New state of matter between solid and liquid

 Physicists found a strange new phase between solid and liquid 

In atomically thin materials, melting does not follow the familiar rules. New observations reveal a fleeting, exotic state between solid and liquid, reshaping scientists’ understanding of phase transitions in two dimensions. This corralled supercooled liquid relies on the surprising property that some atoms in a liquid remain stationary regardless of temperature. New research offers clearer insight into how phase transitions unfold at the atomic scale in real materials. Following are the some of the important points:-

A new study finds that the manipulation of stationary atoms in a liquid metal can alter the properties of the resulting solid, even lowering the temperature at which it cools entirely.

The manipulation of metals as they transform from a liquid to a solid can have immense benefits for technologies.

The authors describe this new breakthrough as a “hybrid” form of matter which lies between a solid and a liquid.

Understanding the atomic machinations of metals is more important than ever, as technologies of ever-increasing complexity must get the most out of rare earth metals. One of the most exciting areas of research right now is right at the line between a metallic liquid and solid.         

Typically, we think of matter in three forms: as a gas, liquid, or solid (sometimes plasma if you’re into that sort of astrophysical thing). While we have a pretty good understanding of the properties of gases and solids, it’s the third major form of matter, liquids, which still has the ability to surprise. The new study also revealed an unexpected twist. According to previous theories, the transitions from solid to hexatic and from hexatic to liquid should be continuous. However, the researchers observed that while the transition from solid to hexatic was indeed continuous, the transition from hexatic to liquid was abrupt, similar to the melting of ice into water. “This suggests that melting in covalent two-dimensional crystals is far more complex than previously thought,” says David Lamprecht from the University of Vienna and the Vienna University of Technology (TU Wien). Scientists once again showed why many aspects of liquids remain perplexing mysteries. By performing transmission electron microscopy during the solidification process of molten metal nano-droplets, the scientists observed that some atoms in the liquid remained stationary, an atypical phenomenon, as atoms in a liquid typically mingle about like strangers lost in a crowd.

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. The atoms remained stationary, and bonded to the support materials around a point defect. And by increasing the number defects using an electron beam, scientists could effectively control the number of stationary atoms in the liquid. This is a big deal, because as a liquid turns solid, the number and position of the atoms within can directly influence the solidification pathway. If the stationary atoms are low, the liquid eventually solidifies regardless, but if the number is high, the liquid can actually be prevented from forming a crystalline lattice. This effect was particularly pronounced when the scientists created a ring, or corral, of stationary atoms around a liquid and then dropped the temperature. “Once the liquid is trapped in this atomic corral, it can remain in a liquid state even at temperatures significantly below its freezing point, which for platinum can be as low as 350 degrees Celsius, which is more than 1,000 degrees below what is typically expected.” Andrei Khlobystov from the University of Nottingham, said. “Our achievement may herald a new form of matter combining characteristics of solids and liquids in the same material.”

Researchers 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. In everyday experience, melting is sudden. Ice, metals, minerals and other three-dimensional materials lose their orderly structure as soon as the melting point is reached, becoming disordered liquids almost immediately. This rapid transformation has long been considered a universal feature of melting in bulk materials. The researchers were able to perform the experiment thanks to the Sub-Angstrom Low-Voltage Electron (SALVE) microscope, whose main mission is to study radiation-sensitive materials at ultra-high resolution via transmission electron microscopy. While most of the atoms in the liquid forms of the various metals did vibrate as expected, the scientists noticed that some stayed perplexingly in place.

The picture changes when materials are reduced to nearly two dimensions. At this extreme thinness, melting can proceed in stages. Between the solid and liquid states, a distinct intermediate form of matter can appear, called the hexatic phase. First proposed in the 1970s, this phase combines properties of both states. Particle spacing becomes irregular, similar to a liquid, while the angles between neighboring particles remain relatively well organized, resembling a solid. “We began by melting metal nanoparticles, such as platinum, gold, and palladium, deposited on an atomically thin support, graphene,” Christopher Leist from the University of Ulm, said. “We used graphene as a sort of hob for this process to heat the particles, and as they melted, their atoms began to move rapidly, as expected. However, to our surprise, we found that some atoms remained stationary.” The Protochips Fusion heating stage and chip used in the Nion electrical module, which enabled the scientists to conduct controlled high-temperature studies in the vacuum of the microscope. Until now, evidence for the hexatic phase had been limited to large-scale model systems, such as tightly packed plastic spheres. Whether the same behavior could exist in real materials held together by strong chemical bonds was an open question. The team has now answered it. By observing atomically thin silver iodide (AgI) crystals as they melted, the researchers demonstrated for the first time that the hexatic phase can occur in a covalently bonded material. This resolves a long-standing scientific puzzle and reveals new details about how melting works in two dimensions.

Strangely, the liquid will still eventually solidify, not into a typical crystalline structure, but into an “amorphous solid” that is highly unstable. Once the ring of stationary atoms is disrupted, this unstable solid forms a normal crystal. This breakthrough could help transform the use of rare earth metals in clean energy conversion and storage while also allowing for significant improvements to platinum-on-carbon catalysts which are often used in fuel cells. Using an advanced scanning transmission electron microscope (STEM) equipped with a heating stage, the team slowly raised the temperature to more than 1100 °C. This approach made it possible to record the melting process directly, capturing atomic-scale changes as they happened. By combining these high-resolution images with neural network analysis, the researchers were able to track how the solid crystal passed through the hexatic phase before fully becoming a liquid, offering a rare, real-time view of melting at the level of individual atoms. “Without the use of AI tools such as neural networks, it would have been impossible to track all these individual atoms,” explains Kimmo Mustonen. The team trained the network with huge amounts of simulated data sets before processing the thousands of high-resolution images generated by the experiment.

The analysis yielded a remarkable result: within a narrow temperature window, approximately 25 °C below the melting point of AgI, a distinct hexatic phase occurred. Supplementary electron diffraction measurements confirmed this finding and provided strong evidence for the existence of this intermediate state in atomically thin, strongly bound materials. This discovery not only challenges long-standing theoretical predictions, but also opens up new perspectives in the study of materials at the atomic level. “Kimmo and his colleagues have once again demonstrated how powerful atomic-resolution microscopy can be,” says Jani Kotakoski, head of the research group. The results of the study not only deepen our understanding of melting in two dimensions, but also highlight the potential of advanced microscopy and AI in exploring the frontiers of materials science around us.