Magnetism-Vanishing Stabilizes the Pyrite-Type 3d Transition Metal Peroxides at High Pressures

Abstract

Extensive AO 2 -type transition metal (TM) oxides are experimentally determined or theoretically predicted to adopt the pyrite-type (Pa3̅ ) structure at different pressures, but their physical and chemical properties are largely unknown. Our theoretical calculations demonstrate that the interpolyhedral O−O bond is formed in pyrite-type CoO 2 with low-spin Co 2+ similar to the case of FeO 2 , whereas ferromagnetic CrO 2 and antiferromagnetic MnO 2 belong to dioxides with separate O 2− anions. The oxidation state of the cation and the potential formation of O−O bonding are affected by the crystal-field splitting, charge transfer between the cation and the anion, but primarily by the magnetic coupling among 3d electrons in a pyrite-type 3d TMO 2 . The spin crossover-induced magnetism-vanishing at high pressures contributes to a smaller cell volume and a weaker electronic correlation, facilitating the formation of an O−O bond and a reduction of the oxidation state of the cation. These results provide critical physical insights into the formation mechanism of 3d TM peroxides with O 2 2− dimers. Based on this conclusion, we have further theoretically predicted the formation of the hydrogen-bearing cobalt peroxide at high pressures. This shows another example for exploring the nature of hydrogen-bearing TM peroxides and sheds light on synthesizing novel TM peroxide materials under high pressure.