Browsing by Author "Lock, N"

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    Elucidating negative thermal expansion in MOF-5
    (American Chemical Society, 2010-9-09) Lock, N; Wu, Y; Christensen, M; Cameron, L; Peterson, VK; Bridgeman, AJ; Kepert, CJ; Iversen, BB
    Multi-temperature X-ray diffraction studies show that twisting, rotation, and libration cause negative thermal expansion (NTE) of the nanoporous metal−organic framework MOF-5, Zn4O(1,4-benzenedicarboxylate)3. The near-linear lattice contraction is quantified in the temperature range 80−500 K using synchrotron powder X-ray diffraction. Vibrational motions causing the abnormal expansion behavior are evidenced by shortening of certain interatomic distances with increasing temperature according to single-crystal X-ray diffraction on a guest-free crystal over a broad temperature range. Detailed analysis of the atomic positional and displacement parameters suggests two contributions to cause the effect: (1) local twisting and vibrational motion of the carboxylate groups and (2) concerted transverse vibration of the linear linkers. The vibrational mechanism is confirmed by calculations of the dynamics in a molecular fragment of the framework. © 2010, American Chemical Society
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    Negative thermal expansion in the metal-organic framework material Cu-3(1,3,5-benzenetricarboxylate)(2)
    (Wiley-VCH Verlag Berlin, 2008-08-08) Wu, Y; Kobayashi, A; Halder, GJ; Peterson, VK; Chapman, KW; Lock, N; Southon, PD; Kepert, CJ
    The metal–organic framework [Cu3(btc)2] displays negative thermal expansion (NTE) over a broad temperature range. This property arises from two coincident mechanisms, each of which are unique for NTE systems: the concerted transverse vibration of triangular organic linkers, and the local dynamic distortion of dinuclear metal centers within the framework lattice. © 2008, Wiley-VCH Verlag Berlin
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    Scrutinizing negative thermal expansion in MOF-5 by scattering techniques and ab initio calculations
    (Royal Society of Chemistry, 2012-09-14) Lock, N; Christensen, M; Wu, Y; Peterson, VK; Thomsen, MK; Piltz, RO; Ramirez-Cuesta, AJ; McIntyre, GJ; Noren, K; Kutteh, R; Kepert, CJ; Kearley, GJ; Iversen, BB
    Complementary experimental techniques and ab initio calculations were used to determine the origin and nature of negative thermal expansion (NTE) in the archetype metal-organic framework MOF-5 (Zn4O(1,4-benzenedicarboxylate)3). The organic linker was probed by inelastic neutron scattering under vacuum and at a gas pressure of 175 bar to distinguish between the pressure and temperature responses of the framework motions, and the local structure of the metal centers was studied by X-ray absorption spectroscopy. Multi-temperature powder- and single-crystal X-ray and neutron diffraction was used to characterize the polymeric nature of the sample and to quantify NTE over the large temperature range 4-400 K. Ab initio calculations complement the experimental data with detailed information on vibrational motions in the framework and their correlations. A uniform and comprehensive picture of NTE in MOF-5 has been drawn, and we provide direct evidence that the main contributor to NTE is translational transverse motion of the aromatic ring, which can be dampened by applying a gas pressure to the sample. The linker motion is highly correlated rather than local in nature. The relative energies of different framework vibrations populated in MOF-5 are suggested by analysis of neutron diffraction data. We note that the lowest-energy motion is a librational motion of the aromatic ring which does not contribute to NTE. The libration is followed by transverse motion of the linker and the carboxylate group. These motions result in unit-cell contraction with increasing temperature. © 2012, Royal Society of Chemistry