Phase diagram up to 105 GPa and mechanical strength of HfO2

Using high-resolution synchrotron powder x-ray diffraction, we have investigated the stability and equation of state (EOS) of hafnia HfO2 phases under high pressures before and after laser heating to high temperatures. We observe three phases with increasing pressure: baddeleyite (monoclinic, MI), orthorhombic I (OI), and cotunnite (orthorhombic, OII). The OII phase is stable up to a pressure of at least 105 GPa before and after laser heating to similar to 1800 (+/- 200) K. We provide experimental EOSs for the observed phases. The present results for MI-HfO2 EOS are distinct from previous measurements yielding an ambient-pressure volume (V-0) of 34.50 (+/- 0.04) angstrom(3)/f.u. and an ambient-pressure bulk modulus K-0 of 185 (+/- 23) GPa, assuming K-0'=4. In contrast, the experimental EOSs of OI and OII are in good agreement with previous studies. The measured EOSs are consistent with our density-functional theory calculations. The large volume decrease across the OI -> OII phase transition as obtained from both our experiments and calculations is similar to 9%. Despite the large increase in density and high bulk modulus of OII-HfO2, we find, using scaling relations, that all HfO2 phases show similar mechanical hardness (H) of similar to 10-12 GPa, too low for HfO2 to be considered a superhard material.