Sound velocity of α-quartz under pressure: First-principles calculations

As the most abundant substance on the earth's crust, the behavior of \ensuremath{\alpha}-quartz under pressure has always been a research hotspot, which has inherent crystallographic significance and is of significance to understanding the rocky parts of the earth. This paper provides the discussion about the stability of \ensuremath{\alpha}-quartz under pressure by analyzing its sound velocity based on first-principles calculation. It is discovered that under pressure, the mechanical instability (24.5 GPa) of \ensuremath{\alpha}-quartz precedes the dynamic instability (40 GPa). The results show that the trend of increasing and then decreasing for compressed sound velocity in different directions occurs before the structural change, and the pressure point that starts to decrease is about 20 GPa. The mechanical instability at 24.5 GPa is caused by amorphization of \ensuremath{\alpha}-quartz. The propagation velocities of the plastic wave in crystal after structural change under pressure are discussed. It is considered that the plastic wave first propagates along the direction $K$ to $G$ in the $a$ axis, and the velocity is about 429 m/s.