Ref Arxiv: cond-mat/0306332
Ref. & Cit.: NASA ADS
Résumé:

We have determined the vibrational properties of a sodium tetrasilicate(Na$_{2}$Si$_{4}$O$_{9}$) glass model generated by molecular dynamicssimulations. The study has been carried out using a classical valence forcefields approach as well as an {\it ab initio} approach in the framework of thedensity functional theory. The total and partial vibrational densities ofstates (VDOS) are presented, as well as some characteristics of the vibrationalmodes (participation ratios, correlation lengths). For the low-frequency bandsbelow 500 cm${}^{-1}$, we find that the shapes of the two calculated VDOS aswell as those of their corresponding partial VDOS are quite similar. For theintermediate- and high-frequency ranges, we observe larger discrepanciesbetween the two calculations. Using the eigenmodes of the dynamical matrix wealso calculate the polarized Raman spectra within the bond-polarizabilityapproximation. We find an overall agreement between the calculated parallelpolarized (VV) Raman spectra and the corresponding experimental spectrum.Regarding the perpendicular depolarized (VH) Raman spectrum, the comparison ofthe calculated spectra to the experimental data indicates a need for anadjustment of the VH bond-polarizability parameters.

Commentaires: 17 pages, 4 EPS figures, submitted to J. Non. Cryst.-Sol., corrected version following referee's comments

Ref HAL: hal-00133199_v1
Ref Arxiv: cond-mat/0403253
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
Résumé:

We use {\it ab initio} molecular dynamics simulations to study a sample of liquid silica containing 3.84 wt.% H$_2$O.We find that, for temperatures of 3000 K and 3500 K,water is almost exclusively dissolved as hydroxyl groups, the silica network is partially broken and static and dynamical properties of the silica network change considerably upon the addition of water.Water molecules or free O-H groups occur only at the highest temperature but are not stable and disintegrate rapidly.Structural properties of this system are compared to those of pure silica and sodium tetrasilicate melts at equivalent temperatures. These comparisons confirm the picture of a partially broken tetrahedral network in the hydrous liquid and suggest that the structure of the matrix is as much changed by the addition of water than it is by the addition of the same amount (in mole %) of sodium oxide. On larger length scales, correlations are qualitatively similar but seem to be more pronounced in the hydrous silica liquid. Finally, we study the diffusion mechanisms of the hydrogen atoms in the melt. It turns out that HOSi$_2$ triclusters and SiO dangling bonds play a decisive role as intermediate states for the hydrogen diffusion.

Commentaires: 25 pages, 18 figures. submitted

Ref HAL: hal-00012958_v1
Ref Arxiv: cond-mat/0305227
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
Résumé:

The amorphous aluminium silicate (Al2O3)2(SiO2) [AS2] is investigated by means of large scale molecular dynamics computer simulations. We consider fully equilibrated melts in the temperature range 6100K >= T >= 2300K as well as glass configurations that were obtained from cooling runs from T=2300K to 300K with a cooling rate of about 10^12K/s. Already at temperatures as high as 4000K, most of the Al and Si atoms are four-fold coordinated by oxygen atoms. Thus, the structure of AS2 is that of a disordered tetrahedral network. The packing of AlO4 tetrahedra is very different from that of SiO4 tetrahedra in that Al is involved with a relatively high probability in small-membered rings and in triclusters in which an O atom is surrounded by four cations. We find as typical configurations two-membered rings with two Al atoms in which the shared O atoms form a tricluster. On larger length scales, the system shows a microphase separation in which the Al-rich network structure percolates through the SiO2 network. The latter structure gives rise to a prepeak in the static structure factor at a wavenumber q=0.5\\AA^{-1}. The comparison of experimental X-ray data with the results from the simulation shows a good agreement for the structure function. The diffusion dynamics in AS2 is found to be much faster than in SiO2. We show that the self-diffusion constants for O and Al are very similar and that they are by a factor of 2-3 larger than the one for Si.

Commentaires: 30 pages of Latex, 13 figures

Ref HAL: hal-00113270_v1
Ref Arxiv: cond-mat/0309025
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
Résumé:

We present the results of molecular dynamics computer simulations of a binary Lennard-Jones liquid confined between two parallel rough walls. These walls are realized by frozen amorphous configurations of the same liquid and therefore the structural properties of the confined fluid are identical to the ones of the bulk system. Hence this setup allows us to study how the relaxation dynamics is affected by the pure effect of confinement, i.e. if structural changes are completely avoided. We find that the local relaxation dynamics is a strong function of z, the distance of the particles from the wall, and that close to the surface the typical relaxation times are orders of magnitude larger than the ones in the bulk. Because of the cooperative nature of the particle dynamics, the slow dynamics also affects the dynamics of the particles for large values of z. Using various empirical laws, we are able to parameterize accurately the z-dependence of the generalized incoherent intermediate scattering function F_s(q,z,t) and also the spatial dependence of structural relaxation times. These laws allow us to determine various dynamical length scales and we find that their temperature dependence is compatible with an Arrhenius law. Furthermore, we find that at low temperatures time and space dependent correlation function fulfill a generalized factorization property similar to the one predicted by mode-coupling theory for bulk systems. For thin films and/or at sufficiently low temperatures, we find that the relaxation dynamics is influenced by the two walls in a strongly non-linear way in that the slowing down is much stronger than the one expected from the presence of only one confining wall. ....

Commentaires: 22 pages of Latex

Commentaires: AMERICAN PHYSICAL SOC

Résumé:

We present extensive molecular dynamics simulations of the motion of a single linear rigid molecule in a two-dimensional random array of fixed overlapping disklike obstacles. The diffusion constants for the center of mass translation, D-CM, and for rotation, D-R, are calculated for a wide range of the molecular length, L, and the density of obstacles, rho. The obtained results follow a master curve Drho(mu)similar to(L(2)rho)(-nu) with an exponent mu=-3/4 and 1/4 for D-R and D-CM, respectively, that can be deduced from simple scaling and kinematic arguments. The nontrivial positive exponent nu shows an abrupt crossover at L(2)rho=zeta(1). For D-CM we find a second crossover at L(2)rho=zeta(2). The values of zeta(1) and zeta(2) correspond to the average minor and major axis of the elliptic holes that characterize the random configuration of the obstacles. A violation of the Stokes-Einstein-Debye relation is observed for L(2)rho>zeta(1), in analogy with the phenomenon of enhanced translational diffusion observed in supercooled liquids close to the glass transition temperature. (C) 2004 American Institute of Physics.

Commentaires: English Article 830HG

Résumé:

We present molecular dynamics simulations of the motion of a single rod in a two-dimensional random static array of discs. For long rods the mean-squared displacement of the centre of mass shows a cage effect similar to that observed in supercooled liquids or dense colloidal systems. We have determined the time-dependence of the non-Gaussian parameter for different rod lengths. It is found that the long-time regime is strongly non-Gaussian even on length scales of the order of 10-15 times the rod length, thus showing the heterogeneity of the dynamics on such length scales.

Commentaires: English Article 821OC