It is known that CH3NH3PbI3 is particularly promising for next-generation solar devices; therefore, molecular perovskite structures have recently received extraordinary attention from the academic community because of their potential in producing unique physical properties. However, although great efforts have been made, molecular ferroelectrics with three-dimensional (3D) perovskite structures are still rare. So far, reported perovskite-like molecular ferroelectrics are basically one- or two-dimensional, significantly deviating from the inorganic perovskite ferroelectrics. Thus, their ferroelectric properties have to be greatly improved to meet the requirements of practical applications. Here, we report a 3D molecular perovskite ferroelectric: (3-ammoniopyrrolidinium)RbBr3 [(AP)RbBr3], with a high Curie temperature (Tc = 440 K) beyond that of BaTiO3. To the best of our knowledge, such above-room-temperature ferroelectricity in the 3D molecular perovskite compound is unprecedented. Furthermore, (AP)RbBr3 has great potential for applications due to its high thermal stability, ultrafast polarization reversal (greater than 20 kHz), and fascinating multiaxial characteristic. This finding opens a new avenue to the design and controllable synthesis of molecular ferroelectric perovskites, where the metal ion, halogen ion, and organic cation can be easily tuned.

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

In addition to the appealing semiconducting properties displayed by layered SnII or PbII halidometallates, intriguing phase-transition-induced properties, such as dielectric and ferroelectric properties, have recently been found by us in (benzylammonium)2PbCl4 and (cyclohexylammonium)2PbBr4–4xI4x (x = 0–1). The simple molecular structures of the layered compounds allow further chemical design for the modification of the physical properties. Thus, we extended the study to layered compounds templated by structurally similar organic cations to achieve future functions. Here, we report the design of a layered perovskite templated by cyclohexylmethylammonium cations, namely, bis[(cyclohexylmethyl)ammonium] tetrabromidolead(II). We found that this compound exhibits not only photoluminescence with a remarkably high efficiency (quantum yield = 21 %) but also interesting phase-transition-related physical properties, such as striking dielectric responses. These results reveal the great application potential of organic–inorganic hybrid compounds in the field of phase-transition multifunctional materials. More interesting phase-transition crystals are expected to be tailored in the future by taking advantage of the richness of layered perovskites.

We investigated the synthesis and structural phase transition of a two-dimensional layered perovskite-type organic–inorganic hybrid compound, (cyclohexylmethylammonium)2SnI4. By systematic characterizations, we found that this compound undergoes a reversible first-order phase transition from the high-temperature space group Cmca to the low-temperature space group P21/c at around Tc = 319 K. Accompanying the phase transition, dielectric bistability was also detected. The phase transition is ascribed to the order–disorder transition of the cyclohexylmethylammonium cations. It is known, the interesting electronic and optical properties in the two-dimensional Sn(II) and Pb(II) halide perovskite structures are dominated by the inorganic part. The phase transition and related physical properties observed in this work arise from the organic parts. This will enrich the chemistry of the hybrid structures by tailoring the organic moieties, and accordingly, may lead to novel multifunctional materials such as ferroelectric materials.Compound 1 undergoes a reversible structural phase transition from the high-temperature space group Cmca to the low-temperature space group P21/c at around Tc = 319 K. The driving force of the phase transition is ascribed to the order–disorder transformation of the organic cations.Download high-res image (74KB)Download full-size image

Besides the single crystals, ferroelectric materials are actually widely used in the forms of the polycrystals like ceramics. Multiaxial ferroelectrics with multiple equivalent polarization directions are preferable for such applications, because more equivalent ferroelectric axes allow random spontaneous polarization vectors to be oriented along the electric field to achieve a larger polarization after poling. Most of ceramic ferroelectrics like BaTiO3 have equivalent ferroelectric axes no more than three. We herein describe a molecular-ionic ferroelectric with 12 equivalent ferroelectric axes: tetraethylammonium perchlorate, whose number of axes is the most in the known ferroelectrics. Appearance of so many equivalent ferroelectric axes benefits from the plastic phase transition, because the plastic phase usually crystallizes in a highly symmetric cubic system. A perfect macroscopic ferroelectricity can be obtained on the polycrystalline film of this material. This finding opened an avenue constructing multiaxial ferroelectrics for applications as polycrystalline materials.

Following our recent finding of interesting dielectric and ferroelectric properties in semiconducting organic–inorganic layered perovskites (benzylammonium)2PbCl4 and (cyclohexylammonium)2PbBr4–4xI4x (x = 0–1), we designed a new layered perovskite-type crystal (cyclohexylmethylammonium)2CdCl4. By systematic characterizations, including differential scanning calorimetry measurements, dielectric measurements, and variable-temperature structural analysis, this crystal was found to undergo a reversible first-order phase transition at around Tc = 342 K. The origin of the phase transition is associated with the order–disorder change of the organic cation as expected. The phase transition is accompanied by the anticipated large dielectric constant change and remarkable dielectric anisotropy. These dielectric performances reveal potential application of the crystal as a high-temperature dielectric material. More interesting dielectric crystals are expected to be tailored in the future by taking advantage of the richness of layered perovskites.

A novel zigzag chain organic–inorganic hybrid compound of the general formula R2MI5, [n-C3H7NH3]2[SbI5] (1), was successfully synthesized, in which the n-propylammonium cations were located in the free cavities between the one-dimensional zigzag chains. Systematic characterization was performed to investigate the phase transition of 1. A pair of sharp peaks at 211.8 K (heating) and 203.7 K (cooling) with a hysteresis 8.1 K were observed in the differential scanning calorimetry (DSC) curve, indicating the first-order phase transition behavior of 1. The temperature dependence dielectric measurement demonstrated a step-like change at around 211.8 K, which makes 1 a potential switchable dielectric material. Frequency dependence measurement revealed that the frequency exerts a weak influence on the dielectric permittivity. Further structural analysis shows that both anionic and cationic moieties contribute to the phase transition, accompanied by weak hydrogen bond interactions between cations and the [SbI5]n2− chains.

The pyridazine hexafluorophosphate [C4H5N2]+[PF6]− (1) undergoes a reversible phase transition around 140 K, which was confirmed by the DSC measurement. Variable-temperature crystal structures determined at 293 K and 93 K show that the compound crystallizes in the same space group P21/c, indicating that 1 undergoes an iso-structural phase transition. As the temperature decreases, dielectric measurement of the title compound shows no significant change around the phase transition temperature. Classic hydrogen bonds are found between molecules at 293 K and 93 K with similar packing arrangement. The most distinct difference between the low temperature and room temperature structures is the order–disorder transition of the hexafluorophosphate anion, which is probably the driving force of the phase transition.The most distinct difference between the low-temperature and room-temperature structures is the order–disorder transition of the hexafluorophosphate anion, which is probably the driving force of the phase transition.

•A phase transition compound has been synthesized.•DSC and dielectric analyses confirm the phase transition at about 197 K.•The order–disorder transition of the 1, 2, 3, 6-tetrahydropyridinium cation is the driving force.The title compound, catena-(1,2,3,6-tetrahydropyridine tris(μ2-chloro)-cadmium(II)), has a hexagonal perovskite-type structure with formula AMX3. It undergoes a reversible dielectric transition at about 197 K. The transition was accompanied by significant dielectric anomaly, and was confirmed by DSC measurements. The variable-temperature single crystal X-ray diffraction analysis reveals that the transition is a process of the doubling of the cell volume from P21/m to P21/a. The transition is caused by the order–disorder change of the 1,2,3,6-tetrahydropyridinium cation.A phase transition compound, {[Thpy]·[CdCl3]} (1), has been synthesized. Differential scanning calorimetry measurement shows a pair of sharp peaks at 196.4 K (heating) and 183.1 K (cooling), indicating the phase transition is first-order. The phase transition is derived from the order-disorder transition of the 1,2,3,6-tetrahydropyridinium cation.

The temperature-dependent polymorphic crystal structure of the 1:1 co-crystal of 1,4-diazabicyclo-[2.2.2]octane (DABCO) and hydroquinone in the low-temperature phase was determined at 93(2) K. DSC measurement confirms that the co-crystal undergoes a reversible phase transition at about 158 K from monoclinic C2/c to monoclinic P21/n without distinctly changing the cell parameters. The crystal structural analysis of data collected at 298(2) and 93(2) K shows that in both structures, in addition to van der Waals' forces, conventional intermolecular N–H⋯O hydrogen bonds are the key molecular interactions, and the hydrogen bonding interactions show no notable changes. The lack of the two-fold axis in the low temperature structure is the most important difference between the structural forms. Contrary to the equal orientation of the molecules in RT phase, the multiplicity of the orientation in LT phase suggests that the LT structure is a supper-lattice for the RT structure void of the particular two-fold axis. The LT phase transition is simply the ordering of dynamically disordered molecules about this two-fold axis at RT, the increase in intramolecular bond distances is also attributed to a reduction in tilting motional disorder and the phase transition is the disorder-order type. In the specific system with narrow hysteresis and minor structural changes across the Tc, no distinct dielectric anomaly was observed in frequency ranges of 10–1000 kHz.

A novel zigzag chain organic–inorganic hybrid compound of the general formula R2MI5, [n-C3H7NH3]2[SbI5] (1), was successfully synthesized, in which the n-propylammonium cations were located in the free cavities between the one-dimensional zigzag chains. Systematic characterization was performed to investigate the phase transition of 1. A pair of sharp peaks at 211.8 K (heating) and 203.7 K (cooling) with a hysteresis 8.1 K were observed in the differential scanning calorimetry (DSC) curve, indicating the first-order phase transition behavior of 1. The temperature dependence dielectric measurement demonstrated a step-like change at around 211.8 K, which makes 1 a potential switchable dielectric material. Frequency dependence measurement revealed that the frequency exerts a weak influence on the dielectric permittivity. Further structural analysis shows that both anionic and cationic moieties contribute to the phase transition, accompanied by weak hydrogen bond interactions between cations and the [SbI5]n2− chains.