Jia-Zhen Ge

Supramolecular bola-like ferroelectric: 4-methoxyanilinium tetrafluoroborate-18-crown-6.

Molecular motion is one of the structural foundations for the development of functional molecular materials such as artificial motors and molecular ferroelectrics. Herein, we show that pendulum-like motion of the terminal group of a molecule causes a ferroelectric phase transition. Complex 4-methoxyanilinium tetrafluoroborate-18-crown-6 ([C(7)H(10)NO(18-crown-6)](+)[BF(4)](-), 1) shows a second-order ferroelectric phase transition at 127 K, together with an abrupt dielectric anomaly, Debye-type relaxation behavior, and the symmetry breaking confirmed by temperature dependence of second harmonic generation effect. The origin of the polarization is due to the order-disorder transition of the pendulum-like motions of the terminal para-methyl group of the 4-methoxyanilinium guest cation; that is, the freezing of pendulum motion at low temperature forces significant orientational motions of the guest molecules and thus induces the formation of the ferroelectric phase. The supramolecular bola-like ferroelectric is distinct from the precedent ferroelectrics and will open a new avenue for the design of polar functional materials.

Diisopropylammonium Chloride: A Ferroelectric Organic Salt with a High Phase Transition Temperature and Practical Utilization Level of Spontaneous Polarization

A simple organic salt, diisopropylammonium chloride, shows the highest ferroelectric phase transition temperature among molecule-based ferroelectrics with a large spontaneous polarization, making it a candidate for practical technological applications.

Discovery of New Ferroelectrics: [H2dbco]2·[Cl3]·[CuCl3(H2O)2]·H2O (dbco = 1,4-Diaza-bicyclo[2.2.2]octane)

Compound [H(2)dbco](2) x [Cl(3)] x [CuCl(3)(H(2)O)(2)] x H(2)O undergoes a sharp dielectric anomaly and a paraelectric-to-ferroelectric phase transition at approximately -23 degrees C with a spontaneous polarization of 1.04 microC cm(-2), being the first molecular metal coordination compound ferroelectrics with a large dielectric response involving a 2 orders of magnitude enhancement and distinct Curie phase transition point. This work has proved an effective way for exploration of new ferroelectrics based on a five-coordinated divalent metal through the combination of crystal engineering and Landau phase transition theory.

The first homochiral coordination polymer with temperature-independent piezoelectric and dielectric properties

Two homochiral coordination polymers [Mn2(D-cam)2(2-Hpao)4]n (1) and [Co2(D-cam)2(3-abpt)2(H2O)3]n·5nH2O (2) were prepared with D-(+)-camphoric acid, and 1-D complex 1 featured good temperature-independent piezoelectric (6.9 pC N−1) and dielectric properties.

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.

Organic salt of hydrogen L-tartaric acid: a novel wide-temperature-range ferroelectrics with a reversible phase transition

4-Ethylanilinium hydrogen (2R,3R)-tartrate, a novel wide-temperature-range ferroelectric was synthesized. DSC measurement discloses that the homochiral organic salt undergoes an isosymmetric reversible phase transition at about 186 K with a sharply narrow heat hysteresis of 0.7 K. The heat capacity Cp obtained from the calorimetric measurement exhibits a sharp peak at 185.8 K, characteristic of a first-order phase transition. However temperature-dependence dielectric constant measurements reveal no dielectric anomaly near the phase transition point. The measurement of the unit cell parameters except for c axis versus temperature suggests that the values change abruptly and remarkably between 180 and 190 K with the cell volume doubled. The crystal structures determined at 123(2) K (a = 7.461 A, b = 11.930 A, c = 14.873 A, α = 95.34°, β = 91.95°, γ = 107.92°) and 298(2) K (a = 6.078 A, b = 7.478 A, c = 14.951 A, α = 87.66°, β = 82.69°, γ = 71.80°) also show that the phase transition could be a type of isosymmetric change with the same triclinic space group P1 (No. 1). Structural analysis shows that the different modes of hydrogen bonds probably affect the configurations of the phenyl rings from the cations, consequently leading to a reversible structural phase transition.

Structural phase transition due to the flexible supramolecule of (4-cyanomethylanilinium)([18]crown-6) in [Ni(dmit)2]− crystal

A flexible hydrogen-bonding supramolecular cation of 4-cyanomethylanilinium (CMAni+)-[18]crown-6 was introduced into the [Ni(dmit)2]− salt (dmit2− = 2-thioxo-1,3-dithiole-4,5-dithiolate) as a counter cation to form (CMAni+)([18]crown-6)[Ni(dmit)2]−. The rotation of the cyanomethyl group of the CMAni+ cation is associated with the structural phase transition at 153 K, at which temperature the space group was changed from Pbcn (high-temperature phase) to P212121 (low-temperature phase). The temperature dependent dielectric constants and potential energy calculations were consistent with the rotation of the cyanomethyl group of CMAni+.

Novel pure Pnma–P212121 ferroelastic phase transition of 1,4-diisopropyl-1,4-diazonia-bicyclo[2.2.2]octane tetra-chlorobromo-M(II) (M = Mn and Co)

Two novel phase transition compounds Dip-DABCO tetra-chlorobromo-M(II) (M = Mn and Co), (Dip-DABCO = 1,4-diisopropyl-1,4-diazonia-bicyclo[2.2.2]octane) C12H26N2·MnBrCl3 (1) and (C12H26N2)4·(CoBr1.25Cl2.75)4 (2) were synthesized, and their structures have been determined by means of single-crystal X-ray diffraction. The two compounds are isomorphous. Differential scanning calorimetry (DSC) measurements indicated that compounds 1 and 2 underwent a reversible phase transition at ca. 245.2 K with a hysteresis width of 4.4 K and at ca. 222.3 K with a hysteresis width of 5.5 K, respectively. The variable-temperature single-crystal X-ray diffraction data suggests that the phase transition was from high crystallographic symmetry with a space group of Pnma to a low-symmetry state with a space group of P212121. That is, Landau symmetry breaking occurred with a pure GPT (mmm–222). The ordering of twisting motions of the 1,4-diazoniabicylo[2,2,2]octane ring may have driven the phase transition.

High temperature structural phase transition and dielectric relaxation in an organic–inorganic hybrid compound: (4-methylpiperidinium)CdCl3

A new organic–inorganic hybrid perovskite-type compound, (4-MPD)CdCl3 (1, 4-MPD = 4-methylpiperidinium), demonstrating high temperature phase transition at around 364 K and prominent dielectric relaxation behavior, can be considered as a potential high temperature relaxor-type dielectric material. Systematic characterization techniques have been performed including variable-temperature structural analyses, differential scanning calorimetry (DSC) measurements, dielectric measurements and powder X-ray diffraction (PXRD) measurements. The structural phase transition coupled with the dielectric relaxation property is mainly ascribed to the order–disorder changes of organic cations. The studies of dielectric relaxation between 25 kHz and 1000 kHz disclose a significant Debye relaxation process above room temperature, which is well fitted by the Cole–Cole function.

Prominent dielectric transitions in layered organic–inorganic hybrids: (isoamyl-ammonium)2CdX4 (X = Cl and Br)

Layered organic–inorganic hybrids represent a class of technologically important materials due to their promising physical properties. In this report, two new layered organic–inorganic hybrid compounds (IAA)2CdX4 (IAA = isoamyl-ammonium cation, X = Cl, 1 and Br, 2) have been successfully synthesized and characterized. 1 undergoes a reversible solid-state phase transition at 230 K and 2 demonstrates three successive phase transitions and both of them exhibit prominent temperature-dependent dielectric behaviors. Detailed characterization was performed to investigate their phase transitions, including differential scanning calorimetry (DSC) measurements, variable-temperature structural analyses, variable-temperature powder X-ray diffraction (VT-PXRD) and dielectric measurements. The obvious difference in the phase transitions between 1 and 2 is originated from a bridged halogen effect, which makes different spaces between the inorganic framework layers where the organic IAA cation resides, resulting in different dynamic motions of the organic cations. These findings might be helpful to search for new dielectric materials based on layered organic–inorganic hybrid perovskites through exchange of the halogen atom.