Pressure-induced phase transformations inHfO2to 50 GPa studied by Raman spectroscopy

High-pressure Raman studies were carried out at room temperature on ${\mathrm{HfO}}_2$ 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 dω/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 ${\mathrm{ZrO}}_2$ 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 ${\mathrm{ZrO}}_2$ and ${\mathrm{HfO}}_2$. The almost identical molar volumes, structural correspondence, and high-pressure behavior of ${\mathrm{ZrO}}_2$ and ${\mathrm{HfO}}_2$ suggest that the bulk modulus ${\mathit{K}}_0$ of ${\mathrm{HfO}}_2$ should be nearly the same as that of ${\mathrm{ZrO}}_2$ (${\mathit{K}}_0$=182±20 GPa). A value for ${\mathit{K}}_0$ of 185±20 GPa is suggested for ${\mathrm{HfO}}_2$.