Exchange between interstitial oxygen molecules and network oxygen atoms in amorphousSiO2studied byO18

Amorphous SiO2 (a-SiO2) thermally annealed in an oxygen atmosphere incorporates oxygen molecules (O2) in interstitial voids. When the thermal annealing is performed in 18O2 gas, interstitial 18O2 as well as interstitial 16O18O and 16O2 are formed due to the oxygen exchange with the a-SiO2 network. The a1Δg(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 18O 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 18O 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 O2