Origin of metal-insulator transitions in correlated perovskite metals

The mechanisms that drive metal-to-insulator transitions (MIT) in correlated solids are not fully understood, though intricate couplings of charge, spin, orbital, and lattice degrees of freedom have been implicated. For example, the perovskite SrCoO₃ is a ferromagnetic metal, while the oxygen-deficient (n-doped) brownmillerite SrCoO_2.5 is an antiferromagnetic insulator. Given the magnetic and structural transitions that accompany the MIT, the driving force for such a MIT transition is unclear. We also observe that, interestingly, the perovskite metals LaNiO3, SrFeO3, and SrCoO3 also undergo MIT when n-doped via high-to-low valence compositional changes, i.e., Ni3+→Fe4+, Sr2+→La3+, and Sr2+→La3+, respectively. On the other hand, pressurizing the insulating brownmillerite SrCoO2.5 phase drives a gap closing. Here we demonstrate that the ABO3 perovskites most prone to MIT are self-hole-doped materials, reminiscent of a negative charge-transfer metal, using a combination of density functional and fixed-node diffusion quantum Monte Carlo calculations. Upon n doping the negative charge-transfer metallic phase, an underlying charge-lattice (or electron-phonon) coupling drives the metal to a charge and bond-disproportionated gapped insulating state, thereby achieving ligand-hole passivation at certain sites only. The size of the band gap is linearly correlated with the degree of hole passivation at these ligand sites. Further, metallization via pressure is also stabilized by a similar increase in the ligand hole, which in turn stabilizes the ferromagnetic coupling. These results suggest that the interaction that drives the band-gap opening to realize a MIT even in correlated metals is the charge-transfer energy, while it couples with the underlying phonons to enable the transition to the insulating phase. Other orderings (magnetic, charge, orbital etc.) driven by weaker interactions may assist gap openings at low doping levels, but it is the charge-transfer energy that predominantly determines the band gap, with a negative energy preferring the metallic phase. This n doping can be achieved by modulations in oxygen stoichiometry or metal composition or pressure. Hence, controlling the amount of the ligand hole, set by the charge-transfer energy, is the key factor in controlling MIT.