelectric materials exhibit large pyroelectric coefficients, their pyroelectric figures of merit (FOMs) are severely limited by their high dielectric constants because of the inverse relationship between FOMs and dielectric constant. Here, we report the molecular ferroelectric [Hdabco]ClO4 and [Hdabco]BF4 (dabco = diazabicyclo[2.2.2]octane) exhibiting improper ferroelectric behavior and pyroelectric FOMs outperforming the current ferroelectrics. Concurrently, the improper molecular ferroelectrics have pyroelectric coefficients that are more than one order of magnitude greater than the state-of-the-art pyroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 Our first-principles and thermodynamic calculations show that the strong coupling between the order parameters, i.e., the rotation angle of anions and polarization, is responsible for the colossal pyroelectric coefficient of the molecular ferroelectrics. Along with the facile preparation and self-poling features, the improper molecular ferroelectrics hold great promise for high-performance pyroelectric devices.

Unique cation-template three-dimensional hybrid material demonstrates dielectric switchable response

The strict tolerance space of three-dimensional (3D) crystalline structures is still a significant challenge in the area of switching dielectrics in comparison with lower-dimensional structures. Generally, the function of crystalline materials can be given or adjusted by controlling the environment in which synthesis takes place or the packing rearrangement. Using this method, special functional enhancements or changes in the dielectrics can be realized by improving the synthetic strategies. Here, a 3D switchable dielectric compound [MeHdabco]K(BF4)3 was achieved by employing the temperature selective effect. In particular, its structure is completely different from the usual 3D perovskite structure, which is constructed using two different cation-template frameworks. Moreover, the 3D [MeHdabco]K(BF4)3 shows a structural phase transition at 358 K. The thermal analysis (differential scanning calorimetry (DSC)) and X-ray diffractometry results provided evidence of these phase changes. This work provides a feasible strategy that can be used to achieve the different structures of an 'isomer', and enrich the method used for designing diverse functional materials.

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

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

A high-Tc organic-ionic phase transition crystal obtained from a trivalent cation

The organic-ionic crystal of [1,4,7-triazacyclononammonium] Cl₃, containing a trivalent cation, shows a high-temperature phase transition coupled with dielectric switching.

Ferroelectricity and switching polarization on the C–H⋯π bond in a pure organic molecular crystal – 1,3,5-trimethylnitrobenzene

The ferroelectric pure molecular crystal with spontaneous polarization related to flexibility of the C–H⋯π hydrogen bonds (molecular joints).

Structural insights into a new family of three-dimensional thiocyanate-bridged molecular double perovskites

Four new three-dimensional thiocyanate-bridged molecular double perovskites with bent Cd–S–C angles in a narrow distribution range reveal highly distorted frameworks with a relatively strong structural rigidity.

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

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

Methylation Design Strategy to Trigger a Dual Dielectric Switch and Improve the Phase Transition Temperature

Phase transitions of hybrid materials have aroused widespread concern and call for an in-depth study on its structure design, because the structure and characteristics are closely related, which promote potential applications in the field of temperature sensors, dielectric switches, and actuators. However, designing materials with multiple phase transitions and a high phase transition temperature (Tr) remains a huge challenge. In order to deal with this key hurdle, we tried to regulate the structural components and successfully synthesized [MASD]₂[CdCl₄] (1, MASD = 8-methyl-5-azoniaspiro[4,5]decane), which displays multiple phase transitions occurring at 273.8 K and 395.9 K separately. The Tr has significantly increased compared with the parent compounds reported previously. As the temperature sensitivity of compound 1 is constant at different frequencies, it can be applied for detectors or sensors under frequency-independent or wide frequency conditions. Moreover, methylation design strategy evidently triggered the dual dielectric switch and improved the Tr, which opens a new path for finding and adjusting ideal materials of multiple phase transition.

Two-Dimensional Hybrid Perovskite Ferroelectric Induced by Perfluorinated Substitution

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

Ferroelectric Metal-Organic Framework as a Host Material for Sulfur to Alleviate the Shuttle Effect of Lithium-Sulfur Battery

Ferroelectricity has an excellent reversible polarization conversion behavior under an external electric field. Herein, we propose an interesting strategy to alleviate the shuttle effect of lithium-sulfur battery by utilizing ferroelectric metal-organic framework (FMOF) as a host material for the first time. Compared to other MOF with same structure but without ferroelectricity and commercial carbon black, the cathode based on FMOF exhibits a low capacity decay and high cycling stability. These results demonstrate that the polarization switching behaviors of FMOF under the discharge voltage of lithium-sulfur battery can effectively trap polysulfides by polar-polar interactions, decrease polysulfides shuttle and improve the electrochemical performance of lithium-sulfur battery.

Designing disorder into crystalline materials

Crystals are a state of matter characterized by periodic order. Yet, crystalline materials can harbour disorder in many guises, such as non-repeating variations in composition, atom displacements, bonding arrangements, molecular orientations, conformations, charge states, orbital occupancies or magnetic structure. Disorder can sometimes be random but, more usually, it is correlated. Frontier research into disordered crystals now seeks to control and exploit the unusual patterns that persist within these correlated disordered states in order to access functional responses inaccessible to conventional crystals. In this Review, we survey the core design principles that guide targeted control over correlated disorder. We show how these principles - often informed by long-studied statistical mechanical models - can be applied across an unexpectedly broad range of materials, including organics, supramolecular assemblies, oxide ceramics and metal-organic frameworks. We conclude with a forward-looking discussion of the exciting link between disorder and function in responsive media, thermoelectrics and topological phases.

A 3D-printed molecular ferroelectric metamaterial

Molecular ferroelectrics combine electromechanical coupling and electric polarizabilities, offering immense promise in stimuli-dependent metamaterials. Despite such promise, current physical realizations of mechanical metamaterials remain hindered by the lack of rapid-prototyping ferroelectric metamaterial structures. Here, we present a continuous rapid printing strategy for the volumetric deposition of water-soluble molecular ferroelectric metamaterials with precise spatial control in virtually any three-dimensional (3D) geometry by means of an electric-field-assisted additive manufacturing. We demonstrate a scaffold-supported ferroelectric crystalline lattice that enables self-healing and a reprogrammable stiffness for dynamic tuning of mechanical metamaterials with a long lifetime and sustainability. A molecular ferroelectric architecture with resonant inclusions then exhibits adaptive mitigation of incident vibroacoustic dynamic loads via an electrically tunable subwavelength-frequency band gap. The findings shown here pave the way for the versatile additive manufacturing of molecular ferroelectric metamaterials.

Geo-inspired crystallization engineering: multifunctional materials design and fabrication at nanoscale and beyond

Crystallization engineering aims to design and develop solutions for the optimum conversion of natural resources for use by humans, by using crystallization. Crystallization is a cross-scale process, from atoms, ions and molecules in microscale to bulk crystals in macroscale. Fabricating nanomaterials with desired performances is an open issue with multiscale challenges during crystallization. For innovation in crystallization engineering, geology may provide various sources of inspiration such as structures, compositions and formation conditions, if mineral materials can be regarded as novel artificial materials. This review shows us some geo-inspirations that enable people to create and engineer novel materials with satisfactory performance.

Organometallic-Based Hybrid Perovskite Piezoelectrics with a Narrow Band Gap

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

Crystal structures and phase transitions of imidazolium hypodiphosphates

Two imidazolium hypodiphosphates, (C₃H₅N₂)(H₃P₂O₆) (I) and (C₃H₅N₂)₂(H₂P₂O₆) (II), have been synthesized and structurally characterized. In both metal-free organic-inorganic hybrids (I) and (II), the hypodiphosphate mono- and dianions, (H₃P₂O₆)^- and (H₂P₂O₆)^2-, form hydrogen-bonded frameworks of different types, to which the organic cations are linked via N-H...O and C-H...O hydrogen bonds. The purity of the compounds was confirmed by powder X-ray diffraction. Differential scanning calorimetry of compound (I) revealed two structural phase transitions: continuous at 311.8 K [cooling/heating; from high-temperature phase (HTP) to room-temperature phase (RTP)] and a discontinuous one at 287.9/289.2 K [RTP → low-temperature phase (LTP)]. Compound (I) is characterized in a wide temperature range by single-crystal and powder X-ray diffraction methods. Crystal structures of high- and low-temperature phases are determined, which show orthorhombic (HTP, Pnna, No. 52) → monoclinic (LTP, P2₁/n11, No. 14, a-axis doubled) structural change on cooling with an intermediate incommensurately modulated phase (RTP). Dynamic properties of polycrystalline (I) were studied by means of dielectric spectroscopy. The dielectric behaviour is explained by the motion of imidazolium cations.

Metal-Free Halide Perovskite Single Crystals with Very Long Charge Lifetimes for Efficient X-ray Imaging

Metal-free halide perovskites, as a specific category of the perovskite family, have recently emerged as novel semiconductors for organic ferroelectrics and promise the wide chemical diversity of the ABX₃ perovskite structure with mechanical flexibility, light weight, and eco-friendly processing. However, after the initial discovery 17 years ago, there has been no experimental information about their charge transport properties and only one brief mention of their optoelectronic properties. Here, growth of large single crystals of metal-free halide perovskite DABCO-NH₄ Br₃ (DABCO = N-N'-diazabicyclo[2.2.2]octonium) is reported together with characterization of their instrinsic optical and electronic properties and demonstration, of metal-free halide perovskite optoelectronics. The results reveal that the crystals have an unusually large semigap of ≈16 eV and a specific band nature with the valence band maximum and the conduction band minimum mainly dominated by the halide and DABCO²⁺ , respectively. The unusually large semigap rationalizes extremely long lifetimes approaching the millisecond regime, leading to very high charge diffusion lengths (tens of μm). The crystals also exhibit high X-ray attenuation as well as being lightweight. All these properties translate to high-performance X-ray imaging with sensitivity up to 173 μC Gy_air ^-1 cm^-2 . This makes metal-free perovskites novel candidates for the next generation of optoelectronics.

Prediction of an Extended Ferroelectric Clathrate

Using first-principles calculations, we predict a lightweight room-temperature ferroelectric carbon-boron framework in a host-guest clathrate structure. This ferroelectric clathrate, with composition ScB_{3}C_{3}, exhibits high polarization density and low mass density compared with widely used commercial ferroelectrics. Molecular dynamics simulations show spontaneous polarization with a moderate above-room-temperature T_{c} of ∼370  K, which implies large susceptibility and possibly large electrocaloric and piezoelectric constants at room temperature. Our findings open the possibility for a new class of ferroelectric materials with potential across a broad range of applications.

Record Enhancement of Phase Transition Temperature Realized by H/F Substitution

A high transition temperature (T_c ) is essential for the practical application of ferroelectrics as electronic devices under extreme thermal conditions in the aerospace, automotive, and energy industries. In recent decades, the isotope effect and strain engineering are found to effectively modulate T_c ; however, these strategies are limited to certain systems. Developing simple, universal, and practical methods to improve T_c has become an imminent challenge for expanding the applications of ferroelectrics. Here, by adopting a molecular design strategy involving H/F substitution on an organic-inorganic hybrid perovskite (1-azabicyclo[2.2.1]heptane)CdCl₃ at a T_c of 190 K, the successful synthesis of a multiaxial, ferroelectric hybrid perovskite (4-fluoro-1-azabicyclo[2.2.1]heptane)CdCl₃ is reported, which demonstrates a large spontaneous polarization of 11.2 µC cm^-2 (greater than that of polyvinylidene difluoride) and a T_c of 419 K (greater than that of BaTiO₃ ). This temperature enhancement (229 K) is the largest reported for molecular ferroelectrics, far exceeding the reported enhancements induced by the isotope effect and other techniques. This pioneering technique provides an effective and universal method for improving T_c in ferroelectrics and represents an important step toward the development of high-performance ferroelectric technology.

Dynamics of Chiral Cations in Two-Dimensional CuX4 and PbX4 Perovskites (X = Cl and Br)

Chiral organic ammonium cations ((R)-2-methylphenethylammonium (R-MPhA) and (R)-3,7-dimethyloctylammonium (R-DMOA)) cations were combined with [MX₄]^2- anions (M = Cu and Pb, X = Cl and Br) to form two-dimensional (2D) perovskites: (R-MPhA)₂CuCl₄ (1a), (R-MPhA)₂CuBr₄ (1b), (R-DMOA)₂CuCl₄ (2a), (R-DMOA)₂CuBr₄ (2b), (R-DMOA)₂PbCl₄ (2c), and (R-DMOA)₂PbBr₄ (2d). The point shearing of the MX₄ octahedron formed 2D perovskite layers, which were sandwiched by the bilayer molecular assembly of chiral organic ammonium cations. We found that the flexible and polar organic R-MPhA and R-DMOA cations in the 2D perovskites played an important role in the phase transition behavior and dielectric responses. Salts 2a-2d showed similar solid-solid (S1-S2) phase transitions, for which the temperatures decreased in the order of CuCl₄ (2a) > PbCl₄ (2c) > CuBr₄ (2b) > PbBr₄ (2d). The occupation volume of one R-DMOA per MX₄ octahedron determined the dynamic crystalline space for the motional freedom of chiral ammonium in the 2D perovskite layer. Although thermally activated dielectric fluctuations were observed in salts 2a, 2b, and 2c, only an order-disorder-type dielectric phase transition was observed in salt 2d. Interband optical transitions were observed in the CuCl₄ and CuBr₄ 2D perovskites, whereas sharp exciton absorptions were observed in the 2D PbCl₄ and PbBr₄ layers in perovskite salts 2c and 2d.

Superior Transverse Piezoelectricity in a Halide Perovskite Molecular Ferroelectric Thin Film

Piezoelectric materials with inherent mechanical-electric coupling effect are a crucial family of functional materials in high-end information technology. For practical applications, the transverse piezoelectric performance (d₃₁ or d₃₂) is mainly considered, because this parameter is a vitally important index to characterize the performance of piezoelectric thin films. However, the transverse piezoelectricity of the thin films as a key figure of merit is seldom mentioned in molecular ferroelectrics. Herein, we report that a new 1D halide perovskite ferroelectric N,N-dimethylallylammoniumCdCl₃ (DMAACdCl₃) exhibits an above room-temperature ferroelectric phase transition with a saturated polarization of 1.9 μC cm^-2 and a coercive field of 5.0 kV cm^-1. The thin film of DMAACdCl₃ is successfully fabricated using an easy processing spinning method and maintains well ferroelectric properties verified by piezoresponse force microscopy (PFM). More significantly, the ferroelectric thin film offers superior transverse piezoelectricity with an in-plane piezoelectric response of about 41 pC N^-1, which is about twice that of well-known piezoelectric polymer PVDF (21 pC N^-1). Transverse piezoelectricity has been scarcely studied in molecular ferroelectrics, and its exploitation would play an important role in the design of next-generation smart piezoelectric devices.

Physical vapor deposited organic ferroelectric diisopropylammonium bromide film and its self-powered photodetector characteristics

Organic diisopropylammonium bromide (DIPAB) is a promising material with superior ferroelectric characteristics. However, the DIPAB continuous film, which is essential to explore its application potential, is challenging because its crystallization kinetics favors island-like microcrystalline growth. In this work, the continuous and uniform deposition of organic ferroelectric DIPAB film on a single crystalline Si(100) substrate is demonstrated by a thermal evaporation process. Structural and optical studies reveal that the film is c-axis oriented with an optical bandgap of 3.52 eV. The topographic image displays well-connected grain-like surface morphology with ∼2 nm roughness. The ferroelectric domain studies illustrate the in-plane orientation of the domains, which is in accordance with c-axis oriented film where polarization is along the in-plane b-axis. The phase and amplitude responses of the domains display hysteresis and butterfly characteristics, respectively and thereby endorse the ferroelectric nature of the film. Importantly, it is demonstrated that the DIPAB film exhibits remarkable self-powered UV-Vis photodetector characteristics with responsivity of 0.66 mA W^-1 and detectivity of 2.20 × 10⁹ Jones at 11.45 mW cm^-2 light intensity. The fabricated DIPAB film reported in this work can widen its application potential in self-powered photodetector and other optoelectronic devices.

Metaelectric multiphase transitions in a highly polarizable molecular crystal

Metaelectric transition, i.e. an abrupt increase in polarization with an electric field is just a phase change phenomenon in dielectrics and attracts increasing interest for practical applications such as electrical energy storage and highly deformable transducers. Here we demonstrate that both field-induced metaelectric transitions and temperature-induced phase transitions occur successively on a crystal of highly polarizable bis-(1H-benzimidazol-2-yl)-methane (BI2C) molecules. In each molecule, two switchable polar subunits are covalently linked with each other. By changing the NH hydrogen location, the low- and high-dipole states of each molecule can be interconverted, turning on and off the polarization of hydrogen-bonded molecular ribbons. In the low-temperature phase III, the tetragonal crystal lattice comprises orthogonally crossed arrays of polar ribbons made up of a ladder-like hydrogen-bond network of fully polarized molecules. The single-step metaelectric transition from this phase III corresponds to the forced alignment of antiparallel dipoles typical of antiferroelectrics. By the transition to the intermediate-temperature phase II, the polarity is turned off for half of the ribbons so that the nonpolar and polar ribbons are orthogonal to each other. Considering also the ferroelastic-like crystal twinning, the doubled steps of metaelectric transitions observed in the phase II can be explained by the additional switching at different critical fields, by which the nonpolar ribbons undergo "metadielectric" molecular transformation restoring the strong polarization. This mechanism inevitably brings about exotic phase change phenomena transforming the multi-domain state of a homogeneous phase into an inhomogeneous (phase mixture) state.