Bond length-bond angle correlation in densified silica—Results from 17O NMR spectroscopy

Abstract

Pressure induced correlated evolution of the distributions of the Si–O distance and Si–O–Si inter-tetrahedral bond angle in vitreous silica quenched from pressures of up to ∼14 GPa at ambient temperature is measured in unprecedented detail using two-dimensional dynamic-angle-spinning 17O nuclear magnetic resonance spectroscopy. The results demonstrate that, in contrast to the conventional wisdom, vitreous silica undergoes irreversible structural changes even at pressures as low as ∼8 GPa. These structural changes at the short range involve a progressive reduction in the mean Si–O–Si angle and a broadening of the corresponding distribution, with increasing pressure. This bond angle reduction is accompanied by a concomitant monotonic increase in the mean Si–O distance. The mean values of the Si–O–Si angle and Si–O distance at various pressures closely follow the minimum in the corresponding potential energy surface calculated for the H6Si2O7 dimer molecule.