PRESSURE-INDUCED PHASE-TRANSITION IN CRISTOBALITE - AN X-RAY-POWDER DIFFRACTION STUDY TO 4.4 GPA

The structural behavior of cristobalite, SiO2, has been studied under hydrostatic conditions in a diamond-anvil cell to 4.4 GPa, using high- resolution synchrotron X-ray powder diffraction. On increasing pressur e, we observed a phase transition at P(c) is similar to 1.5 GPa, chara cterized by the onset of twinning and the splitting of powder diffract ion lines. This transition is reversible and first-order in character. The high-pressure phase, referred to here as cristobalite II, can be indexed according to a monoclinic unit cell with a = 9.124(5), b = 4.6 25(3), c = 8.394(5) angstrom, beta = 124.91(5)degrees, and V = 290.5(2 ) angstrom3 at P = 3.1 GPa. The transition from tetragonal alpha crist obalite to monoclinic cristobalite 11 involves a doubling of the unit- cell size and must therefore be induced by a zone-boundary instability . The resulting components of the spontaneous strain tensor are analyz ed in terms of the change in point group symmetry from 422 to 2 and of coupling with the macroscopic order parameter. There is a significant non-symmetry-breaking (volume) strain. The actual symmetry-breaking p rocess is a shear parallel to [101BAR] in the tetragonal (101) planes, corresponding to slip on the {111} tetrahedral sheets of the high- T cubic beta-cristobalite phase.