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We present the first, to our knowledge, study of solid state oxygen translational diffusion by quasielastic neutron scattering. Such studies in the past might have been precluded by relatively low diffusivities of oxygen anions in the temperature range amenable to neutron scattering experiments. To explore the potential of the quasielastic scattering technique, which can deduce atomic diffusion jump length of oxygen anions through the momentum transfer dependence of the scattering signal, we have selected the fastest known oxygen conductor, bismuth oxide. We have found the oxygen anion jump length in excellent agreement with the nearest oxygen-vacancy distance in the anion sublattice of the fluorite-related structure of bismuth oxide. [ABSTRACT FROM AUTHOR]

The properties of fluids can be significantly altered by the geometry of their confining environments. While there has been significant work on the properties of such confined fluids, the properties of fluids under ultraconfinement, environments where, at least in one plane, the dimensions of the confining environment are similar to that of the confined molecule, have not been investigated. This paper investigates the dynamic properties of water in beryl (BeiAliSifiOis), the structure of which contains approximately 5-Â-diam channels parallel to the c axis. Three techniques, inelastic neutron scattering, quasielastic neutron scattering, and dielectric spectroscopy, have been used to quantify these properties over a dynamic range covering approximately 16 orders of magnitude. Because beryl can be obtained in large single crystals we were able to quantify directional variations, perpendicular and parallel to the channel directions, in the dynamics of the confined fluid. These are significantly anisotropic and somewhat counterintuitively, show that vibrations parallel to the c-axis channels are significantly more hindered than those perpendicular to the channels. The effective potential for vibrations in the c direction is harder than the potential in directions perpendicular to it. There is evidence of single-file diffusion of water molecules along the channels at higher temperatures, but below 150 K this diffusion is strongly suppressed. No such suppression, however, has been observed in the channel-perpendicular direction. Inelastic neutron scattering spectra include an intramolecular stretching O-H peak at ~465 meV. As this is nearly coincident with that known for free water molecules and approximately 30 meV higher than that in liquid water or ice, this suggests that there is no hydrogen bonding constraining vibrations between the channel water and the beryl structure. However, dielectric spectroscopic measurements at higher temperatures and lower frequencies yield an activation energy for the dipole reorientation of 16.4 ± 0.14 kJ/mol, close to the energy required to break a hydrogen bond in bulk water. This may suggest the presence of some other form of bonding between the water molecules and the structure, but the resolution of the apparent contradiction between the inelastic neutron and dielectric spectroscopic results remains uncertain. [ABSTRACT FROM AUTHOR]

Abstract: Using quasielastic neutron scattering, we compare dynamics in single-element liquids, glass-forming selenium and non glass-forming gallium. There is a single jump-diffusion process in gallium, whereas in selenium there is also a faster, spatially localized process. The fast and slow processes describe β- and α-relaxation, respectively. We then analyze an archetypical glass-former, glycerol, to show that the two-component fit, with β- and α-relaxations explicitly separated, yields the correct value for the translational diffusion coefficient and provides information on the spatial localization of the β-relaxation that is not experimentally accessible otherwise. [Copyright &y& Elsevier]

Abstract: The diffusion of methane confined in nano-porous carbon aerogel with the average pore size 48Å and porosity ∼60% was investigated as a function of pressure at T =298K using quasi-elastic neutron scattering (QENS). The diffusivity of methane shows a clear effect of confinement: it is about two orders of magnitude lower than in bulk at the same thermodynamic conditions and is close to the diffusivity of liquid methane at 100K (i.e. ∼90K below the liquid–gas critical temperature T C ≈191K). The diffusion coefficient (D) of methane initially increases with pressure by a factor of ∼2.5 from 3.47±0.41×10−10 m2 s−1 at 0.482MPa to D =8.55±0.33×10−10 m2 s−1 at 2.75MPa and starts to decrease at higher pressures. An explanation of the observed non-monotonic behavior of the diffusivity in the confined fluid is based on the results of small-angle neutron scattering experiments of the phase behavior of methane in a similar carbon aerogel sample. The initial increase of the diffusion coefficient with pressure is explained as due to progressive filling of bigger pores in which molecular mobility in the internal pore volume is less affected by the sluggish liquid-like molecular mobility in the adsorbed phase. Subsequent decrease of D, is associated with the effect of intermolecular collisions, which result in a lower total molecular mobility with pressure, as in the bulk state. The results are compared with the available QENS data on the methane diffusivity in zeolites, metal organic frameworks, and porous silica as well as with the molecular dynamics simulations of methane in nano-porous carbons and silica zeolites. [Copyright &y& Elsevier]

Abstract: Monophase thorium–zirconium hydrides (ThZr2H x ) have been fabricated starting from a metallic alloy and the hydrogen stoichiometry determined by X-ray diffraction. Incoherent Quasielastic Neutron Scattering (IQNS) on the hydrides was conducted over the temperature range 650–750K at the Backscattering Silicon Spectrometer (BASIS) at the Spallation Neutron Source (SNS) at ORNL. The isotropic Chudley–Elliott model was utilized to analyze the quasielastic linewidth broadening data as function of momentum transfer. The diffusion coefficient and average jump distance of hydrogen atoms in ThZr2H5.6 and ThZr2H6.2 were extracted from the measurements. [Copyright &y& Elsevier]

The high-temperature motion of hydrogen in near stoichiometric yttrium dihydride (YH x , x = 1.62 and 1.87 at.%) was investigated using incoherent quasi-elastic neutron scattering and Density Functional Theory (DFT) calculations as a function of hydrogen stoichiometry. Translational motion (diffusivity) of hydrogen in yttrium dihydride was only observed in a temperature range of 1073–1173 K under vacuum environment. The hydrogen motion was found to be limited to the tetrahedral sublattice, and diffusivity of hydrogen was observed to increase with decreasing hydrogen stoichiometry. The same behavior was also supported with DFT calculations. The DFT results also indicated that certain migration paths with smaller energy barriers favored H jump resulting in higher diffusivities. [ABSTRACT FROM AUTHOR]

We used quasielastic neutron scattering (QENS) to studythe dynamicsof phenanthrenequinone (PQ) on the surface of onion-like carbon (OLC),or so-called carbon onions, as a function of surface coverage andtemperature. For both the high- and low-coverage samples, we observedtwo diffusion processes; a faster process and nearly an order of magnitudeslower process. On the high-coverage surface, the slow diffusion processis of long-range translational character, whereas the fast diffusionprocess is spatially localized on the length scale of ∼4.7Å. On the low-coverage surface, both diffusion processes arespatially localized; on the same length scale of ∼4.7 Åfor the fast diffusion and a somewhat larger length scale for theslow diffusion. Arrhenius temperature dependence is observed exceptfor the long-range diffusion on the high-coverage surface. We attributethe fast diffusion process to the generic localized in-cage dynamicsof PQ molecules, and the slow diffusion process to the long-rangetranslational dynamics of PQ molecules, which, depending on the coverage,may be either spatially restricted or long-range. On the low-coveragesurface, uniform surface coverage is not attained, and the PQ moleculesexperience the effect of spatial constraints on their long-range translationaldynamics. Unexpectedly, the dynamics of PQ molecules on OLC as a functionof temperature and surface coverage bears qualitative resemblanceto the dynamics of water molecules on oxide surfaces, including practicallytemperature-independent residence times for the low-coverage surface.The dynamics features that we observed may be universal across differentclasses of surface adsorbates. [ABSTRACT FROM AUTHOR]

Quasielastic neutron scattering (QENS) was used to investigate the diffusion dynamics of hydration water on the surface of futile (TiO2) nanopowder. The dynamics measurements utilizing two inelastic instruments, a backscattering spectrometer and a disk chopper spectrometer, probed the fast, intermediate, and slow motions of the water molecules on the time scale of picoseconds to more than a nanosecond. We employed a model-independent analysis of the data collected at each value of the scattering momentum transfer to investigate the temperature dependence of several diffusion components. All of the probed components were present in the studied temperature range of 230-320 K, providing, at a first sight, no evidence of discontinuity in the hydration water dynamics. However, a qualitative change in the elastic scattering between 240 and 250 K suggested a surface freezing-melting transition, when the motions that were localized at lower temperatures became delocalized at higher temperatures. On the basis of our previous molecular dynamics simulations of this system, we argue that interpretation of QENS data from such a complex interfacial system requires at least qualitative input from simulations, particularly when comparing results from spectrometers with very different energy resolutions and dynamic ranges. [ABSTRACT FROM AUTHOR]

Diffusion dynamics of adsorbate particles confined on a surface strongly depends on the adsorbate surface coverage. Our molecular dynamics simulation reveals that the average lateral diffusivity of the Lennard-Jones adsorbate on a flat substrate varies non-monotonically with the surface coverage. Given that the adsorbate becomes stratified into distinct layers, we propose a novel approach to analyze the lateral diffusion of the adsorbate particles in different layers, via dividing the total lateral diffusion into consecutive lateral diffusion processes, separated by the layer boundary crossing events. A new definition of the lateral diffusivity in the specific layer has been introduced, and it is revealed that the total lateral diffusivity is equal to the sum of lateral diffusivities in different layers. Moreover, the non-monotonic behavior of the average lateral diffusivity with the surface coverage is found to result from the difference in behavior of the lateral diffusivity between the first and other layers. [ABSTRACT FROM AUTHOR]

Abstract: A diffusive behavior of Li+ ion in a garnet-type oxide, Li5+ x La3Zr x Nb2−x012 with x =0–2, has been investigated by both a positive muon-spin relaxation (μ +SR) and quasi-elastic neutron scattering (QENS) technique using powder samples. The μ +SR results revealed that Li+ ions start to diffuse above ~150K for the whole samples measured. The activation energy of Li diffusion (Ea ) estimated from the μ +SR data was in good agreement with Ea obtained by the QENS measurements. However, both Ea s were about a half of Ea of ionic conductivity of Li+ (σ Li), which was evaluated by AC-impedance measurements on sintered pellets. This indicated the serious effect of grain boundary and/or surface on Ea . Furthermore, since the diffusion coefficient of Li+ estimated by μ +SR is roughly independent of x, the number of mobile Li+ in the garnet lattice was found to be the predominant parameter to determine σ Li. [Copyright &y& Elsevier]

Quasielastic neutron scattering of H2 and D2 in the same nanoporous carbon at 10-40 K demonstrates extreme quantum sieving, with D2 diffusing up to 76 times faster. D2 also shows liquidlike diffusion while H2 exhibits Chudley-Elliott jump diffusion, evidence of their different relationships with the local lattice of adsorption sites due to quantum effects on intermolecular interactions. The onset of diffusion occurs at 22-25 K for H2and 10-13 K for D2. At these temperatures, H2 and D2 have identical thermal de Broglie wavelengths that correlate with the dominant pore size. [ABSTRACT FROM AUTHOR]

The translational dynamics of hydration water confined in calcium-silicate-hydrate (C-S-H) gel was studied by quasielastic neutron scattering spectroscopy in the temperature range from 280 to 230 K. The stretch exponent β, the self-diffusion constant D, the average translational relaxation time <τ>, and the temperature dependence of confinement radius α extracted from the elastic fraction of immobile water molecules p(Q) were obtained from the analyses of the low- Q spectra according to the relaxing cage model. Measurements were made using C-S-H of three different water contents, 10%, 17%, and 30%. Among the three samples of C-S-H gel with different water contents, the values of β decrease with increasing water contents, while α increases. The values of D and <τ> are insensitive to temperature for the two lower water contents, as opposed to the 30% case where a slight variation is observed. The trend for violation of the Stokes-Einstein relation is only visible in the case of 30% water content. [ABSTRACT FROM AUTHOR]

Dynamics of hydration water is essential for the function of biomacromolecules. Previous studies have demonstrated that water molecules exhibit subdiffusion on the surface of biomacromolecules; yet the microscopic mechanism remains vague. Here, by performing neutron scattering, molecular dynamics simulations, and analytic modeling on hydrated perdeuterated protein powders, we found water molecules jump randomly between trapping sites on protein surfaces, whose waiting times obey a broad distribution, resulting in subdiffusion. Moreover, the subdiffusive exponent gradually increases with observation time towards normal diffusion due to a many-body volume-exclusion effect. [ABSTRACT FROM AUTHOR]