  The mechanisms that drive metal-to-insulator transitions (MIT) in correlated
solids are not fully understood. For example, the perovskite (PV) SrCoO3 is a
FM metal while the oxygen-deficient (n-doped) brownmillerite (BM) SrCoO2.5 is
an anti-ferromagnetic (AFM) insulator. Given the magnetic and structural
transitions that accompany the MIT, the driver for such a MIT transition is
unclear. We also observe that the perovskite metals LaNiO3, SrFeO3, and SrCoO3
also undergo MIT when n-doped via high-to-low valence compositional changes.
Also, pressurizing the insulating BM SrCoO2.5 phase, drives a gap closing.
Using DFT and correlated diffusion Monte Carlo approaches we demonstrate that
the ABO3 perovskites most prone to MIT are self hole-doped materials,
reminiscent of a negative charge-transfer system. Upon n-doping away from the
negative-charge transfer metallic phase, an underlying charge-lattice (or
e-phonon) coupling drives the system to a bond-disproportionated gapped state,
thereby achieving ligand hole passivation at certain sites only, leading to
charge-disproportionated states. The size of the gap opened is correlated with
the size of the hole-filling at these ligand sites. This suggests that the
interactions driving the gap opening to realize a MIT even in correlated metals
is the charge-transfer energy, but it couples with the underlying phonons to
enable the transition to the insulating phase. Other orderings (magnetic,
charge, etc.) driven by weaker interactions are secondary and may assist gap
openings at small dopings, but its the charge-transfer energy that
predominantly determines the bandgap, with a negative energy preferring the
metallic phase. This n-doping can be achieved by modulations in stoichiometry
or composition or pressure. Hence, controlling the amount of the ligand-hole is
key in controlling MIT. We compare our predictions to experiments where
possible.
