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Charge-transfer insulators are a class of materials predicted to be conductors following conventional band theory, but which are in fact insulators due to a charge-transfer process. Unlike Mott insulators, where the insulating properties arise from electrons hopping between unit cells, the electrons in charge-transfer insulators move between atoms within the unit cell. In the Mott–Hubbard case, it's easier for electrons to transfer between two adjacent metal sites ; here we have an excitation corresponding to the Coulomb energy U with
d n d n → d n − 1 d n + 1 , Δ E = U = U d d {\displaystyle d^{n}d^{n}\rightarrow d^{n-1}d^{n+1},\quad \Delta E=U=U_{dd}}.
In the charge-transfer case, the excitation happens from the anion p level to the metal d level with the charge-transfer energy Δ:
d n p 6 → d n + 1 p 5 , Δ E = Δ C T {\displaystyle d^{n}p^{6}\rightarrow d^{n+1}p^{5},\quad \Delta E=\Delta _{CT}}.