Ultrafast Polarization Switching in a Biaxial Molecular Ferroelectric Thin Film: [Hdabco]ClO4

[C7H14NO][ClO4]: order–disorder structural change induced sudden switchable dielectric behaviour at room temperature

Static-to-dynamic transition in organic–inorganic hybrid [C₇H₁₄NO][ClO₄] exhibits switch-type characteristics during the dielectric transitions between high- and low-dielectric states.

A high-temperature molecular ferroelastic phase transition and switchable dielectric response in the trimethylbromomethylammonium salt [C4H11NBr] [PF6]

A new molecular ferroelastic compound [TMBMA] [PF₆] was successfully synthesized, wherein the origin of the phase transition and dielectric anomalies is related to the order–disorder processes of the TMBMA cations.

High-temperature reversible phase transitions and exceptional dielectric anomalies in cobalt(ii) based ionic crystals: [Me3NCH2X]2[CoX4] (X = Cl and Br)

Two isostructural cobalt(ii) based ionic crystals with exceptional dielectric anomalies have been designed as new high-temperature phase transition materials.

High-temperature sequential structural transitions with distinct switchable dielectric behaviors in two organic ionic plastic crystals: [C4H11NBr] [ClO4] and [C4H11NBr] [BF4]

[C₄H₁₁NBr] [ClO₄] and [C₄H₁₁NBr] [BF₄] undergo phase transitions from an ordered phase to plastic phase accompanied by switchable dielectric anomalies.

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.

The Narrowest Band Gap Ever Observed in Molecular Ferroelectrics: Hexane-1,6-diammonium Pentaiodobismuth(III)

Narrow band gaps and excellent ferroelectricity are intrinsically paradoxical in ferroelectrics as the leakage current caused by an increase in the number of thermally excited carriers will lead to a deterioration of ferroelectricity. A new molecular ferroelectric, hexane-1,6-diammonium pentaiodobismuth (HDA-BiI₅ ), was now developed through band gap engineering of organic-inorganic hybrid materials. It features an intrinsic band gap of 1.89 eV, and thus represents the first molecular ferroelectric with a band gap of less than 2.0 eV. Simultaneously, low-temperature solution processing was successfully applied to fabricate high-quality ferroelectric thin films based on HDA-BiI₅ , for which high-precision controllable domain flips were realized. Owing to its narrow band gap and excellent ferroelectricity, HDA-BiI₅ can be considered as a milestone in the exploitation of molecular ferroelectrics, with promising applications in high-density data storage and photovoltaic conversion.

Multifunctional Material with Efficient Optoelectronic Integrated Molecular Switches Based on a Flexible Thin Film/Crystal

Switchable materials, due to their potential applications in the fields of sensors, photonic devices, digital processing, etc., have been developed drastically. However, they still face great challenges in effectively inducing multiple molecular switching. Herein organic-inorganic hybrid compounds, an emerging class of hydrosoluble optoelectronic-active materials, welcome a new member with smart unique optical/electrical (fluorescence/dielectric) dual switches (switching ON/OFF), that is, [C₅H₁₃NBr][Cd₃Br₇] (1) in the form of both a bulk crystal and an ultraflexible monodirectional thin film, which simultaneously exhibits fast dielectric/fluorescent dual switching triggered by an optical/thermal/electric signal with a high signal-to-noise ratio of 35 (the highest one in the known optical/dielectric dual molecular switches). Additionally, the exceptional stability/fatigue resistance as well as the fantastic extensibility/compactness of thin films (more than 10000 times folding over 90°), makes 1 an ideal candidate for single-molecule intelligent wearable devices and seamlessly integrated optoelectronic multiswitchable devices. This opens up a new route toward advanced light/electric high-performance switches/memories based on organic-inorganic hybrid compounds.

A room temperature reversible phase transition containing dielectric switching of a host-guest supramolecular metal-halide compound

Following our recent findings on dielectric materials, we synthesized a new host-guest supramolecular metal-halide compound, [(2-AMPD)(18-crown-6)]CuCl₄ (1, 2-AMPD = 2-aminomethylpiperidinium). Systematic characterization techniques such as variable-temperature crystal structure analyses, differential scanning calorimetry (DSC) measurements, temperature-dependent dielectric measurements and powder X-ray diffraction (PXRD) measurements demonstrate that 1 undergoes a reversible phase transition at room temperature, accompanied by switchable dielectric responses and remarkable anisotropy along three different crystallographic axes. The structural phase transition mechanism is triggered by the order-disorder transition of the 18-crown-6 molecules. We believe that these findings might further promote the application of a host-guest inclusion compound in the field of switchable dielectric materials.

A Multiaxial Molecular Ferroelectric with Highest Curie Temperature and Fastest Polarization Switching

The classical organic ferroelectric, poly(vinylidene fluoride) (PVDF), has attracted much attention as a promising candidate for data storage applications compatible with all-organic electronics. However, it is the low crystallinity, the large coercive field, and the limited thermal stability of remanent polarization that severely hinder large-scale integration. In light of that, we show a molecular ferroelectric thin film of [Hdabco][ReO₄] (dabco = 1,4-diazabicyclo[2.2.2]octane) (1), belonging to another class of typical organic ferroelectrics. Remarkably, it displays not only the highest Curie temperature of 499.6 K but also the fastest polarization switching of 100k Hz among all reported molecular ferroelectrics. Combined with the large remanent polarization values (∼9 μC/cm²), the low coercive voltages (∼10 V), and the unique multiaxial ferroelectric nature, 1 becomes a promising and viable alternative to PVDF for data storage applications in next-generation flexible devices, wearable devices, and bionics.

A Molecular Polycrystalline Ferroelectric with Record-High Phase Transition Temperature

An outstanding advantage of inorganic ceramic ferroelectrics is their usability in the polycrystalline ceramic or thin film forms, which has dominated applications in the ferroelectric, dielectric, and piezoelectric fields. Although the history of ferroelectrics began with the molecular ferroelectric Rochelle salt in 1921, so far there have been very few molecular ferroelectrics, with lightweight, flexible, low-cost, and biocompatible superior properties compared to inorganic ceramic ferroelectrics, that can be applied in the polycrystalline form. Here, a multiaxial molecular ferroelectric, guanidinium perchlorate ([C(NH₂ )₃ ]ClO₄ ), with a record-high phase transition temperature of 454 K is presented. It is the rectangular polarization-electric field (P-E) hysteresis loops recorded on the powder and thin film samples (with respective large P_r of 5.1 and 8.1 µC cm^-2 ) that confirm the ferroelectricity of [C(NH₂ )₃ ]ClO₄ in the polycrystalline states. Intriguingly, after poling, the piezoelectric coefficient (d₃₃ ) of the powder sample shows a significant increase from 0 to 10 pC N^-1 , comparable to that of LiNbO₃ single crystal (8 pC N^-1 ). This is the first time that such a phenomenon has been observed in molecular ferroelectrics, indicating the great potential of molecular ferroelectrics being used in the polycrystalline form like inorganic ferroelectrics, as well as being viable alternatives or supplements to conventional ceramic ferroelectrics.

A near-room-temperature organic–inorganic hybrid ferroelectric: [C6H5CH2CH2NH3]2[CdI4]

The features of synchronously switchable dielectric constant, SHG, and pyroelectric current provide a promising multifunctional switching material applied in the field of electrical/optical switches and sensors at near room temperature.