Topochemical Oxidation of Ruddlesden–Popper Nickelates Reveals Distinct Structural Family: Oxygen-Intercalated Layered Perovskites

摘要

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.

材料

• La₂NiO₄+δ
• La₃Ni₂O₇+δ
• La₄Ni₃O₁₀+δ
• La₅Ni₄O₁₃+δ

方法

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

关键词

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

亮点

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

结论

• 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.

主要论断

• 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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

研究流程

• Thin film synthesis — Epitaxial Ruddlesden-Popper nickelate films with n=1-4 are successfully grown.
  • 材料: Lan+1NinO₃n+1 (n=1-4) on NdGaO₃(110) substrates; also on LaAlO₃(100) and SrTiO₃
  • 方法: ozone-assisted molecular-beam epitaxy (MBE)
  • 观察: as-grown films are Ruddlesden-Popper phase
• Topochemical oxidation — Ozone annealing drives oxygen intercalation into rock salt spacer layers, causing large c-axis expansion.
  • 方法: ozone annealing
  • 观察: 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.
  • 方法: synchrotron surface X-ray diffraction; coherent Bragg rod analysis (COBRA); reciprocal space mapping
  • 观察: 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.
  • 方法: X-ray absorption spectroscopy (XAS); DFT calculations; temperature-dependent resistivity
  • 观察: 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.
  • 方法: aging in nitrogen desiccator; water immersion; air annealing
  • 观察: minor structural degradation after 6 months; stable in water; air annealing restores parent Ruddlesden-Popper phase