Influence of rotational distortions on Li⁺- and Na⁺- intercalation in anti-NASICON Fe₂(MoO₄)₃

Anti-NASICON Fe₂(MoO4)₃ (P2₁/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.