Topotactic Reduction‐Driven Crystal Field Excitations in Brownmillerite Manganite Thin Films

Abstract Topotactic reduction of perovskite oxides offers a powerful approach for discovering novel phenomena, such as superconducting infinite‐layer nickelates and polar metallicity, and is commonly accompanied by the emergence of multiple valence states and/or complex crystal fields of transition metals. However, understanding the complex interplay between crystal chemistry, electronic structure, and physical properties at the spin‐ and orbital‐resolved levels in these reduced systems remains elusive. Here, x‐ray absorption spectroscopy, resonant inelastic x‐ray scattering (RIXS), and density functional theory calculations are used to uncover topotactic metal‐insulator transition and orbital‐specific crystal field excitations in brownmillerite La 0.67 Ca 0.33 MnO 2.5 thin films. The Mn valence states are found to be Mn 2+ /Mn 3+ , along with their corresponding populations at octahedral and tetrahedral sites, which effectively weaken the Mn‐O hybridization compared to the parent perovskite phase. As a result, La 0.67 Ca 0.33 MnO 2.5 films exhibit an antiferromagnetic insulating ground state. Moreover, by combining the RIXS measurements on selected single‐valence manganites, specifically MnO, LaMnO 3 , and CaMnO 3 , with orbital‐ and spin‐resolved density‐of‐states calculations, the study identifies the dd excitations of octahedrally and tetrahedrally coordinated Mn 2+ /Mn 3+ ions, directly linking the microscopic electronic structure to the macroscopic magnetic/electrical properties.