Abstract <p>Li_1+<i>x</i>Zr_2–<i>x</i>La_<i>x</i> (PO₄)₃ (where <i>x</i> = 0.05, 0.10, 0.15) solid electrolytes with a NASICON structure are synthesized using the sol-gel technique. The impact of replacing Zr⁴⁺ with La³⁺ on the morphology, ionic conductivity and structure of the parent compound LiZr₂(PO₄)₃ (LZP) is examined. X-ray diffraction data of the prepared powder indicates that substituting La³⁺ stabilizes LZP in the high conductive rhombohedral R3^– c phase at normal room temperature. The introduction of lanthanum into the LZP resulted in a noticeable shift of high-intensity bands towards lower wavenumbers, along with an enhancement in the peak intensities at 472 and 490 cm^–1 in the Raman spectra. A significant increase in grain size and a reduction in porosity were observed, leading to a notable enhancement in relative density when La³⁺ was substituted for Zr⁴⁺ were noticed by SEM analysis. The La³⁺ substituted LZP reveals improved ionic conductivities, achieving a higher value of 0.52 × 10^–4 S/cm at normal room temperature for the composition LiZrLa(PO₄)₃ (<i>x</i> = 0.15). Further, measured at low operating temperatures exhibited a hike in its ionic conductivity of 0.54 × 10^–3 S/cm at 350°C. The enhancement in conductivity of LZP with the trivalent ion La³⁺, which has a similar ionic radius to Zr⁴⁺ (0.72 Ǻ), is also discussed. Moreover, the activation energy of about 0.4 eV for the La³⁺ substituted LZP with <i>x</i> = 0.15 which confirms the excellent ionic conducting properties of this solid electrolyte. Notably, the NASICON type LZP with La³⁺ substitution (<i>x</i> = 0.15) shows commendable chemical and structural stability after being exposed to water, air, Li metal, and both acidic and basic solutions.</p>