Researchers have discovered that not all atoms in a liquid are in motion and that some remain stationary regardless of the temperature, significantly impacting the solidification process, including the formation of an unusual state of matter—a corralled supercooled liquid.
Atoms in liquids move in a complex way, resembling a jostling crowd of people. They constantly and rapidly pass by each other while still interacting with one another. Studying the behaviour of atoms in liquids can be challenging, especially during the critical stage when the liquid starts to solidify. This stage is crucial because it determines the structure and many of the material's functional properties.
Dr Christopher Leist, who performed transmission electron microscopy experiments at Ulm using the unique low-voltage SALVE instrument, said, 'We began by melting metal nanoparticles, such as platinum, gold, and palladium, deposited on an atomically thin support—graphene. 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 researchers found that stationary atoms are strongly bonded to the support material at locations of point defects, even at very high temperatures. They were able to increase the number of defects by focusing the electron beam and so control the number of stationary atoms within the liquid.
Professor Ute Kaiser, who estabilished the SALVE centre at Ulm University, said, 'Our experiments have surprised us as we directly observe the wave-particle duality of electrons in the electron beam. We visualise the material using electrons as waves. At the same time, electrons behave like particles, delivering discrete bursts of momentum that can either move or, surprisingly, even fix atoms at the edge of a liquid metal. This remarkable observation has allowed us to discover a new phase of matter.'
The team previously reported films of chemical reactions involving individual molecules, including the first instance of a chemical bond breaking and forming in real time. Their method enables the observation of chemistry at the atomic level.
In this study, the researchers found that stationary atoms have an influence on the solidification process. When there is a small number of them, a crystal forms directly from the liquid and continues to grow until the entire particle solidifies. However, when the number of stationary atoms is high, the solidification process is significantly disrupted, preventing any crystal from forming.
The effect is particularly striking when stationary atoms create a ring that surrounds the liquid. 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-that is more than 1,000 degrees below what is typically expected.
Below a certain temperature, the corralled liquid solidifies, not into a crystalline form but as an amorphous solid. This amorphous form of metal is highly unstable, maintained only by the confinement of stationary atoms. When the confinement is disrupted, the tension is released, allowing the metal to transform into its normal crystalline structure.
The discovery of a new hybrid state of metal is significant. Since platinum on carbon is one of the most widely used catalysts globally, finding a confined liquid state with non-classical phase behaviour could change our understanding of how catalysts work. This advancement may lead to the design of self-cleaning catalysts with improved activity and longevity.
So far, corralling at the nanoscale has been achieved only for photons and electrons; this work is the first time that atoms have been corralled. Professor Andrei Khlobystov said, 'Our achievement may herald a new form of matter combining characteristics of solids and liquids in the same material.'
The researchers hope that manipulation of the positions of pinned atoms on the surface may create more extended and complex corral shapes. This could pave the way for more efficient use of rare metals in clean technologies, such as energy conversion and storage.
This work is funded by the EPSRC Programme Grant 'Metal atoms on surfaces and interfaces (MASI) for sustainable future' www.masi.ac.uk addressing the challenges of sustainable use of rare elements in the future.
