Two-Dimensional Melting Has a Secret In-Between State—Science Catches the Hexatic Phase On Camera

Thuy An Bui and an international team at the University of Vienna have shown that 2D melting is not a single leap but a two-step process: solid to hexatic, hexatic to liquid. In covalent 2D crystals, the hexatic phase exists, not just in model systems.

In atomically thin silver iodide encapsulated in graphene, researchers heated the sample above 1100°C inside a scanning transmission electron microscope to watch the transformation unfold. The result is a direct observation of an intermediate hexatic phase that lasts a narrow 25°C window, before the material crosses into the liquid state. For broader context, see the ScienceDaily coverage and the formal report in Science.

The AI Lens That Made It Visible

A neural network was trained to track thousands of atoms in real time, turning microscopic flickers into measurable evidence. This AI-enabled atomic tracking was central to resolving the hexatic window and mapping how order and disorder shift as melting proceeds.

Today, the study’s University of Vienna team and collaborators demonstrate that computational observers can translate visual data into quantitative phase diagrams, a trend echoed in the broader AI-in-science movement.

From Observation to Design: What Comes Next

The discovery challenges the assumption that melting in 2D crystals must be abrupt and suggests design principles where intermediate states are engineered for specific properties. The implications span graphene-based materials, flexible electronics, and sensors, where controlling a hexatic window could tune mechanical and electrical behavior.

As AI-augmented microscopy becomes routine, researchers will increasingly treat hidden states as levers for materials design rather than curiosities of phase transitions. The era of the \’device\’ is ending, and we are entering an era where in-between phases become deliberate tools for building the next generation of 2D materials.

• Hexatic phase is an intermediate state during 2D melting, now demonstrated in covalent silver iodide between graphene.
• Two-step melting means solid → hexatic (gradual) → liquid (abrupt).
• AI-enabled atomic tracking turns atom-scale motion into actionable data, enabling real-time phase mapping.