Influence of Rotational Distortions on Li⁺- and Na⁺-Intercalation in Anti-NASICON Fe₂(MoO₄)₃

Anti-NASICON Fe 2 (MoO 4 ) 3 ( P 2 1 / c ) shows significant structural and electrochemical differences in the intercalation of Li + and Na + ions. To understand the origin of this behavior, we have used a combination of in situ X-ray and high-resolution neutron diffraction, total scattering, electrochemical measurements, density functional theory calculations, and symmetry-mode analysis. We find that for Li + -intercalation, which proceeds via a two-phase monoclinic-to-orthorhombic ( Pbcn ) phase transition, the host lattice undergoes a concerted rotation of rigid polyhedral subunits driven by strong interactions with the Li + ions, leading to an ordered lithium arrangement. Na + -intercalation, which proceeds via a two-stage solid solution insertion into the monoclinic structure, similarly produces rotations of the lattice polyhedral subunits. However, using a combination of total neutron scattering data and density functional theory calculations, we find that while these rotational distortions upon Na + -intercalation are fundamentally the same as for Li + -intercalation, they result in a far less coherent final structure, with this difference attributed to the substantial difference between the ionic radii of the two alkali metals.