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.