First-principles investigation of Zr-O compounds, their crystal structures, and mechanical properties

First-principles evolutionary simulations are used to systematically predict stable compounds in the Zr-O system at pressures up to 120 GPa. Zr-O compounds and Hf-O compounds share many similarities, but four new phases Cmmm-Zr3O, R3¯c-Zr3O2, Pmma-ZrO, and Fe2P-type ZrO2 (P6¯2m) appear in the Zr-O system: the latter two phases appear in the Hf-O system at higher pressure. OII ZrO2 (Pnma) transforms into Fe2P-type ZrO2 at 102 GPa. Meanwhile, Fe2P-type ZrO2 and P6¯2m-ZrO have similar structures based on ω-Zr. However, the calculated Vickers hardness of Fe2P-type ZrO2 (5.6 GPa) is inferior to that of P6¯2m-ZrO (14.1 GPa). The hardness of P6¯2m-ZrO (14.1 GPa) is lower than that of P6¯2m-HfO (16.1 GPa) and P6¯2m-TiO (16.6 GPa). On the whole, Zr-O compounds exhibit lower hardnesses and bulk moduli than Hf-O compounds.