NiO is a Mott-Hubbard insulator and increasing oxygen concentration leads to anion excess and oxidation of Nickel so electron hopping is more favourable so electron repulsion is reduced and it becomes metallic. This makes sense to me but I think it can also be explained as a p-type semiconductor since you're essentially adding Ni(III). So you'd expect to get a band structure with a very narrow empty band above a filled band. However, I don't see how these ideas match up?

ZnO has ten d electrons so isn't a Mott Hubbard insulator so can only be a n-type semiconductor since it won't be oxidised. This leaves a narrow filled band below and empty band right? But what is the empty band? The s band? Is my reasoning okay?


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