The observation of $\mathrm{high}\text{−}{T}_{\mathrm{c}}$ superconductivity in bilayer nickelate ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under high pressure provides a new venue for exploring novel unconventional superconductors and elucidating the mechanism of $\mathrm{high}\text{−}{T}_{\mathrm{c}}$ superconductivity. Subsequently, numerous chemical substitution studies have been reported, aiming to stabilize superconductivity at ambient pressure or significantly reduce the pressure threshold required for its occurrence. Here, we report the comprehensive study of sodium (Na) doping in the Ruddlesden-Popper nickelate ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_{7+\delta }$, where $\mathrm{N}{\mathrm{a}}^{+}$ substitutes for $\mathrm{L}{\mathrm{a}}^{3+}$ at the A-site with varying doping concentrations $\left(x\right)$. The structural, thermal, magnetic, and electronic transport properties of as-synthesized polycrystalline samples were systematically investigated. X-ray diffraction analysis reveals that the dominant structure has changed from the 327 Amam phase to the 4310 Bmab phase when $x\ge 0.075$, which is further corroborated by thermogravimetric analysis measurements. Substitution of $\mathrm{L}{\mathrm{a}}^{3+}$ with $\mathrm{N}{\mathrm{a}}^{+}$ within the 327 phase gives rise to a gradual expansion of the lattice. Meanwhile, resistivity measurements indicate that the resistivity anomaly associated with a density-wave-like (DW-like) transition is marginally suppressed and metallicity is significantly enhanced. Upon application of pressure, the DW-like transition can be further suppressed, whereas the low-T insulating behavior remains insensitive to pressure. These results offer critical insights into the roles of elemental substitution and charge carrier doping in steering the competing electronic phases in layered nickelates.

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• Doping effects
• Electrical conductivity
• Magnetic susceptibility
• Superconductivity
• Transition temperature
• Nickelates
• Superconductors
• Resistivity measurements
• X-ray diffraction