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Angewandte Chemie International Edition 06/2011; 50(26):5847-50. · 13.45 Impact Factor
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ABSTRACT: Transitions associated with orientational order-disorder phenomena are found in a wide range of materials and may have a significant impact on their properties. In this work, specific heat and (1)H NMR measurements have been used to study the phase transition in the metal-organic framework (MOF) compound [(CH(3))(2)NH(2)]Zn(HCOO)(3). This compound, which possesses a perovskite-type architecture, undergoes a remarkable order-disorder phase transition at 156 K. The (DMA(+)) cationic moieties that are bound by hydrogen bonds to the oxygens of the formate groups (N─HO ∼ 2.9 Å) are essentially trapped inside the basic perovskite cage architecture. Above 156 K, it is the orientations of these moieties that are responsible for the disorder, as each can take up three different orientations with equal probability. Below 156 K, the DMA(+) is ordered within one of these sites, although the moiety still retains a considerable state of motion. Below 40 K, the rotational motions of the methyl groups start to freeze. As the temperature is increased from 4 K in the NMR measurements, different relaxation pathways can be observed in the temperature range approximately 65-150 K, as a result of a "memory effect." This dynamic behavior is characteristic of a glass in which multiple states possess similar energies, found here for a MOF. This conclusion is strongly supported by the specific heat data.
Proceedings of the National Academy of Sciences 04/2011; · 9.68 Impact Factor
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Nature 03/2010; 464(7286):160. · 36.28 Impact Factor
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ABSTRACT: Multiferroic behavior in perovskite-related metal-organic frameworks of general formula [(CH(3))(2)NH(2)]M(HCOO)(3), where M = Mn, Fe, Co, and Ni, is reported. All four compounds exhibit paraelectric-antiferroelectric phase transition behavior in the temperature range 160-185 K (Mn: 185 K, Fe: 160 K; Co: 165 K; Ni: 180 K); this is associated with an order-disorder transition involving the hydrogen bonded dimethylammonium cations. On further cooling, the compounds become canted weak ferromagnets below 40 K. This research opens up a new class of multiferroics in which the electrical ordering is achieved by means of hydrogen bonding.
Journal of the American Chemical Society 10/2009; 131(38):13625-7. · 9.91 Impact Factor
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ABSTRACT: [(CH3)2NH2]Zn(HCOO)3, 1, adopts a structure that is analogous to that of a traditional perovskite, ABX3, with A = [(CH3)2NH2], B = Zn, and X = HCOO. The hydrogen atoms of the dimethyl ammonium cation, which hydrogen bond to oxygen atoms of the formate framework, are disordered at room temperature. X-ray powder diffraction, dielectric constant, and specific heat data show that 1 undergoes an order-disorder phase transition on cooling below 156 K. We present evidence that this is a classical paraelectric to antiferroelectric phase transition that is driven by ordering of the hydrogen atoms. This sort of electrical ordering associated with order-disorder phase transition is unprecedented in hybrid frameworks and opens up an exciting new direction in rational synthetic strategies to create extended hybrid networks for applications in ferroic-related fields.
Journal of the American Chemical Society 08/2008; 130(32):10450-1. · 9.91 Impact Factor
