Ren-Gen Xiong

Ferroelectric metal-organic frameworks.

Xuan Zhang Advisor: Prof. Kim R. Dunbar Apr. 16, 2012 Room 2102 Ever since its appearance about 17 years ago, 1 the term Metal-Organic Frameworks (MOFs) has been gaining tremendous attention in chemistry and crystal engineering because of the fascinating solid state materials it pertains to. These MOFs, also known as porous coordination polymers (PCPs) are characterized by their high porosity, large surface area, and easy tunability of surface and structural properties. And they are extensively investigated for the potential applications in gas storage and separation, catalysis and biomedical imaging. 3 Due to their unique structural motifs, the surface properties and the macroproperties can also be readily adjusted to afford functionalities such as ferroelectricity, electron conductivity, magnetism and optical properties. 3 Among these less well-developed functionalities, the ferroelectric properties of some lately reported MOFs will be presented in this talk, covering their triggering mechanism, designing strategies and potential applications as multifunctional materials.

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.

Exceptional Dielectric Phase Transitions in a Perovskite-Type Cage Compound

Progress in metal–organic framework (MOF) research has recently opened up new possibilities to realize hybrid materials with unique solid-state electric properties, such as ferroelectricity, piezoelectricity, and dielectricity. Compared with conventional pure inorganic/organic compounds, MOFs take advantage of structural tunability and multifunctionality to develop polarizable molecular materials with rich dielectric properties. Among them, switchable molecular dielectrics, which undergo transitions between high and low dielectric states, are promising materials with potential applications especially in data communication, signal processing, and sensing. However, reports of such MOFs have remained scarce owing to a lack of knowledge regarding control of the motions of the dipole moments in the crystal lattice. From the microscopic point of view, the tunable dielectric permittivity closely relates to the positional freedom of molecular dipole moments. For instance, polar molecules in the liquid state show larger dielectric permittivities than in the solid state owing to the “melting” and “freezing” of the molecular reorientations. With regard to MOFs, the dipole moments are rigidly fixed in the crystal structures in most cases, usually resulting in small and almost temperatureindependent dielectric permittivities. Fortunately, there is still much room for the integration of flexible units into the frameworks; that is, the introduction of a polarization rotation unit in the form of a solid-state molecular rotator or host–guest systems, such as porous compounds. Cage compounds, which are assembled by the inclusion of guest species into the well-matched host cages, is a very promising class of switchable molecular dielectrics. The reorientations of the polar guests in the carefully designed cage compounds may give rise to large dielectric permittivities, which are characterized by a multidimensional liquidlike state, and their freezing will lead to low-dielectric systems. Herein, we present a novel organic–inorganic hybrid cage compound (HIm)2[KFe(CN)6] (1; HIm = imidazolium) with a perovskite-type structure, in which the order– disorder behavior of the HIm polar guests give rise to striking dielectric anomalies. The (HIm)2[KFe(CN)6] crystals were grown from an aqueous solution of K3[Fe(CN)6] and (HIm)Cl salts by slow evaporation at room temperature as large red hexagonal plate perpendicular to the c axis. The existence of HIm and CN groups in 1 is verified by IR spectra. The CN group in 1 exhibits several vibrations in the range 2102–2143 cm , distinct from a single peak of 2118 cm 1 in K3[Fe(CN)6]. Thermal analysis reveals that 1 undergoes two phase transitions, at 187 K (T1) and 158 K (T2). For convenience, we label the phase above T1 as the high-temperature phase (HTP), the phase between T1 and T2 as intermediate-temperature phase (ITP), and the phase below T2 as low-temperature phase (LTP). Variable-temperature X-ray diffraction analysis reveals that 1 crystallizes in the centrosymmetric space group R3̄m at 293 K and 173 K as the HTP and ITP, respectively, and in C2/c at 83 K as the LTP. The common structural feature of the compound is the anionic cage formed by Fe CN K units in which the HIm cation resides. The metal–cyanide bond is strong and covalent in the fragment {Fe(CN)6} (Fe C = 1.9 ) and much weaker and ionic in the fragment {K(NC)6} (K N = 2.9 ; Figure 1). In the HTP, the cation reorients around the threefold c axis perpendicular to the ring plane. The cation consists of three carbon and two nitrogen atoms, which were all refined as carbon atoms. The five atoms of the

Tunable and Switchable Dielectric Constant in an Amphidynamic Crystal

The inclusion compound [(CH3)2NH2]2[KCo(CN)6] exhibits a marked temperature-dependent dielectric constant and can be considered as a model of tunable and switchable dielectric materials. Crystal structure and solid-state NMR studies reveal a switchable property between low and high dielectric states around 245 K. This originates from an order-disorder phase transition of the system, changing the dynamics of the polar dimethylammonium (DMA) cation. Furthermore, the tuning of the dielectric constant at temperatures below the phase transition point is related to increasing angular pretransitional fluctuations of the dipole moment of DMA.

Hydrogen-Bonded Ferroelectrics Based on Metal−Organic Coordination

Metal-organic coordination (MOC)-type ferroelectrics, cobalt(II) (R)-2-methylpiperazine (MPPA) trichloride [Co(II)Cl(3)(H-MPPA)], was constructed through hydrogen bonds. It is a good ferroelectric candidate with a P(s) = 6.8 microC.cm(-2) as high as almost twice that of triglycine sulfate (P(s) = 3.5 microC.cm(-2)) and significantly larger than that of KH(2)PO(4) at the low-temperature ferroelectric phase Fdd2. [Co(II)Cl(3)(H-MPPA)] is the first example of ferroelectric MOC that can really reach the spontaneous polarization status and opens up a new avenue to explore novel MOC-based ferroelectrics.

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.

A lead-halide perovskite molecular ferroelectric semiconductor

Inorganic semiconductor ferroelectrics such as BiFeO3 have shown great potential in photovoltaic and other applications. Currently, semiconducting properties and the corresponding application in optoelectronic devices of hybrid organo-plumbate or stannate are a hot topic of academic research; more and more of such hybrids have been synthesized. Structurally, these hybrids are suitable for exploration of ferroelectricity. Therefore, the design of molecular ferroelectric semiconductors based on these hybrids provides a possibility to obtain new or high-performance semiconductor ferroelectrics. Here we investigated Pb-layered perovskites, and found the layer perovskite (benzylammonium)2PbCl4 is ferroelectric with semiconducting behaviours. It has a larger ferroelectric spontaneous polarization Ps=13 μC cm−2 and a higher Curie temperature Tc=438 K with a band gap of 3.65 eV. This finding throws light on the new properties of the hybrid organo-plumbate or stannate compounds and provides a new way to develop new semiconductor ferroelectrics.

3D Framework Containing Cu4Br4 Cubane as Connecting Node with Strong Ferroelectricity

The methanolothermal reaction of (S)-1,4-diallyl-2-methylpiperazine (DAMP) with an excess CuBr affords a novel homochiral 3D framework (DAMP)3(Cu4Br4)2(H2O)3 (1) in which Cu4Br4 cubane acts as a connecting node to mimic the pure inorganic role in the ferroelectricity to enhance the remnant polarization value which is comparable to that of BaTiO3 synthesized by peptide-assisted synthesis.