Post-stishovite transition in hydrous aluminous SiO$_{2}$

Incorporation of aluminum and some water into SiO$_{2}$ significantly reduces the post-stishovite transition pressure in SiO$_{2}$. This behavior suggests that the ferroelastic post-stishovite transition in subducted Earth's crust could be the source of seismic anomalies with low shear velocities observed in the mid to upper lower mantle. Using ab initio static calculations and molecular dynamics with inter-atomic potentials, we show that hydrogen bonds play a crucial role in lowering the transition pressure. A cooperative redistribution of hydrogen atoms is the main mechanism responsible for the transition pressure reduction in hydrous aluminous stishovite. The effect is enhanced by increasing the water content and suggests a relationship between the depth of these seismic anomalies and degree of hydration of stishovite in the subducted crust.