Thermal relaxation of silica phases densified under electron irradiation

The thermal relaxation of the silica metamict phase obtained after electron irradiation of 11 GGy Suprasil SiO2 glass was characterized using Raman spectroscopy and compared to silica densified under high Pressure and high temperature irradiated at 107 Gy. In parallel, we studied the densification relaxation kinetic by step isochronal annealing treatments. Starting from the metamict phase, local density indicators, D1 and D2 bands decreased quasi-monotonously under thermal treatment together with the glass density. However, for 107 Gy samples, we evidenced an anomalous sharp increase of D1 and D2 bands between 800°C and 1000°C, which could be interpreted as a transition from low-density amorphous (LDA) to a high-density amorphous configurational state (HDA). Interestingly after an annealing at 1000°C, the overall Raman signature indicates that the “relaxed metamict phase” exhibits a higher configurational disorder than pristine silica whereas 107 Gy samples simply relaxed towards pristine silica suggesting a low density amorphous megabasin with several minima.