A 29Si MAS NMR study of sub-Tg amorphization of stishovite at ambient pressure |
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| Authors: | Xianyu Xue Jonathan F Stebbins Masami Kanzaki |
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| Institution: | 1. Department of Geology, Stanford University, 94305, Stanford, CA, USA
2. Department of Inorganic Materials, Tokyo Institute of Technology, 152, Tokyo, Japan
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| Abstract: | 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. |
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