Oxygen interstitials in radiation processes in glassy SiO2

Full text: Among optical glasses, glassy silicon dioxide (silica) shows a superior resistance against particle-, ionizing and ultraviolet radiations. Similarly to many other oxide materials, the oxygen vacancy is one of the most studied defects in silica. In contrast, the properties of its complementary defect, the interstitial oxygen atom, and its role in radiation processes in silica are relatively poorly understood. The purpose of this study was to demonstrate that interstitial oxygen atoms play an important part in defect annealing processes in silica typically occurring in the 300- 400 C temperature region. Interstitial oxygen atoms are hard to observe spectroscopically in a direct way: they (probably) have only a weak optical absorption band in vacuum UV part of the spectrum. This band was predicted theoretically and is only tentatively confirmed experimentally. Therefore, it was detected indirectly, by monitoring three forms of superstoichiometric oxygen in SiO2: 1) interstitial O2 molecules, 2) oxygen dangling bonds ≡ Si-O· and 3) peroxy radicals ≡ Si-O-O ·. Interstitial O atoms were created in two ways: by F2 laser (7.9 eV) photolysis of interstitial O2 and by 5eV (KrF laser or Xe lamp) photolysis of peroxy radicals. It was found that when the samples prepared in this way were heated above 300C, a growth of the peroxy radicals takes place simultaneously with a commensurate decrease of the concentration of dangling oxygen bonds. This process is reversed by a subsequent 5 eV irradiation and can be repeated many times. Up to now it was generally believed that the creation of peroxy radicals is due to a diffusion-controlled reaction between interstitial O2 molecules and silicon dangling bonds (O2 + ≡ Si· → ≡ Si-O-O·). The results of the present work show that the dominant reaction responsible for the creation of peroxy radicals occurs between oxygen dangling bonds and interstitial oxygen atoms. The published calculations indicate that interstitial oxygen atoms in silica glass network should exist as peroxy linkages ≡Si-O-O-0Si≡ bond). The observed low mobility of interstitial O at room temperatures is in agreement with such structural assignment