# Black & Green Nickel Oxide-Supplemental

A previous question (April 2017) asked about black $$\ce{NiO}$$ did not supply any answer. Sadly, such a referral only deters colleagues from commenting on the current question.

There is a great deal of literature on cation defects in non-stoichiometric nickel (II) oxide. There is much on charge-transfer in defective crystal structures. But, I can't find any attempt to marry the two together - which may explain how the deep colour (black) is observed. Of course, it may simply be that I have not discovered it yet. I did find a question on how/why copper (II) oxide is black. This one has semiconductor properties; so, $$\ce{CuO}$$ and non-stoichiometric $$\ce{NiO}$$ may be black for similar reasons? Can anyone comment?