PRESSURE-INDUCED PHASE-TRANSFORMATIONS IN HFO2 TO 50 GPA STUDIED BY RAMAN-SPECTROSCOPY

High-pressure Raman studies were carried out at room temperature on HfO2 single-crystal samples up to 50 GPa in the diamond anvil cell, with a methanol-ethanol pressure medium. From the changes in the Raman spectral features and the pressure dependence domega/dP of the Raman modes, three pressure-induced phase transitions were found to occur at 4.3+/-0.2 GPa, 12+/-0.5 GPa, and 28+/-2 GPa. The first two high-pressure phases are nonquenchable while the high-pressure phase after the 28 GPa transition is quenchable. The high-pressure phase after the 4.3-GPa transition has been determined as orthorhombic belonging to the space group Pbcm, from earlier high-pressure Raman and x-ray studies. The 12-GPa transition and the 28-GPa transition, newly found in this study, are believed to have the same structure as that of the high-pressure polymorphs of ZrO2 that occur at nearly the same pressures. For these, other orthorhombic and tetragonal structures, respectively, are proposed from the close similarity in high-pressure Raman behavior between ZrO2 and HfO2. The almost identical molar volumes, structural correspondence, and high-pressure behavior of ZrO2 and HfO2 suggest that the bulk modulus K0 of HfO2 should be nearly the same as that of ZrO2 (K0 = 182+/-20 GPa). A value for K0 of 185+/-20 GPa is suggested for HfO2.