A ²⁹Si MAS NMR study of sub-T_g amorphization of stishovite at ambient pressure

Stishovite, a high-pressure SiO₂ polymorph in which each Si is coordinated by six O atoms, transforms to an amorphous phase when undergoing heat treatment below the glass transition temperature at ambient pressure. We have applied ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) to study this amorphization process. We found that the amorphous phase generated after heating stishovite for up to 3 days at around 600 ° C consisted exclusively of four-coordinate Si, similar to glasses quenched from melts at ambient pressure. Furthermore, our data suggest that there are subtle structural differences between the amorphous phase transformed from stishovite at 600 ° C and glasses quenched from melts at ambient pressure: the amorphous phase from stishovite had a smaller mean Si-O-Si angle initially, and it gradually relaxed toward the latter with increasing heating time. There was no detectable change in the stishovite structure even after about 80% of it had been converted to the amorphous phase. The mechanism of the amorphization of stishovite is discussed in light of these results.