Pressure-induced novel compounds in the Hf-O system from first-principles calculations

Using first-principles evolutionary simulations, we have systematically investigated phase stability in the Hf-O system at pressure up to 120 GPa. New compounds Hf₅O₂,Hf₃O₂, HfO, and HfO₃ are discovered to be thermodynamically stable at certain pressure ranges. Two new high-pressure phases are found for Hf₂O: one with space group Pnnm and anti-CaCl₂-type structure, another with space group I4₁/amd. Pnnm-HfO₃ shows interesting structure, simultaneously containing oxide O^2- and peroxide [O-O]^2- anions. Remarkably, it is P6̄2m-HfO rather than OII-HfO₂ that exhibits the highest mechanical characteristics among Hf-O compounds. Pnnm-Hf₂O, Imm2-Hf₅O₂,P3̄1m-Hf₂O, and P4̄m2-Hf₂O₃ phases also show superior mechanical properties; theoretically these phases become metastable phases to ambient pressure and their properties can be exploited.