SHORT- AND MEDIUM-RANGE ORDER IN BISMUTH-SILICATE GLASSES: A MOLECULAR DYNAMICS STUDY

Abstract

We report on the results of classical molecular dynamics (MD) simulations of structure of amorphous 15 Bi₂0₃ 85 SiO₂ [% mol] and 40 Bi₂O₃ 60 SiO₂ [% mol], and their totally reduced forms, 15 Bi₂ 85 SiO₂ [% mol], and 40 Bi₂ 60 SiO₂ [% mol], respectively. The simulations have been performed in the isobaric-isothermal ensemble, using a two-body interaction potential. The set of the potential parameters was constructed as a suitable combination of the parameters which were previously proposed for pure Bi₂O₃, and SiO₂. Both unreduced, and reduced systems were initially prepared as well equilibrated hot melts, and then slowly cooled down to 300K. The structural information from the MD simulations was obtained from radial and angular distribution functions, static structural factors, Voronoi polyhedra statistics, and ring analysis.

The simulation results can be summarised as follows. In unreduced glass with 15 Bi₂O₃ [%mol] contents, the silicon structural units (mainly regular tetrahedra) form continuous network, whereas in 40 Bi₂O₃ [%mol] glass these units are disconnected. In both unreduced systems Bi ions have mainly sixfold oxygen coordination, and no dominating structural unit can be individuated. However, the distorted bismuth units form a continuous network. In both totally reduced glasses (15 Bi₂ 85 SiO₂, and 40 Bi₂ 60 SiO₂ [% mol]), the silica network is built entirely from comer sharing SiO₄ tetrahedra. The structure of the silica subsystem is similar to that of pure α-SiO₂. After the reduction, the Bi- Bi coordination significantly increases, whereas the first neighbour distance decreases. Moreover, partial static structural factors for Bi-Bi pairs indicate that the medium-range order in reduced glasses exhibits greater periodicity than in unreduced glasses. Neutral Bi atoms form small clusters within the silica matrix.