<p>A novel conducting lithium NASICON series Li_1.3+xAl_0.3La_xTi_1.7-x(PO₄)₃ (LALTP), with 0 ≤ <i>x</i> ≤ 0.2, has been prepared using the conventional solid-state reaction route at 900&#xa0;°C. The prepared samples have been characterized with X-ray diffraction (XRD), Raman, Nuclear Magnetic Resonance (NMR), Scanning Electron Microscopy (SEM) and Impedance Spectroscopy (IS). In these compounds, the effect of the partial substitution of Ti⁴⁺ by trivalent La³⁺ and Al³⁺ cations, on structural, electrical, and electrochemical properties of NASICON phases has been investigated. In NASICON phases, structural refinements showed that the maximum amount of trivalent (La + Al)³⁺ cations remains below 0.4/structural formula, being Al³⁺ miscibility against Ti⁴⁺ higher than that of La³⁺. In Al and La-richer samples, secondary AlPO₄ and LaPO₄ phases were detected in XRD patterns and SEM images. The distribution of Al³⁺ and La³⁺ was analyzed by ²⁷Al and ³¹P MAS-NMR spectroscopy. The maximum conductivity ~ 1.9 × 10⁻³ S·cm⁻¹ and the minimum activation energy ~ 0.37&#xa0;eV were obtained in the Li_1.5Al_0.3La_0.2Ti_1.5(PO₄)₃ sample, which is a good candidate for solid electrolytes in all-solid-state batteries (ASSB). The ⁶Li MAS-NMR spectroscopy showed that lithium mobility decreases when the La content increases above 0.1 value. The presence of two “bulk” conductivities was associated with chemical heterogeneity at the external surfaces of Li_1.3Al_0.3Ti_1.7(PO₄)₃ NASICON particles. The formation of small LaPO₄ at the particle surface improved the ceramic microstructure, enhancing “overall” conductivity of ceramic pellets.</p> Graphical Abstract <p></p>