Probing La-based nickelates with Ni 1s core-level photoelectron spectroscopy

Summary

This study investigates the electronic structures of La₃Ni₂O₇, Nd₃Ni₂O₇, and LaNiO₃ by comparing Ni 2p and Ni 1s core-level photoelectron spectra. Owing to the severe overlap of La 3d with Ni 2p levels and the presence of La high-energy satellite peaks, conventional Ni 2p spectra fail to reliably extract the intrinsic signal from La-based nickelates. Using hard X-ray photoelectron spectroscopy to probe the deeper Ni 1s core level, which is free of spin–orbit coupling and has negligible multiplet interactions, provides a clean perspective on charge-transfer excitations. The results show that the Ni 1s spectra can clearly distinguish the perovskite LaNiO₃ from the bilayer Ruddlesden–Popper phases and reveal that compared to Nd₃Ni₂O₇, La₃Ni₂O₇ exhibits a broadened main peak with reduced intensity and an enhanced satellite peak. Combined with DFT+DMFT calculations, these spectral changes are attributed to alterations in the charge-transfer energy and hybridization strength, where the tensile strain in La₃Ni₂O₇ weakens the Ni–ligand hybridization. This approach demonstrates the sensitivity of Ni 1s core-level spectroscopy to subtle electronic-structure variations and offers an effective means for systematically characterizing nickelates with different strains, doping levels, or layer numbers.

Materials

• La₃Ni₂O₇
• Nd₃Ni₂O₇
• LaNiO₃
• La₂CuO₄

Methods

• hard X-ray photoelectron spectroscopy (HAXPES)
• Ni 1s core-level spectroscopy
• Ni 2p core-level spectroscopy
• density functional theory plus dynamical mean-field theory (DFT+DMFT)

Keywords

• charge transfer excitations
• hybridization strength
• la 3d overlap
• spin orbit coupling free
• epitaxial strain
• ruddlesden popper nickelates
• satellite peaks

Highlights

• Ni 1s core-level spectroscopy provides a clean probe free of interference from La 3d levels, enabling reliable comparison of nickelate electronic structures.
• The technique reveals that tensile strain in La₃Ni₂O₇ weakens Ni–ligand hybridization, altering the charge-transfer satellite intensity relative to the compressive-strained Nd₃Ni₂O₇.
• This approach offers a systematic way to characterize nickelates with different strains, doping levels, or layer numbers in the Ruddlesden-Popper series.

Conclusions

• Reliable extraction of the intrinsic Ni 2p line shape is not feasible for La-based nickelates due to overlap with La 3d core levels and the presence of La-derived satellites.
• Ni 1s core-level photoelectron spectroscopy is a suitable alternative, free of spin-orbit coupling and multiplet interactions, and can resolve subtle differences in electronic structure among nickelates.
• Spectral differences between La₃Ni₂O₇ and Nd₃Ni₂O₇, including a broadened main peak and enhanced satellite, are attributed to changes in charge-transfer energy and hybridization strength due to tensile strain.
• DFT+DMFT calculations confirm that reduced hybridization broadens the main line and enhances the satellite, consistent with experimental observations.

Main claims

• Ni 2p core-level photoemission spectra of La-based nickelates are severely compromised by overlap with La 3d core levels and intrinsic high-energy La satellites, preventing reliable analysis.
  • Evidence: Figure 1 shows Ni 2p and La 3d spectra of La₃Ni₂O₇ and LaNiO₃ with overlapping features; La 3d of Ni-free La₂CuO₄ reveals high-energy satellites in the same energy range as Ni 2p.
• Deep Ni 1s core-level spectroscopy provides a clean, overlap-free view of charge-transfer excitations, allowing clear distinction between different nickelates (e.g., perovskite vs. bilayer, La₃Ni₂O₇ vs. Nd₃Ni₂O₇).
  • Evidence: Figure 2 and Figure 3 show Ni 1s spectra with clearly resolved main and satellite peaks, free from La interference; LaNiO₃ displays a markedly different line shape from the bilayer compounds.
• The broader main peak and enhanced satellite in La₃Ni₂O₇ relative to Nd₃Ni₂O₇ result from weakened Ni–ligand hybridization due to tensile strain, as confirmed by DFT+DMFT simulations that reproduce the spectral changes when hybridization is reduced.
  • Evidence: Figure 4 shows DFT+DMFT simulations where reduced hybridization yields a broader main peak and enhanced satellite, matching the experimental La₃Ni₂O₇ spectrum.

Workflow

• Synthesis of nickelate samples — High-quality samples of La₃Ni₂O₇, Nd₃Ni₂O₇, LaNiO₃, and reference La₂CuO₄ were synthesized for comparative core-level spectroscopy.
  • Materials: La₃Ni₂O₇ thin film; Nd₃Ni₂O₇ thin film; LaNiO₃ single crystal; La₂CuO₄ single crystal (reference); LaAlO₃ substrate; NdGaO₃ substrate
  • Methods: Reactive ozone-assisted molecular beam epitaxy (thin films); Floating zone technique under 125 bar O₂ (LaNiO₃); Floating-solvent traveling-zone method (La₂CuO₄)
  • Observations: Thin films and single crystals produced
• HAXPES measurements — HAXPES provided bulk-sensitive Ni 1s and Ni 2p spectra of the nickelates.
  • Materials: Synthesized nickelate samples; HAXPES endstation at BL₁₂XU (SPring-8); MB Scientific A-1 HE analyzer
  • Methods: Hard x-ray photoelectron spectroscopy (HAXPES) at 8 keV and 10 keV photon energies
  • Observations: Ni 1s, Ni 2p, and La 3d core-level spectra recorded for all samples
• Comparative spectral analysis — Conventional Ni 2p spectroscopy is unreliable for La-based nickelates; Ni 1s provides an overlap-free, clean probe of intrinsic electronic excitations.
  • Materials: Ni 2p and Ni 1s spectra of Nd₃Ni₂O₇, La₃Ni₂O₇, LaNiO₃; La 3d spectra of La₂CuO₄ and La-based nickelates
  • Methods: Line-shape comparison; Examination of La 3d features using Ni-free La₂CuO₄ reference; Assessment of spectral overlaps and satellites
  • Observations: Ni 2p strongly overlaps with La 3d; La 3d shows high-energy satellites extending into Ni 2p region; La 3d core-level shapes differ between materials, preventing simple subtraction; Ni 1s spectra are free from La overlap, show a clear main peak and charge-transfer satellite; Ni 1s clearly distinguishes perovskite LaNiO₃ from bilayer Ruddlesden–Popper phases; La₃Ni₂O₇ shows a broader main peak and enhanced satellite compared to Nd₃Ni₂O₇
• DFT+DMFT simulation and interpretation — Tensile strain in La₃Ni₂O₇ weakens Ni–ligand hybridization, causing the observed broader main peak and stronger satellite; Ni 1s is sensitive to such electronic structure changes.
  • Materials: DFT+DMFT electronic structure of Nd₃Ni₂O₇; Artificial hybridization densities
  • Methods: DFT+DMFT calculations of core-level spectra; Systematic variation of charge-transfer energy Δ and hybridization strength
  • Observations: Reducing hybridization broadens the main peak and enhances the satellite; Reducing Δ narrows the main peak; Simulated trend for reduced hybridization matches experimental La₃Ni₂O₇ spectra