Intermediate range order in vitreous silica from a partial structure factor analysis

By combining the methods of isotopic substitution in neutron diffraction and high-energy x-ray diffraction, we have determined partial structure factors of vitreous SiO2. A discussion of the effect of systematic and statistical errors is presented. The experimental results are found to be in good (but not exact) agreement with existing ab initio and classical molecular-dynamics simulations. No first sharp diffraction peak (FSDP) is observed in the concentration-concentration partial structure factor, ruling out the void-cluster-based model as a possible explanation for the origin of intermediate range order. However, the data are consistent with a model in which the intermediate range order arises from the periodicity of boundaries between a succession of small cages in the network, and the second diffraction peak is associated with chemical ordering of SiO4 tetrahedra within continuous regions of the network between cages. The cage model is used to explain compositional trends in the FSDP height for neutron and x-ray data on BeO-SiO2 glasses.