In 1937 Jan Hendrik de Boer and Evert Verwey noted that transition-metal oxides such as CoO and NiO are insulators despite having partially filled 3d bands, and Nevill Mott and Rudolf Peierls explained the failure via electron-electron interactions.
In Mott's 1949 model for NiO, an insulating gap E_gap = U − 2zt opens when on-site Coulomb repulsion U between 3d electrons exceeds the tight-binding hopping bandwidth 2zt.
Varying carrier density, pressure, temperature, or doping can trigger a Mott metal-insulator transition when the electron density n and effective Bohr radius a_0* cross the Mott criterion n^(1/3) a_0* ≈ 0.2.