When reading about the Mott metal-insulator transition, it has not become clear to me if the transition from a metal to an insulator occurs with increasing or decreasing density of valence electrons.

For example, look at fig.3 in [1]. On the left, there is V203 doped with Ti (lower density of valence electrons) and on the right there is V203 doped with Cr (higher density of valence electrons). Also, the figure indicates that the metal phase is on the left and the insulator phase on the right. This suggests that the metal-insulator transition occurs as the density of valence electrons increases. This is also what Wikipedia [2] says.

"The physical origin of the Mott transition is the interplay between the Coulomb repulsion of electrons and their degree of localization (band width). Once the carrier density becomes too high (e.g. due to doping), the energy of the system can be lowered by the localization of the formerly conducting electrons (band width reduction), leading to the formation of a band gap, e.g. by pressure (i.e. a semiconductor/insulator)."

On the other hand, look at fig. 4 in [3]. As the concentration of Na (thus also the concentration of electrons) increases, the conductivity increases as well. This suggests that the metal-insulator transition occurs as the density decreases. This view is also supported by for example [4]:

"Mott showed that there is a critical density of electrons nc above which the valence electrons are no longer bound by individual nuclei and are free to roam the crystal. This critical density marks the transition to the metallic state, and is given by the Mott criterion."

So which one is it? Or am I oversymplifying the situation and ignoring other effects?

[1] McWhan, D. B., T. M. Rice en J. P. Remeika (1969). “Mott transition in Cr-Doped V2O3”. English.In:Physical Review Letters23.24, p. 1384–1387.issn: 00319007.

[2] https://en.wikipedia.org/wiki/Mott_transition

[3] Likal’ter, A. (1997). “The metal-insulator transition and the phase transition in metal-ammonia so-lutions”. eng. In:Journal of Experimental and Theoretical Physics84.3, p. 516–521.issn:1063-7761.doi:10.1134/1.558170.

[4] https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Book%3A_Introduction_to_Inorganic_Chemistry/10%3A_Electronic_Properties_of_Materials_-_Superconductors_and_Semiconductors/10.02%3A_Metal-Insulator_Transitions



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