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Piezoelectric sensors that can work under various conditions with superior performance are highly desirable with the arrival of the Internet of Things. For practical applications, a large piezoelectric voltage coefficient gand a high Curie temperature Tcare critical to the performance of piezoelectric sensors. Here, we report a two-dimensional perovskite ferroelectric (4-aminotetrahydropyran)2PbBr4[(ATHP)2PbBr4] with a saturated polarization of 5.6 μC cm–2, high Tcof 503 K [above that of BaTiO3(BTO, 393 K)], and extremely large g33of 660.3 × 10–3V m N–1[much beyond that of Pb(Zr,Ti)O3(PZT) ceramics (20 to 40 × 10–3V m N–1), more than 2 times higher than that of poly(vinylidene fluoride) (PVDF, about 286.7 × 10–3V m N–1)]. Combined with the advantages of molecular ferroelectrics, such as light weight, easy and environmentally friendly processing, and mechanical flexibility, (ATHP)2PbBr4would be a competitive candidate for next-generation smart piezoelectric sensors in flexible devices, soft robotics, and biomedical devices.

Molecular ferroelectrics as alternatives to the conventional inorganic ferroelectrics have been greatly developed in past decades; many of these have been discovered and designed through various chemical means due to their structural adjustability. However, it is still a huge challenge to obtain high (above room temperature) Curie temperature (Tc) molecular ferroelectrics to meet the application requirements. Here, we present a new organic–inorganic hybrid molecular ferroelectric, [cyclopentylammonium]2CdBr4(1), showing a moderate above-room-temperature Tcof 340.3 K. The mechanism of the ferroelectric phase transition from Pnamto Pna21in 1is ascribed to the order–disorder transition of both the organic cations and inorganic anions, affording a spontaneous polarization of 0.57 μC/cm2for the ferroelectric phase. Using piezoresponse force microscopy (PFM), we clearly observed the antiparallel 180° stripe domains and realized the polarization switching, unambiguously establishing the existence of room-temperature ferroelectricity in the thin film. These attributes make it attractive for use in flexible devices, soft robotics, biomedical devices, and other applications.

The solid structure of the crystalline 1:1 complexes of 18-crown-6 (1,4,7,10,13,16-hexaoxacyclo-octadecane) with 4-ethoxyanilinium perchlorate [1] has been determined at 298 and 93 K, respectively. The measurements of the unit cell parameters versus temperature show that the values of aaxis and crystal volume change abruptly and remarkably at ∼163 K, indicating that this phase character may be a first-order. The crystal structural analysis of both two temperatures shows that in addition to van der Waals’ forces, conventional N−H···O, C−H···O hydrogen bonds, and C−H···π interaction are the main molecular interactions. The most distinct differences between the structures in room-temperature phase and low-temperature phase are of the twist of molecules viewing along a-axis. DSC measurements further confirm that this inclusion compound undergoes a reversible structural phase transition at about 163 K with a thermal hysteresis of 3.6 K in which relatively large latent heat (H) makes 1a good candidate for PCMs (phase change materials). As usual, the physical properties, such as dielectric constant and specific heat (Cp), will change a lot near phase transition temperature (Tc). No distinct dielectric constant anomaly was observed in frequency and temperature ranges of 10−1000 kHz and 100−298 K.

Supramolecular cationic structures of (CHDA2+)(DCH[18]crown-6)2were introduced into [Ni(dmit)2]−salts (where CHDA2+, DCH[18]crown-6, and dmit2−represent cis-cyclohexane-1,4-diammonium, meso-dicyclohexano[18]crown-6, and 2-thioxo-1,3-dithiole-4,5-dithiolate, respectively). The crystal polymorphs of (CHDA2+)(DCH[18]crown-6)2[Ni(dmit)2]2(crystals 1and 2) within the same crystallization batch were classified in the space groups P21/n(1) and P1̅ (2). The N−H+∼O hydrogen-bonding interactions between the axial- and equatorial-ammonium moieties of the CHDA2+cation and the oxygen atoms of DCH[18]crown-6 yielded a sandwich-type 1:2 adduct of the supramolecular cation in 1and 2. Although the overall cationic structures in 1and 2resembled one another, the [Ni(dmit)2]−anion arrangements were distinctively different from each other. The lateral [Ni(dmit)2]−dimer along the short axis of the anion was observed in 1, whereas two types of [Ni(dmit)2]−π-dimer existed independently in 2. The weak intermolecular interactions between the [Ni(dmit)2]−anions in 1was reflected by the temperature dependent magnetic susceptibility, which followed the Curie−Weiss model. For 2, two different magnetic exchange energies arising from the different strengths of the π-dimer units dominated the magnetic properties.

The past decade has witnessed much progress in designing molecular ferroelectrics, whose intrinsic mechanical flexibility, structural tunability, and easy processability are desirable for next-generation flexible and wearable electronic devices. However, an obstacle in expanding their promising applications in nonvolatile memory elements, capacitors, and sensors is effectively modulating the Curie temperature (Tc). Here, taking advantage of fluorine substitution on the reported molecular ferroelectric, (pyrrolidinium)MnCl3, we present enantiomeric perovskite ferroelectrics, namely, (R)-and (S)-3-(fluoropyrrolidinium)MnCl3. The close van der Waal’s radii and the similar steric parameters between H and F atoms ensure the minimum disruption of the crystal structure, while their different electronegativity and polarizability can trigger significant changes in the physical and chemical properties. As expected, the Tcgets successfully increased from 295 K in (pyrrolidinium)MnCl3to 333 K in these two homochiral compounds. Such a dramatic enhancement of 38 K signifies an important step toward designing high-Tcmolecular ferroelectrics. In the light of the conceptually new idea of fluorine substitution, one could look forward to a continuous succession of new molecular ferroelectric materials and technology developments.

Quasi-spherical molecules have recently been developed as promising building blocks for constructing high-performance molecular ferroelectrics. However, although the modification of spherical molecules into quasi-spherical ones can efficiently lower the crystal symmetry, it is still a challenge to precisely arouse a low-symmetric polar crystal structure. Here, by introducing directional hydrogen-bonding interactions in the molecular modification, we successfully reduced the cubic centrosymmetric Pm3̅mspace group of [quinuclidinium]ClO4at room temperature to the orthorhombic polar Pna21space group of [3-oxoquinuclidinium]ClO4. Different from the substituent groups of −OH, −CH3, and ═CH2, the addition of a ═O group with H-acceptor to [quinuclidinium]+forms directionally N–H···O═C hydrogen-bonded chains, which plays a critical role in the generation of polar structure in [3-oxoquinuclidinium]ClO4. Systematic characterization indicates that [3-oxoquinuclidinium]ClO4is an excellent molecular ferroelectric with a high Curie temperature of 457 K, a large saturate polarization of 6.7 μC/cm2, and a multiaxial feature of 6 equiv ferroelectric axes. This work demonstrates that the strategy of combining quasi-spherical molecule building blocks with directional intermolecular interactions provides an efficient route to precisely design new eminent molecular ferroelectrics.