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.

• The paper demonstrates that topotactic reduction of LCMO3 produces brownmillerite LCMO2.5 thin films with ordered oxygen vacancies, triggering a metal-insulator transition and antiferromagnetism.
• High-resolution XAS, RIXS, and DFT techniques are used to elucidate site-specific Mn valence states and distinct dd excitations indicative of electronic localization.
• The findings establish a quantitative framework for tailoring magneto-electronic functionalities in complex oxides via precise oxygen vacancy engineering.

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

Introduction

The paper "Topotactic Reduction-Driven Crystal Field Excitations in Brownmillerite Manganite Thin Films" (2503.10373) establishes a comprehensive electronic, structural, and spectroscopic characterization of La$_{0.67}$Ca$_{0.33}$MnO$_{2.5}$ (LCMO2.5) thin films derived from perovskite La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO3) through topotactic reduction. Distinct from conventional perovskites, brownmillerite LCMO2.5 incorporates periodically ordered oxygen vacancies, manifesting as alternately stacked MnO$_6$ octahedral and MnO$_4$ tetrahedral layers. The authors employ high-resolution x-ray absorption spectroscopy (XAS), resonant inelastic x-ray scattering (RIXS), and density functional theory (DFT) to correlate the microstructural evolution with the emergence of electronic localization, valence-state redistribution, and corresponding functional properties.

Structural and Phase Transformation

LCMO2.5 crystallizes in the orthorhombic $Pbnm$ space group, evidenced by XRD, reciprocal space mapping, and atomic-resolution HAADF-STEM. The transformation from LCMO3 to LCMO2.5 yields a doubling of $c$ due to the alternation of MnO$_6$ and MnO$_4$ layers. The maintenance of coherent epitaxy on NdGaO$_3$(001) enables precise mapping of induced strain, which is critical for stabilizing the brownmillerite phase under significant compressive and tensile distortions. The ordered oxygen vacancy superstructure induces volumetric expansion, modifies local atomic arrangements, and produces alternating Mn–O bond environments requisite for local symmetry breaking and electronic redistribution.

Electronic Structure: Ground State Evolution

Topotactic reduction induces a metal-insulator transition (MIT). While LCMO3 displays half-metallic ferromagnetism facilitated by double exchange between Mn$^{3+}$/Mn$^{4+}$ ions, LCMO2.5 reveals insulating and antiferromagnetic (AFM) behavior. DFT calculations confirm a G-type AFM state with a calculated bandgap $\sim1.25$ eV, in agreement with the suppression of electronic transport. The effective mass anisotropy, substantiated by calculated band dispersions, directly translates to transport anisotropy.

Site-Selective Valence States

Using XAS at both Mn-$L_{2,3}$ and O-$K$ edges, the Mn valence distribution is elucidated. Mn$^{2+}$ is localized at all tetrahedral sites and one-third of octahedral sites, while Mn$^{3+}$ is confined to the remaining octahedral sites. The presence of Mn$^{2+}$ is directly corroborated by the 639.5 eV feature in XAS, and diminished Mn–O hybridization is inferred from the suppression and shift of the O-$K$ pre-edge. This redistribution attenuates the eg–O 2p overlap, enhancing electron localization and suppressing itinerant ferromagnetism.

Geometrically and chemically, charge balance requires a $2:1$ population of Mn$^{3+}$:Mn$^{2+}$ among the octahedral sites, as supported by the absence of spectroscopic signatures of Mn$^{4+}$. The configuration excludes the possibility of breathing-mode–induced Mn$^{2+}$/Mn$^{4+}$ coexistence characteristic of half-doped compounds.

Orbital- and Site-Resolved dd Excitations

High-resolution Mn-$L_{3}$ RIXS, benchmarked against single-valence standards (MnO, LaMnO$_3$, CaMnO$_3$), permits deconvolution of collective dd excitations from localized Mn$^{2+}$ and Mn$^{3+}$ multiplets within distinct local geometries. The broad multiplet at 1–2 eV in LCMO2.5 is attributed to Mn$^{3+}$ (octahedral) dd excitations, as in LaMnO$_3$. The sharper features above 3 eV correspond to Mn$^{2+}$ (octahedral and tetrahedral) dd transitions, consistent with MnO. The fine structure above 4 eV is unique to the mixed-coordination brownmillerite, indicating multiplet contributions from both octahedral and tetrahedral Mn$^{2+}$. Band-resolved DFT reproduces the unoccupied density of states and aligns with the experimental dd excitation energy hierarchy, confirming the assignments.

Notably, the absence of Mn$^{4+}$ spectral features (as observed in CaMnO$_3$) in LCMO2.5 excludes the presence of significant Mn$^{4+}$ content post-reduction. The dominant energy scales extracted—Jahn-Teller splitting, intra-atomic Hund's coupling, and Mott-Hubbard $U$—define the energy window of observed excitations and the strong localization regime.

Implications and Prospects

This work demonstrates that topotactic reduction enables access to electronic and structural ground states inaccessible in bulk or oxygen-stoichiometric perovskites. The ability to spatially and chemically engineer oxygen content at unit-cell precision affords independent tuning of crystal field environments and valence state populations, strongly influencing electronic (e.g., MIT) and magnetic (e.g., AFM) responses.

The protocol for site- and valence-specific dd excitation identification via combined RIXS and DFT is broadly extendable to complex oxides with mixed site occupancy, disordered or ordered oxygen vacancies, and correlated electron phenomena. This is especially relevant for the rational design of functional materials for spintronics, solid-state ionic devices, and systems wherein controllable electronic and magnetic anisotropies are required.

The elucidated mechanisms—valence reduction via topotactic oxygen extraction, crystal field symmetry breaking, electronic bandwidth tuning, and the suppression of Mn–O hybridization—deliver a robust platform for tailoring material behavior across a wide parameter space. The demonstrated approach can be systematically generalized to other transition metal oxide families (nickelates, cobaltates, ferrites), enabling modular engineering of heterovalent and coordinatively diverse quantum oxides.

Conclusion

The authors present a rigorous paradigm for decoding the interplay of structure, valence, and electronic excitations in topotactically reduced brownmillerite manganite thin films. By integrating element- and site-selective RIXS measurements with state-of-the-art DFT, they establish a quantitative, multiparametric framework for understanding the emergent phenomena in reduced perovskite derivatives. The study offers pathways for the controlled design of correlated oxides with tailored magneto-electronic functionalities, with significant implications for next-generation quantum materials systems.