Regulating oxygen content and superconductivity in La₃Ni₂O₇+δ

Summary

B_y systematically tuning the oxygen content of La₃Ni₂O₇₊δ samples, this study synthesized materials with varying phase compositions, including pure bilayer phase, a mixed phase of bilayer and monolayer–bilayer hybrid, and a predominant bilayer phase containing trilayer intergrowths. High-pressure transport measurements revealed that these phases correspond to distinct superconducting transition temperatures (T_c), with the bilayer phase exhibiting superconductivity at approximately 80 K, while the hybrid and trilayer-intergrowth phases show lower T_c values. Oxygen content not only influences phase purity but also directly modulates the upper critical field (H_c2) of the bilayer superconductivity, with the pure bilayer phase displaying a higher H_c2. B_y constructing a phase diagram of T_c and H_c2 as functions of oxygen content, this study achieves precise control over oxygen stoichiometry in Ruddlesden–Popper nickelates, providing critical experimental insights for understanding the high-pressure superconducting mechanism.

Materials

• La₃Ni₂O₇+δ

Methods

• high-pressure transport
• X-ray diffraction
• compositional analysis

Keywords

• oxygen content
• phase purity
• upper critical field
• intergrowth phases
• bilayer phase

Highlights

• Establishes a phase diagram of T_c and Hc₂ as functions of oxygen content in La₃Ni₂O₇+δ.

Conclusions

• Oxygen content governs phase purity, controlling the presence of hybrid phases and trilayer intergrowths.
• Superconducting transitions with contrasting T_c values correspond to different phase fractions.
• The upper critical field (Hc₂) of the bilayer phase is directly modulated by oxygen content.

Main claims

• Oxygen content in La₃Ni₂O₇₊δ controls the formation of intergrowth phases: low oxygen promotes hybrid-1212, high oxygen promotes trilayer inclusions.
  • Evidence: NPD and STEM show hybrid-1212 phase in oxygen-deficient sample (δ=-0.14); trilayer intergrowths in oxygen-rich samples (δ>0)
• Different RP phases show distinct superconducting transition temperatures: bilayer ≈80 K, hybrid-1212 ≈70 K, trilayer ≈4-6 K.
  • Evidence: High-pressure transport at 25.5 GPa shows multiple transitions with these T_c values
• The upper critical field H_c2 of the bilayer superconductivity is strongly modulated by oxygen content, peaking near stoichiometric composition.
  • Evidence: Ginzburg-Landau fitting of transport under field shows that H_c2 increases with oxygen content up to δ=0, then decreases for higher δ

Workflow

• sample_synthesis — Systematic control of oxygen content in La₃Ni₂O₇₊δ.
  • Materials: La₂O₃; NiO; Ni(NO₃)₂·6H₂O
  • Methods: solid-state reaction; sol-gel method; controlled annealing in H₂/N₂, O₂, and high-pressure O₂
  • Observations: six samples with oxygen content δ from -0.34 to 0.08
• structural_and_compositional_analysis — Oxygen content governs NiO₆ tilting and formation of intergrowth phases.
  • Materials: polycrystalline La₃Ni₂O₇₊δ
  • Methods: XAFS; NPD; SXRD; STEM; TGA
  • Observations: oxygen content correlates with Ni valence and octahedral distortion; intergrowth phases identified
• high_pressure_electronic_transport — Different RP phases exhibit distinct T_c values under pressure.
  • Materials: polycrystalline samples in DAC
  • Methods: four-probe resistivity measurement under pressure with KBr pressure medium
  • Observations: superconducting transitions with onset T_c ≈80 K for bilayer, ≈70 K for hybrid-1212, ≈5 K for trilayer
• upper_critical_field_analysis — Oxygen content directly modulates H_c2 of bilayer superconductivity.
  • Materials: transport data at 25.5 GPa
  • Methods: Ginzburg-Landau fitting
  • Observations: H_c2 is modulated by oxygen content, peaking near stoichiometric composition