Dual roles of oxygen-annealing in modulating the superconductivity of La₃Ni₂O_7+δ

Oxygen deficiency is known to critically influence the superconductivity of La₃Ni₂O_7–δ. However, precise control of oxygen content to mitigate such deficiencies remains a significant challenge. In this work, we synthesized high-oxygen-content La₃Ni₂O_7+δ polycrystals via high-pressure oxygen annealing with the oxygen stoichiometry (δ) successfully tuned by varying the amount of KClO₄. The obtained samples La₃Ni₂O_7.16 and La₃Ni₂O_7.38 exhibit metallic behavior at ambient pressure. We further conducted a comprehensive investigation into the pressure-induced superconductivity and atomic structure. STEM imaging revealed large-area bilayer-phase stacking in La₃Ni₂O_7.16, while La₃Ni₂O_7.38 showed noticeable intergrowth with other Ruddlesden–Popper (R–P) phases. Notably, the critical pressure in La₃Ni₂O_7.16 is substantially reduced, though its critical temperature (T_c) is lower than that of as-grown samples. In contrast, only a weak superconducting signal was detected in higher oxygen content sample La₃Ni₂O_7.38 likely due to the intergrowth with other R–P phases, non-negligible interstitial oxygen, or an increased fraction of the tetragonal phase. Our findings provide a viable pathway for optimizing nickelate superconductivity and offer insights into the fundamental mechanisms governing superconductivity in these materials.