MD Refinement of Topologically Simulated Irradiation Cascades in Silica
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MD Refinement of Topologically Simulated Irradiation Cascades in Silica Xianglong Yuan†, Vinay Pulim‡, and Linn W. Hobbs† Department of Materials Science and Engineering ‡ Laboratory for Computer Science Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. †
ABSTRACT Refinement of several topologically generated displacement cascades in silica has been conducted using molecular dynamics (MD) simulation. Several metastable amorphous silicas with substantially different medium-range order (as characterized by ring topologies) were obtained. However, their total correlation functions were found scarcely distinguishable. Major structural reconstruction was observed when the refinement took place above a glass transition temperature, below which the cascades largely retained their original topological ring structures. Attempts are made to correlate topological ring distributions with the first sharp diffraction peak, which may in turn provide some insight into the medium range structures of irradiated silicas. INTRODUCTION Radiation effects in silicas and silicate glasses have long been of interest, because of their wide usage in many technological applications, such as in fiber optics, in flat panel displays, as insulating layers in metal-oxide-semiconductor (MOS) devices, and as nuclear waste storage media. Studies of neutron irradiated crystalline and vitreous silicas [1-3] have reported substantial structural alteration by irradiation, in particular reduction to non-crystalline states of common terminal density known geologically as metamict [4]. Though apparently comprising the same constituent vertex-sharing [SiO4] tetrahedral units, these metamict structures differ from vitreous silica, and from pressure-densified silica of comparable density, both spectroscopically and energetically [5-7]. While the structures of crystalline polymorphs are well documented [8], the topologically disordered structure of even exhaustively studied vitreous silica remains largely enigmatic, let alone the structures of other amorphous silicas. Diffraction unfortunately provides only average information about short-range order and is relatively insensitive to medium-range order differences in such atom arrangements [9]. A useful alternative way to envisage the possibilities for the tetrahedral arrangements in silicas is topologically [10]. Crystalline silicas are easy to amorphize by irradiation because they possess sufficient topological freedom to be only marginally rigid [11]. Many forms of topological disorder can result because multiple stochastic reconstructive paths are available following the initial radiation-induced disordering. Topological modeling of irradiation collision cascades in a range of silica structures has been previously carried out using rules-based assembly and global maximal matching procedures [1214]. In this study, we focus on the refinement of these radiation-induced topologically disordered models, using molecular dynamics (MD) simulation techniques, and investigate model stabilities at diffe
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