When metals are heated strongly, their crystal structure changes. Now, for the first time, researchers have succeeded in stabilizing and making visible an intermediate stage of this so-called martensite transition in a material. This is exciting because this structural change is responsible, among other things, for the properties of steel and other important materials. The crystal superlattices created using silver nanoparticles also showed unusual optical features, as the team reports in Science.
In metals, the atoms typically arrange themselves in one of two lattice structures: face-centered cubic (FCC) or body-centered cubic (BCC). In both cases, metal atoms are located in all corners of the cube-shaped basic unit of this lattice. In the face-centered lattice, the remaining atoms are placed in the center of the cube faces. This FCC lattice structure is particularly dense and stable, but often brittle. In the space-centered variant, the surfaces remain empty; there is only another atom in the center of the lattice cube. This BCC grid is more compliant and makes metals malleable.
Intermediate stages have so far only been postulated theoretically
Such atomic lattice structures are important because they shape the properties of important metal materials, such as steel. It partially changes into the FCC structure, the so-called martensite. This gives it its increased strength. “Materials researchers have long been interested in how to control the proportion of FCC and BCC in metals, but the transitions between these phases have been difficult to study because they are so unstable,” explains co-author Timothy Moore from the University of Michigan.
There are various theoretical models that simulate the atomic rearrangements and intermediate stages of this so-called martensite transition. However, it has not yet been possible to observe these short-lived intermediate stages more closely experimentally. Whether the transformation really takes place like this and what the intermediate forms look like remained unclear – until now.
Silver nanoparticles as analogues
The team led by Moore and first author Yasutaka Nagaoka from Brown University have now succeeded for the first time in experimentally demonstrating one of these previously only theoretically postulated intermediate states during the transition from the FCC to the BCC lattice structure. However, they did not achieve this with iron or another solid metal, but rather with the help of silver nanoparticles. These served as a model for individual metal atoms; through a coating with “sticky”, chain-shaped molecules, these particles interact in a similar way to atoms in a metal lattice: they also assemble to form a crystal lattice.
The highlight: For their experiments, the researchers produced silver nanoparticles with slightly different shapes. Depending on the temperature during synthesis, more spherical or more angular shapes were formed. The latter resembled octahedra with cut corners – so-called mecons. The team then observed which lattice structures these different variants assembled into.
“A symmetry-breaking phase region that has never been observed before”
It turned out that, depending on the shape, the silver nanoparticles formed a face-centered cubic lattice, a body-centered lattice or intermediate stages of both variants. “We observed a gradual evolution from an FCC to a BCC grid,” report Nagaoka and his colleagues. A new, stable grid variant also emerged. Their less symmetrical structure corresponds to one of the intermediate stages that the model of the so-called Nishiyama-Wassermann path for the FCC-BCC transition theoretically predicts.
“This ensemble forms a symmetry-breaking phase region that has never been observed before,” write the researchers. Despite its low symmetry, this novel structure is stable enough to be examined in more detail. “Our work therefore offers an experimental analogue that can be used to explore the martensitic transformation paths in metallic systems.” This could help optimize steel types and other metal materials.
New approaches for materials research
According to Nagaoka and his colleagues, such superlattices made of nanoparticles open up completely new possibilities for materials research. They can be used to develop tailor-made materials with specifically adjustable properties. “Our work is a bit like children playing with LEGO bricks,” explains senior author Ou Chen from Brown University. “We synthesize unique nanoscale building blocks and stack them into interesting structures.”
This makes it possible to find lattice variants that do not exist in nature – or, as in this case, are too short-lived to be directly observed. For example, the newly discovered intermediate state in the FCC-BCC transition exhibits extraordinary optical and quantum physical properties even at room temperature. Materials based on this structural model can therefore be useful for quantum computing or other quantum information systems, as the team explains.
Source: Yasutaka Nagaoka (Brown University, Providence, Rhode Island) et al., Science, 2026; doi: 10.1126/science.ady6472