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This book is a unique reference work in the area of atomic-scale simulation of glasses. For the first time, a highly selected panel of about 20 researchers provides, in a single book, their views, methodologies and applications on the use of molecular dynamics as a tool to describe glassy materials. The book covers a wide range of systems covering "traditional" network glasses, such as chalcogenides and oxides, as well as glasses for applications in the area of phase change materials. The novelty of this work is the interplay between molecular dynamics methods (both at the classical and first-principles level) and the structure of materials for which, quite often, direct experimental structural information is rather scarce or absent. The book features specific examples of how quite subtle features of the structure of glasses can be unraveled by relying on the predictive power of molecular dynamics, used in connection with a realistic description of forces.

First-principles molecular dynamics simulations within the density functional theory framework were performed to generate amorphous models of the Ga4Sb6Te3 phase change alloy by quenching from the melt. We find that Ga-Sb and Ga-Te are the most abundant bonds with only a minor amount of Sb-Te bonds participating to the alloy network. Ga and four-coordinated Sb atoms present a tetrahedral-like geometry, whereas three-coordinated Sb atoms are in a pyramidal configuration. The tetrahedral-like geometries are similar to those of the crystalline phase of the two binary compounds GaTe and GaSb. A sizable fraction of Sb-Sb bonds is also present, indicating a partial nanoscale segregation of Sb. Despite the fact that the composition Ga4Sb6Te3 lies on the pseudobinary GaSb-Sb2Te3 tie line, the amorphous network can be seen as a mixture of the two binary compounds GaTe and GaSb with intertwined elemental Sb.

Structural assignments are determined for 77Se and 73Ge chemical shifts through density-functional NMR calculations for GeSe2, Ge4 Se9, and GeSe crystals. In particular, a very good agreement between calculated and measured 77Se isotropic chemical shifts and anisotropies is found for the GeSe2 crystal, for which experimental data are available. These assignments provide a consistent interpretation of experimental 77Se spectra of Gex Se1-x glasses, indicating that the contribution from Ge-Se-Se linkages overlaps with that from Ge-Se-Ge linkages in corner-sharing tetrahedral arrangements, thereby dismissing the occurrence of a bimodal phase. © 2010 The American Physical Society.

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The structural, electronic, and vibrational properties of glassy GexSe1-x are studied using density-functional-based molecular dynamics. The focus is on four compositions (x = 10%, 20%, 25%, 33%) spanning the rigidity transitions and representing typical compositions of flexible, intermediate, and stressed rigid systems. We investigate structural properties including structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions and compare our results with experimental findings, when available. Most noticeable is the excellent agreement found in the reproduction of the structure in real and reciprocal space which allows tracking the effect of Ge composition on the structure. Ring statistics and ring correlations are examined and followed across the rigidity transition, and the details of typical small rings show a much more complex picture than established previously. A comparison is made with simple bond models and their validity is discussed. Topological constraint analysis is performed and shows that the onset of rigidity changes substantially the angular motion inside the Ge tetrahedra, which displays increased soft bending motions in the stressed rigid phase. We then investigate the vibrational properties via the vibrational density of states and the dielectric function (infrared absorption), and discuss them with respect to experimental findings. Finally, the electronic properties are computed and show an excellent agreement with respect to previous first-principles simulations and to experiments.

The structural properties of glassy Ge2Se3 were studied in the framework of first-principles molecular dynamics by using the Becke-Lee-Yang-Parr scheme for the treatment of the exchange-correlation functional in density functional theory. Our results for the total neutron structure factor and the total pair distribution function are in very good agreement with the experimental results. When compared to the structural description obtained for liquid Ge2Se3, glassy Ge2Se3 is found to be characterized by a larger percentage of fourfold coordinated Ge atoms and a lower number of miscoordinations. However, Ge-Ge homopolar bonds inevitably occur due to the lack of Se atoms available, at this concentration, to form GeSe4 tetrahedra. Focusing on the family of glasses GexSe1-x, the present results allow a comparison to be carried out in reciprocal and real space among three prototypical glassy structures. The first was obtained at the stoichiometric composition (glassy GeSe2), the second at a Se-rich composition (glassy GeSe4) and the third at a Ge-rich composition (glassy Ge2Se3). All networks are consistent with the “8 - N” rule, in particular, glassy GeSe4, which exhibits the highest degree of chemical order. The electronic structure of glassy Ge2Se3 has been characterized by using the Wannier localized orbital formalism. The analysis of the Ge environment shows the presence of dangling, ionocovalent Ge-Se, and covalent bonds, the latter related to Ge-Ge connections. DOI: 10.1103/PhysRevB.86.224201 This work was granted access by GENCI (Grand Equipement National de Calcul Intensif) under allocation 2011095071 to the HPC resources of CCRT/CINES/IDRIS

ISSN 1293-2558