SIMULATION EVIDENCE FOR THE ORIGIN OF THE FIRST SHARP DIFFRACTION PEAK

Calculations of the distinct neutron scattering function, i(Q), for a model of vitreous silica (v-SiO2), whose voids were filled with various concentrations of atoms of Li (having a negative scattering length) and Na (having a positive scattering length) revealed changes in the first sharp diffraction peak (FSDP). The FSDP intensity increases as the concentration of Li atoms increases; by contrast, the FSDP intensity decreases with increasing concentration of Na. Moreover, the FSDP intensity was found to vary linearly with the concentration of the extrinsic atoms. This behaviour is in agreement with the predictions of the void-based model, assuming that the FSDP in the structure factor of covalent glasses is a pre-peak in the concentration-concentration factor due to the chemical ordering of interstitial voids around cation-centred clusters in the structure.