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

New high-pressure measurements reveal the coupling of strain and order para meter in the pressure-induced ferroelastic transition in dense SiO2. Single -crystal X-ray diffraction measurements in quasi-hydrostatic media reversib ly probe the spontaneous strain in the vicinity of the P4(2)/mnm -> Pnnm tr ansition near 50 GPa, and indicate weak first-order character. A Landau mod el is developed that quantitatively relates all of the spectroscopic, elast ic, structural, and thermodynamic data for the system. The elastic instabil ity at the transition gives rise to anomalies in the Raman spectrum, which are expected to be a general feature of such pressure-induced transitions. (C) 2000 Elsevier Science Ltd. All rights reserved.