Mott insulator and Charge transfer insulator

Mott insulators are a class of materials that should conduct electricity under conventional band theories, but are insulators when measured (particularly at low temperatures). This effect is due to electron–electron interactions, which are not considered in conventional band theory.

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.

The bandgap in a Mott insulator exists between bands of like character, such as 3d character, whereas the bandgap in charge transfer insulators exists between anion and cation states,^[1] such as between O 2p and Ni 3d bands in NiO.


References:

http://www.physics.udel.edu/~bnikolic/teaching/phys624/lectures/mott_insulators.pdf

http://www.icmm.csic.es/lenibascones/mottphysics1talk.pdf

http://budker.berkeley.edu/Physics141_2013/Mott%20Metal-Insulator%20Transitions_%20Satej%20Soman,%20Robert%20Tang-Kong.pdf