Molecular Dynamics Simulations of Glassforming Network Fluids
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1048-Z09-01
Molecular Dynamics Simulations of Glassforming Network Fluids Kurt Binder1, Juergen Horbach1,2, and Michael Hawlitzky1 1 Institut fuer Physik, Johannes-Gutenberg Universitaet, Staudinger Weg 7, Mainz, 55099, Germany 2 Institut fuer Materialphysik im Weltraum, Deutsches Zentrum fuer Luft- und Raumfahrt e.V. (DLR), Linder Hoehe, Koeln, 51147, Germany
ABSTRACT Molecular Dynamics simulations of molten oxides, such as fluid silicon dioxide and germanium dioxide, based on simple classical pair potentials, are compared with corresponding Car-Parrinello “ab initio” Molecular Dynamics (CPMD) work and with experiment. It is shown that CPMD provides a significantly better account for properties on short length scales, but classical MD is still indispensable to deal with larger scales of length and time. The behavior of the mean square displacement of the particles as well as the incoherent intermediate scattering function is compatible with a mode coupling description, at least at very high temperatures, while the diffusion constants show a crossover to Arrhenius behavior near the mode coupling critical temperature of these systems. Finally, the results for the network forming liquids are compared to those from simulations of binary metallic alloys such as Al80Ni20, which form a structure similar to densely packed hard spheres. INTRODUCTION It is still a challenge to understand the structure-property relations of glasses and glassforming fluids in detail [1-3]. One major classification considers the nature of the chemical bonding and resultant atomic packing [1,3]: the one extreme is metallic bonding, and the resulting structural picture is dense random packing of spheres (of different size, in systems containing several components). The other extreme is covalent bonding, as it occurs in the silica-like network glasses, the structural picture being the continuous random network [1,3]. To some extent, this classification correlates to the dynamics of glassy freezing of the supercooled liquid: when one considers the Angell plot [4], where the logarithm of the viscosity η is plotted versus inverse temperature, normalized by the glass transition temperature Tg , one finds that the network glassformer SiO2 and GeO2 simply fall on straight lines; fluids formed from small molecules which belong to the class of dense random packing glassformers exhibit a lot of curvature on such a plot, however. Note that Tg in this context is defined by the condition that η (T = Tg ) = 1013 Poise. The glassformers yielding the straight line on the Angell plot (reflecting Arrhenius behavior) are called “strong”, the others are called “fragile” [3,4]. Metallic glasses are typically moderately fragile. A basic question about the glass transition is the issue of universality. One does not know whether distinctions such as “strong vs. fragile” are really fundamental, or only two sides of the same coin. One does not know to what extent such distinctions of a macroscopic property like η correlate with the structure and dynamics on the atomisti
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