Oxygen-excess amorphous SiO2 with 18O-labeled interstitial oxygen molecules

Exchange between oxygen molecules embedded in amorphous SiO2 (interstitial O2) and oxygen atoms in the a-SiO2 network is found to be remarkably slow at 500 °C. Thermal loading of 18O2 at this temperature yields a-SiO2 containing 18O-labeled interstitial O2 whose 18O fraction is as high as ~ 90%. The 18O fraction of interstitial O2 in this sample is quickly decreased by thermal annealing at or above 700 °C because of the oxygen exchange accompanied by the release of 16O from the a-SiO2 network. This finding indicates that the oxygen exchange starts at much lower temperatures than indicated by previous works, based on monitoring of the isotopic composition of oxygen atoms in the a-SiO2 network.