Investigation of the Electronic Structure and Spin-State Crossover in LaCoO3 Using Photoemission Spectroscopy

Photoemission spectroscopy is a powerful technique for studying electronic structure and spin-state transitions, as it reveals changes in the orbital configuration accompanying a spin-state crossover. In this report, we combine excitation-energy-, temperature-, and geometry-dependent photoemission measurements to probe the electronic structure of LaCoO3 across its thermally driven spin-state transition. By systematically comparing valence-band spectra across a wide photon-energy window - from surface-sensitive soft x-ray photoemission spectroscopy (SXPS) to bulk-sensitive hard x-ray photoemission spectroscopy (HAXPES) - we identify the Co 3d-derived feature (A) along with the O 2p-dominated features (B and C), and explain their relative evolution in terms of photon-energy-dependent photo-ionization cross-section ratios. The thermally induced spin-state crossover is demonstrated using temperature-dependent SXPS valence-band spectra, which show a progressive suppression of the feature A with heating. Geometry-dependent HAXPES measurements further clarify how the signature of the spin-state transition in LaCoO3 is intricately linked to the orbital-selective response of the t2g and eg states. Additionally, angular-dependent photo-ionization cross-section analysis provides a consistent description of the polarization dependence observed in HAXPES. Finally, configuration-interaction analysis of the Co 2p core-level spectra reveals that LaCoO3 evolves from a predominantly low-spin ground state at low temperature to a mixed low-spin/high-spin configuration at elevated temperatures, with the high-spin fraction reaching about 30 percent at 400 K. The temperature evolution of the core-level line shape thus establishes Co 2p photoemission as a sensitive quantitative probe of spin-state transitions in LaCoO3.

💡 Research Summary

In this work the authors investigate the thermally driven spin‑state crossover in the perovskite oxide LaCoO₃ by combining soft‑X‑ray photoemission spectroscopy (SXPS) and hard‑X‑ray photoemission spectroscopy (HAXPES). Polycrystalline LaCoO₃ was prepared by a citric‑gel route and confirmed to adopt the rhombohedral R 3̅c structure. SXPS measurements were performed with monochromatic Al Kα radiation (hν = 1486.6 eV) while HAXPES was carried out at the PETRA‑II P22 beamline using 6 keV photons. Both techniques were employed over a wide temperature range (≈160 K to 400 K) and, for HAXPES, in two geometries: parallel (P) and perpendicular (S) polarization relative to the sample surface.

The valence‑band spectra reveal three dominant features: A at ~1 eV, B at ~3 eV and C at ~5 eV binding energy. Feature A is primarily Co 3d derived, whereas B and C stem from O 2p states. By consulting tabulated photo‑ionization cross sections, the authors show that the relative intensity of A versus B/C is consistent with the expected Co 3d/O 2p ratios for both SXPS and HAXPES. However, a striking geometry dependence appears in the HAXPES data: the S‑polarized spectra display an enhanced A relative to the P‑polarized spectra, contrary to the simple cross‑section prediction. This discrepancy is resolved by noting that La 5p states contribute strongly to the HAXPES valence region; S‑polarization suppresses the La 5p matrix element far more than that of Co 3d, effectively amplifying the Co‑derived signal. Thus, polarization‑dependent matrix elements and orbital hybridization must be considered when interpreting bulk‑sensitive spectra.

Temperature‑dependent measurements show a systematic reduction of feature A with increasing temperature in both SXPS and HAXPES. At low temperature LaCoO₃ is a non‑magnetic insulator with Co³⁺ in the low‑spin (t₂g⁶ e_g⁰) configuration, giving a valence band dominated by fully occupied t₂g states. Heating populates higher‑energy spin states (high‑spin t₂g⁴ e_g² or intermediate‑spin t₂g⁵ e_g¹), which depopulates t₂g and fills e_g orbitals. Because the photo‑ionization cross section for t₂g orbitals is larger than for e_g, the net intensity of the Co 3d‑derived peak A diminishes across the crossover. The effect is observed in surface‑sensitive SXPS as well as bulk‑sensitive HAXPES, confirming that the spin‑state transition is an intrinsic bulk phenomenon rather than a surface artifact.

To quantify the spin‑state evolution, the Co 2p core‑level spectra were analyzed using full‑multiplet configuration‑interaction (CI) cluster calculations (Quanty). The model treats a CoO₆ octahedron in O_h symmetry, includes 3d⁶, 3d⁷L and 3d⁸L² configurations, and incorporates on‑site Coulomb interactions (U_dd), 2p‑3d interactions (U_pd), crystal‑field splitting (10 Dq), charge‑transfer energy (Δ), and hybridization parameters (V_eg, V_t2g). By adjusting 10 Dq to simulate different crystal‑field strengths, the authors reproduce the low‑temperature Co 2p spectrum with a pure low‑spin component, while the 400 K spectrum requires a mixture of low‑spin and high‑spin contributions. The best fit yields a high‑spin fraction of ~30 % at 400 K, manifested as additional satellite intensity around 786 eV and an asymmetric main line. The calculated spectra, broadened with realistic Gaussian (instrumental) and Lorentzian (core‑hole lifetime) functions, match the experimental data across the full temperature range.

Overall, the study delivers three key insights: (i) photon‑energy and polarization dependent matrix elements critically shape the observed valence‑band intensities, especially the La 5p contribution in hard‑X‑ray measurements; (ii) the progressive suppression of the Co 3d‑derived valence‑band feature directly tracks the LS→HS (or LS→IS) crossover, reflecting orbital‑selective changes in t₂g/e_g occupancy; and (iii) Co 2p core‑level photoemission, when interpreted with rigorous CI multiplet theory, provides a quantitative probe of the spin‑state population, revealing a mixed LS/HS state with ~30 % HS at 400 K. By integrating surface‑sensitive SXPS, bulk‑sensitive HAXPES, polarization analysis, and advanced theoretical modeling, the work establishes a comprehensive framework for probing spin‑state physics in correlated transition‑metal oxides.