High-density ZrO2 and HfO2: Crystalline structures and equations of state

On the basis of Raman spectra and x-ray-diffraction patterns, we show that both dense zirconium and hafnium dioxides adopt the same structural phases, in similar pressure ranges. Furthermore both dioxides may be quenched at r oom conditions from high pressure to give polycrystalline solids which pres ent optical transparency. From energy dispersive and angle dispersive x-ray diffraction using synchrotron radiation, the structure of the quenched pha se is confirmed to be of the nine-coordinated PbCl2-type structure (cotunni te), space group Pnma (Z=4), as proposed earlier. Equations of state of all phases encountered in the pressure range investigated are also given. It i s shown that the volume-pressure relationship for the quenched phase at roo m temperature is not well modeled by a usual equation of state based on the finite strain theory, as the room conditions bulk moduli are very high and the first derivative of the bull; moduli are small for both oxides. Nevert heless values for the bulk modulus of the high-density solids recovered at room conditions from at high pressure in the cotunnite phase, 444+/-15 GPa and 340+/-10 GPa, for ZrO2 and HfO2, respectively, indicate a very low comp ressibility for both dioxides. If the correlation between hardness and high bulk modulus in materials is admitted, then high density cotunnite ZrO2 an d HfO2 can be considered as candidates for potentially ultrahard materials. [S0163-1829(99)11813-7].