Pressure-induced phase transformations in HfO2 to 50 GPa studied by Raman spectroscopy.

Abstract

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 d\ensuremath{\omega}/dP of the Raman modes, three pressure-induced phase transitions were found to occur at 4.3\ifmmode\pm\else\textpm\fi{}0.2 GPa, 12\ifmmode\pm\else\textpm\fi{}0.5 GPa, and 28\ifmmode\pm\else\textpm\fi{}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\ifmmode\pm\else\textpm\fi{}20 GPa). A value for ${\mathit{K}}_{0}$ of 185\ifmmode\pm\else\textpm\fi{}20 GPa is suggested for ${\mathrm{HfO}}_{2}$.