Exchange between interstitial oxygen molecules and network oxygen atoms in amorphousSiO2studied byO18isotope labeling and infrared photoluminescence spectroscopy

Abstract

Amorphous ${\mathrm{SiO}}_{2}$ ($a$-${\mathrm{SiO}}_{2}$) thermally annealed in an oxygen atmosphere incorporates oxygen molecules (O${}_{2}$) in interstitial voids. When the thermal annealing is performed in $^{18}\mathrm{O}$${}_{2}$ gas, interstitial $^{18}\mathrm{O}$${}_{2}$ as well as interstitial $^{16}\mathrm{O}$$^{18}\mathrm{O}$ and $^{16}\mathrm{O}$${}_{2}$ are formed due to the oxygen exchange with the $a$-${\mathrm{SiO}}_{2}$ network. The ${a}^{1}{\ensuremath{\Delta}}_{g}(v=0)\ensuremath{\rightarrow}{X}^{3}{\ensuremath{\Sigma}}_{g}^{\ensuremath{-}}(v=1)$ infrared photoluminescence band of interstitial ${\mathrm{O}}_{2}$ was utilized to quantitatively analyze the oxygen exchange, taking into account the influences of common network modifiers in synthetic $a$-${\mathrm{SiO}}_{2}$ (SiOH, SiF, and SiCl groups). The presence of network modifiers does not significantly change the average rate of $^{18}\mathrm{O}$ transfer from interstitial ${\mathrm{O}}_{2}$ to the $a$-${\mathrm{SiO}}_{2}$ 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 $^{18}\mathrm{O}$ atoms and the modification of the $a$-${\mathrm{SiO}}_{2}$ 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 ${\mathrm{O}}_{2}$ in $a$-${\mathrm{SiO}}_{2}$. Furthermore, the average exchange-free diffusion length of interstitial ${\mathrm{O}}_{2}$ below 900 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C ($\ensuremath{\gtrsim}$1 $\ensuremath{\mu}$m) is far larger than the scale of the interstitial voids in $a$-${\mathrm{SiO}}_{2}$ ($\ensuremath{\lesssim}$1 nm). These observations confirm that the oxygen exchange is not necessarily involved in the permeation of interstitial ${\mathrm{O}}_{2}$.