Charge-transfer insulators

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 (on-site Coulomb interaction U); in the charge-transfer case, it's easier from the anion to the metal (charge-transfer energy Δ). U is determined by repulsive/exchange effects between the cation valence electrons. Δ is tuned by the chemistry between the cation and anion.