PRESSURE-INDUCED PHASE-TRANSITIONS AND VOLUME CHANGES IN HFO2 UP TO 50 GPA

The phase transformations and pressure-volume dependence of HfO2 have been investigated at room temperature by angle-dispersive powder x-ray diffraction under high pressure to 50 GPa in a diamond anvil cell. The phase transformation from the monoclinic I (baddeleyite) to orthorhombic phase II was observed around 10 GPa. This phase is stable up to 26 GPa where it transforms to a new phase III with another orthorhombic unit cell. At about 42 GPa, a third phase transition occurs to phase IV of tetragonal symmetry. The pressure dependences of the cell parameters and volume have been determined. The successive volume discontinuities are 2.5%, 2.5%, and 5%, respectively. The bulk moduli of all the phases have been calculated from Birch's equation of state and are discussed. The high-pressure phases were found to be metastable at normal pressure. No orthorhombic cotunnite-type structure was observed under pressure at room temperature. Although the structural properties of HfO2 and ZrO2 are similar at lower pressures, their evolutions are different above 20 GPa.