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The mechanisms that drive metal-to-insulator transitions (MIT) in correlatedsolids are not fully understood. For example, the perovskite (PV) SrCoO3 is aFM metal while the oxygen-deficient (n-doped) brownmillerite (BM) SrCoO2.5 isan anti-ferromagnetic (AFM) insulator. Given the magnetic and structuraltransitions that accompany the MIT, the driver for such a MIT transition isunclear. We also observe that the perovskite metals LaNiO3, SrFeO3, and SrCoO3also 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 thatthe ABO3 perovskites most prone to MIT are self hole-doped materials,reminiscent of a negative charge-transfer system. Upon n-doping away from thenegative-charge transfer metallic phase, an underlying charge-lattice (ore-phonon) coupling drives the system to a bond-disproportionated gapped state,thereby achieving ligand hole passivation at certain sites only, leading tocharge-disproportionated states. The size of the gap opened is correlated withthe size of the hole-filling at these ligand sites. This suggests that theinteractions driving the gap opening to realize a MIT even in correlated metalsis the charge-transfer energy, but it couples with the underlying phonons toenable the transition to the insulating phase. Other orderings (magnetic,charge, etc.) driven by weaker interactions are secondary and may assist gapopenings at small dopings, but its the charge-transfer energy thatpredominantly determines the bandgap, with a negative energy preferring themetallic phase. This n-doping can be achieved by modulations in stoichiometryor composition or pressure. Hence, controlling the amount of the ligand-hole iskey in controlling MIT. We compare our predictions to experiments wherepossible.

Origin of Metal-Insulator Transitions in Correlated Perovskite Metals