(Image Source: https://www.chemeurope.com/en/news/1185490/breakthrough-in-materials-research-the-metal-that-does-not-expand.html)
Most metals expand as they heat up. A well-known example is the Eiffel Tower, which stands 10 to 15 centimeters taller in summer due to thermal expansion. However, this phenomenon is undesirable for many technical applications, prompting researchers to seek materials that maintain a stable length regardless of temperature changes. One such material is Invar, an iron-nickel alloy with an exceptionally low thermal expansion rate. Despite its long-standing use, the exact mechanism behind this property remained unclear—until now.
A recent collaboration between theoretical researchers at the Vienna University of Technology (TU Wien) and experimental scientists at the University of Science and Technology Beijing has led to a major breakthrough. Using advanced computer simulations, they have unraveled the invar effect at an atomic level and developed a new material, a pyrochlore magnet, with even better thermal stability than Invar. This alloy exhibits an astonishingly low thermal expansion across a temperature range exceeding 400 Kelvin, changing in length by only about one ten-thousandth of a percent per Kelvin. “The higher the temperature in a material, the more the atoms tend to move – and when the atoms move more, they need more space. The average distance between them increases,” explains Dr Sergii Khmelevskyi from the Vienna Scientific Cluster (VSC) Research Centre at TU Wien. “This effect is the basis of thermal expansion and cannot be prevented. But it is possible to produce materials in which it is almost exactly balanced out by another, compensating effect.”
Using sophisticated simulations, Khmelevskyi and his team studied how magnetic materials behave at different temperatures on an atomic scale. They discovered that invar’s minimal expansion is due to certain electrons shifting their states as the temperature increases. This weakens the magnetic order in the material, leading to slight contraction—an effect that nearly cancels out thermal expansion. While it was already known that magnetism played a role in Invar’s properties, this study provides a much deeper understanding. For the first time, a theoretical model can predict and guide the design of new materials with near-zero thermal expansion.
The Pyrochlore Magnet: A Multi-Element Solution
To validate these predictions, Khmelevskyi collaborated with experimentalists led by Prof. Xianran Xing and Assoc. Prof. Yili Cao at the University of Science and Technology Beijing. Their research resulted in the pyrochlore magnet, a material with an extremely low thermal expansion coefficient over an unprecedented temperature range.
Unlike conventional invar alloys, which contain only two elements, the pyrochlore magnet is a quaternary alloy composed of zirconium, niobium, iron, and cobalt. Its exceptional stability is linked to its complex, non-uniform structure. Unlike a perfectly ordered crystal lattice, this material exhibits local variations in composition—some areas have slightly more cobalt, while others have less. These differences allow different regions of the material to respond to temperature changes in complementary ways, effectively neutralizing overall thermal expansion. This novel alloy holds great promise for applications requiring dimensional stability under extreme temperature variations, such as aerospace, precision instrumentation, and high-accuracy electronics.
Reference: “Local chemical heterogeneity enabled superior zero thermal expansion in nonstoichiometric pyrochlore magnets” by Yanming Sun, Ruohan Yu, Sergii Khmelevskyi, Kenichi Kato, Yili Cao, Shixin Hu, Maxim Avdeev, Chin-Wei Wang, Chengyi Yu, Qiang Li, Kun Lin, Xiaojun Kuang and Xianran Xing, 17 December 2024, National Science Review.
DOI: 10.1093/nsr/nwae462
