Nickel(III) oxide (Ni2O3) has been referred to in the literature but is not a well characterised compound. The substance black nickel oxide is sometimes described as being Ni2O3 however the composition quoted by suppliers has a nickel content of around 77% by weight whereas Ni2O3 would have 70.98% Ni by weight, and may be non-stoichiometric NiO.
There are references in the literature to traces of Ni2O3 on nickel surfaces, and as intermediates in nickel oxidation, e.g. .
or in double oxidation of 3-butenoicid to fumaric acid: