The filamentary nature of high-pressure superconductivity in La₃Ni₂O₇ remained an open question [1-3] with the absence of a bulk Meissner effect largely attributed to microscale stoichiometric inhomogeneity [4], and nanoscale defects, such as oxygen vacancies [5] and stacking faults [6]. Motivated by the successful sol-gel synthesis of homogeneous La₂PrNi₂O₇ which displayed bulk Meissner effect [7], we systematically explored the influence of the synthesis methods on the formation of bilayer nickelates. Using homogenous starting precursors prepared by an improved sol-gel method, we managed to obtain single-phase La₃Ni₂O₇ and its isovalently doped derivatives. Similar to its isovalently doped counterparts -where smaller A-site ions are thought to promote the formation of the bilayer structure [7]-the non-doped La₃Ni₂O₇ exhibits comparable level of crystallinity along the 00l direction as confirmed by the XRD in Debye-Scherrer geometry. Physical properties characterization reveals that this crystallinity-improved La₃Ni₂O₇ shows an enhanced conductivity and a metal-insulator transition at 150K, distinct from most previous reports. Furthermore, magnetic susceptibility measurements confirm the absence of low-temperature spin-glass behavior, suggesting reduced magnetic inhomogeneities. Finally, we demonstrate that high-pressure oxygen post-annealing of La₃Ni₂O₇ yields a pure oxygen-rich phase in the Fmmm space group. This phase displays Fermi-liquid behavior, exhibiting physical property evolutions analogous to those observed in the over-hole-doped region of cuprates.

[1] H. Sun et al., Nature 621, 493 (2023).

[2] Y. Zhou et al., Matter and Radiation at Extremes 10 (2025).

[3] Y. Wang, K. Jiang, J. Ying, T. Wu, J. Cheng, J. Hu, and X. Chen, National Science Review 12, nwaf373 (2025).

[4] S. Mandyam et al., Nature 651, 54 (2026).

[5] Z. Dong et al., Nature 630, 847 (2024).

[6] P. Puphal et al., Physical Review Letters 133, 146002 (2024).

[7] N. Wang et al., Nature 634, 579 (2024).