Origin of metal-insulator transitions in correlated perovskites – a combined DFT+U and QMC investigation

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 (PV) SrCoO₃ is a FM metal and the oxygen-deficient (n-doped) brownmillerite SrCoO_2.5 is an AFM insulator. Given the magnetic and structural transitions that accompany the MIT, the driving force for the transition is unclear. Interestingly, the PV metals LaNiO₃, SrFeO_3, and SrCoO₃ also undergo MIT when n-doped via high-to-low valence compositional changes. We posit that the ABO₃ PV's most prone to MIT are self hole-doped negative charge transfer materials. Upon n-doping, ligand hole passivation at certain sites occurs, leading to a bond-disproportionated gapped state due to charge-lattice coupling. Other orderings (magnetic, charge, orbital etc.) are secondary and may assist gap openings at small dopings. We use DFT methods along with explicitly correlated diffusion Monte Carlo to test these hypotheses and compare to experiments where possible.