Correlated structural distributions in silica glass

Abstract

The two-dimensional $^{17}\mathrm{O}$ dynamic-angle spinning solid-state nuclear magnetic resonance spectrum of silica glass produced from the melt was measured. From this spectrum a three-dimensional histogram correlating $^{17}\mathrm{O}$ chemical shift, quadrupolar coupling constant, and quadrupolar coupling asymmetry parameter for the bridging oxygen was obtained. Using existing correlations between NMR parameters and local structure, the distribution in quadrupolar coupling parameters was mapped into two-dimensional histograms correlating the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Si}$ angle with $\mathrm{Si}\text{\ensuremath{-}}\mathrm{O}$ distance, the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Si}$ angle with $\mathrm{Si}\text{\ensuremath{-}}\mathrm{Si}$ distance, and the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{O}$ distance with $\mathrm{Si}\text{\ensuremath{-}}\mathrm{Si}$ distance. While the peak values for the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Si}$ angle, the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{O}$ distance, and $\mathrm{Si}\text{\ensuremath{-}}\mathrm{Si}$ distance distributions, at $147\ifmmode^\circ\else\textdegree\fi{}$, $1.59\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, and $3.05\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, respectively, are consistent within the precision of the NMR measurement with previous diffraction studies, the distribution widths are narrower than previous diffraction studies. The two-dimensional histogram reveals an unexpected strong positive correlation between the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Si}$ angle and $\mathrm{Si}\text{\ensuremath{-}}\mathrm{O}$ distance in the glass, running opposite to the trend generally found in crystalline silica polymorphs.