From perovskite to infinite-layer nickelates: hole concentration from x-ray absorption

Summary

This study systematically investigated the evolution of PrNiO_x thin films at various intermediate stages of topological reduction (x = 2–3) using soft X-ray absorption spectroscopy. B_y comparing Ni L-edge experimental spectra with single-cluster and double-cluster ligand-field model calculations, it was found that none of the samples exhibited a pure d⁹ electronic configuration. Quantitative analysis based on the charge sum rule revealed that even in the most reduced films, the average number of Ni 3d holes remained 1.35, while superconducting samples displayed higher hole counts, challenging previous assumptions regarding the hole doping limit. Concurrent changes in the O K-edge absorption spectra during reduction indicated the presence of O 2p holes even in the most reduced films. Collectively, these results suggest that a complex hole doping mechanism arises from the interplay between self-doping effects and oxygen non-stoichiometry.

Materials

• PrNiOx
• NdNiO₂
• NiO
• Nd_1-ySr_yNiO₂

Methods

• Soft X-ray absorption spectroscopy (XAS)
• Ligand-field model calculations (single cluster and double cluster)
• Charge sum rule analysis
• soft X-ray absorption spectroscopy (XAS)
• ligand-field calculations (single and double cluster)
• charge sum rule analysis
• topotactic reduction
• molecular beam epitaxy (MBE)
• PLD

Keywords

• hole doping
• self doping
• oxygen non stoichiometry
• charge transfer
• orbital polarization
• hole concentration
• ni 3d hole count
• mott hubbard
• intermediate phases

Highlights

• Our results challenge previous findings regarding the doping range in which superconductivity occurs in infinite-layer nickelates.
• The possibility to observe superconductivity at higher hole doping in nickelates does not necessarily contradict the analogy to cuprates.
• The oxygen K-edge spectra indicate the presence of oxygen 2p holes even in the most reduced films.
• The charge sum rule analysis challenges the nominal doping range of the superconducting dome in infinite-layer nickelates.
• The hole doping mechanism is more complex than simple hole injection, involving self-doping and oxygen non-stoichiometry.

Conclusions

• None of the samples exhibit a pure d9 electronic configuration.
• Even in the most reduced films, the average number of Ni 3d holes remains 1.35, and superconducting samples have even higher values.
• Changes in O K-edge spectra indicate the presence of O 2p holes even in the most reduced films, suggesting a complex interplay of self-doping effects and oxygen non-stoichiometry.
• None of our samples exhibit a pure d9 configuration.
• Even when films are maximally reduced, the averaged number of nickel 3d holes is 1.35.
• Superconducting samples have even higher values, calling into question the previously assumed limit of hole doping.
• The results suggest a complex interplay of hole doping mechanisms resulting from self-doping effects and oxygen non-stoichiometry.

Main claims

• None of the PrNiO_x samples exhibit a pure Ni d9 configuration; even maximally reduced films have an average Ni 3d hole count of 1.35.
  • Evidence: Abstract: 'none of our samples exhibit a pure d9 configuration… even when films are maximally reduced, the averaged number of nickel holes is 1.35'
• Superconducting samples have higher hole counts than previously assumed doping limit, suggesting complex interplay of self-doping and oxygen non-stoichiometry.
  • Evidence: Abstract: 'Superconducting samples have even higher values, calling into question the previously assumed limit of hole doping'
• None of the samples at any intermediate reduction stage exhibit a pure d⁹ configuration; even maximally reduced films have an average of 1.35Ni 3d holes.
  • Evidence: abstract: 'none of our samples exhibit a pure d⁹ configuration' and 'the averaged number of nickel 3d holes is 1.35',full_text Fig. 3: integral values converted to nh ≈ 1.35 for most reduced sample
• Superconducting samples have even higher hole concentrations than the most reduced films, challenging the previously assumed hole-doping limit.
  • Evidence: abstract: 'Superconducting samples have even higher values, calling into question the previously assumed limit of hole doping',full_text: superconducting PLD-2 sample has nh ≈ 1.55, higher than most reduced MBE-2 sample
• Oxygen K-edge spectra show that oxygen 2p holes persist even in the most reduced films, indicating finite hybridization with Ni states.
  • Evidence: abstract: 'Concomitant changes in the oxygen K-edge absorption spectra upon reduction indicate the presence of oxygen 2p holes',full_text Fig. 6 shows spectral weight changes even at highest reduction
• The hole doping mechanism in infinite-layer nickelates is complex, arising from both self-doping effects and oxygen non-stoichiometry.
  • Evidence: abstract: 'complex interplay of hole doping mechanisms resulting from self-doping effects and oxygen non-stoichiometry',full_text: 'three scenarios: cation non-stoichiometry, oxygen off-stoichiometry, and self-doping'

Workflow

• Sample preparation and reduction
  • Materials: PrNiO_x thin films on NdGaO₃ and SrTiO₃ substrates
  • Methods: Molecular beam epitaxy (MBE); Pulsed laser deposition (PLD); Topotactic reduction with CaH₂
  • Observations: Samples show different Ni-Ni distances and crystallinity
• X-ray absorption spectroscopy — Complex hole doping from self-doping and oxygen non-stoichiometry
  • Materials: PrNiO_x films at different reduction stages
  • Methods: Soft X-ray absorption at Ni L-edge and O K-edge; Linear polarization dependence
  • Observations: None of the samples exhibit pure d9 configuration; Even most reduced films have Ni holes ≈1.35
• Model calculations — Deviation from ideal oxygen stoichiometry (O₂.0) explains discrepancies
  • Materials: XAS spectra
  • Methods: Single-cluster and double-cluster ligand-field model calculations; Charge sum rule analysis
  • Observations: Superconducting samples have higher hole counts than previously assumed limit
• sample_preparation — Stepwise reduction enables tracking of electronic structure evolution.
  • Materials: PrNiO_x thin films on NdGaO₃ (110) and SrTiO₃
  • Methods: molecular beam epitaxy (MBE); pulsed laser deposition (PLD); topotactic reduction with CaH₂
  • Observations: Samples span perovskite to infinite-layer phases
• XAS_measurements — XAS reveals changes in electronic configuration.
  • Materials: PrNiO_x films at various reduction stages
  • Methods: Ni L-edge and O K-edge X-ray absorption spectroscopy (XAS); linear polarized X-rays; total electron yield and partial fluorescence yield
  • Observations: Evolution of spectral shape and dichroism with reduction; Shift of Ni L-edge to lower energies
• model_calculations — Calculations quantify hole concentration and orbital polarization.
  • Materials: single and double cluster ligand-field models
  • Methods: Quanty software; charge sum rule analysis
  • Observations: Simulated spectra reproduce experimental features