Phase relations and hardness trends of ZrO2 phases at high pressure

We use high-resolution synchrotron x-ray powder diffraction and density-functional theory (DFT) to investigate the phase stability, equations of state (EOSs), and mechanical hardness of zirconia (ZrO2) up to similar to 54 and 160 GPa, respectively. For the equilibrium phase at ambient conditions (MI), we provide an experimental EOS that is comparable to results obtained from room-pressure Brillouin scattering experiments and bulk modulus-volume systematics but different from previous high-pressure experiments. The experimental second-order Birch-Murnaghan EOS parameters of MI-ZrO2 are: ambient-pressure volume (V-0) of 35.15(+/-0.03) angstrom(3)/f.u. with an ambient-pressure bulk modulus K-0 of 210(+/-28) GPa. For the high-pressure OI phase, we find that the K-0=290(+/-11) GPa, which is 19%-32% higher than previously determined, and V-0=33.65(+/-0.07) angstrom(3)/f.u. The small volume decrease of 3.4% across the MI -> OI transition at similar to 10 GPa is associated with a 38% increase in the bulk modulus consistent with our DFT calculations that predict a similar to 36% and 39% increase in K-0 for the generalized gradient and local density approximations, respectively. In contrast to the EOS of MI and OI, we find that our experimental EOS for the high-pressure OII phase is in good agreement with previous measurements. The large volume decrease across the OI -> OII phase transition as obtained from both our experiments and calculations is similar to 10%. Our estimates, using scaling relations, indicate that this phase, while dense and quenchable, may have a comparatively low mechanical hardness of similar to 10 GPa.