The potentially hard oxides ZrO 2 and HfO 2 are investigated using ab initio electronic structure calculations and structural properties of the lowest phases compared. In general there are strong similarities between some phases apart from the softer monoclinic baddeyelite phase, which in HfO 2 is the lowest energy phase with a bulk modulus almost twice that of ZrO 2 . Other differences relate to the formation of the first orthorhombic phase, especially the inter-relation between Pbc21 and Pbca structures. The bulk moduli of the high-pressure, cotunnite-type phases are in good agreement with experimental results. The present calculations confirm that these phases are highly incompressible and are thus good candidates for hard materials.

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