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.