Rhombohedral NASICON compounds with general formula Li_1+xTi_2-xSc_x(PO₄)₃ (0 ≤ x ≤ 0.5) have been prepared using a conventional solid-state reaction and characterized by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and impedance spectroscopy. The partial substitution of Ti⁴⁺ by Sc³⁺ and Li⁺ in pristine LiTi₂(PO₄)₃ increases unit-cell dimensions and the number of charge carriers. In Sc-rich samples, the analysis of XRD data and⁶Li/⁷Li,³¹P, and⁴⁵Sc MAS NMR spectra confirms the presence of secondary LiScO₂ and LiScP₂O₇ phases that reduce the amount of lithium incorporated in the NASICON phase. In samples with x <0.3, electrostatic repulsions between Li ions located at M1 and M3 sites increase Li mobility. For x ≥ 0.3, ionic conductivity decreases because of secondary nonconducting phases formed at grain boundaries of the NASICON particles (core-shell structures). For x = 0.2, high bulk conductivity (2.5 × 10^-3 S·cm^-1) and low activation energy (E_a = 0.25 eV) measured at room temperature make Li_1.2Ti_1.8Sc_0.2(PO₄)₃ one of the best lithium ionic conductors reported in the literature. In this material, the vacancy arrangement enhances Li conductivity.

The NASICON series, with formula Ba_x/2Li_1-xTi₂(PO₄)₃ (0.4 ≤ x ≤ 1), has been prepared by solid-state reaction and characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman, nuclear magnetic resonance (NMR) and impedance spectroscopy (IS). XRD patterns of samples indicated the formation of single phases with rhombohedral structure (space group R-3c). The Rietveld analysis of XRD patterns was performed to deduce location of Li and Ba ions. FTIR, Raman, and NMR techniques showed the only presence of isolated PO₄ groups in analyzed phosphates. ³¹P MAS-NMR spectra were used to investigate Li and Ba distribution and ⁷Li MAS-NMR spectra to discriminated Li ions with different mobility in conduction paths. A maximum total conductivity of 2.5 × 10⁻⁷ S cm⁻¹ and a minimum activation energy of 0.47 eV were obtained at room temperature for Ba_0.3Li_0.4Ti₂(PO₄)₃ (x = 0.6).