Ions Move Through Solid Crystals Like Liquid Flow

Superionic conductors blur the boundary between solids and liquids by enabling certain ions to move extraordinarily fast while the material’s overall crystal framework remains intact. This remarkable behavior is the basis for high-performance solid-state batteries, yet the underlying physics has remained stubbornly material-specific—until now.

A team led by the University of Osaka, in collaboration with AIST, RIKEN, and Institute of Science Tokyo, tackled the problem by stripping superionic conduction down to its essentials. They built a chemically neutral minimal model: a rigid “host” lattice stabilized by strong short-range steric repulsion, plus smaller mobile “carrier” particles occupying interstitial spaces.

In their simulation, carriers initially remain ordered at low temperatures, forming a crystalline substructure within the host’s fixed scaffold. As temperature rises, the carriers lose long-range order while the host lattice stays crystalline—an effect known as sublattice melting. Crucially, this is not a simple picture of independent hops between static sites.

Near the transition, transport becomes cooperative and spatially heterogeneous. Carrier motion organizes into transient, string-like patterns, suggesting that ion conduction is driven by collective rearrangements rather than isolated diffusion events. This cooperative behavior provides a mechanistic bridge between structural change and rapid ionic transport.

The researchers further found that the host lattice’s vibrational properties matter. Increasing anharmonicity—departures from ideal spring-like motion—locally softens the environment surrounding carriers. That softening lowers the barrier to collective mobility and promotes the onset of sublattice melting.

Adjusting particle density shifts the temperature range where carriers transition from ordered to liquid-like behavior, linking thermodynamics directly to conduction readiness. To test robustness, the team extended the approach to a three-dimensional silver iodide model and reproduced similar transport regimes.

Overall, the study offers a unified, broadly applicable explanation for superionic conduction that does not depend on the chemistry of any single compound. By highlighting the roles of sublattice melting, anharmonic lattice vibrations, and heterogeneous cooperative transport, the work points toward concrete design principles for next-generation energy-conversion and battery materials with higher ionic conductivity.

The paper, published in Proceedings of the National Academy of Sciences, emphasizes that starting from a minimal physical model can expose universal mechanisms hidden inside real materials’ complexity—turning “superionic” from an observed phenomenon into an engineering target.

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Subject of Research:
Not applicable
Article Title:
Probing Anharmonic and Heterogeneous Carrier Dynamics Across Sublattice Melting in a Minimal Model Superionic Conductor

News Publication Date:
7-Jul-2026

Web References:
http://dx.doi.org/10.1073/pnas.2605867123

References:
10.1073/pnas.2605867123

Keywords

Molecular physics; Molecular dynamics; Soft matter physics; Ions