From a Mott-Anderson Insulator to an Itinerant Metal in LaCo_1-x Ni _x O₃: Charge Transfer, Spin-State Percolation, and...

The perovskite solid solution LaCo_1-x Ni _x O₃ is a promising candidate for high-efficiency energy conversion, yet the microscopic evolution of its electronic structure across the Ni concentration-dependent insulator-to-metal transition (IMT) remains a subject of debate. Here, we comprehensively investigate the electronic phase diagram by combining element-specific X-ray spectroscopies (X-ray absorption and photoelectron spectroscopy as well as resonant photoemission spectroscopy) with density-functional theory plus Hubbard U (DFT + U) calculations. We identify a three-stage mechanism for the IMT: (1) a Mott-Anderson insulating phase (x ≤ 0.3) governed by disorder-induced localization within narrow impurity bands; (2) a spin-state-assisted percolation (x ≈ 0.5) triggered by the formation of high-spin Co³⁺ bridges that interconnect itinerant Ni³⁺ clusters; and (3) a bandwidth-controlled metallic regime (x > 0.5) stabilized by the structural straightening of B-O-B bond angles. By elucidating how Ni-substitution modulates metal-oxygen covalency and the Co/Ni valence manifolds, this work establishes a comprehensive electronic-structure framework. This framework provides valuable insights for optimizing these materials in future energy applications.