When a substance is heated, its atoms or molecules move faster and faster. This movement creates disorder, also known as entropy. This is what we observe when ice melts: the water molecules, neatly arranged in ice, start moving freely in liquid water.
But chemists have discovered surprising behavior: in certain materials, atoms can freeze even as temperature increases! A phenomenon that seems to contradict classical physics laws. Yet this discovery is very real, and it could pave the way for materials capable of retaining special properties—like being magnetized or producing electricity under pressure—even at room temperature.
Generally, the hotter a material gets, the more its atoms or molecules move in all directions. This movement causes disorder, and thus an increase in entropy. Conversely, when a material is cooled, its atoms can stabilize into a precise position: we then say they have "frozen."
It is in these well-ordered states, achieved at low temperatures, that certain interesting properties emerge, such as ferroelectricity (the ability to produce an electric voltage under pressure) or magnetization. The problem is that these effects usually only work in cold conditions, limiting their applications.
However, researchers from Rennes and Bordeaux have just shown that in certain special materials, atoms can freeze when temperature rises! An observation that appears to contradict thermodynamics laws requiring increased disorder with temperature, but which scientists have managed to explain.
The material they studied exhibits two stable magnetic states depending on its temperature. At low temperatures, electrons pair up, and the magnetic state is considered ordered. Heating to room temperature leads to a disordered magnetic state where electrons are no longer paired.
Thus, heating promotes electronic disorder (or magnetic entropy), which competes with the entropy related to atomic positions. The study shows that the system's total entropy does increase with temperature, as thermodynamics laws dictate, and that magnetic entropy dominates, allowing atoms to maintain their "frozen" low-temperature positions even at high temperatures.
These results, published in
Materials Horizons, show that combining electronic disorder with atomic order could lead to new materials for sensors, memory devices, and other applications.
Reference:
Francisco Javier Valverde-Muñoz, Ricardo Guillermo Torres Ramírez, Elzbieta Trzop, Thierry Bataille, Nathalie Daro, Dominique Denux, Philippe Guionneau, Hervé Cailleau, Guillaume Chastanet, Boris Le Guennic & Eric Collet.
Stabilizing low symmetry-based functions of materials at room temperature through isosymmetric electronic bistability
Materials Horizons 2025
https://doi.org/10.1039/D4MH01318B