Browsing by Author "Chen, YS"

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    Atmospheric iron deposition: global distribution, variability, and human perturbations
    (Annual Reviews, 2009-01) Mahowald, NM; Engelstaedter, S; Luo, CW; Sealy, A; Artaxo, P; Benitez-Nelson, C; Bonnet, S; Chen, YS; Chuang, PY; Cohen, DD; Dulac, F; Herut, B; Johansen, AM; Kubilay, N; Losno, R; Maenhaut, W; Paytan, A; Prospero, JM; Shank, LM; Siefert, RL
    Atmospheric inputs of iron to the open ocean are hypothesized to modulate ocean biogeochemistry. This review presents an integration of available observations of atmospheric iron and iron deposition, and also covers bioavailable iron distributions. Methods for estimating temporal variability in ocean deposition over the recent past are reviewed. Desert dust iron is estimated to represent 95% of the global atmospheric iron cycle, and combustion sources of iron are responsible for the remaining 5%. Humans may be significantly perturbing desert dust (up to 50%). The sources of bioavailable iron ire less well understood than those of iron, partly because we do not know what speciation of the iron is bioavailable. Bioavailable iron can derive from atmospheric processing of relatively insoluble desert dust iron or from direct emissions of soluble iron from combustion sources. These results imply that humans could be substantially impacting it-on and bioavailable iron deposition to ocean regions, but there are large uncertainties in our understanding. © 2009, Annual Reviews
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    Intermolecular interactions in solid-state metalloporphyrins and their impacts on crystal and molecular structures
    (ACS Publications, 2014-10-23) Hunter, SC; Smith, BA; Hoffmann, CM; Wang, XP; Chen, YS; McIntyre, GJ; Xue, ZL
    A variable-temperature (VT) crystal structure study of [Fe(TPP)Cl] (TPP2– = meso-tetraphenylporphyrinate) and Hirshfeld surface analyses of its structures and previously reported structures of [M(TPP)(NO)] (M = Fe, Co) reveal that intermolecular interactions are a significant factor in structure disorder in the three metalloporphyrins and phase changes in the nitrosyl complexes. These interactions cause, for example, an 8-fold disorder in the crystal structures of [M(TPP)(NO)] at room temperature that obscures the M–NO binding. Hirshfeld analyses of the structure of [Co(TPP)(NO)] indicate that the phase change from I4/m to P1̅ leads to an increase in void-volume percentage, permitting additional structural compression through tilting of the phenyl rings to offset the close-packing interactions at the interlayer positions in the crystal structures with temperature decrease. X-ray and neutron structure studies of [Fe(TPP)Cl] at 293, 143, and 20 K reveal a tilting of the phenyl groups away from being perpendicular to the porphyrin ring as a result of intermolecular interactions. Structural similarities and differences among the three complexes are identified and described by Hirshfeld surface and void-volume calculations.© 2014, American Chemical Society.
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    Narrowed bandgaps and stronger excitonic effects from small boron nitride nanotubes
    (Elsevier, 2009-07-16) Yu, J; Yu, DH; Chen, YS; Chen, H; Lin, MY; Cheng, BM; Li, J; Duan, W
    The bandgap of boron nitride nanotubes (BNNTs) is generally considered to be independent on tube radius and chirality. However, we have observed that the bandgaps of BNNTs do depend on the tube size. Photoluminescence excitation spectroscopy with variable photon energies in vacuum ultraviolet (VUV) range revealed that the bandgap becomes smaller when the tube diameter decreases. This is consistent with red-shifted luminescent emissions. The strong interactions between excitons and phonons are possibly responsible for the bandgap narrowing as the function of nanotube size. © 2009, Elsevier Ltd.
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    Re-investigation of the structure and crystal chemistry of the Bi2O3-W2O6 'type (Ib)' solid solution using single-crystal neutron and synchrotron x-ray diffraction
    (International Union of Crystallography, 2010-04) Sharma, N; Macquart, RB; Avdeev, M; Christensen, M; McIntyre, GJ; Chen, YS; Ling, CD
    Single crystals of composition Bi35.66W4.34O66.51 (or Bi8.2WO15.3, bismuth tungsten oxide), within the type (Ib) solid-solution region of the Bi2O3-WO3 system, were synthesized using the floating-zone furnace method. Synchrotron X-ray and neutron single-crystal diffraction data were used to confirm the previously tentative assignment of the room-temperature space group as I41. Fourier analysis of the combined X-ray and neutron datasets was used to elucidate and refine fully the cation and anion arrays for the first time. The mixed cation site M1 is shown to be coordinated by eight O atoms in an irregular cube when M = Bi, and by six O atoms in an octahedron when M = W. The resulting disorder in the average structure around M1 is discussed in the context of experimentally observed oxide-ion conductivity. © 2010, International Union of Crystallography
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    Structure and crystal chemistry of fluorite-related Bi38Mo7O78 from single crystal x-ray diffraction and ab initio calculations
    (Elsevier, 2009-06) Sharma, N; Macquart, RB; Christensen, M; Avdeev, M; Chen, YS; Ling, CD
    The floating-zone furnace method was used to synthesize single crystals of the fluorite-related delta-Bi2O3-type phase Bi(3)gMo(7)O(78) for the first time. Single crystal synchrotron X-ray diffraction data, in conjunction with ab initio (density functional theory) calculations, were used to solve, optimize, and refine the 5 x 3 x 3 commensurate superstructure of fluorite-type delta-Bi2O3 in Pbcn (a = 28.7058(11) angstrom, b = 16.8493(7) angstrom and c = 16.9376(6) angstrom, Z = 4, R-F= 11.26%, wR(I) = 21.67%). The structure contains stepped channels of Mo6+ in tetrahedral environments along the b axis and chains of Mo6+ in octahedral environments along the ac plane. The role of the stepped channels in oxide ion conduction is discussed. The simultaneous presence of both tetrahedral and octahedral coordination environments for Mo6+, something not previously observed in Mo6+-doped delta-Bi2O3-type phases, is supported by charge balance considerations in addition to the results of crystallographic and ab initio analysis. © 2009, Elsevier Ltd.