Harvest the Polyanion Rotation in Sodium Superionic Conductors?

The existing design principles and screening strategies of superionic conductors predominantly focus on the perspective of a static crystal structure. However, the dynamic mechanism involving anion rotational motion as well as its interaction with cation translational motion has received less exploration, especially in the realm of the accelerated discovery of new fast ionic conductors with these strong dynamic couplings. Herein, we design a multistep density functional theory molecular dynamics (DFT-MD) high-throughput workflow based on the structural feature of an isolated framework to rapidly screen Na superionic conductors with polyanion rotation. Interpretable machine-learning classification indicates that structures with larger atomic volumes and smaller polyanions tend to exhibit anion rotational behavior. Building on the observation of persistent, large-angle anion reorientation and the time-spatial correlation of Na hops and polyanion rotations, we identified polyanion rotation behavior for the first time in 10 new compounds and quantified the contribution of polyanion rotation to Na diffusion, among which three are novel Na superionic conductors with significant cation-anion dynamics coupling, including NaGaBr4, NaNbCl6, and Na4SiSe4. Additionally, we developed highly accurate moment tensor potentials for NaNbCl6 and Na4SiSe4. Long time scale machine-learning molecular dynamics simulations (MLMD) at 300 K revealed that the anion rotation still exists in NaNbCl6 at room temperature, while Na4SiSe4 displays non-Arrhenius behavior. This study provides valuable guidance for the development of new fast ion conductors utilizing polyanion rotation.