Source zotero
Authors Not available in this batch.
arXiv / Publisher https://pubs.acs.org/doi/full/10.1021/jacs.5c12712
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Published 2026-02-05
Research paradigm Experimental
Sample form Thin Film

Summary

Layered perovskites─including the Dion–Jacobson, Ruddlesden–Popper, and Aurivillius families─exhibit a wide range of correlated electron phenomena, from high-temperature superconductivity to multiferroicity. Here, we report a new family of layered perovskites realized through topochemical oxidation of Lan+1NinO3n+1+δ (n = 1–4) Ruddlesden–Popper nickelate thin films. Postgrowth ozone annealing induces a substantial c-axis expansion─17.8% for La2NiO4+δ (n = 1)─that monotonically decreases with increasing n. Surface synchrotron X-ray diffraction and coherent Bragg rod analysis (COBRA) reveal that this structural expansion arises from the intercalation of approximately δ ≈ 0.7–1.0 oxygen atoms into interstitial sites within the rock salt spacer layers, far exceeding the previous record of δ ≈ 0.3 for any Ruddlesden–Popper oxide. These oxygen-intercalated phases form a new class of layered perovskites with a spacer layer composition intermediate between the Ruddlesden–Popper and Aurivillius phases. Furthermore, oxygen intercalation induces metallicity, enhances nickel–oxygen hybridization, and suppresses oxygen octahedral rotations, a feature associated with high-temperature superconductivity in Ruddlesden–Popper nickelates. Our work establishes topochemical oxidation as a powerful approach to accessing highly oxidized, metastable phases across a broad range of layered oxide systems, offering new platforms to engineer electronic properties via intercalation chemistry.

Materials

Methods

  • topochemical oxidation (ozone annealing)
  • surface synchrotron X-ray diffraction
  • coherent Bragg rod analysis (COBRA)

Keywords

  • oxygen intercalation
  • metallicity
  • nickel oxygen hybridization
  • suppression of oxygen octahedral rotations
  • new class of layered perovskites

Highlights

  • Oxygen intercalation amount far exceeds previous record (δ≈0.3) for Ruddlesden-Popper oxides.
  • Provides new platform to engineer electronic properties via intercalation chemistry.

Conclusions

  • Topochemical oxidation of Ruddlesden-Popper nickelate thin films leads to oxygen intercalation (δ≈0.7-1.0) in rock salt layers, creating a new structural family of layered perovskites with enhanced metallicity and Ni-O hybridization, and suppressed octahedral rotations.

Main claims

  • Ozone annealing of Ruddlesden-Popper nickelate thin films induces a pronounced c-axis expansion (17.8% for n=1) due to intercalation of ≈0.7-1.0 oxygen atoms per formula unit into rock salt spacer layers.
    • Evidence: Figure 2: clear leftward peak shifts in (0,0,L) CTRs after oxidation,Figure 3: COBRA electron density profiles show additional electron density in spacer layers,Table S1: lattice constants increase
  • The oxygen-intercalated phases form a new structural family intermediate between Ruddlesden-Popper and Aurivillius, with spacer layer composition A2Oδ (δ ≈ 0.7-1.0) and ≈35-50% interstitial site occupancy.
    • Evidence: Figure 4: comparison of spacer layer structures and compositions,Figures S12, S13: integrated electron density gives δ values
  • Oxygen intercalation induces metallicity in otherwise insulating or semiconducting Ruddlesden-Popper nickelates, enhances Ni-O hybridization, and suppresses oxygen octahedral rotations.
    • Evidence: Figure 5(d-g): resistivity drops and becomes metallic after oxidation,Figure 5(b): O-K prepeak increases and shifts to lower energy (enhanced hybridization),Figure S18: half-order CTR peaks vanish for n=2, suppressed for n=3
  • The oxidation is reversible via air annealing, and the oxygen-intercalated phases are stable under ambient conditions for months.
    • Evidence: Figure S11: air annealing restores original Ruddlesden-Popper structure,Figure S9: only minor degradation after 6 months in nitrogen desiccator,Figure S10: stable after 18 h in distilled water

Workflow

  • Thin film synthesis — Epitaxial Ruddlesden-Popper nickelate films with n=1-4 are successfully grown.
    • Materials: Lan+1NinO3n+1 (n=1-4) on NdGaO3(110) substrates; also on LaAlO3(100) and SrTiO3
    • Methods: ozone-assisted molecular-beam epitaxy (MBE)
    • Observations: as-grown films are Ruddlesden-Popper phase
  • Topochemical oxidation — Ozone annealing drives oxygen intercalation into rock salt spacer layers, causing large c-axis expansion.
    • Methods: ozone annealing
    • Observations: c-axis expansion: 17.8% for n=1, decreasing with n; absolute expansion ≈2.1 Å for all n
  • Structural characterization — Intercalated oxygen occupies interstitial sites in rock salt layers, yielding δ ≈0.7-1.0, and suppresses oxygen octahedral rotations.
    • Methods: synchrotron surface X-ray diffraction; coherent Bragg rod analysis (COBRA); reciprocal space mapping
    • Observations: electron density appears in spacer layers (interstitial oxygen); interstitial occupancy 35-50%; half-order peaks vanish for n=2, suppressed for n=3
  • Electronic structure and transport characterization — Oxygen intercalation induces hole doping, enhances Ni-O hybridization, and drives semiconductor-to-metal transitions.
    • Methods: X-ray absorption spectroscopy (XAS); DFT calculations; temperature-dependent resistivity
    • Observations: O-K prepeak increases and shifts to lower energy; Ni-L2 edge shifts to higher energy (beyond Ni3+ reference); metallization in n=1,2; CDW suppression in n=3
  • Stability and reversibility tests — Oxygen-intercalated nickelates are stable and the oxidation process is reversible.
    • Methods: aging in nitrogen desiccator; water immersion; air annealing
    • Observations: minor structural degradation after 6 months; stable in water; air annealing restores parent Ruddlesden-Popper phase