Oxygen deficiency mechanism of La₃Ni₂O₇−δ under pressure

摘要

The recent discovery of superconductivity in pressurized bilayer nickelate La₃Ni₂O₇ has triggered tremendous research interest. However, the experimentally observed oxygen deficiency implies that obtaining perfect stoichiometric single crystals is still challenging. The influence of oxygen deficiency on physical properties remains unexplained. Here, we construct a chemical potential phase diagram to characterize the stability of La₃Ni₂O₇. The narrow stable region explains the difficulty of synthesizing pure samples. First, oxygen defect studies reveal that the interlayer apical oxygen vacancy has the highest defect concentrations and is responsible for oxygen deficiency. Second, unfolding band structures show as the oxygen-deficient variant increases, Ni 3d_;z2bands shift toward a lower energy position under the Fermi level at Γ point, which is adverse to the metallization of Ni 3d_;z2bands. Third, high-pressure calculations indicate that oxygen vacancy would destroy the hybridization of interlayer Ni 3d_;z2orbitals, and the larger the oxygen deficiency, the higher the pressure needed to metalize the Ni 3d_;z2bands. Thus, the oxygen deficiency would suppress the emergence of superconductivity in La₃Ni₂O₇−δ. Our results elucidate the mechanism of oxygen deficiency for superconductivity in La₃Ni₂O₇−δ and provide instructive guidance to the experimental research.

材料

• La₃Ni₂O₇-δ

方法

• chemical potential phase diagram
• band structure unfolding
• high-pressure calculations

关键词

• oxygen deficiency
• apical oxygen vacancy
• ni 3dz2 bands
• interlayer hybridization
• suppression of superconductivity

亮点

• Explains difficulty in synthesizing pure stoichiometric samples due to narrow stable region.
• Provides instructive guidance for experimental research on oxygen deficiency.

结论

• Oxygen deficiency, particularly interlayer apical oxygen vacancies, in La₃Ni₂O₇−δ shifts Ni 3dz2 bands to lower energy and destroys interlayer hybridization, suppressing superconductivity; larger deficiency requires higher pressure to metallize the Ni 3dz2 bands.

主要论断

• The stable growth region for stoichiometric La₃Ni₂O₇ is extremely narrow, making synthesis of pure crystals challenging.
  • 证据: Figure 1: chemical potential phase diagram shows a small stable region bounded byLa₂O₃, NiO, Ni₃O₄,The region spans only ≈0.3 eV in μLa and ≈0.23 eV in μNi
• Interlayer apical oxygen vacancy (VO₁) has the highest defect concentration among all oxygen vacancies and is the primary source of oxygen deficiency.
  • 证据: Figure 2(b): VO₁ has lowest formation energy under all chemical potential conditions,Table 1: concentration of VO₁ is orders of magnitude higher than others (e.g., 2.86×1019 vs 7.11×107 cm-3 under 7 Pa, 600°C),Figure 2(c): difference >106
• Oxygen vacancies, particularly VO₁, disrupt the interlayer Ni dz2 orbital hybridization and shift the σ-band away from the Fermi level, hindering metallization.
  • 证据: Figure 4(a): for δ=0.5, VO₁ pushes Ni and O bands away from EF at Γ point,Figure 5(a-d): as δ increases from 0.071 to 0.25, Ni dz2 bands shift to lower energy and become flatter at Γ,Figure 5(e): electron density redistributes from oxygen vacancy to neighboring Ni atoms
• Larger oxygen deficiency requires higher applied pressure to restore Ni dz2 band metallization, and for δ≥0.25, superconductivity may be suppressed even under high pressure.
  • 证据: Figure S₅: at 30 GPa, La₃Ni₂O₆.929 and La₃Ni₂O₆.9 show Ni dz2 crossing EF; La₃Ni₂O₆.875 does not; at 50 GPa, La₃Ni₂O₆.875 crosses but La₃Ni₂O₆.75 does not,Consistent with experimental reports that superconductivity vanishes for δ > ≈0.21

研究流程

• Chemical potential phase diagram construction — The narrow stable region explains the experimental difficulty of growing perfect stoichiometric La₃Ni₂O₇ single crystals.
  • 材料: La₃Ni₂O₇; impurity phases: La₂O₃, NiO, Ni₃O₄, La₂NiO₄, La₄Ni₃O₁₀
  • 方法: DFT calculations with GGA+U (U=4 eV); formation enthalpy from Materials Project
  • 观察: stable growth region is very narrow, bounded mainly byLa₂O₃ and NiO; La₄Ni₃O₁₀ is the most active impurity
• Oxygen vacancy formation energy and concentration calculations — The interlayer apical oxygen vacancy (VO₁) is the dominant oxygen defect and the main source of oxygen deficiency in La₃Ni₂O₇−δ.
  • 材料: La₃Ni₂O₇ supercells (192 atoms, 432 atoms for convergence check)
  • 方法: DFT+U; formation energy formula; defect concentration from Boltzmann statistics
  • 观察: interlayer apical oxygen vacancy (VO₁) has the lowest formation energy under all conditions; VO₁ concentration is orders of magnitude higher than other vacancies; realistic growth conditions give much higher vacancy concentrations than O-rich assumption
• Electronic structure analysis — Oxygen vacancies, especially interlayer apical vacancies, disrupt the hybridization of interlayer Ni dz2 orbitals and suppress metallization of the σ-band, which is detrimental to superconductivity.
  • 材料: supercells with various oxygen-deficient variants δ (0.071, 0.1, 0.125, 0.25, 0.5)
  • 方法: band-unfolding technique; projected density of states
  • 观察: VO₁ pushes Ni dz2 bands away from Fermi level at Γ point; band flattening with increasing δ; electron density redistribution from oxygen vacancy to nearest Ni atoms; structural distortion (bond distances and angles) around vacancy
• Pressure-dependent analysis — The larger the oxygen deficiency, the higher the pressure needed to restore Ni dz2 band metallization; extreme oxygen deficiency can suppress superconductivity entirely.
  • 方法: DFT+U with applied pressure
  • 观察: larger oxygen deficiency requires higher pressure to metalize Ni dz2 bands; for δ=0.125, 30 GPa insufficient; need 50 GPa; for δ=0.25, even 50 GPa not enough