Opening new windows on the structure of glass

When a liquid is cooled fast enough, without crystalizing and until it appears rigid, its apparent state is known as glass. It is commonly assumed that glass is frozen at the atomic level with an immutable structure, but recent research featuring metallic glasses suggests otherwise, revealing some significant local mobility of atoms.

Egami and Ryu showcased their observations from recent simulation studies to explore glass phenomena and the implications of atomic mobility in glass.

“Many atoms in ‘frozen’ glass are in fact locally mobile, particularly in metallic glasses — a relatively new class of glasses — partly because of quantum effects,” said author Takeshi Egami. “This local mobility has a minimal effect on most macroscopic properties, which is why this has not been recognized until recently. But for new applications, for instance at very high frequencies in electronic devices, it can be important.”

The researchers examined the atomic mobility in glasses at low temperatures, the potential energy landscapes of glasses, and the nature of the glass transition. They observed that energy dissipation caused by atomic mobility in glasses is not like that of crystalline solids with lattice defects and suggested that a more holistic view is required for a better understanding of glass and other amorphous solids.

“In crystals, local structural changes are readily recognized as deviations from the lattice periodicity, whereas in glasses, local structural changes are very difficult to recognize,” said Egami. “Yet they can have unexpected consequences. This changes the fundamental view on glasses, and lets us recognize that in complex matter, such as glasses, the macroscopic world does not straightforwardly reflect the microscopic world.”

Source: “Perspective: Are glasses really frozen?,” by Takeshi Egami and Chae Woo Ryu, Journal of Applied Physics (2025). The article can be accessed at https://doi.org/10.1063/5.0246799 .