Scientists have directly visualized a long-mysterious intermediate phase during melting, upending decades-old theory. A research team from the University of Vienna captured the most detailed view ever of how ultra-thin materials melt—revealing behavior that challenges the fundamental understanding of phase transitions.

In most everyday materials—such as ice, metals, or plastics—melting happens abruptly. Once the melting point is reached, the crystal structure rapidly collapses into a liquid. But when materials are thinned down to nearly two dimensions, the physics changes dramatically. These 2D materials, seen as crucial for next-generation flexible electronics, melt differently because atoms are confined to a flat plane and can’t rearrange in three dimensions.

Figure 1. Transitional Phase.

This unique environment gives rise to an unusual state known as the hexatic phase. In this state, the spacing between atoms becomes disordered like a liquid, while the angles between neighboring atoms remain orderly, as in a solid. Essentially, the material becomes a shaky, partially ordered crystal—not fully solid and not fully liquid. Predicted in the 1970s, this hexatic phase has never been directly observed in a naturally bonded material—until now. Figure 1 shows Transitional Phase.

First real-world observation of 2D melting

Past experiments detected this phase only in artificial model systems—such as floating plastic beads—rather than real atomic materials. Theory predicts that 2D materials melt in two gradual stages: first from solid to hexatic, and then from hexatic to liquid.

To test this, the Vienna team studied an atom-thick layer of silver iodide (AgI) sealed between two graphene sheets. The graphene acted as both a stabilizer and heat shield, preventing the thin crystal from deforming [1]. Using a scanning transmission electron microscope, they heated the sample to 1100°C (2012°F) while capturing atomic-resolution video—something previously considered impossible. AI-powered neural networks then analyzed thousands of frames, tracking every atom’s movement.

A surprising twist to the theory

The team confirmed part of the existing model: the hexatic phase formed about 25°C (77°F) below the melting point. But the next stage defied expectations. Instead of melting gradually from hexatic to liquid, the transition happened suddenly—more like conventional 3D melting than the predicted smooth 2D behavior.

This abrupt shift overturns long-standing theories of 2D melting and suggests that the true behavior of ultrathin materials is far more complex than believed.

References:

1. https://interestingengineering.com/science/scientists-track-every-atom-during-melting

Cite this article:

Keerthana S (2025), Scientists Trace Atoms in Real Time as They Melt, Uncovering A Never-Before-Seen Transitional Phase, AnaTechMaz, pp.311