Can anyone explain to me why most articles describe chromium as an acceptor in TiO2? In TiO2, titanium has the charge state Ti4+ and oxygen has the charge state O2-. When Cr substitutes for Ti, it does so as Cr3+. Now, at first glance, Cr has atomic number 24 and Ti 22. Cr therefore has two more valence electrons and is a donor. In TiO2, Cr3+ actually has three more valence electrons than Ti4+ ([Ar]3d34s0 vs [Ar]3d04s0). It should therefore be a donor. The thing is, it forms a deep impurity level near the valence band. TiO2 has an energy gap of around 3.2 eV, and the impurity state is about 1.0 eV from the valence band maximum. To me, that makes it a deep donor. For some reason, journals almost always describe it as an acceptor. Can someone help me make sense of this?