When ice turns into water, the change happens almost instantly. Once the melting temperature is reached, the rigid structure of the solid collapses and becomes a flowing liquid. This abrupt shift is typical for most materials in three dimensions. Extremely thin materials, however, behave very differently. Instead of switching directly from solid to liquid, they can pass through an unusual intermediate state known as the hexatic phase.
Researchers at the University of Vienna have now directly observed this rare state in an atomically thin crystal. By combining advanced electron microscopy with neural network analysis, the team recorded a silver iodide crystal, protected by graphene, as it melted. These ultra-thin, two-dimensional materials made it possible to watch atomic-scale melting as it unfolded. The results deepen scientific understanding of phase transitions and, unexpectedly, challenge earlier theoretical predictions. The work has now been published in Science.
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