[Un article de The Conversation écrit par Éric Collet – Professeur de Physique, expert en sciences des matériaux, Université de Rennes]
But, unlike intuition, we discovered a material with a change of magnetic state, for which X -ray crystallography measurements have shown that the atoms are freezing … by heating! This first surprised us, but we found an explanation, which we detail in our recent publication.
Some electronic devices only operate low temperature
The atoms positions usually freeze when you lower your temperature – this is the case, for example, when the water freezes in the freezer or when melted sugar crystallizes while cooling.
This phenomenon also exists in a solid state in many materials. Even in a solid, atoms vibrate between equivalent positions by symmetry (for example between left and right) – they only freeze in one of these positions when the temperature decreases.
For some materials, such as sugar or piezoelectrics used on sonars or sensors for ultrasound, atoms are frozen at room temperature. But for many molecular materials, this only occurs -20 ° C, -100 ° C or -200 ° C, for example.
The change in symmetry associated with the ordering of atoms which are freezing according to certain positions is illustrated in the figure above.
On the right, the atoms are messy and vibrate at high temperature. There is a mirror symmetry here and the positions of atoms on one side of the mirror are equivalent to those on the other side.
At low temperature, the atoms are freezing. For example, red atoms approach blue atoms on the right and move from blue atoms to the left. This modifies certain physical properties of materials and, for example, loads (+ and -) appear on the surface.
If we press such a material, the loads change, and this is what is the basis of piezoelectric sensors, for example. A simple press, like a sound, can modulate these charges and then be detected. This is how devices for ultrasound or sonars in submarines, for example: the sound wave that is reflected on an object is detected by the piezoelectric sensor through an electrical signal.
Other materials are also ferroelectric. It is then possible to return the atomic positions with an electric field and therefore reverse the loads. It is this device that is the basis of ram ferroelectric memories.
Unfortunately, for many molecular materials, this type of properties linked to the change of symmetry appear only in low temperature. It is then necessary to cool the materials to obtain the property, sometimes at -200 ° C. This constraint therefore limits the application of these materials, because many applications require devices operating at room temperature, because it is too complex and expensive to integrate cooling devices.
A surprising discovery: a material that freezes at high temperature
In the majority of materials, the atoms that constitute them set in motion with the rise in temperature. This thermal agitation creates a disorder, which is measured by a thermodynamic quantity called “entropy”.
The laws of physics stipulate that the more the temperature increases, the more disorder and therefore entropy increases. Thus, the disorder is larger at high temperature, with agitated atoms, at low temperature where the atoms are frozen. Conversely, at low temperature, disorder and, therefore, entropy decrease, as well as symmetry.
In our study, we observe the opposite phenomenon: the material we study is more symmetrical below -40 ° C than above. In other words, the molecules are on right/left disorderly positions at low temperature and ordered at high temperature and therefore, here, at room temperature.
Several types of disorder in competition
This phenomenon is made possible thanks to “electronic disorder”.
Indeed, in the material studied, the high and low temperature states also correspond to two magnetic states.
At low temperature, the material is in the state called “diamagnetic”, that is to say that the electrons live as a couple and their spins (their magnetic moments) are opposed – it is a constraint imposed by quantum mechanics. This corresponds to an ordered electronic state, because there is only a possible configuration: a spin up, the other down.
At high temperature, on the contrary, the material is in the “paramagnetic” state, that is to say that the electrons are single and their spins Can be oriented freely, which gives rise to several configurations (a few up, the others down, as illustrated by the red arrows in the figure above).
By heating, we promote “electronic” disorder (the large number of configurations of spins). This disorder competes with the ordering of atoms positions.
The gain in entropy linked to electronic disorder (which goes from a single configuration to five) is then greater than the cost to entropy linked to the ordering of atoms (from two configurations to one). Other phenomena also contribute to this increase in entropy.
In the end, global entropy, including atomic and electronic disorder, therefore increases well with temperature as imposed on the laws of physics. It is therefore the disorder of the electrons which authorizes to freeze the positions of the molecules.
Consequently, this new concept, combining electronic disorder and atomic order, opens the way to the development of new materials for devices such as sensors, memories, transducers or actuators operating at room temperature, without recourse to low temperatures.
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
