How ions flow like a liquid through a solid crystal
Researchers in Japan used a simple physical model and revealed a fundamental connection between sublattice melting and cooperative ion transport
image:
Basic structure of the superionic conductor identified in this study: Host particles form a stable crystalline framework through strong steric repulsion. Carrier particles, on the other hand, exist at low density in the interstitial spaces between the host particles. Owing to weak long-range interactions (dashed lines), they form a crystalline structure at low temperatures. As the temperature increases, however, we found that the carrier particles melt prior to the host particles while the host crystalline framework remains intact; this process is known as sublattice melting.
view moreCredit: Takeshi Kawasaki
Osaka, Japan - A research team led by the University of Osaka, working with The National Institute of Advanced Industrial Science and Technology (AIST), RIKEN, and the Institute of Science Tokyo has uncovered a fundamental mechanism behind superionic conduction, in which ions move rapidly through a solid while its crystalline framework remains intact. Using a simple physical model, the researchers connected “sublattice melting” with cooperative and spatially heterogeneous ion transport. The findings offer a unified explanation for superionic conduction and could help guide the design of next-generation solid-state batteries.
Superionic conductors are solid materials in which certain ions move almost as freely as they do in a liquid, making them attractive for solid-state batteries. They have traditionally been studied on a material-by-material basis since real materials often possess complex crystal structures and chemical compositions. This has made it difficult to identify the essential physical mechanism underlying superionic conduction, independent of any single material’s chemistry.
The team constructed a chemically neutral model containing a rigid lattice of host particles and smaller mobile carrier particles. It retained only the interactions considered essential for superionic conduction: strong, short-range repulsion that stabilizes the host framework and softer, longer-range interactions between carriers.
As the temperature increased, the carriers lost their ordered arrangement and began moving like a liquid while the host lattice remained crystalline. This selective loss of order is known as sublattice melting. Near the transition, carriers did not simply hop independently between fixed sites. Instead, they moved cooperatively in spatially heterogeneous, string-like patterns.
The researchers also found that increasingly anharmonic or non-spring-like lattice vibrations softened the carriers’ local environment and promoted collective motion. Adjusting particle density shifted the onset of sublattice melting, while simulations using a three-dimensional model of silver iodide reproduced similar transport regimes.
Because the model captures the essential physics without relying on any specific chemistry, its conclusions apply broadly across many materials. The findings are expected to provide design principles for next-generation solid-state battery and energy-conversion materials with high ionic conductivity, contributing to more efficient materials development.
“Superionic conduction has long been difficult to understand because of the complexity of real materials,” says senior author Takeshi Kawasaki. “By deliberately starting from a simple model, we identified broadly applicable physics that could guide the design of new ion-conducting materials.”
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The article, “Probing Anharmonic and Heterogeneous Carrier Dynamics Across Sublattice Melting in a Minimal Model Superionic Conductor
,” was published in Proceedings of the National Academy of Sciences of the United States of America at DOI: https://doi.org/10.1073/pnas.2605867123
About The University of Osaka
The University of Osaka was founded in 1931 as one of the seven imperial universities of Japan and is now one of Japan's leading comprehensive universities with a broad disciplinary spectrum. This strength is coupled with a singular drive for innovation that extends throughout the scientific process, from fundamental research to the creation of applied technology with positive economic impacts. Its commitment to innovation has been recognized in Japan and around the world. Now, The University of Osaka is leveraging its role as a Designated National University Corporation selected by the Ministry of Education, Culture, Sports, Science and Technology to contribute to innovation for human welfare, sustainable development of society, and social transformation.
Website: https://resou.osaka-u.ac.jp/en
Journal
Proceedings of the National Academy of Sciences
Method of Research
Computational simulation/modeling
Subject of Research
Not applicable
Article Title
Probing Anharmonic and Heterogeneous Carrier Dynamics Across Sublattice Melting in a Minimal Model Superionic Conductor
Article Publication Date
17-Jul-2026
The trajectories of carrier particles in the superionic conducting regime in our model: The colors of the trajectory distinguish individual particles. The carrier particles move in a liquid-like manner through the interstitial spaces between the host particles forming the crystalline framework, clearly demonstrating their rapid transport.
Credit
Takeshi Kawasaki
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