On the Discrepancy between Local and Average Structure in the Fast Na⁺ Ionic Conductor Na_2.9Sb_0.9W_0.1S₄

Aliovalent substitution is a common strategy to improve the ionic conductivity of solid electrolytes for solid-state batteries. The substitution of SbS₄^3- by WS₄^2- in Na_2.9Sb_0.9W_0.1S₄ leads to a very high ionic conductivity of 41 mS cm^-1 at room temperature. While pristine Na₃SbS₄ crystallizes in a tetragonal structure, the substituted Na_2.9Sb_0.9W_0.1S₄ crystallizes in a cubic phase at room temperature based on its X-ray diffractogram. Here, we show by performing pair distribution function analyses and static single-pulse ¹²¹Sb NMR experiments that the short-range order of Na_2.9Sb_0.9W_0.1S₄ remains tetragonal despite the change in the Bragg diffraction pattern. Temperature-dependent Raman spectroscopy revealed that changed lattice dynamics due to the increased disorder in the Na⁺ substructure leads to dynamic sampling causing the discrepancy in local and average structure. While showing no differences in the local structure, compared to pristine Na₃SbS₄, quasi-elastic neutron scattering and solid-state ²³Na nuclear magnetic resonance measurements revealed drastically improved Na⁺ diffusivity and decreased activation energies for Na_2.9Sb_0.9W_0.1S₄. The obtained diffusion coefficients are in very good agreement with theoretical values and long-range transport measured by impedance spectroscopy. This work demonstrates the importance of studying the local structure of ionic conductors to fully understand their transport mechanisms, a prerequisite for the development of faster ionic conductors.