Competitive Halogen Bond in the Molecular Ferroelectric with Large Piezoelectric Response

Molecular piezoelectrics are attracting tremendous interest because of their easy processing, light weight, low acoustical impedance, and mechanical flexibility. However, reports of molecular piezoelectrics with a piezoelectric coefficient <i>d</i>₃₃ comparable to piezoceramics such as barium titanate (BTO, 90–190 pC/N) have been scarce. Here, we present a uniaxial molecular ferroelectric, trimethylchloromethylammonium tribromocadmium­(II) (TMCM-CdBr₃), in which the halogen bonding might be a possible critical point for the stabilization of one-dimensional (1D) {CdBr₃}⁻ chain and further reservation of its ferroelectricity in such organic–inorganic hybrid crystalline systems. It has a large <i>d</i>₃₃ of 139 pC/N, 1 order of magnitude higher than those of most classically uniaxial ferroelectrics such as LiNbO₃ (6–16 pC/N) and Rochelle salt (∼7 pC/N), and comparable with those of multiaxial ferroelectrics such as BTO and trimethylbromomethylammonium tribromomanganese­(II) (112 pC/N). Moreover, the simple single-crystal growth and easy-to-find polar axis enable it to hold a great potential for applying in the single-crystal form. In light of the strong, specific, and directional halogen-bonding interactions, this work provides possibilities to explore new classes of molecular piezoelectrics and contribute to further developments

Competitive Halogen Bond in the Molecular Ferroelectric with Large Piezoelectric Response

Molecular piezoelectrics are attracting tremendous interest because of their easy processing, light weight, low acoustical impedance, and mechanical flexibility. However, reports of molecular piezoelectrics with a piezoelectric coefficient <i>d</i>₃₃ comparable to piezoceramics such as barium titanate (BTO, 90–190 pC/N) have been scarce. Here, we present a uniaxial molecular ferroelectric, trimethylchloromethylammonium tribromocadmium­(II) (TMCM-CdBr₃), in which the halogen bonding might be a possible critical point for the stabilization of one-dimensional (1D) {CdBr₃}⁻ chain and further reservation of its ferroelectricity in such organic–inorganic hybrid crystalline systems. It has a large <i>d</i>₃₃ of 139 pC/N, 1 order of magnitude higher than those of most classically uniaxial ferroelectrics such as LiNbO₃ (6–16 pC/N) and Rochelle salt (∼7 pC/N), and comparable with those of multiaxial ferroelectrics such as BTO and trimethylbromomethylammonium tribromomanganese­(II) (112 pC/N). Moreover, the simple single-crystal growth and easy-to-find polar axis enable it to hold a great potential for applying in the single-crystal form. In light of the strong, specific, and directional halogen-bonding interactions, this work provides possibilities to explore new classes of molecular piezoelectrics and contribute to further developments

Competitive Halogen Bond in the Molecular Ferroelectric with Large Piezoelectric Response

Molecular piezoelectrics are attracting tremendous interest because of their easy processing, light weight, low acoustical impedance, and mechanical flexibility. However, reports of molecular piezoelectrics with a piezoelectric coefficient <i>d</i>₃₃ comparable to piezoceramics such as barium titanate (BTO, 90–190 pC/N) have been scarce. Here, we present a uniaxial molecular ferroelectric, trimethylchloromethylammonium tribromocadmium­(II) (TMCM-CdBr₃), in which the halogen bonding might be a possible critical point for the stabilization of one-dimensional (1D) {CdBr₃}⁻ chain and further reservation of its ferroelectricity in such organic–inorganic hybrid crystalline systems. It has a large <i>d</i>₃₃ of 139 pC/N, 1 order of magnitude higher than those of most classically uniaxial ferroelectrics such as LiNbO₃ (6–16 pC/N) and Rochelle salt (∼7 pC/N), and comparable with those of multiaxial ferroelectrics such as BTO and trimethylbromomethylammonium tribromomanganese­(II) (112 pC/N). Moreover, the simple single-crystal growth and easy-to-find polar axis enable it to hold a great potential for applying in the single-crystal form. In light of the strong, specific, and directional halogen-bonding interactions, this work provides possibilities to explore new classes of molecular piezoelectrics and contribute to further developments

Large Piezoelectric Effect in a Lead-Free Molecular Ferroelectric Thin Film

Piezoelectric materials have been widely used in various applications, such as high-voltage sources, actuators, sensors, motors, frequency standard, vibration reducer, and so on. In the past decades, lead zirconate titanate (PZT) binary ferroelectric ceramics have dominated the commercial piezoelectric market due to their excellent properties near the morphotropic phase boundary (MPB), although they contain more than 60% toxic lead element. Here, we report a lead-free and one-composition molecular ferroelectric trimethylbromomethylammonium tribromomanganese­(II) (TMBM-MnBr₃) with a large piezoelectric coefficient <i>d</i>₃₃ of 112 pC/N along polar axis, comparable with those of typically one-composition piezoceramics such as BaTiO₃ along polar axis [001] (∼90 pC/N) and much greater than those of most known molecular ferroelectrics (almost below 40 pC/N). More significantly, the effective local piezoelectric coefficient of TMBM-MnBr₃ films is comparable to that of its bulk crystals. In terms of ferroelectric performance, it is the low coercive voltages, combined with the multiaxial characteristic, that ensure the feasibility of piezo film applications. Based on these, along with the common superiorities of molecular ferroelectrics like light weight, flexibility, low acoustical impedance, easy and environmentally friendly processing, it will open a new avenue for the exploration of next-generation piezoelectric devices in industrial and medical applications

Competitive Halogen Bond in the Molecular Ferroelectric with Large Piezoelectric Response

Molecular piezoelectrics are attracting tremendous interest because of their easy processing, light weight, low acoustical impedance, and mechanical flexibility. However, reports of molecular piezoelectrics with a piezoelectric coefficient <i>d</i>₃₃ comparable to piezoceramics such as barium titanate (BTO, 90–190 pC/N) have been scarce. Here, we present a uniaxial molecular ferroelectric, trimethylchloromethylammonium tribromocadmium­(II) (TMCM-CdBr₃), in which the halogen bonding might be a possible critical point for the stabilization of one-dimensional (1D) {CdBr₃}⁻ chain and further reservation of its ferroelectricity in such organic–inorganic hybrid crystalline systems. It has a large <i>d</i>₃₃ of 139 pC/N, 1 order of magnitude higher than those of most classically uniaxial ferroelectrics such as LiNbO₃ (6–16 pC/N) and Rochelle salt (∼7 pC/N), and comparable with those of multiaxial ferroelectrics such as BTO and trimethylbromomethylammonium tribromomanganese­(II) (112 pC/N). Moreover, the simple single-crystal growth and easy-to-find polar axis enable it to hold a great potential for applying in the single-crystal form. In light of the strong, specific, and directional halogen-bonding interactions, this work provides possibilities to explore new classes of molecular piezoelectrics and contribute to further developments

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) (<b>1</b>), 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, <b>1</b> becomes a promising and viable alternative to PVDF for data storage applications in next-generation flexible devices, wearable devices, and bionics

Multiaxial Molecular Ferroelectric Thin Films Bring Light to Practical Applications

Though dominating most of the practical applications, inorganic ferroelectric thin films usually suffer from the high processing temperatures, the substrate limitation, and the complicated fabrication techniques that are high-cost, energy-intensive, and time-consuming. By contrast, molecular ferroelectrics offer more opportunities for the next-generation flexible and wearable devices due to their inherent flexibility, tunability, environmental-friendliness, and easy processability. However, most of the discovered molecular ferroelectrics are uniaxial, one major obstacle for improving the thin-film performance and expanding the application potential. In this Perspective, we overview the recent advances on multiaxial molecular ferroelectric thin films, which is a solution to this issue. We describe the strategies for screening multiaxial molecular ferroelectrics and characterizations of the thin films, and highlight their advantages and future applications. Upon rational and precise design as well as optimizing ferroelectric performance, the family of multiaxial molecular ferroelectric thin films surely will get booming in the near future and inject vigor into the century-old ferroelectric field

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

Molecular ferroelectrics are attracting much attention as valuable complements to conventional ceramic ferroelectrics owing to their solution processability and nontoxicity. Encouragingly, the recent discovery of a multiaxial molecular ferroelectric, tetraethylammonium perchlorate, is expected to be able to solve the problem that in the technologically relevant thin-film form uniaxial molecular ferroelectrics have been found to perform considerably more poorly than in bulk. However, it can show good polarization–electric field (<i>P</i>–<i>E</i>) hysteresis loops only at very low frequency, severely hampering practical applications such as ferroelectric random access memory. Here, we present a biaxial molecular ferroelectric thin film of [Hdabco]­ClO₄ (dabco = 1,4-diazabicyclo[2.2.2]­octane) (<b>1</b>), where a perfect ferroelectric hysteresis loop can be observed even at 10 kHz. It is the first example of a molecular ferroelectric thin film whose polarization can be switched at such a high frequency. Moreover, using piezoresponse force microscopy, we clearly observed the coexistence of 180° and non-180° ferroelectric domains and provided direct experimental proof that 180° ferroelectric switching and non-180° ferroelastic switching are both realized; that is, a flexible alteration of the polarization axis direction can occur in the thin film by applying an electric field. These results open a new avenue for applications of molecular ferroelectrics and will inspire further exploration of high-performance multiaxial molecular ferroelectric thin films

Molecular Ferroelectric with Most Equivalent Polarization Directions Induced by the Plastic Phase Transition

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

High-Temperature Ferroelectricity and Photoluminescence in a Hybrid Organic–Inorganic Compound: (3-Pyrrolinium)MnCl₃

Coupling of ferroelectricity and optical properties has become an interesting aspect of material research. The switchable spontaneous polarization in ferroelectrics provides an alternative way to manipulate the light–matter interaction. The recent observation of strong photoluminescence emission in ferroelectric hybrid organic–inorganic compounds, (pyrrolidinium)­MnX₃ (X = Cl or Br), is an attractive approach to high efficiency luminescence with the advantages of ferroelectricity. However, (pyrrolidinium)­MnX₃ only displays ferroelectricity near or below room temperature, which limits its future applications in optoelectronics and multifunctional devices. Here, we rationally designed and synthesized high-temperature luminescent ferroelectric materials. The new hybrid compound (3-pyrrolinium)­MnCl₃ has a very high Curie temperature, <i>T</i>_c = 376 K, large spontaneous electronic polarization of 6.2 μC/cm², and high fatigue resistance, as well as high emission efficiency of 28%. This finding is a further step to the practical use of ferroelectric luminescence based on organic–inorganic compounds