Pressure-induced bond-angle variation in amorphousSiO2

Magic-angle spinning nuclear magnetic resonance measurements have been performed on ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ nuclei in amorphous silica before and after densification ∼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° to 138° 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.