Insights into the Fast Sodium Conductor NASICON and the Effects of Mg²⁺ Doping on Na⁺ Conductivity

Na super ionic CONductor (NASICON), Na1+xZr2SixP3–xO12, is an excellent solid-state electrolyte on account of its high sodium-ion conductivity, which can be further enhanced through chemical doping. However, the underlying sodium diffusion mechanism and how it is affected by chemical doping are not fully understood. This, in part, stems from the ambiguity between reported average structure models, where there is variation in the number and location of the sodium crystallographic sites. In this work, we present a neutron scattering study on monoclinic NASICON and Mg2+-doped NASICON, with a focus on the characterization of the sodium sites. Our difference Fourier maps show that the sodium sites have limited long-range order at temperatures as low as 3 K. In addition, our characterization of Mg2+-doped NASICON suggests that although Mg2+ preferentially dopes the secondary Na3PO4 phase rather than the bulk NASICON phase, its NASICON sodium sites are comparatively more diffuse. Furthermore, quasi-elastic neutron scattering data show broader spectra for the Mg-doped NASICON compared to NASICON at 300 K, supporting a more dynamic environment. We propose that the more diffuse character of the sodium sites arising from the introduction of Mg2+ contributes to the improvement of ionic conductivity in Mg-doped NASICON near room temperature.