| Using high-resolution synchrotron powder x-ray diffraction (XRD) and density-functional theory (DFT), I investigate the high-pressure behavior of transition metal dioxides titania (TiO2), zirconia (ZrO2) and hafnia (HfO2). In this study, I determine the equation of state (EOS), the phase diagram and estimate the mechanical strength of each high-pressure dioxide phase.;In this work, new experimental EOSs are provided for each observed phase, some of the EOSs are significantly different from previous work; in particular, for the high-pressure orthorhombic OII-TiO2 phase and the ambient-pressure monoclinic MI-ZrO2 and MI-HfO2 phases. The OII-TiO 2 phase is observed to be much denser than previously observed, but in agreement with first-principles calculations performed on TiO2, and consistent with my and other previous measurements on similar dioxides ZrO2 and HfO2. The possibility of other recently proposed phases for ZrO2 system has been also investigated and have been found to be energetically unfavorable consistent with my experimental measurements.;I compute the mechanical strengths of the OII phases of ZrO2 and HfO2, which had been previously suggested to be superhard. Using scaling relations, the estimates show that the OII phase of both ZrO 2 and HfO2, while dense and quenchable, have a comparatively low mechanical hardness (H) of ∼10 GPa and thus do not qualify as superhard (H > 40 GPa). The low-pressure phases (MI and OI) show similar mechanical strength values.;In this study, I investigate the EOS, phase stability, and the mechanical strength to provide a better understanding of the relationship between phase and its mechanical properties. This work shows that the shear modulus correlates with hardness between phases better than the bulk modulus. |
