Molecular Dynamics Study of the Influence of a Surface on a Simplified Nuclear Glass Structure and on Displacement Casca
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A three body term is applied on the O-Si-O, Si-O-Si, O-AI-O and O-B-O angles to improve the local environment around formers. Formulae and parameter values of the three body terms are given in reference [14]. We prepared two different cubic cells with a side length equal to 40 A or 80 A, and containing respectively 5184 and 41472 atoms. First, simulation cells with no surface are calculated by a previously described procedure [17]. The surface is then created by removing the periodic boundary conditions (PBC) in the z direction. So we introduce two parallel surfaces. The two cells with and without surface are relaxed during 10 ps at ambient temperature. Structural changes are determined from atomic concentration profiles, angular distributions, coordination number evolution versus depth and Voronoi volumes. Cascadeinitiation Cascade initiation is performed by accelerating one atom in the cell. Primary knock-on atom (PKA) energy is limited to keep the damage volume inside the simulation box and avoid PBC artifacts. BMH potential is not able to reproduce energetic interactions which occur during collision events. So a Ziegler-Biersack-Littmark potential (ZBL) [18] is added for interatomic distances less than 0.9 A. The connection between ZBL and BMH potentials is performed between 0.9 and 1.0 A (1.1 and 1.3 A for U-O interactions) by a polynomial expression. The time step used for cascade simulations varies from 10s 7 S at the beginning to 10-15 s at the end. This time step adjustment is introduced to ensure precise dynamics. At the beginning, atomic velocities can be large and a small time step is required. The progressive increase of the time step is controlled by the largest atomic velocity. The maximum atomic displacement between two successive time steps is thus limited to 0.01 A. The system is embedded in a thermal bath to absorb the thermal agitation created by the cascade (i.e. outward layers up to 3 A are maintained at ambient temperature by periodically rescaling the atomic velocities). RESULTS Structure Radial distributionfunction The first characterisation is done by calculating the radial distribution functions (RDF) to enlighten the surface influence on the local atomic environments. No clear modifications of RDF have been observed between RDF in cells with or without surface, even if we only take into account near-surface atoms (i.e. the first 5 A). Concentrationprofile A direct visualisation of the simulated surface profile shows a glass expansion and sodium accumulation on the surface. This observation is confirmed by plotting concentration profiles of atomic species (fig. 1). The origin corresponds to the initial location of the created surface. Each point represents the concentration of the atomic species in a 1.4 A thick slice. The sodium profile shows an accumulation in outward layers with a depletion in layers just under the surface. We can also note that sodium concentration in the bulk is not modified. So this phenomenon is localised in the surface region (i.e. the first 4 A). Voronoz Volume V
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