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.