Direct Observation of Ag-Ion Diffusion in a Superionic Conductor Using Ultrafast Electron Diffraction

Understanding how ions move through solid materials is critical for fields ranging from solid-state electrolytes to thermoelectric materials and energy storage systems. Despite decades of study, directly observing ionic diffusion inside crystalline solids has remained difficult because most experimental techniques cannot simultaneously resolve atomic-scale structure and ultrafast dynamics.

Researchers recently addressed this challenge by using ultrafast electron diffraction (UED) to directly visualize Ag-ion diffusion in the superionic conductor AgCrSe₂. Their work captured structural evolution on picosecond timescales and revealed that ionic transport in the superionic phase is not completely random, as traditionally assumed.

Instead, the study showed that Ag ions move through dynamically correlated and transiently ordered pathways during diffusion.

What Researchers Directly Observed

The researchers found that Ag ions rapidly became mobile after excitation, driving AgCrSe₂ into a superionic phase within only a few picoseconds. During this transition, the Ag-ion sublattice underwent substantial structural rearrangement while maintaining transient short-range ordering.

This was one of the study’s most important discoveries. The superionic state was not fully disordered. Instead, local correlations between Ag ions persisted during diffusion, creating dynamically evolving transport pathways.

The experiments showed that:

• Ag-ion diffusion occurred collectively rather than through isolated random hopping

• transient short-range ordered structures formed during transport

• local Ag–Ag bond contraction emerged during the superionic transition

• these transient structures reduced migration barriers and promoted fast ion diffusion

The study suggests that ionic conductivity in superionic conductors can emerge from cooperative structural dynamics rather than from static diffusion channels alone.

“For the first time, ion diffusion in a solid was directly observed in both space and time.”

Schematic of the ultrafast-electron-diffraction pump-probe setup used to measure the ultrafast-laser-induced phase transition of AgCrSe2. The left panel depicts the low-temperature-ordered phase in which Ag atoms exclusively occupy 𝛼 sites, and the right panel illustrates the high-temperature-disordered phase wherein Ag atoms randomly occupy both 𝛼 and 𝛽 sites.

How Ultrafast Electron Diffraction Revealed the Mechanism

The discovery was enabled through ultrafast electron diffraction, which combines femtosecond laser excitation with time-resolved diffraction measurements capable of tracking structural evolution at atomic length scales.

After laser excitation triggered the superionic transition, researchers monitored changes in diffraction intensity and diffuse scattering in real time. This allowed them to directly track how the Ag-ion arrangement evolved during diffusion.

Unlike conventional equilibrium diffraction methods, UED captured transient nonequilibrium structural states as they formed. This provided direct experimental evidence for ionic diffusion behavior that had previously been inferred primarily through molecular dynamics simulations and theoretical modeling.

The ability to observe both temporal and spatial evolution simultaneously represents a significant advance for studying superionic conductors and other solid-state ionic conductors.

Why the Discovery Matters

Superionic conductors are often described using models in which ions diffuse randomly through a disordered lattice. The findings from AgCrSe₂ suggest a more complex picture in which ionic transport remains dynamically structured even during highly mobile superionic states.

The study demonstrated that:

• transient short-range order survives during diffusion

• dynamically correlated motion contributes to ionic conductivity

• local structural fluctuations create favorable diffusion pathways

These observations shift the understanding of ionic diffusion away from purely stochastic transport models and toward mechanisms involving cooperative ion dynamics and evolving local order.

The findings may also help explain why some superionic conductor materials exhibit exceptionally fast ionic transport despite maintaining an overall crystalline framework.

“Superionic transport emerges from dynamically correlated ion motion rather than completely random disorder.”

Implications for Solid-State Ionics and Energy Materials

The discovery has important implications for the study of solid electrolyte materials, where ionic conductivity depends strongly on how ions move through solid materials.

Understanding how transient ordering and correlated motion influence ion transport may support future efforts to design:

• solid-state electrolytes with enhanced ionic conductivity

• superionic conductors with lower activation energy barriers

• materials for advanced energy storage and electrochemical systems

The work may also influence research into thermoelectric materials, where superionic behavior can strongly affect thermal transport and lattice dynamics.

Nanoscale Structure and Ionic Transport

The study further highlights the importance of local structural behavior during diffusion. Short-range ordering, transient transport pathways, and lattice fluctuations can significantly influence ionic conductivity at nanometer length scales. This makes nanomaterials increasingly relevant for investigating and tailoring ionic transport behavior in solid-state ionic conductors and related functional materials.

“The superionic state retained transient local order even during rapid Ag-ion diffusion.”

Final Thoughts

The direct observation of Ag-ion diffusion in AgCrSe₂ represents a major experimental advance in the study of ionic transport. By capturing transient structural evolution in real time, researchers demonstrated that superionic conduction is governed by dynamically correlated motion rather than fully random disorder.

Beyond providing new insight into superionic conductors, the work establishes ultrafast electron diffraction as a powerful tool for investigating nonequilibrium ionic transport mechanisms in solid materials. The findings offer a more detailed framework for understanding how fast ionic conductivity emerges and how future ionic conductors may be studied and designed.

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Sources:

1. Yang, J., Xie, L., Hu, M., Qi, Y., Li, J., Jia, B., Feng, J., Chen, Z., Bosman, M., Xiang, D., & He, J. (2026). Tracking ultrafast ion diffusion dynamics in AGCRSE 2 superionic Conductor. Physical Review X, 16(2). https://doi.org/10.1103/s6rh-7219