Half-Metallic CoO2 and Semiconducting NiO2 at High Pressures

The recent discovery of stable FeO2 under high pressures has aroused great interest in materials science, suggesting the existence of the 4+ oxidation state of late transition metals in their binary oxides. Compared with other cobalt and nickel compounds, reports on CoO2 and NiO2 are limited, and even the oxidation states of Co and Ni in CoO2 and NiO2 are the subject of debate in theoretical works. Herein, PbO2-type CoO2 and pyrite-type NiO2 are predicted using density functional theory calculations. The equations of state and relative enthalpies with known compounds are determined, showing that the PbO2-type CoO2 and pyrite-type NiO2 can be synthesized above 40 and 43 GPa, respectively. The calculations of elasticity and phonon spectra have confirmed their mechanical and dynamical stabilities at 0 GPa. The band structures and densities of states show that PbO2-CoO2 is half metallic and pyrite-NiO2 is semiconducting.