Exchange between interstitial oxygen molecules and network oxygen atoms in amorphous SiO₂ studied by O18 isotope labeling and infrared photoluminescence spectroscopy

Amorphous SiO₂ (a-SiO₂) thermally annealed in an oxygen atmosphere incorporates oxygen molecules (O₂) in interstitial voids. When the thermal annealing is performed in O18₂ gas, interstitial O18₂ as well as interstitial O16O18 and O16₂ are formed due to the oxygen exchange with the a-SiO₂ network. The a1Δ_g(v=0)→X3Σg-(v=1) infrared photoluminescence band of interstitial O₂ was utilized to quantitatively analyze the oxygen exchange, taking into account the influences of common network modifiers in synthetic a-SiO₂ (SiOH, SiF, and SiCl groups). The presence of network modifiers does not significantly change the average rate of O18 transfer from interstitial O₂ to the a-SiO₂ network and its activation energy, suggesting that the network modifiers themselves do not serve as preferential oxygen exchange sites. When the concentration of SiOH groups is low, the oxygen exchange rate is distributed, indicating that only a small part of the network oxygen atoms participates in the oxygen exchange. However, the distribution of the oxygen exchange rate is distinctly narrow in the sample with high SiOH concentration. It is attributed to the redistribution of the network O18 atoms and the modification of the a-SiO₂ network topology caused by reactions with mobile interstitial water molecules, which are transiently formed by dehydroxylation of paired SiOH groups. The activation energy for the average oxygen exchange rate is larger than that of the permeation of interstitial O₂ in a-SiO₂. Furthermore, the average exchange-free diffusion length of interstitial O₂ below 900 ^°C (1 μm) is far larger than the scale of the interstitial voids in a-SiO₂ (1 nm). These observations confirm that the oxygen exchange is not necessarily involved in the permeation of interstitial O ₂.