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

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

First principles calculations of the vibrational, thermodynamic and mechanical properties of the Ni-Ti-Sn Heusler and half-Heusler compounds have been performed. First, we have calculated the Raman and infrared spectra of NiTiSn, providing benchmark theoretical data directly useful for the assignments of its experimental spectra and clarifying the debate reported in the literature on the assignment of its modes. Then, we have discussed the significant vibrational density-of-states of Ni2TiSn at low-frequencies. These states are at the origin of (i) its smaller free energy, (ii) its higher entropy, and (iii) its lower Debye temperature, with respect to NiTiSn. Finally, we have reported the mechanical properties of the two compounds. In particular, we have found that the half-Heusler compound has the largest stiffness. Paradoxically, its bulk modulus is also the smallest. This unusual behavior has been related to the Ni-vacancies that weaken the structure under isostatic compression. Both compounds show a ductile behavior.Comment: 22 pages, 7 figure

Crystallization in a model glass: influence of the boundary conditions

Using molecular dynamics calculations and the Voronoi tessellation, we study the evolution of the local structure of a soft-sphere glass versus temperature starting from the liquid phase at different quenching rates. This study is done for different sizes and for two different boundary conditions namely the usual cubic periodic boundary conditions and the isotropic hyperspherical boundary conditions for which the particles evolve on the surface of a hypersphere in four dimensions. Our results show that for small system sizes, crystallization can indeed be induced by the cubic boundary conditions. On the other hand we show that finite size effects are more pronounced on the hypersphere and that crystallization is artificially inhibited even for large system sizes.Comment: 11 pages, 2 figure