Strain/order parameter coupling in the ferroelastic transition in dense SiO2

New high-pressure measurements reveal the coupling of strain and order parameter in the pressure-induced ferroelastic transition in dense SiO2. Single-crystal X-ray diffraction measurements in quasi-hydrostatic media reversibly probe the spontaneous strain in the vicinity of the P42/mnm→Pnnm transition near 50 GPa, and indicate weak first-order character. A Landau model is developed that quantitatively relates all of the spectroscopic, elastic, structural, and thermodynamic data for the system. The elastic instability at the transition gives rise to anomalies in the Raman spectrum, which are expected to be a general feature of such pressure-induced transitions.