Shock-induced disproportionation of mullite (3Al₂O₃ 2SiO₂)

Shock-recovery experiments have been performed on high-purity mullite polycrystals in the pressure range up to 72 GPa. The recovered samples have been examined by an x-ray diffraction method, nuclear magnetic resonance spectroscopy, and transmission electron microscopy. The samples shocked above 65 GPa contain an amorphous Si O₂ phase and a γ- Al₂ O₃ phase, indicating that a rapid disproportionation reaction of mullite is induced by shock compression. The recovered amorphous Si O ₂ has a mean Si-O-Si bond angle roughly 7° narrower than that of the fused Si O₂ glass, indicating the formation of the densified amorphous Si O₂ phase. The γ- Al₂ O₃ phase is crystallized as very fine particles with grain sizes less than 10 nm in the matrix of the desified amorphous Si O₂ phase. The crystallization of γ- Al₂ O₃ is expected to occur during a pressure-release process owing to the crystal size effect concerning the phase stability of Al₂ O₃.