Structural properties of various sodium thiogermanate glasses through DFT-based molecular dynamics simulations

We present a study of the structural properties of (x)Na$_2$S-(1-x)GeS$_2$ glasses through DFT-based molecular dynamics simulations, at different sodium concentrations ($0<x<0.5$). We computed the radial pair correlation functions as well as the total and partial structure factors. We also analyzed the evolution of the corner- and edge-sharing intertetrahedral links with the sodium concentration and show that the sodium ions exclusively destroy the former. With the increase of the sodium concentration the ``standard'' FSDP disappears and a new pre-peak appears in the structure factor which can be traced back in the Na-Na partial structure factor. This self organization of the sodium ions is coherent with Na-rich zones that we find at high modifier concentration.Comment: 9 pages, 7 figures; to be published in Phys. Rev.

Influence of the cooling-rate on the glass transition temperature and the structural properties of glassy GeS2: an ab initio molecular dynamics study

Using density-functional molecular dynamics simulations we analyzed the cooling-rate effects on the physical properties of GeS$_2$ chalcogenide glasses. Liquid samples were cooled linearly in time according to $T(t) = T_0 - \gamma t$ where $\gamma$ is the cooling rate. We found that our model leads to a promising description of the glass transition temperature $T_g$ as a function of $\gamma$ and gives a correct $T_g$ for experimental cooling rates. We also investigated the dependence of the structural properties on the cooling rate. We show that, globally, the properties determined from our simulations are in good agreement with experimental values and this even for the highest cooling rates. In particular, our results confirm that, in the range of cooling rates studied here, homopolar bonds and extended charged regions are always present in the glassy phase. Nevertheless in order to reproduce the experimental intermediate range order of the glass, a maximum cooling rate should not be exceeded in numerical simulations.Comment: 12 pages, 6 figures. To appear in J. Phys.: C

Classical molecular dynamics simulations of amorphous silica surfaces

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 2-fold 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.Comment: 7 pages,5 figures - To be published in J. Phys. C.

Physical properties of the thermoelectric cubic lanthanum chalcogenides La3-yX4 (X=S,Se,Te) from first-principles

We report ab-initio calculations of the stability, lattice dynamics, electronic and thermoelectric properties of cubic La3-yX4 (X=S,Se,Te) materials in view of analyzing their potential for thermoelectric applications. The lanthanum motions are strongly coupled to the tellurium motions in the telluride, whereas the motions of both types of atoms are decoupled in the sulfides. Nevertheless, this has no impact on their thermal properties because experimentally all compounds have low thermal conductivity. We believe that this is due to Umklapp scattering of the acoustical modes, notably by the low energy optical modes at about 7-8 meV found in all three chalcogenides, as in cage compounds such as skutterudites or clathrates, even though there are no cages in the cubic Th3P4 structure. We find that the energy bandgap increases from the telluride to the sulfide in good agreement with the experiments. However, due to their similar band structure, we find that all three compounds have almost identical thermoelectric properties. Our results agree qualitatively with the experiments, especially in the case of the telluride for which a great amount of data exists. All our results indicate that the sulfides have strong potential for thermoelectricity and could replace the tellurides if the charge carrier concentration is optimized. Finally, we predict also a larger maximum ZT for the p-type doped materials than for the n-type doped ones, even though compounds with p-doping have still to be synthesized. Thus our results indicate the possibility to make high temperature performing thermo-generators based only on La3X4 compounds.Comment: 37 pages, 12 figure