An incredible shrinking material: Engineers reveal how scandium trifluoride contracts with heat

Nov 07, 2011 by Marcus Woo
Heat causes the atoms in ScF3 to vibrate, as captured in this snapshot from a simulation. Fluorine atoms are in green while scandium atoms are in yellow. [Credit: Caltech/C. Li et al.]

(PhysOrg.com) -- They shrink when you heat 'em. Most materials expand when heated, but a few contract. Now engineers at the California Institute of Technology (Caltech) have figured out how one of these curious materials, scandium trifluoride (ScF3), does the trick—a finding, they say, that will lead to a deeper understanding of all kinds of materials. 

The researchers, led by graduate student Chen Li, published their results in the November 4 issue of Physical Review Letters (PRL).

that don't expand under heat aren't just an oddity. They're useful in a variety of applications—in mechanical machines such as clocks, for example, that have to be extremely precise. Materials that contract could counteract the expansion of more conventional ones, helping devices remain stable even when the heat is on.

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"When you heat a solid, most of the heat goes into the vibrations of the atoms," explains Brent Fultz, professor of materials science and applied physics and a coauthor of the paper. In normal materials, this vibration causes atoms to move apart and the material to expand. A few of the known shrinking materials, however, have unique crystal structures that cause them to contract when heated, a property called negative thermal expansion. But because these crystal structures are complicated, scientists have not been able to clearly see how heat—in the form of atomic vibrations—could lead to contraction.

But in 2010 researchers discovered negative thermal expansion in ScF3, a powdery substance with a relatively simple crystal structure. To figure out how its atoms vibrated under heat, Li, Fultz, and their colleagues used a computer to simulate each atom's quantum behavior. The team also probed the material's properties by blasting it with neutrons at the Spallation Neutron Source at Oak Ridge National Laboratory (ORNL) in Tennessee; by measuring the angles and speeds with which the neutrons scattered off the atoms in the crystal lattice, the team could study the atoms' vibrations. The more the material is heated the more it contracts, so by doing this scattering experiment at increasing temperatures, the team learned how the vibrations changed as the material shrank.

a. The crystal structure of ScF3. Scandium atoms are green while fluorine atoms are white. b. You can imagine the bonds connecting the atoms as springs. In the so-called quartic oscillations of ScF3, a fluorine atom (white) vibrates more in the vertical direction, as seen in this diagram. With each shake, the fluorine pulls the two scandium atoms (green) together. [Credit: Modified from fig. 1 of C. Li et al., Phys. Rev. Lett. 107, 195504 (2011). Copyright APS]

The results paint a clear picture of how the material shrinks, the researchers say. You can imagine the bound scandium and fluorine atoms as balls attached to one another with springs. The lighter fluorine atom is linked to two heavier scandium atoms on opposite sides. As the temperature is cranked up, all the atoms jiggle in many directions. But because of the linear arrangement of the fluorine and two scandiums, the fluorine vibrates more in directions perpendicular to the springs. With every shake, the fluorine pulls the scandium toward each other. Since this happens throughout the material, the entire structure shrinks.

The surprise, the researchers say, was that in the large fluorine vibrations, the energy in the springs is proportional to the atom's displacement—how far the atom moves while shaking—raised to the fourth power, a behavior known as a quartic oscillation. Most materials are dominated by quadratic (or harmonic) oscillations—characteristic of the typical back-and-forth motion of springs and pendulums—in which the stored energy is proportional to the square of the displacement.

"A nearly pure quantum quartic oscillator has never been seen in atom vibrations in crystals," Fultz says. Many materials have a little bit of quartic behavior, he explains, but their quartic tendencies are pretty small. In the case of ScF3, however, the team observed the quartic behavior very clearly. "A pure quartic oscillator is a lot of fun," he says. "Now that we've found a case that's very pure, I think we know where to look for it in many other materials." Understanding quartic oscillator behavior will help engineers design materials with unusual thermal properties. "In my opinion," Fultz says, "that will be the biggest long-term impact of this work."

Explore further: Physicists advance understanding of transition metal oxides used in electronics

More information: The other authors of the PRL paper, "The structural relationship between negative thermal expansion and quartic anharmonicity of cubic ScF3," are former Caltech postdoctoral scholars Xiaoli Tang and J. Brandon Keith; Caltech graduate students Jorge Muñoz and Sally Tracy; and Doug Abernathy of ORNL. The research was supported by the Department of Energy.

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Macksb
1 / 5 (1) Nov 07, 2011
This research may have much greater importance than the investigators realize. The knowledge gained here may be relevant to high temp superconductivity. Specifically, this work might explain how the perovskite structure of the hts cuprates could contribute to superconductivity. (Despite the prevailing belief that phonons (lattice vibrations) are not involved in high temp superconductivity.)

Ignore expansion or contraction, though that knowledge is important. Consider instead synchronized oscillations within a lattice. Synchronized lattice oscillations (synchronized phonons) could be part of the mechanism behind high temp superconductivity. The perovskite structure of the cuprates is much like a horse and rider system. If the rider is perfectly synchronized with the moving legs of the horse, and that synchrony becomes pervasive throughout the cuprate, that would create a powerful and perfect organization. Such a synchrony may break out as the cuprate temperature is reduced.

rawa1
not rated yet Nov 07, 2011
Ignore expansion or contraction, though that knowledge is important. Consider instead synchronized oscillations within a lattice.
You're just promoting your usual "synchronized oscillators" stuff, do you? Unfortunately, if this stuff has nothing to do with expansion or contraction, then it has nothing to do with research presented and it's simply OT.
Macksb
1 / 5 (1) Nov 07, 2011
In a nutshell, Rawa, I plead guilty to your charge, but maintain that my comment is directly and deeply on topic. Note that this research is all about vibrations--which are oscillations. Note also that the special behavior here involves an unusual type of oscillation--quartic oscillations, not quadratic. It is a pure quartic oscillator, created and driven by changes in temp, producing an interesting result: contraction. But the story is not just expand or contract; that view is important but too limited. Expansion and contraction are the outcomes (the "what")of the internal organization of the lattice (the "why"), which is driven by oscillations and their interactions. I merely suggest that it is worth considering other potential consequences of organized oscillations. My post compliments the authors by focusing on the "why" of their work, tracking the words of Prof. Fultz. So my post draws attention to their work and is quite on topic--deeply so. To the fourth power.
hush1
not rated yet Nov 07, 2011
Pressure-induced expansion has expanded (pun intended) beyond the zeolites. The opposite of what is described above.
Isaacsname
not rated yet Nov 07, 2011
Can they make lattices with this material that include materials that exhibit the opposite effects , ie, is it possible to compose a lattice of materials that both expand and contract with thermal energy ?
dnatwork
not rated yet Nov 07, 2011
Isaacsname, I was thinking the same thing. Any temperature change should be magnified. Then pair that up with some piezolectric materials, and generate some extra power from waste heat.
Isaacsname
not rated yet Nov 07, 2011
Isaacsname, I was thinking the same thing. Any temperature change should be magnified. Then pair that up with some piezolectric materials, and generate some extra power from waste heat.


I wonder if it could lead to a new type of Skutterudite ?
rawa1
not rated yet Nov 08, 2011
Can they make lattices with this material that include materials that exhibit the opposite effects , ie, is it possible to compose a lattice of materials that both expand and contract with thermal energy ?
Every material will start to expand with increasing temperature at the sufficient temperature (providing that it will not decompose during it). It's similar to negative differential resistance of thermistors, which work in limited range too.
Isaacsname
not rated yet Nov 08, 2011
Can they make lattices with this material that include materials that exhibit the opposite effects , ie, is it possible to compose a lattice of materials that both expand and contract with thermal energy ?
Every material will start to expand with increasing temperature at the sufficient temperature (providing that it will not decompose during it). It's similar to negative differential resistance of thermistors, which work in limited range too.


Yes..thank you...I realize this....what I am asking is if you can build a lattice with materials that exhibit both positive AND negative thermal expansion ? Something like Invar ?
Macksb
1 / 5 (1) Nov 09, 2011
As Rawa notes above, I post frequently about "synchronized oscillators." (Glad to see you are a fan, Rawa.) In my posts,I almost always cite Art Winfree's work, circa 1967. Art, who later won a MacArthur Prize, developed a mathematical model showing that limit cycle oscillators have a tendency to synchronize, and when they do, certain exact patterns are allowed--which he identified--and no others.

Several people have extended his work, including Kuramoto, Steve Strogatz of Cornell (Sync) and Mirollo. Strogatz wrote a great summary of Winfree's work in Scientific American, late 1993--copy available on Strogatz website (cornell.edu).

Winfree applied his model to biology, not physics. Strogatz and others have argued that it should apply to physics, but physicists remain oblivious.

I like this article because it shows that Winfree's ideas do apply to physics. The article is about oscillations (vibrations), and their interactions. It's a three oscillator system....
Macksb
1 / 5 (1) Nov 09, 2011
Continuing...it's a three oscillator system comprised of two scandiums and one fluorine, organized by the interactions of their oscillations. The pattern that emerges is precisely one of the "allowable" patterns that Winfree specified for a three oscillator system: two that are exactly antisynchronous, and one that is exactly twice the frequency of the other two. Two at power of two, one at power of four. The two scandiums are exactly antisychronous (converging, then diverging). This article thus shows it all: smart physicists who appear to be unaware of Winfree's model; a system of oscillators that interact; an exact pattern that emerges from such interactions; and the pattern is one predicted (abstractly) by Winfree.

As I have said in the past, Winfree's 1967 model cries out to be applied to physics. It may be the key to understanding phases of matter (solid, liquid, gas); phase transitions; superconductivity, superfluidity, and more. It's inherently quantum as well.

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