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An international research team has investigated how nano-droplets of metals such as platinum, gold, and palladium solidify when deposited on an ultrathin support and observed with high-resolution transmission electron microscopy. Using a specially designed microscope, they could follow the transition from liquid to solid atom by atom, discovering that not all atoms behave as expected in a conventional liquid.

In a typical liquid, atoms are in constant motion, whereas in a solid they stay in fixed positions forming an ordered lattice. However, in these metallic nano-droplets the scientists detected a subset of “stationary” atoms that remain strongly anchored to defects in the support material, even at very high temperatures. By controlling the number and distribution of these defects with the microscope’s electron beam, the team can tune how many atoms become “pinned” inside the liquid metal.

When the number of fixed atoms is small, the metal solidifies in a fairly normal way: a tiny crystal forms and then grows until the whole droplet becomes a crystalline solid. But when the amount of stationary atoms increases significantly and, above all, when they organize into a ring around the liquid region, the process changes completely. This ring of immobile atoms acts as an “atomic corral” that confines the liquid and prevents it from crystallizing in the usual way, creating a situation of extreme supercooling.

In this corralled state, platinum can remain liquid at about 350 degrees Celsius, more than 1,000 degrees below the temperature at which this metal would typically be expected to solidify. This coexistence of fixed atoms reminiscent of a solid with a central region that still behaves like a liquid defines a new hybrid state of matter, distinct from the classical solid, liquid, or gas. When the temperature drops further, the trapped liquid eventually solidifies, but not as an ordered crystal; instead, it becomes an amorphous, glass-like metallic solid, maintained only by the atomic confinement. When that corral breaks, the metal’s atoms rearrange and recover their usual crystalline structure.

The potential applications of this finding are especially relevant for catalysis. Platinum supported on carbon is one of the most widely used catalysts worldwide, for example in automotive systems and fuel cells. Understanding and exploiting a confined liquid state with non-classical phase behavior could change how catalysts are designed, enabling “self-cleaning” surfaces that retain their activity and stability for longer. In the medium term, the ability to “design” more complex atomic corrals opens the door to using rare metals more efficiently in clean energy technologies, from energy conversion to storage, reducing the consumption of critical materials while boosting device performance.

Main scientific source: ACS Nano article, synthesis news at Phys.org
https://phys.org/news/2025-12-molten-metal-nano-droplets-reveal.html