Identity of Oxygen-Rich Nickel Oxides as Oxosuperoxides and Oxoperoxides and Their Heterostructures

The chemical identity of oxygen-rich nickel oxides was investigated employing Density Functional Theory calculations performed for Ni-substituted transition-metal pentoxides M₂O₅, which have the potential to host M⁵⁺ species, while the ground-state structure was predicted using evolutionary algorithms for crystal structure prediction. Our results have shown that Ni⁵⁺ is unlikely to stabilize in the oxide environment and will immediately reduce to more stable nickel oxidation states (Ni⁴⁺-Ni²⁺) through the formation of molecular oxygen species, while both superoxide and peroxide species are equally likely. The resulting oxoperoxide and oxosuperoxide phases represent oxygen-enriched versions of the already well-known or long-suspected binary nickel oxides (NiO, Ni₂O₃, NiO₂) and their heterostructures including O₂-decorated rock salt NiO, O-enriched layered CdI₂-type NiO₂, a der Waals heterostructure of NiO₂ and NiO(O₂) as well as the monoclinic C2/c form (high-pressure V₂O₅ type structure), which is common to all known pentoxides, and in the case of nickel takes the form of oxosuperoxide Ni₂O₃(O₂). All predicted models, although dynamically stable, were found to be highly unstable in relation to reduction to nickel monoxide, the most stable nickel oxide phase, which provides one explanation for why oxygen-rich nickel oxide phases beyond NiO₂ have not yet been observed.