When ice melts, it happens in an instant: solid crystals suddenly transform into liquid water. But for ultra-thin materials, the rules of melting are far stranger. A team of
researchers at the University of Vienna has for the first time directly observed an unusual intermediate state—known as the 'hexatic phase'—in an atomically thin crystal. Their findings, published today in *Science*, challenge long-held theories about how materials change from solid to liquid.
A liquid that behaves like a solid
The hexatic phase was first predicted in the 1970s. In this exotic state, a material flows like a liquid, with irregular distances between atoms, yet retains some of the angular order typical of solids. Until now, this behavior had only been seen in large-scale model systems, such as packed polystyrene beads. It remained unclear whether everyday covalently bonded materials could exhibit the same behavior—until now.
Using cutting-edge scanning transmission electron microscopy (STEM) and artificial intelligence, the researchers captured the melting of a single layer of silver iodide (AgI) protected by graphene. This “graphene sandwich” allowed the fragile crystal to remain stable while being gradually heated to over 1100 °C, revealing the atomic-scale dynamics of melting in real time.
AI meets atomic melting
Tracking thousands of atoms in motion would have been impossible without neural networks. The team trained AI on vast simulated datasets, enabling it to analyze the high-resolution images and identify the hexatic phase. Within a narrow temperature window—roughly 25 °C below AgI’s melting point—the team confirmed the presence of this elusive intermediate state. Supplementary electron diffraction measurements reinforced the finding.
A twist in melting theory
Interestingly, the transitions didn’t behave exactly as predicted. While the solid-to-hexatic transition was smooth, the hexatic-to-liquid shift occurred abruptly, more like ice melting into water. “Melting in covalent two-dimensional crystals is far more complex than previously thought,” said David Lamprecht of the University of Vienna and TU Wien.
The study not only confirms that hexatic phases exist in real, strongly bonded materials but also opens new doors for exploring phase transitions at the atomic level. “Kimmo Mustonen and colleagues have once again shown the power of atomic-resolution microscopy combined with AI,” said Jani Kotakoski, head of the research group at the University of Vienna.
Implications for materials science
By observing these transitions directly, scientists are gaining unprecedented insight into how ultra-thin materials behave at the atomic scale. These discoveries could guide the design of next-generation nanomaterials and electronic devices, where understanding precise phase transitions is critical.
In brief:
- Ultra-thin silver iodide crystals do not melt instantly; they pass through a hexatic phase.
- Researchers embedded the crystal in a graphene “sandwich” and used AI-assisted electron microscopy to track atomic motion.
- The solid-to-hexatic transition is continuous, but hexatic-to-liquid is abrupt, challenging prior theories.
- This work deepens our understanding of two-dimensional materials and highlights the potential of AI in advanced microscopy.
Reference: Thuy An Bui, David Lamprecht, et al., Hexatic phase in covalent two-dimensional silver iodide, ‘Science’, 2025, DOI: 10.1126/science.adv7915
Photo by C.Stadler/Bwag, Wikimedia commons.