Novel compounds in the Zr–O system, their crystal structures and mechanical properties
With the motivation of exploring new high-strength ceramics, ab initio evolutionary simulations are performed to search for all the stable compounds in the Zr–O system. We have found that not only the traditional compound ZrO2, but also the ordered suboxides R-Zr6O, Rc-Zr3O, P1m-Zr2O and P2m-ZrO are stable at zero pressure. The crystal structure of semimetallic P2m-ZrO consists of Zr-graphene layers and can be described as an intercalated version of the ω-Zr structure. An interesting massive Dirac cone is found in the three-dimensional (3D) band structure of P2m-ZrO at the Γ-point. The elastic properties, the hardness and the correlation between the mechanical properties of Zr–O compounds and the oxygen content have been systematically investigated. Surprisingly, the hardest zirconium oxide is not ZrO2, but ZrO. Both P2m-ZrO and P1m-Zr2O exhibit relatively high hardness values of 14 GPa and 10 GPa, respectively. The anisotropic Young's modulus E, torsion shear modulus Gt and linear compressibility β have been derived for P2m-ZrO and P1m-Zr2O. Further analyses of the density of states, the band structure and the crystal orbital Hamilton population indicate that the electronic structure of Zr–O compounds is directly related to their mechanical properties. The simultaneous occurrence of the 3D-framework of Zr–O and the strong Zr–Zr bonds in P2m-ZrO explains its high hardness.