Reviewed by Lexie CornerFeb 4 2025
A study published in The Journal of Physical Chemistry C reports a collaboration between theoretical researchers at TU Wien (Vienna) and experimentalists at the University of Science and Technology Beijing. Using advanced computer simulations, the researchers analyzed the invar effect in detail and developed a pyrochlore magnet—an alloy with improved thermal expansion properties compared to invar.
Metal usually expands when heated. Image Credit: TU Wien
Most metals expand with increasing temperature. For example, the Eiffel Tower extends by approximately 10 to 15 centimeters between winter and summer due to thermal expansion. However, thermal expansion is undesirable in many technological applications.
Research has focused on materials that maintain a constant length regardless of temperature. Invar, an iron-nickel alloy, exhibits exceptionally low thermal expansion, but the underlying physical mechanism has remained unclear.
Over a temperature range exceeding 400 Kelvin, Invar’s length changes by only approximately 0.0001 % per Kelvin.
Thermal Expansion and its Antagonist
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. 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.
Dr. Sergii Khmelevskyi, Senior Researcher, Vienna Scientific Cluster (VSC) Research Centre, TU Wien
Sergii Khmelevskyi and his team developed advanced computer simulations to analyze the behavior of magnetic materials at finite temperatures at the atomic scale.
Dr. Khmelevskyi explained, “This enabled us to better understand the reason why invar hardly expands at all. The effect is due to certain electrons changing their state as the temperature rises. The magnetic order in the material decreases, causing the material to contract. This effect almost exactly cancels the usual thermal expansion.”
While the invar effect was previously attributed to the material’s magnetic order, the simulations conducted in Vienna provided a detailed understanding of the underlying mechanism, allowing for predictive modeling of other materials.
“For the first time, a theory is available that can make concrete predictions for the development of new materials with vanishing thermal expansion,” stated Sergii Khmelevskyi.
The Pyrochlore Magnet with Kagome Planes
Sergii Khmelevskyi collaborated with Prof. Xianran Xing and Assoc. Prof. Yili Cao from the Institute of Solid State Chemistry at the University of Science and Technology Beijing to experimentally validate these predictions. The outcome of this collaboration is the development of a pyrochlore magnet.
Unlike conventional invar alloys, which typically contain two metals, the pyrochlore magnet consists of four: zirconium, niobium, iron, and cobalt.
It is a material with an extremely low coefficient of thermal expansion over an unprecedentedly wide temperature range.
Yili Cao, University of Science and Technology Beijing
The material's thermal stability is attributed to its heterogeneous composition. Unlike crystalline materials with a perfectly repeating lattice structure, the pyrochlore magnet exhibits compositional variations, with localized differences in cobalt concentration.
These variations lead to differential thermal responses within the material. The distinct subsystems compensate for each other’s expansion and contraction, resulting in an overall thermal expansion close to zero.
This property makes the material a potential candidate for applications requiring stability under extreme temperature fluctuations or high-precision measurement conditions, such as in aerospace, aviation, and electronic components.
Journal References:
Khmelevskyi, S. and Steiner, S. (2025) Predictive Theory of Anomalous Volume Magnetostriction in Fe–Ni Alloys: Bond Repopulation Mechanism of the Invar Effect. The Journal of Physical Chemistry C. doi.org/10.1021/acs.jpcc.3c07037
Sun, Y. et. al. (2025) Local chemical heterogeneity enabled superior zero thermal expansion in nonstoichiometric pyrochlore magnets. National Science Review. doi.org/10.1093/nsr/nwae462