Pressure-induced bond-angle variation in amorphous SiO2.

Abstract

Magic-angle spinning nuclear magnetic resonance measurements have been performed on $^{29}\mathrm{Si}$ nuclei in amorphous silica before and after densification \ensuremath{\sim}16% by pressure compaction at 50 kbar and 600?deC. The pressure-induced shift in the Si---O---Si bond-angle distribution maximum from 143\ifmmode^\circ\else\textdegree\fi{} to 138\ifmmode^\circ\else\textdegree\fi{} is consistent with results obtained from x-ray scattering and Raman spectroscopy. Variation in the bond-angle distribution is discussed in terms of recent models for the structure of amorphous ${\mathrm{SiO}}_{2}$. It is concluded that the important parameter modified by densification is the oxygen--second-nearest-neighbor-oxygen smallest distance of approach. This in turn leads to a variation in the nature of the short-range order.