🔬 Observation of a new phase of matter, at the boundary between solid and liquid

The melting of materials seems like a simple phenomenon. However, when their thickness is reduced to just a few atoms, established rules waver. In this two-dimensional mode, new states emerge between solid and liquid.

A team from the University of Vienna has managed to directly observe one of these rare states within an extremely thin crystal. By coupling a state-of-the-art electron microscope with neural network analysis, the scientists monitored in real time the melting of a silver iodide crystal wrapped in graphene. This method allowed them to capture atomic-scale transformations in real time.


For all of us, the transition from the solid to the liquid state seems instantaneous: the solid becomes liquid. Nevertheless, for materials reduced to almost two dimensions, the process can be more gradual. A transient state, called the hexatic phase, can then manifest. It blends attributes of solids and liquids.

Until now, this phase had mainly been examined in simplified model systems. The international team demonstrated that it also occurs in real materials, held together by strong chemical bonds. By gradually heating the silver iodide crystal, they recorded its passage through the hexatic phase before complete melting.

The results indicate that the transition from solid to the hexatic phase is continuous, while the passage from this phase to liquid is sudden, contradicting some previous theoretical expectations.

This discovery shows the contribution of hybrid technologies, such as electron microscopy and artificial intelligence, to push the boundaries of materials science. It refines our perception of fundamental behaviors under extreme conditions, with potential implications for several technological fields.

The hexatic phase: a transient state

The hexatic phase is a state of matter that lies between solid and liquid, particularly visible in two-dimensional materials. While solids exhibit a rigid and regular atomic arrangement and liquids exhibit great disorder, this phase displays partial order. The distances between particles become irregular, but the angles between neighbors retain a certain organization, generating a singular structure.

This state was proposed theoretically in the 1970s, but its detection in real materials remained difficult. It was mainly analyzed in model systems, such as assemblies of plastic spheres, where interactions are simpler. The hexatic phase constitutes an important step in understanding phase transitions, indicating that melting can be a multi-step process rather than an abrupt switch.

For ultra-thin materials, such as atomically thin crystals, the hexatic phase appears within a limited temperature range.