Xiu-Ni Hua

Metal-free three-dimensional perovskite ferroelectrics

Perovskites go organic The perovskite structure accommodates many different combinations of elements, making it attractive for use in a wide variety of applications. Building perovskites out of only organic compounds is appealing because these materials tend to be flexible, fracture-resistant, and potentially easier to synthesize than their inorganic counterparts. Ye et al. describe a previously unknown family of all-organic perovskites, of which they synthesized 23 different family members (see the Perspective by Li and Ji). The compounds are attractive as ferroelectrics, including one compound with properties close to the well-known inorganic ferroelectric BaTiO3. Science, this issue p. 151; see also p. 132 A family of all-organic perovskites has attractive ferroelectric properties. Inorganic perovskite ferroelectrics are widely used in nonvolatile memory elements, capacitors, and sensors because of their excellent ferroelectric and other properties. Organic ferroelectrics are desirable for their mechanical flexibility, low weight, environmentally friendly processing, and low processing temperatures. Although almost a century has passed since the first ferroelectric, Rochelle salt, was discovered, examples of highly desirable organic perovskite ferroelectrics are lacking. We found a family of metal-free organic perovskite ferroelectrics with the characteristic three-dimensional structure, among which MDABCO (N-methyl-N-diazabicyclo[2.2.2]octonium)–ammonium triiodide has a spontaneous polarization of 22 microcoulombs per square centimeter [close to that of barium titanate (BTO)], a high phase transition temperature of 448 kelvins (above that of BTO), and eight possible polarization directions. These attributes make it attractive for use in flexible devices, soft robotics, biomedical devices, and other applications.

Unusual two-step sequential reversible phase transitions with coexisting switchable nonlinear optical and dielectric behaviors in [(CH3)3NCH2Cl]2[ZnCl4]

A novel material with coexisting switchable nonlinear optical (NLO) and dielectric properties, [trimethylchloromethyl ammonium]2[ZnCl4] [(TMCM)2-ZnCl4], which undergoes two reversible solid state phase transitions at 337 K (Tc1) and 258 K (Tc2), respectively, has been successfully synthesized and grown as block crystals. Exceptionally, variable-temperature X-ray single crystal structural analyses of (TMCM)2-ZnCl4 reveal that the unusual space group changes from P212121 at 223 K to P21/c between Tc2 and Tc1 to Pmcn above Tc1. As expected, the dielectric value of (TMCM)2-ZnCl4 exhibits two remarkable step-like enlargements around Tc1 and Tc2, indicating that it exhibits a low dielectric state, an intermediate dielectric state and a high dielectric state. In particular, the second harmonic generation (SHG) efficiency shows a sharp step-like increase from almost zero to 0.47, and its SHG activities in a high-NLO state can be recovered after several cycles without any attenuation. The transitions of (TMCM)2-ZnCl4 could be intrinsically ascribed to the sequential order–disorder transformations of both the cations and anions. All these results demonstrate its potential application as a switchable and tunable molecular dielectric and NLO material.

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

Two new organic ionic plastic crystals (OIPCs), [TMBM] [ClO4] (1) and [TMBM] [BF4] (2) (TMBM = trimethylbromomethyl ammonium), were successfully synthesized. Systematic characterization via differential scanning calorimetry (DSC), variable-temperature single-crystal X-ray diffraction, variable-temperature powder X-ray diffraction (PXRD) and dielectric measurements demonstrated that 1 and 2 undergo reversible high-temperature sequential phase transitions accompanied by distinct stepwise switchable dielectric anomalies (1 at 358 K and 388 K and 2 at 346 K and 379 K). The structural relatively ordered-disordered transitions of ClO4−, BF4− and [TMBM]+ contributed to the switching of the dielectric constant from the low-dielectric state to the high-dielectric state. Thus, OIPCs consisting of spherical ammonia cations and inorganic acid anions are proven to be promising in the search for high-temperature multi-step switchable dielectric materials.

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

By introducing halogens to 16 symmetric tetramethylammonium, one new molecular ferroelastic compound [TMBMA] [PF6] (1) (TMBMA = trimethylbromomethylammonium) was successfully synthesized. Differential scanning calorimetry (DSC) measurements demonstrated that 1 underwent a typical first-order phase transition at ca. 380 K with a large thermal hysteresis of 30 K. Single crystal X-ray diffraction data, the variable-temperature powder X-ray diffraction (PXRD) patterns and the complex domain-wall orientations suggested that the phase transition from monoclinic (P21/c) to cubic (Pmm) belongs to a ferroelastic phase transition with an Aizu notation of mmF2/m. Dielectric studies show that 1 exhibits distinct switchable dielectric behavior and satisfactory thermal stability, revealing its potential application in dielectric switches. As expected, the TMBMA cations bearing an ordered–disordered motion turn out to be crucial in the generation of the ferroelastic phase transition. Therefore, this molecular design strategy of introducing halogens to tetraalkylammoniums can not only reduce the symmetry of the tetraalkylammonium molecules at room temperature, but can also maintain the highly disordered characteristics at high temperatures. This provides an efficient route for us to find new molecular ferroelastics and dielectric switches.

A Room-Temperature Hybrid Lead Iodide Perovskite Ferroelectric

Organic-inorganic hybrid perovskite, [CH3NH3]PbI3, holds a great potential for next-generation solar devices. However, whether the ferroelectricity exists in [CH3NH3]PbI3 and results in the ultrahigh performance is not at present clear. Beyond that, no hybrid lead iodide perovskite ferroelectric has yet been found. Here, using precise molecular modifications, we successfully designed a room-temperature hybrid perovskite ferroelectric, [(CH3)3NCH2I]PbI3. Because of the high-symmetry and nearly spherical shape of [(CH3)4N]+ cation, [(CH3)4N]PbI3 crystallizes in a centrosymmetric space group P63/ m at room temperature and undergoes a structural phase transition at 184 K. Accompanied by the introduction of halogen atoms on the cation from F to I, the phase transition temperature gradually increases to 312 K and the space group transforms into a polar C2 at room temperature. The strongest halogen bond energy of [(CH3)3NCH2I]-I and the largest volume of [(CH3)3NCH2I]+ among these compounds might be possible reasons for the stabilization of ordered [(CH3)3NCH2I]+ cation array and further reservation of its ferroelectricity at relatively high temperature. This work provides an efficient molecular design strategy toward the targeted harvest of room-temperature organic-inorganic perovskite ferroelectrics, and should inspire further exploration of the interplay between structure and ferroelectricity. The discovery of lead iodide perovskite ferroelectric also offers a foothold to the possibility for the existence of ferroelectricity in [CH3NH3]PbI3.

Switchable dielectric phase transition behaviors in two organic-inorganic copper(II) halides with distinct coordination geometries

Two new organic–inorganic chlorocuprates(II), [(ClCH2)(CH3)3N]2CuCl4 (1), which contains a discrete [CuCl4]2− tetrahedron, and [(ClCH2)(CH3)3N]CuCl3 (2), which comprises one-dimensional [CuCl3]− chains of an edge-sharing CuCl5 pyramid, have been synthesized. Compound 1 exhibits a symmetry-breaking phase transition at 335.2 K, while 2 undergoes an isostructural phase transition at 333.8 K, as disclosed by differential scanning calorimetry measurements and variable-temperature X-ray diffraction analyses. The dielectric measurements reveal that both compounds 1 and 2 show prominent switchable dielectric activities between high and low dielectric states in the vicinity of the phase transition temperature. In addition, the contrast between the two dielectric states in 1 is much higher than that in 2, which is related to the more remarkable dynamic change of the [(ClCH2)(CH3)3N]+ cation in 1 during the phase transition process. It is believed that this work will pave a new avenue for the design of multifunctional phase transition materials by exploring the area of Cu2+-based organic–inorganic metal halides.

A new two-dimensional polymeric cadmium(II) complex containing dicyanamide bridging ligands

As part of an exploration of new coordination polymers, a cadmium-dicyanamide complex, namely poly[benzyltriethylammonium [tri-μ-dicyanamido-κ6N1:N5-cadmium(II)]], {(C13H22N)[Cd(C2N3)3]}n, has been synthesized by the reaction of benzyltriethylammonium bromide, cadmium nitrate tetrahydrate and sodium dicyanamide in aqueous solution, and characterized by single-crystal X-ray diffraction at room temperature. In the crystal structure, each CdII cation is coordinated by six nitrile N atoms from six anionic dicyanamide (dca) ligands to furnish a slightly distorted octahedral geometry. Neighbouring CdII cations are linked by dicyanamide bridges to construct a two-dimensional anionic layer coordination polymer. One amide N atom in the bridging dca ligand is disordered over two sites. The cations lie between the anionic frameworks and there are no hydrogen-bond interactions between the cations and anions. The organic cations are not involved in the formation of the supramolecular network.