关键词: oxygen-annealing at high pressure, La$_{3}$Ni$_{2}$O$_{7}$, high temperature superconductivity, high pressure measurements

Abstract: Oxygen deficiency is known to critically influence the superconductivity of La$_{3}$Ni$_{2}$O$_{7-\delta }$. However, precise control of oxygen content to mitigate such deficiencies remains a significant challenge. In this work, we synthesized high-oxygen-content La$_{3}$Ni$_{2}$O$_{7+\delta } $ polycrystals via high-pressure oxygen annealing with the oxygen stoichiometry ($\delta $) successfully tuned by varying the amount of KClO$_{4}$. The obtained samples La$_{3}$Ni$_{2}$O$_{7.16}$ and La$_{3}$Ni$_{2}$O$_{7.38}$ 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 La$_{3}$Ni$_{2}$O$_{7.16}$, while La$_{3}$Ni$_{2}$O$_{7.38}$ showed noticeable intergrowth with other Ruddlesden-Popper (R-P) phases. Notably, the critical pressure in La$_{3}$Ni$_{2}$O$_{7.16}$ is substantially reduced, though its critical temperature ($T_{\rm c}$) is lower than that of as-grown samples. In contrast, only a weak superconducting signal was detected in higher oxygen content sample La$_{3}$Ni$_{2}$O$_{7.38}$ 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.

Key words: oxygen-annealing at high pressure, La$_{3}$Ni$_{2}$O$_{7}$, high temperature superconductivity, high pressure measurements