SIMULATIONS OF VOID-FILLED VITREOUS SILICA TO INTERPRET THE ORIGIN OF THE 1ST SHARP DIFFRACTION PEAK

It has been proposed that the first sharp diffraction peak (FSDP) in the structure factor of covalent glasses is a prepeak in the concentration-concentration structure factor due to the chemical ordering of interstitial voids around cation-centered clusters in the structure. This void-based model predicts that if voids are occupied by extrinsic atoms the intensity of the FSDP will change. Dramatic changes in the FSDP have been observed by calculating the structure factor for a model of vitreous silica whose interstices were filled with He, Li, and Na. Moreover, a splitting of the FSDP has been observed in the case of the Na-filled model, similar to that observed experimentally for potassium silicate glasses. The origin of this splitting is found to arise from subtle cancellation effects among the partial structure factors.