In-situ observation of relaxation process in f-doped silica glass by raman spectroscopy

Structural changes including relaxation process of 5 mol% fluorine doped silica glass with low initial fictive temperature (T f ) of 700°C has been studied by in-situ Raman spectroscopy at temperatures 27 - 1300°C. With increasing temperature, ω 1 , ω 3 and ω 4 bands, attributed to fundamental skeletal vibrations, monotonously shift in their respective ways, suggesting that both the decreases of average Si-O-Si bond angle and Si-O bond stretching force constant simultaneously occur. On reaching T f , intensity of the D 2 line, thought to be attributed to three-membered ring structure, showed a steady increase with increasing temperature, while all the fundamental vibrations were insensitive to the progress of structural relaxation. From the plot of the D 2 area vs. the inverse "actual" temperature, activation energy of the D 2 formation was estimated to be 0.43 eV, which is very close to the previously reported values estimated from the inverse "fictive" temperature. Furthermore, based on the central-force network model and the Badger's law, the change rate of average Si-O-Si bond angle was estimated from shift of ω 4 frequency to be about -0.02°/°C, which is several times higher than that in T f dependence.