The amorphization and disproportionation of Mullite (3Al$_{2}$O$_{3}$ 2SiO$_{2})$ under shock compression

Shock-recovery experiments have been performed on mullite (3Al$_{2}$O$_{3}$ 2SiO$_{2})$ polycrystals in the pressure range up to 65 GPa. The recovered samples have been examined by using XRD method and TEM observation. In the samples shocked at 40 and 49 GPa, an amorphization of mullite occurs. Mullite nanocrystals ($<$10 nm) are dispersed in an amorphous phase. The direction of crystal axis of these nanocrystals seems to be preserved to that of the starting sample, indicating that the relatively large starting mullite crystal ($\sim $10 $\mu $m) break into nanocrystals accompanied with the amorphization. In contrast, at 65 GPa, the mullite decomposes into amorphous SiO$_{2}$ and $\gamma $-Al$_{2}$O$_{3}$. Very fine $\gamma $-Al$_{2}$O$_{3}$ particles less than 10 nm are observed in the matrix of the amorphous SiO$_{2}$. The electron diffraction reveals that the $\gamma $-Al$_{2}$O$_{3}$ crystals are randomly oriented, suggesting that a rapid disproportionation reaction of mullite have been induced.