X-ray and neutron scattering observations of structural relaxation of vitreous silica

Measurements of the X-ray and neutron scattering of vitreous silica were made before and after annealing in air for up to 60 h. Structural relaxation effects were observed for annealing at temperatures between 1100 and 1550 K. Relaxation enhances the peaks and other features in the X-ray and neutron structure factors, with the most prominent change being an increase and narrowing of the first sharp diffraction peak (FSDP). The largest relaxation effects, corresponding to a decrease of the fictive temperature by about 300 K, increase the FSDP by 14%. This shows that in vitreous silica annealing can lead to a substantial enhancement of the medium-range order. The structure changes are reversed by heating the glass to near the glass transition temperature. In agreement with previous infrared absorption measurements by Tomozawa and co-workers, the structure changes occur predominantly in a 0.3 mm thick layer at the sample surface. Crystals grow on the surface of the sample upon annealing at temperatures above 1350 K. Etching of the sample surface shows that the crystallization of the surface takes place in parallel to the relaxation of the glass below. The results are compared with prior measurements of structure factor changes due to different cooling rates, temperature change and compression. The structure factor changes agree with molecular-dynamics simulations of vitreous silica [Phys. Rev. B 54 (1996) 15808]. The dependence of the height of the FSDP on structure parameters in the models by Gladden is used to estimate the change in the mean Si–O–Si bond angle.