In ordinary three-dimensional materials, ice turns into water instantly: upon reaching the melting point, the rigid structure breaks down and the substance transitions into a liquid state. In atomically thin crystals, just one atom thick, the process unfolds differently — through an unusual intermediate state.
Scientists at the University of Vienna have for the first time directly observed the hexatic phase in a real material — a crystal of silver iodide one atom thick. In this phase, the distances between atoms become disordered, as in a liquid, but the angles between them retain a partial order characteristic of a solid. Such a hybrid state was predicted back in the 1970s, but until now it had only been seen in model systems made of beads.
To protect the fragile crystal, the researchers placed it between two layers of graphene, creating a kind of "sandwich." The sample was then heated in a transmission electron microscope to a temperature above 1100 °C, recording the melting process with atomic resolution. Neural networks helped track the motion of thousands of individual atoms across thousands of frames.
It turned out that the hexatic phase exists within a narrow temperature window — roughly 25 °C below the melting point of silver iodide. The transition from the solid state to the hexatic one occurs smoothly, whereas the transition from the hexatic to the liquid one is abrupt, as in ordinary materials. This contradicts previous theoretical expectations and shows that melting in two-dimensional covalent crystals is more complex than was believed.
The discovery changes our understanding of phase transitions in two-dimensional materials and opens the way to creating new nanostructures with controllable properties. Understanding how atoms behave during melting in extremely thin layers is important for future technologies — from flexible electronics to quantum devices.
The experiment confirmed that even in strongly bound real materials this elusive intermediate state can exist. Physicists now have direct proof and a tool for studying it.
