Insights into the Fast Sodium Conductor NASICON and\nthe Effects of Mg<sup>2+</sup> Doping on Na<sup>+</sup> Conductivity

Na\nsuper ionic CONductor (NASICON), Na_1+<i>x</i>Zr₂Si_<i>x</i>P_3–<i>x</i>O₁₂, is an excellent solid-state electrolyte\non account of its high sodium-ion conductivity, which can be further\nenhanced through chemical doping. However, the underlying sodium diffusion\nmechanism and how it is affected by chemical doping are not fully\nunderstood. This, in part, stems from the ambiguity between reported\naverage structure models, where there is variation in the number and\nlocation of the sodium crystallographic sites. In this work, we present\na neutron scattering study on monoclinic NASICON and Mg²⁺-doped NASICON, with a focus on the characterization of the sodium\nsites. Our difference Fourier maps show that the sodium sites have\nlimited long-range order at temperatures as low as 3 K. In addition,\nour characterization of Mg²⁺-doped NASICON suggests that\nalthough Mg²⁺ preferentially dopes the secondary Na₃PO₄ phase rather than the bulk NASICON phase, its\nNASICON sodium sites are comparatively more diffuse. Furthermore,\nquasi-elastic neutron scattering data show broader spectra for the\nMg-doped NASICON compared to NASICON at 300 K, supporting a more dynamic\nenvironment. We propose that the more diffuse character of the sodium\nsites arising from the introduction of Mg²⁺ contributes\nto the improvement of ionic conductivity in Mg-doped NASICON near\nroom temperature.