Mechanism of Sodium-Ion Diffusion in Alluaudite-Type Na₅Sc(MoO₄)₄ from NMR Experiment and Ab Initio Calculations

The crystal structure, electronic properties, and sodium diffusion mechanism in Na₅Sc(MoO₄)₄ were investigated using the powder X-ray diffraction, nuclear magnetic resonance, and electrical conductivity measurements, as well as ab initio calculations. Na₅Sc(MoO₄)₄ belongs to the family of alluaudite-type oxides NaₓMy(AO₄)₃ (M = In, Sc, Mg, Cd, Zn, Mn, Fe, Co, and Ni; A = Mo, W, P, As, and S), which are now considered as promising materials for sodium-ion batteries. Our results demonstrate a considerable difference in the mechanism of Na⁺ ion transport in Na₅Sc(MoO₄)₄ and in previously studied alluaudite oxides, where one-dimensional sodium diffusion was suggested to occur through channels along the c-axis. The Na⁺ motion in Na₅Sc(MoO₄)₄ is found to be rather two-dimensionally occurring along the bc-plane. We believe that filling of the M-sublattice plays a key role in the mechanism of Na⁺ ion diffusion in alluaudite compounds. In particular, in Na₅Sc(MoO₄)₄ characterized by a low Sc-occupancy of the M-sublattice, the sodium ions located far from scandium are the first to be activated with increasing temperature and the activation energy for their jumps, Eₐ ≤ 0.3 eV, has one of the lowest values among Na-conductive materials.