Our atomistic computer simulations mainly based on classical force fields suggest that the pressure-induced transition from $\alpha$ quartz to quartz II at $21\mathrm{GPa}$ is irreversible. While quartz II is ferroelastic in principle, the transition itself is coelastic, as the shape of the newly formed crystal is determined by the handedness of $\alpha$-quartz. Upon releasing the pressure, our model quartz II remains stable down to $5\mathrm{GPa}$, where it undergoes an isosymmetric transformation into a less dense polymorph. If the classical force field model of quartz II is compressed quickly to $50\mathrm{GPa}$, a yet different post-quartz polymorph results, which can probably best be described as an incommensurate modulation of the quartz II structure. We discuss the equation of state and the thermomechanical stability of all four phases. One of the post-quartz phases can be switched elastically by shear between two symmetrically equivalent shapes; however, $\alpha$-quartz appears as an intermittent phase.

• Received 9 June 2004

DOI:https://doi.org/10.1103/PhysRevB.70.224101