Search results for "neutron diffraction"
showing 10 items of 102 documents
Study of percolation and clustering in supercritical water-CO2 mixtures
2008
The microscopic structure of supercritical water-CO(2) mixture is investigated by neutron diffraction experiments exploiting the isotopic HD substitution. The investigated water reach mixtures are in the liquidlike region of the phase diagram, according to the behavior of the radial distribution functions, yet a reduction of the average number of hydrogen bonds, compared to equivalent states of pure water, is found. As a consequence, the average dimension of water clusters is reduced and the system stays below the percolation threshold. These results, along with the shift of the main peaks of the site-site radial distribution functions, suggest that the excess volume in these supercritical …
Magnetism in Polyoxometalates: Anisotropic Exchange Interactions in the Co Moiety of [Co3W(D2O)2(ZnW9O34)2]12−—A Magnetic and Inelastic Neutron Scatt…
2002
The ground-state properties of a Co moiety encapsulated in a polyoxometalate anion were investigated by combining measurements of specific heat, magnetic susceptibility, and low-temperature magnetization with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na12[Co3W(D2O)2(ZnW9O34)2]⋅40 D2O (Co3). The ferromagnetic Co3O14 cluster core consists of three octahedrally oxo-coordinated CoII ions. According to the single-ion anisotropy and spin–orbit coupling of the octahedral CoII ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co3 spin cluster is anisotropic and is expressed as ℋ=−2Σ(JŜ1αŜ2α+JŜ2αŜ3α), w…
Canted Antiferromagnetism on Rectangular Layers of Fe2+ in Polymorphic CaFeSeO
2017
From stoichiometric amounts of CaO, Fe, and Se, pure powders and single crystals of quaternary Ca[FeSe2/2O2/2]∞2 can be obtained by solid-state reaction and self-flux growth, respectively. The as-synthesized compound exhibits a polymorphic crystal structure, where the two modifications have different stacking sequences of [FeSe2/2O2/2]2−∞2 layers. The two polymorphs have similar unit cells but different crystal symmetries (Cmc21 and Pnma), of which the former is non-centrosymmetric. Fe is divalent (d6) and high-spin, as proven by X-ray spectroscopy, Mossbauer spectroscopy, and powder neutron diffraction data. The latter two, in combination with magnetic susceptibility and specific heat data…
Average versus local structure in K2NiF4-type LaSrAlO4: direct experimental evidence of local cationic ordering
2012
The long-range ordering of a crystalline material can be accurately determined by analyzing the Bragg intensities and positions. In contrast, direct observation of short-range ordering in crystalline materials, which is increasingly considered of fundamental importance to unravel the structure-property relationships that underpin their technological applications, is a challenging task. In this study we have investigated the structure of LaSrAlO4, a representative example of compounds with the K2NiF4-type structure. By the combined use of synchrotron and neutron diffraction, pair distribution function analysis, Al-27 MQMAS NMR and atomistic simulations we have highlighted differences between…
Heat-capacity and neutron-diffraction studies of HD monolayers physisorbed on graphite
1992
Abstract The phase diagram of HD monolayers physisorbed on graphite is studied in detail by using heat-capacity and neutron-diffraction measurements. The following features are observed: At low coverages the system forms a commensurate (C) (√3 × √3)R30° phase and at coverages near monolayer completion an incommensurate (IC) two-dimensional solid phase. The order-disorder transition of the C phase belongs to the three-state Potts universality class. The C-IC transition occurs via two intermediate phases, i.e., the α phase and a novel phase, which is interpreted as an α-IC coexistence region. Though the molecule HD is a strongly incoherent neutron scatterer, for the first time elastic neutron…
Rigid versus Flexible Protein Matrix: Light-Harvesting Complex II Exhibits a Temperature-Dependent Phonon Spectral Density
2018
Dynamics-function correlations are usually inferred when molecular mobility and protein function are simultaneously impaired at characteristic temperatures or hydration levels. In this sense, excitation energy transfer in the photosynthetic light-harvesting complex II (LHC II) is an untypical example because it remains fully functional even at cryogenic temperatures relying mainly on interactions of electronic states with protein vibrations. Here, we study the vibrational and conformational protein dynamics of monomeric and trimeric LHC II from spinach using inelastic neutron scattering (INS) in the temperature range of 20-305 K. INS spectra of trimeric LHC II reveal a distinct vibrational …
The Structure of Cholesterol in Lipid Rafts
2014
Rafts, or functional domains, are transient nano- or mesoscopic structures in the plasma membrane and are thought to be essential for many cellular processes such as signal transduction, adhesion, trafficking and lipid/protein sorting. Observations of these membrane heterogeneities have proven challenging, as they are thought to be both small and short-lived. With a combination of coarse-grained molecular dynamics simulations and neutron diffraction using deuterium labeled cholesterol molecules we observe raft-like structures and determine the ordering of the cholesterol molecules in binary cholesterol-containing lipid membranes. From coarse-grained computer simulations, heterogenous membra…
Systematical, experimental investigations on LiMgZ (Z= P, As, Sb) wide band gap semiconductors
2011
This work reports on the experimental investigation of the wide band gap compounds LiMgZ (Z = P, As, Sb), which are promising candidates for opto-electronics and anode materials for Lithium batteries. The compounds crystallize in the cubic (C1_b) MgAgAs structure (space group F-43m). The polycrystalline samples were synthesized by solid state reaction methods. X-ray and neutron diffraction measurements show a homogeneous, single-phased samples. The electronic properties were studied using the direct current (DC) method. Additionally UV-VIS diffuse reflectance spectra were recorded in order to investigate the band gap nature. The measurements show that all compounds exhibit semiconducting be…
Neutron-diffraction study of end-for-end ordering in commensurate submonolayers of carbon monoxide physisorbed on the graphite basal plane
1997
The head-tail (dipolar) ordering in commensurate submonolayer films of carbon monoxide adsorbed on graphite recently observed in heat-capacity measurements for T < 5.18 K has been investigated by neutron diffraction. It will be shown that the experimental results are consistent with an end-for-end ordered commensurate herringbone structure.
Neutron diffraction studies of the molecular compound [Co 2(bta)]n (H4bta =1,2,4,5-benzenetetracarboxylic acid): In the quest of canted ferromagnetism
2013
The exchange mechanism and magnetic structure of the organic-inorganic layered molecule-based magnet [Co2(bta)]n (1) (H 4bta =1,2,4,5-benzenetetracarboxylic acid) have been investigated through variable-temperature magnetic susceptibility measurements and supported with a series of neutron diffraction experiments. Cryomagnetic studies have shown an antiferromagnetic ordering at a transition temperature of 16 K that is followed by the appearance of a weak ferromagnetism below 11 K. The weak antiferromagnetic interlayer interaction plays an important role in this system in spite of the long interlayer separation. A ferromagnetic ordering is induced by applied magnetic fields greater than 1800…