Another mechanism for the insulator-metal transition observed in Mott insulators

The two widely accepted mechanisms of the insulator-metal Mott--Hubbard transitions which have been considered up until now are driven by the band-filling or bandwidth effects. We found a different mechanism of the Mott--Hubbard insulator-metal transition, which is controlled instead by the changes in the Mott--Hubbard energy $U$. In contrast to the changes in the bandwidth $W$ in the ``bandwidth control'' scenario or to the variations of the band-filling $n$ parameter in the ``band-filling'' scenario, a dramatic decrease in the Mott--Hubbard energy $U$ plays the key role in this mechanism. We have experimentally observed this type of the insulator metal transition in the transition metal oxide $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$. The decrease in the Mott--Hubbard energy is caused by the high-spin--low-spin crossover in the electronic $d$ shell of $3d$ transition metal ion ${\mathrm{Fe}}^{3+}$ with ${d}^{5}$ configuration under high pressure. The pressure-induced spin crossover in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ was investigated and confirmed by synchrotron x-ray diffraction, nuclear forward scattering, and x-ray emission methods. The insulator-metal transition at the same pressures was found by the optical absorption and dc resistivity measurements.