Comment on: High-Pressure Elasticity of alpha-Quartz: Instability and Ferroelastic Transition

We comment on the stability analysis of alpha-quartz under pressure by Gregoryanz et al., Phys. Rev. Lett. 84, 3117 (2000). The proper stability criterion for solids under pressure has to be formulated in terms of Birch coefficients rather than in terms of elastic constants. We show that the fictitious (extrapolated) stable phase of alpha-quartz above a pressure of 22 GPa is much smaller than anticipated by Gregoryanz et al.