Hong-Ling Cai

Coexistence of magnetic and electric orderings in the metal-formate frameworks of [NH4][M(HCOO)3].

A family of three-dimensional chiral metal-formate frameworks of [NH(4)][M(HCOO)(3)] (M = Mn, Fe, Co, Ni, and Zn) displays paraelectric to ferroelectric phase transitions between 191 and 254 K, triggered by disorder-order transitions of NH(4)(+) cations and their displacement within the framework channels, combined with spin-canted antiferromagnetic ordering within 8-30 K for the magnetic members, providing a new class of metal-organic frameworks showing the coexistence of magnetic and electric orderings.

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.

Ferroelectricity Induced by Ordering of Twisting Motion in a Molecular Rotor

A novel mononuclear metal-organic compound, [Cu(Hdabco)(H(2)O)Cl(3)] (1, dabco = 1,4-diazabicyclo[2.2.2]octane) in which the Cu(II) cation adopts a slightly distorted bipyramidal geometry where the three Cl anions constitute the equatorial plane and the Hdabco cation and H(2)O molecule occupy the two axial positions, was synthesized. Its paraelectric-to-ferroelectric phase transition at 235 K (T(c)) and dynamic behaviors were characterized by single crystal X-ray diffraction analysis, thermal analysis, dielectric and ferroelectric measurements, second harmonic generation experiments, and solid-state nuclear magnetic resonance measurements. Compound 1 behaves as a molecular rotor above room temperature in which the (Hdabco) part rotates around the N···N axis as a rotator and the [Cu(H(2)O)Cl(3)] part acts as a stator. In the temperature range 235-301 K, a twisting motion of the rotator is confirmed. Below the T(c), the motions of the rotor are frozen and the molecules become ordered, corresponding to a ferroelectric phase. Origin of the ferroelectricity was ascribed to relative movements of the anions and cations from the equilibrium position, which is induced by the order-disorder transformation of the twisting motion of the molecule between the ferroelectric and paraelectric phases. Study of the deuterated analogue [Cu(Ddabco)(D(2)O)Cl(3)] (2) excludes the possibility of proton ordering as the origin of the ferroelectricity in 1.

Switchable Dielectric, Piezoelectric, and Second‐Harmonic Generation Bistability in a New Improper Ferroelectric above Room Temperature

Imidazolium periodate (IPI) is found to be an improper ferroelectric. It shows bistable properties simultaneously in three channels of dielectricity, piezoelectricity, and second-harmonic generation within the temperature window 300-310 K.

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.

Above-Room-Temperature Magnetodielectric Coupling in a Possible Molecule-Based Multiferroic: Triethylmethylammonium Tetrabromoferrate(III)

A possible above-room-temperature molecular multiferroic, triethylmethylammonium tetrabromoferrate(III) (1), has been discovered. Its ferroelectric and magnetic phase transitions take place at almost the same temperature (∼360 K), resulting in strong magnetodielectric (MD) coupling, with a MD ratio of 18% at 0.6 MHz. Interestingly, 1 also undergoes a low-temperature ferroelectric-ferroelectric phase transition with an Aizu notation of 6mmF6 and small magnetic and dielectric anomalies at 171 K.

Molecule-displacive ferroelectricity in organic supramolecular solids

Ferroelectricity is essential to many forms of current technology, ranging from sensors and actuators to optical or memory devices. In this circumstance, organic ferroelectrics are of particular importance because of their potential application in tomorrows organic devices, and several pure organic ferroelectrics have been recently developed. However, some problems, such as current leakage and/or low working frequencies, make their application prospects especially for ferroelectric memory (FeRAM) not clear. Here, we describe the molecule-displacive ferroelectricity of supramolecular adducts of tartaric acid and 1,4-diazabicyclo[2.2.2]octane N,N′-dioxide. The adducts show large spontaneous polarization, high rectangularity of the ferroelectric hysteresis loops even at high operation frequency (10 kHz), and high performance in polarization switching up to 1 × 106 times without showing fatigue. It opens great perspectives in terms of applications, especially in organic FeRAM.

Reversible structural phase transition of pyridinium-4-carboxylic acid perchlorate

Pyridinium-4-carboxylic acid perchlorate (C6H6NO2·ClO4) was synthesized and separated as crystals. Differential scanning calorimetry measurement shows that this compound undergoes a reversible phase transition at about 122 K with a heat hysteresis of 1.8 K. A dielectric anomaly observed at 127 K further confirms the phase transition. The low-temperature (LT; T = 103 K) structure has space group P21/c and cell parameters a = 17.356 (6), b = 13.241 (3), c = 16.161 (7) A, β = 138.055 (17)°. The high-temperature (HT; T = 298 K) structure has space group P21/c and cell parameters a = 5.5046 (11), b = 13.574 (3), c = 11.834 (2) A, β = 99.35 (3)°, but can be re-described using new axes a′ = a, b′ = b, c′ = −2a + c, V′ = V to give the cell a′ = 5.5046 (11), b′ = 13.574 (3), c′ = 17.424 (3) A, β′ = 137.92 (3)° and space group P21/c. The associated coordinate transformation is x′ = x + 2z, y′ = y, z′ = z and the associated reflection index transformation is h′ = h, k′ = k, l′ = l − 2h. The relationship between the two cells is 3a, b, c (HT) approximates a, b, c (LT). The crystal comprises one-dimensional hydrogen-bonded chains of the pyridinium-4-carboxylic acid cations and perchlorate anions. A precise analysis of the main packing and structural differences as well as the changes in the intermolecular interactions between the HT phase and the LT phase reveals that the disorder–order transition of the perchlorate anions may be the driving force of the transition, and the hydrogen-bonding effect may contribute to the transition as a secondary parameter.