Tracking Ultrafast Ion Diffusion Dynamics in AgCrSe_{2} Superionic Conductor

Superionic conductors (SICs) exhibit liquidlike ionic diffusivities while maintaining a periodic crystalline lattice, making them promising candidates for applications in fuel cells, solid-state electrolytes, and thermoelectric materials. The transient local structures of mobile ions and their cooperative interactions with the host lattice are of pivotal importance regarding both the ion and heat conduction in SICs. However, accurately capturing the structural evolution of mobile ions remains a significant challenge due to the inherent complexities involved. Here, we employed ultrafast electron diffraction (UED) with femtosecond-temporal and angstrom-spatial resolution to resolve the structural evolution of mobile Ag^{+} ions in AgCrSe_{2}. Our experiments identify a critical process originating from localized Ag^{+} vibrations at the long-range-ordered lattice sites to a formation of short-range-correlated Ag^{+} trimer structures through a drastic contraction of the Ag^{+}─Ag^{+} bond from approximately 3.68 to approximately 3.00 Å in 1.97 ps. Combining real-time time-dependent density-functional-theory and molecular dynamics simulation, we further reveal the crucial role of these contracted trimer structures in enabling fast Ag^{+} diffusion by opening up excess free volume and reducing local energy barriers. Such an intimate relation between fast diffusion and local lattice variations is not exclusive and can be extended beyond AgCrSe_{2} to materials such as Li_{10}XP_{2}S_{12} (X=Si, Ge, Sn). The ability to track ion diffusion with UED also provides new avenues for exploring the atomistic mechanisms of fast ion diffusion in next-generation solid-state electrolytes, fuel cells, and ion transport membranes.