Visualization of oxygen vacancies and self-doped ligand holes in La₃Ni₂O₇−δ

Summary

The recent discovery of superconductivity in La₃Ni₂O₇−δ under high pressure with a transition temperature around 80 K (ref. 1) has sparked extensive experimental2–6 and theoretical efforts7–12. Several key questions regarding the pairing mechanism remain to be answered, such as the most relevant atomic orbitals and the role of atomic deficiencies. Here we develop a new, energy-filtered, multislice electron ptychography technique, assisted by electron energy-loss spectroscopy, to address these critical issues. Oxygen vacancies are directly visualized and are found to primarily occupy the inner apical sites, which have been proposed to be crucial to superconductivity13,14. We precisely determine the nanoscale stoichiometry and its correlation to the oxygen K-edge spectra, which reveals a significant inhomogeneity in the oxygen content and electronic structure within the sample. The spectroscopic results also reveal that stoichiometric La₃Ni₂O₇ has strong charge-transfer characteristics, with holes that are self-doped from Ni sites into O sites. The ligand holes mainly reside on the inner apical O and the planar O, whereas the density on the outer apical O is negligible. As the concentration of O vacancies increases, ligand holes on both sites are simultaneously annihilated. These observations will assist in further development and understanding of superconducting nickelate materials. Our imaging technique for quantifying atomic deficiencies can also be widely applied in materials science and condensed-matter physics.

Materials

• La₃Ni₂O₇-δ

Methods

• Energy-filtered multislice electron ptychography
• Electron energy-loss spectroscopy (EELS)

Keywords

• oxygen vacancies
• self doped ligand holes
• charge transfer characteristics
• inner apical oxygen

Highlights

• New imaging technique for quantifying atomic deficiencies developed.
• Direct visualization of oxygen vacancies and correlation with electronic structure.

Conclusions

• Oxygen vacancies primarily occupy the inner apical sites.
• Stoichiometric La₃Ni₂O₇ has strong charge-transfer characteristics with self-doped holes from Ni to O sites.
• Ligand holes reside mainly on inner apical O and planar O, and are annihilated with increasing O vacancies.

Main claims

• Oxygen vacancies predominantly occupy the inner apical oxygen sites in La₃Ni₂O₇-δ.
  • Evidence: MEP phase images show weaker phase at inner apical O columns compared to outer apical and planar O (Fig. 2a-c)
• Stoichiometric La₃Ni₂O₇ has strong charge-transfer characteristics with self-doped ligand holes from Ni to O sites.
  • Evidence: O K-edge EELS shows a strong prepeak attributed to ligand holes, which is sensitive to electron doping by vacancies (Fig. 3a)
• Ligand holes mainly occupy inner apical and planar oxygen sites, while outer apical O has negligible density.
  • Evidence: Atomic-resolved EELS shows prepeak only from inner apical and planar O sites (Fig. 4a-c)

Workflow

• sample_preparation — Samples suitable for atomic-scale imaging and spectroscopy.
  • Materials: La₃Ni₂O₇-δ single crystals
  • Methods: High-pressure floating zone method; TEM sample preparation via powder dispersion and FIB
  • Observations: High-quality single crystals with bilayer structure
• energy_filtered_multislice_electron_ptychography — Energy-filtered MEP enables quantitative imaging of oxygen vacancies.
  • Materials: TEM lamella of La₃Ni₂O₇-δ
  • Methods: 4D-STEM with energy slit (5 eV); Multislice ptychographic reconstruction
  • Observations: Three-dimensional atomic potential map obtained; Oxygen atoms clearly visualized with linear phase response
• oxygen_vacancy_quantification — Oxygen vacancies predominantly occupy inner apical sites, with significant inhomogeneity.
  • Materials: MEP phase images from three regions
  • Methods: Gaussian fitting of atomic columns; Phase normalization to outer apical O
  • Observations: Inner apical O shows lower phase in some regions; Estimated δ = 0.04, 0.17, 0.34 for the three regions
• eels_spectroscopy — Stoichiometric La₃Ni₂O₇ has strong charge-transfer characteristics with self-doped ligand holes.
  • Materials: Same three regions
  • Methods: STEM-EELS at O K-edge
  • Observations: Prepeak at ≈528 eV varies with vacancy concentration; Prepeak intensity correlates with δ
• atomic_resolved_eels — Ligand holes mainly reside on inner apical and planar oxygen sites.
  • Materials: Thin sample region
  • Methods: Low-voltage STEM-EELS (80 kV); Atomic-plane resolved spectra
  • Observations: Prepeak mainly from inner apical and planar O, not outer apical O; Prepeak decreases with increasing vacancies