Philippe Jund

MOLECULAR-DYNAMICS CALCULATION OF THE THERMAL CONDUCTIVITY OF VITREOUS SILICA

We use extensive classical molecular-dynamics simulations to calculate the thermal conductivity of a model silica glass. Apart from the potential parameters, this is done with no other adjustable quantity and the standard equations of heat transport are used directly in the simulation box. The calculations have been done between 10 and 1000 K and the results are in good agreement with the experimental data at temperatures above 20 K. The plateau observed around 10 K can be accounted for by correcting our results taking into account finite-size effects in a phenomenological way.

Channel diffusion of sodium in a silicate glass

We use classical molecular dynamics simulations to study the dynamics of sodium atoms in amorphous

Structural and electronic properties of the sodium tetrasilicate glass Na 2 Si 4 O 9 from classical and ab initio molecular dynamics simulations

{\mathrm{Na}}_{2}\mathrm{O}\ensuremath{-}4{\mathrm{SiO}}_{2}.

Phase stability and physical properties of Ta 5 Si 3 compounds from first-principles calculations

We find that the sodium trajectories form a well connected network of pockets and channels. Inside these channels the motion of the atoms is not cooperative, but rather is given by independent thermally activated hops of individual atoms between the pockets. By determining the probability that an atom returns to a given starting site, we show that such events are not important to the dynamics of this system at high temperatures.

Physical properties of thermoelectric zinc antimonide using first-principles calculations

The structure and the electronic properties of a sodium tetrasilicate

Classical molecular dynamics simulations of amorphous silica surfaces

({\mathrm{Na}}_{2}{\mathrm{Si}}_{4}{\mathrm{O}}_{9})

Structural properties of glassy and liquid sodium tetrasilicate: comparison between ab initio and classical molecular dynamics simulations

glass were studied by combined Car-Parrinello and classical molecular dynamics simulations. The glass sample was prepared using a method recently employed in a study of a silica glass [M. Benoit et al., Euro. Phys. J. B 13, 631 (2000)]. First we generated a NS4 glass by classical molecular dynamics and then we took it as the initial configuration of a first-principles molecular dynamics simulation. In the ab initio molecular dynamics simulation, the electronic structure was computed in the framework of the Kohn\char21{}Sham density functional theory within the generalized gradient approximation using a B-LYP functional. The Car-Parrinello dynamics is remarkably stable during the considered trajectory and, as soon as it is switched on, some significant structural changes occur. The ab initio description improves the comparison of the structural characteristics with experimental data, in particular concerning the Si\char21{}O and Na\char21{}O bond lengths. From an electronic point of view, we find that the introduction of the sodium oxide in the silica network lowers the band gap and leads to a highly nonlocalized effect on the charges of the network atoms.

Physical properties of a GeS 2 glass using approximate ab initio molecular dynamics

We present a study of the thermodynamic and physical properties of Ta5Si3 compounds by means of density functional theory based calculations. Among the three different structures (D8m, D8l, D88), the D8l structure (Cr5B3-prototype) is the low temperature phase with a high formation enthalpy of -449.20kJ/mol, the D8m structure (W5Si3-prototype) is the high temperature phase with a formation enthalpy of -419.36kJ/mol, and the D88 structure (Mn5Si3-prototype) is a metastable phase. The optimized lattice constants of the different Ta5Si3 compounds are also in good agreement with the experimental data. The electronic density of states (DOS) and the bonding charge density have also been calculated to elucidate the bonding mechanism in these compounds and the results indicate that bonding is mostly of covalent nature. The elastic constants of the D8m and D8l structures have been calculated together with the different moduli. Finally, by using a quasiharmonic Debye model, the Debye temperature, the heat capacity, the coefficient of thermal expansion and the Gruneisen parameter have also been obtained in the present work. The transformation temperature (2303.7K) between the D8m and the D8l structures has been predicted by means of the Gibbs energy, and this predicted temperature (2303.7K) is close to the experimental value (2433.5K).

A first-principles investigation of the thermodynamic and mechanical properties of Ni–Ti–Sn Heusler and half-Heusler materials

We report first-principles calculations of the structural, electronic, elastic, and vibrational properties of the semiconducting orthorhombic ZnSb compound. We study also the intrinsic point defects in order to eventually improve the thermoelectric properties of this already very promising thermoelectric material. Concerning the electronic properties, in addition to the band structure, we show that the Zn (Sb) crystallographically equivalent atoms are not exactly equivalent from the electronic point of view. Lattice dynamics, elastic, and thermodynamic properties are found to be in good agreement with the experiments and they confirm the nonequivalency of the zinc and antimony atoms from the vibrational point of view. The calculated elastic properties show a relatively weak anisotropy and the hardest direction is the y direction. We observe the presence of low energy modes involving both Zn and Sb atoms at about 5-6 meV, similar to what has been found in Zn4Sb3, and we suggest that the interactions of these modes with acoustic phonons could explain the relatively low thermal conductivity of ZnSb. Zinc vacancies are the most stable defects, and this explains the intrinsic p-type conductivity of ZnSb.

Multiple scattering calculations of the XANES Si K-edge in amorphous silica

We have adapted classical molecular dynamics to study the structural and dynamical properties of amorphous silica surfaces. Concerning the structure, the density profile exhibits oscillations perpendicularly to the surface as observed in liquid metal surfaces and the pair correlation functions as well as the angle distributions show features (absent in the interior of the films) that can be attributed to the presence of twofold rings which are perpendicular to the surface. From the mean-squared displacement of the non-bridging oxygen atoms we find that in the interior region they move perpendicular to the surface while they move parallel to it in the surface region.