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N-body refinement of an atom pair potential for molecular dynamics simulations of structural and dynamical properties of aluminosilicates. Model and preliminary results
No full textReverse Mössbauer Effect as a Source of “Hot” Protons in Hydrogen Absorbing Metals
No full textAs an attempt to explain some of the many anomalies and unresolved problems, which have been reported about the dynamic behavior of particles and molecules absorbed in crystalline solids, in a recent paper (1) we derived an extension of the Bragg – von Laue scattering law to high-energy colliding particles, which is related also to the Mössbauer effect (2), and was referred to as “reverse Mössbauer effect” (RME). In particular, we proposed an explanation of a specific and well characterized anomalous behavior found in neutron inelastic-scattering spectra (recoiled bands) of methane adsorbed in a zeolite (3).
According to RME, a particle in non-equilibrium state with respect to a crystal (colliding with the crystal or adsorbed/absorbed in it, and, set out of thermal equilibrium with the crystal by some external cause), can be scattered by the whole crystal with momentum proportional to a vector representing a reciprocal lattice point. The scattering is expected to occur with a well-defined probability and the momentum transferable to the particle is expected to follow a predictable distribution.
As hydrogen is lighter than methane and is adsorbed not only in nanoporous materials, but also is absorbed in a number of metals, we wondered if RME could be present also in H-metal systems and could be detected through some anomalous behavior. RME is essentially a non-equilibrium phenomenon occurring in crystals. Therefore, it could be detected, in principle, by looking for behaviors, which for similar systems differ whether the systems are or not in equilibrium, as well as whether they are crystalline or amorphous.
Enhanced diffusion of H under irradiation by ions and electrons (4,5) was observed. Enhanced diffusion was even reported for hydrogen atoms absorbed in bulk Pd in low-temperature scanning tunneling microscopy (6). In a recent paper (7), the interpretation of the anisotropic diffusion of hydrogen in Nickel required unexpectedly high fitting parameters, at least one order of magnitude larger than the values derived from experimental and ab initio studies.
The consideration of all these phenomena encouraged us to study the features of the RME for H absorbed in metals, and, as some examples, we evaluated its general features for H absorbed in Ni, Nb, Pd and Ti, because they are among the most frequently studied systems.
It was then applied to explain, at least in part, the above reported anomalies.
References: 1. P. Demontis, G. B. Suffritti, J. Chem. Phys. 145, 094110 (2016). 2. R. L. Mössbauer, Zeit. Phys. 151, 124 (1958). 3. H. Jobic, Chem. Phys. Lett. 170, 217 (1990). 4. P. Chemov , A.P. Mamontov, Y.I. Tjurin, Y.P. Cherdantsev, J. Nucl. Mater. 233-237, 1118 (1996). 5. I. Chernov, Yu. Tyurin, Yu. P. Cherdantzev, M. Kröning, H. Baumbach, Int. J. Hydrogen Energy 24, 359 (1999) 6. E. C. H. Sykes, L. C. Fernández-Torres, S. U. Nanayakkara, B. A. Mantooth, R. M. Nevin, P. S. Weiss, Proc. Natl. Acad. Sci. U.S.A. 102, 17907 (2005). 7. J. Li, A. Oudriss, A. Metsue, J. Bouhattate, X. Feaugas, Sci. Rep. 7, 45041 (2017)
A CTRW interpretation of simulated single-file diffusion in zeolites
Full text linkSingle-file diffusion of molecules adsorbed in the channels of zeolite is a phenomenon occurring at different time and length scales, which is difficult to simulate by standard methods, because it often occurs for large molecules adsorbed in microporous materials showing parallel non-crossing channels, with large energy barriers to diffusion..
Two interesting examples are given by water adsorbed in two different Li containing zeolites, Li-ABW and bikitaite, which show parallel straight channels where hydrogen-bonded linear chains of water molecules run along the axis of the channels, parallel to regular rows of lithium ions sticking to the channel surface.
Extensive Molecular Dynamics (MD) simulations of the dynamical of water in these zeolites at different loading and temperature were performed by this research group. It resulted that the energy barrier for water molecules to cross one another in the same channel is much larger than kT event at any reasonably high temperature, so that the diffusion resulted single-file in all the simulation conditions.
The simulation details and results are reported in Refs. [3] and [4], were it was shown that for time scales ranging from a few to some hundreds of picoseconds depending on temperature and loading the mean square displacement (MSD) is proportional to the square root of time as expected for single-file diffusion
Problems in developing water-water and water-silica potentials for the simulation of structural and dynamical properties
No full textA Comment on the flexibility of framework in molecular dynamics simulations of zeolites
No full textThe use of the framework flexibility in molecular dynamics (MD) computer simulation of zeolites and related materials is discussed in detail and from different perspectives. Once ascertained that a flexible framework is needed to ensure, in the micro-canonical ensemble, the exact, in principle, conservation of linear momentum of the simulated system, the practical effects of keeping the framework fixed on several simulated quantities and processes are described. In particular, the diffusivity, the activated processes, the framework deformations and the approximations arising from using classical instead of quantum mechanics are considered
Fractional diffusion interpretation of simulated single-file systems in microporous materials
Full text linkThe single-file diffusion of water in the straight channels of two different crystalline microporous aluminosilicates (zeolites bikitaite and Li-ABW) was studied by comparing the results of molecular dynamics computer simulations with the predictions of anomalous diffusion theory modeled by using fractional diffusion equations. At high coverage, the agreement is reasonably good, in particular for sufficiently large displacements and sufficiently long times. At low coverage, interesting phenomena appear in the simulation results, such as multimodal propagators, which could be interpreted on the basis of fractional Fokker-Planck equations. The results are discussed also in view of different theories that have been proposed to model the single-file diffusion process
Capturing the cage-to cage dynamics of adsorbated at arbitrary loadings in the all-silica zeolite ZK4
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