High-pressure elasticity of alpha-quartz: instability and ferroelastic transition

The single-crystal elastic moduli of alpha-quartz were measured to above 20 GPa in a diamond-anvil cell by Brillouin spectroscopy. The behavior of the elastic moduli indicates that the high-pressure phase transition in quartz is ferroelastic in nature and is driven by softening of C44 through one of the Born stability criteria. The trends in elastic moduli confirm theoretical predictions, but there are important differences, particularly with respect to the magnitudes of the B(i). The quartz I-II transition occurs prior to complete softening of the mode and amorphization.