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, we combine x-ray absorption spectroscopy, resonant inelastic x-ray scattering (RIXS), and density functional theory calculations to uncover topotactic metal-insulator transition and orbital-specific crystal field excitations in brownmillerite La0.67Ca0.33MnO2.5 thin films. We reveal the Mn valence states to be predominantly Mn2+/Mn3+, 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, La0.67Ca0.33MnO2.5 films exhibit an antiferromagnetic insulating ground state. Moreover, by combining the RIXS measurements on selected single-valence manganites, specifically MnO, LaMnO3, and CaMnO3, with orbital- and spin-resolved density-of-states calculations, we identify the dd excitations of octahedrally and tetrahedrally coordinated Mn2+/Mn3+ ions, directly linking the microscopic electronic structure to the macroscopic magnetic/electrical properties.