PHASE-TRANSITIONS IN RUTHENIUM DIOXIDE UP TO 40 GPA - MECHANISM FOR THE RUTILE-TO-FLUORITE PHASE-TRANSFORMATION AND A MODEL FOR THE HIGH-PRESSURE BEHAVIOR OF STISHOVITE SIO2

RuO2 was studied up to 40.2 GPa by angle-dispersive x-ray diffraction. Two phase transitions were observed, the first from the rutile-structured ambient phase to an orthorhombic phase below 8.0 GPa with cell parameters a = 4.615(17), b = 4.230(13), and c = 3.112(4) angstrom, Z = 2 at 11.3 GPa. The second transition at just below 13 GPa was to a fluorite-type structure with a = 4.727(4) angstrom, Z = 4 at 40.3 GPa. The first transition appeared to be second order producing a structure that is related to the CaCl2 type. Several reflexions were found to be common to all three phases, indicating that the rutile-to-fluorite phase transformation occurs via a diffusionless mechanism dominated by oxygen displacement in the xy plane of rutile. Using group theory, we propose possible diffusionless pathways for the transformation from the initial rutile structure, via the observed, orthorhombic, CaCl2-like intermediate, to the fluorite structure, based on a series of group-subgroup relationships. Ruthenium dioxide under ambient conditions is isostructural with stishovite and hence provides a model for the high-pressure behavior of SiO2.