Rutile-structured Sn0 2 , Pb0 2 , and Ru0 2 have long been known to transform to cubic high-pressure phases, for which a fluorite structure has been assumed. Rietveld refinement results from x-ray diffraction^ studies indicated that these phases have a modified fluorite structure (space group Pa3). Thus, for metal dioxides, all known cubic, postrutile phases have the Pa3 structure, thereby providing experimental examples of the highpressure structure predicted from theoretical calculations for stishovite (rutile-structured silica). High-pressure transitions in stishovite may have profound implications for the geochemistry of the core-mantle boundary.I he sequence of pressure-induced phase transitions in rutile-structured metal dioxides has been the subject of considerable interest, particularly because these metal dioxides serve as models for the behavior of stishovite at higher pressures. Because of the larger size of the metal cations, the same phase transitions to denser phases as for silica are expected to be observed at lower pressures in metal dioxides.It has been proposed that rutile-structured Sn0 2 , PbO z , and Ru0 2 transform to the cubic, fluorite structure at high pressure, with an increase in cation coordination number from 6 to 8 (J-3). Previous diffraction experiments indicated that SnO z and Pb0 7 undergo transitions first to an a-PbO z -type phase {4,5) and then subsequently to a cubic phase assumed to be fluorite structured (2, 6, 7). The volume changes (AV) are 2 to 3% for the transition from rutile to a-PbO z and 5% for the transition from a-Pb0 7 to the cubic phase. In Ru0 2 , a second-order transition to a CaCl 2 -type phase was observed before the transformation (AV = 6.1%) to the cubic phase (3). The orthorhombic CaCl 2 structure (Pnnm) is a simple distortion of the tetragonal rutile structure (P4 2 /mnm), which involves a slight rotation of the columns of edge-sharing octahedra aligned parallel to zTheoretical calculations indicate, however, that for both SiO z and Ge0 2 the cubic, postrutile phase should have the Pa3 modified fluorite structure (8,9). Both theory and experiment show that stishovite undergoes a first transition to a CaCl 2 -type structure (8,10). Calculations indicate that at pressures of 60 or 150 GPa, respectively, stishovite or CaCl 2 -type SiO z should transform to a_modified fluorite structure (space group Pa3) (8, J J). In this modified structure the oxygen atoms are significantly displaced from their -positions in the fluorite structure (space group Fm3m): The oxygen