Heterogeneous large amplitude atomic motion in supercooled liquids

One of the central questions in glass physics is the dynamic nature of the glass transition. Essential issues are the type of atomic motions involved and their homogeneity or heterogeneity. Previous experimental studies of the dynamic heterogeneity performed by special NMR techniques for times > 10 -6 s and by incoherent neutron scattering were restricted to the α-relaxation process. Here we review the results of neutron scattering studies focused on the picosecond time domain showing that fast β-process corresponds to large amplitude cluster like heterogeneous motion. © 2003 Published by Elsevier Science B.V

Intra-cage dynamics of molecular hydrogen confined in cages of two different dimensions of clathrate hydrates

In porous materials the molecular confinement is often realized by means of weak Van der Waals interactions between the molecule and the pore surface. The understanding of the mechanism of such interactions is important for a number of applications. In order to establish the role of the confinement size we have studied the microscopic dynamics of molecular hydrogen stored in the nanocages of clathrate hydrates of two different dimensions. We have found that by varying the size of the pore the diffusive mobility of confined hydrogen can be modified in both directions, i.e. reduced or enhanced compared to that in the bulk solid at the same temperatures. In the small cages with a mean crystallographic radius of 3.95 Å the confinement reduces diffusive mobility by orders of magnitude. In contrast, in large cages with a mean radius of 4.75 Å hydrogen molecules displays diffusive jump motion between different equilibrium sites inside the cages, visible at temperatures where bulk H 2 is solid. The localization of H 2 molecules observed in small cages can promote improved functional properties valuable for hydrogen storage applications

Impact of the confinement on the intra-cage dynamics of molecular hydrogen in clathrate hydrates

We have studied the diffusive mobility of hydrogen molecules confined in different size cages in clathrate hydrates. In clathrate hydrate H2 molecules are effectively stored by confinement in two different size cages of the nano-porous host structure with accessible volumes of about 0.50 and 0.67 nm diameters, respectively. For the processes of sorption and desorption of the stored hydrogen the diffusive mobility of the molecules plays a fundamental role. In the present study we have focused on the dynamics of the H2 molecules inside the cages as one aspect of global guest molecule mobility across the crystalline host structure. We have found that for the two cage sizes different in diameter by only 34% and in volume by about a factor of 2.4, the dimension can modify the diffusive mobility of confined hydrogen in both directions, i.e. reducing and surprisingly enhancing mobility compared to the bulk at the same temperature. In the smaller cages of clathrate hydrates hydrogen molecules are localized in the center of the cages even at temperatures >100 K. Confinement in the large cages leads to the onset already at T=10 K of jump diffusion between sorption sites separated from each other by about 2.9 Å at the 4 corners of a tetrahedron. At this temperature bulk hydrogen is frozen at ambient pressure and shows no molecular mobility on the same time scale. A particular feature of this diffusive mobility is the pronounced dynamic heterogeneity: only a temperature dependent fraction of the H2 molecules was found mobile on the time scale covered by the neutron spectrometer used. The differences in microscopic dynamics inside the cages of two different sizes can help to explain the differences in the parameters of macroscopic mobility: trapping of hydrogen molecules in smaller pores matching the molecule size can to play a role in the higher desorption temperature for the small cages

BH₄⁻ self-siffusion in liquid LiBH₄

The hydrogen dynamics in solid and in liquid LiBH₄ was studied by means of incoherent quasielastic neutron scattering. Rotational jump diffusion of the BH₄⁻ subunits on the picosecond scale was observed in solid LiBH₄. The characteristic time constant is significantly shortened when the system transforms from the low-temperature phase to the high-temperature phase at 383 K. In the molten phase of LiBH₄ above 553 K, translational diffusion of the BH₄⁻ units is found. The measured diffusion coefficients are in the 10⁻⁵cm²/s range at temperatures around 700 K, which is in the same order of magnitude as the self-diffusion of liquid lithium or the diffusion of ions in molten alkali halides. The temperature dependence of the diffusion coefficient shows an Arrhenius behavior, with an activation energy of Ea = 88 meV and a prefactor of D₀ = 3.1 × 10⁻⁴cm²/s

Modeling the THF clathrate hydrate dynamics by combining molecular dynamics and quasi-elastic neutron scattering

International audienceThe dynamics of the THF molecule encapsulated in the type II clathrate hydrate matches the MD-QENS observation time (typically 0.1–10 ps) between 100 K and 270 K. Spatial and time characteristics of the THF molecule’s dynamics obtained by means of MD simulations are in agreement with those experimentally determined by means of quasielastic neutron scattering. A detailed model of the THF dynamics is then proposed through the calculations of MD-derived properties. Reorientational relaxation has been observed on a timescale of 0.7 ± 0.1 ps at 270 K with activation energy of 3.0 ± 0.3 kJ/mol in addition to a highly damped rotational excitation occurring in the plane of the THF molecule with a period of ca. 2 ps. Moreover, the anisotropic cage energy landscape of the THF clathrate hydrate is revealed through a comprehensive investigation of THF orientational distribution functions, revealing the occurrence of preferred orientation of the THF molecule within the cage