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 I4(1)/amd. Pnnm-HfO3 shows interesting structure, simultaneously containing oxide O2- and peroxide [O-O](2-) anions. Remarkably, it is P (6) over bar 2m-HfO rather than OII-HfO2 that exhibits the highest mechanical characteristics among Hf-O compounds. Pnnm-Hf2O, Imm(2)-Hf5O2, P (3) over bar 1m-Hf2O, and P (4) over barm(2)-Hf2O3 phases also show superior mechanical properties; theoretically these phases become metastable phases to ambient pressure and their properties can be exploited.