The crystal structure, electronic properties, and sodium diffusion mechanism in Na 5 Sc(MoO 4 ) 4 were investigated using the powder X-ray diffraction, nuclear magnetic resonance, and electrical conductivity measurements, as well as ab initio calculations. Na 5 Sc(MoO 4 ) 4 belongs to the family of alluaudite-type oxides Na x M y (AO 4 ) 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 Na 5 Sc(MoO 4 ) 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 Na 5 Sc(MoO 4 ) 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 Na 5 Sc(MoO 4 ) 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, E a ≤ 0.3 eV, has one of the lowest values among Na-conductive materials.

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