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 pressure, we observed a phase transition at Pc ~ 1.5 GPa, characterized by the onset of twinning and the splitting of powder diffraction 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 = 4.625(3), c = 8.394(5) Å, β = 124.91(5)°, and V = 290.5(2) Å3 at P = 3.1 GPa. The transition from tetragonal a cristobalite to monoclinic cristobalite II 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 analyzed 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 process is a shear parallel to [101] in the tetragonal (101) planes, corresponding to slip on the {111} tetrahedral sheets of the high-T cubic β-cristobalite phase.

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