Figure 4. Na-ion diffusion in Na₂Fe₂(SO₄)₃.

(a,b) Equi-value surface of the ΔBVS. The blue and light-blue surfaces are for ΔBVS=0.2 and 0.4, respectively. Inner side of the surface corresponds to accessible spaces for the Na ions. Green and yellow polyhedra are that of FeO₆ and SO₄, respectively. (c) Migration activation energy of Na⁺ ion calculated with DFT. Shown are the values (from left to right) for migrations along the c axis for the Na2 sites, between Na2 and Na1 sites, between Na1 and Na3 sites, and along the c axis for the Na3 sites. Calculations are done at low concentration of Na.