A Dynamically Induced Phase Transition in Na₄P₂S₆─Ultrafast Na⁺ Mobility Triggering Rotor Phase Formation

The physical properties of any crystalline solid, such as the irregular movement of ions or atoms, are closely linked to its structure. Changes in local structure or local defect chemistry are typically attributed to changes in ion hopping. Conversely, one might also ask whether fast ionic diffusion can cause structural changes, finally initiating an overall phase transition. By using high-resolution ²³Na and ³¹P nuclear magnetic resonance (NMR) carried out at temperatures as high as 650 °C, we show that changes of the local Na⁺ environment in the Na⁺-conducting model compound Na₄P₂S₆ indeed precede the transition of the anionic framework from β-Na₄P₂S₆ to the fast-conducting γ-phase. While rapid 2D Na⁺ diffusion governs ionic conductivity in the β-phase of Na₄P₂S₆, the high-temperature γ-phase has been theoretically predicted and experimentally shown to be a rotor phase with high dynamics of both the mobile Na⁺ cations and the anionic framework. Here, we provide evidence that Na⁺ diffusion and the initial transformation of the Na substructure precede the transition of the P₂S₆ units to a rotating framework. NMR spectra and relaxation times of both ²³Na and ³¹P reveal that rapid P₂S₆^4- motions occur in a molten Na⁺ substructure, but these motions do not influence Na⁺ hopping much. Hence, we suggest that Na⁺ hopping while first initiating the transformation to the rotor phase is indeed uncoupled from polyanion rotations at high temperatures. Our study provides a new perspective on the details governing phase transitions in fast-ion conductors and may lead to a deeper understanding of these phenomena.