The energy spectrum of single-hole states in transition metal oxides

The p-d hybridised single-hole states of the transition-met al-oxygen tetrahedron (TMO4) are collectivised due to the direct p-p hopping between oxygens of different clusters. The lowest-lying energy band is always narrow and fully occupied. The first excited band gets occupied as an effect of valence-uncompensated doping, so it can be almost localised. The possible hole excitations to the two higher energy bands, which are wider, may imply the Mott-like hopping form of charge transport in these systems.