Refinement of a random network model for vitreous silicon dioxide

Abstract

We report a refinement of the original Bell–Dean model for vitreous silica using an energy minimization technique with a Keating force-field. Atomic coordinates are obtained for structures with several values of the oxygen bond angle in the range 130–160°. Values of the density, heat of crystallization, X-ray and neutron structure factors, together with the corresponding radial distribution functions, are calculated and compared with recent experimental measurements. On the whole, the Bell–Dean model is confirmed as an acceptable description of vitreous silica, and the best relaxed model provides a more adequate representation of the physical properties of silica than the original structure. However, the elastic strain contained in relaxed models is anomalously low and this appears to be due to their limited size and the high proportion of surface atoms. Consequently, a good fit to neutron-scattering data requires artifically broadened correlation functions. With these qualifications, the best fit to experimental data is obtained for a structure with a mean oxygen bond angle which is 10° lower than that of the original model.