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The secret behind crystals that shrink when heated



  The secret behind crystals that shrink when heated
This animation shows how solid crystals of scandium fluoride shrink when heated. While the bonds between scandium (green) and fluorine atoms (blue) remain relatively rigid, the fluorine atoms oscillate independently along the sides of the cubic crystals, resulting in a wide range of distances between adjacent fluorine atoms. The higher the temperature, the greater the kinking on the sides of the crystals, resulting in the overall effect of contraction (negative thermal expansion). Picture credits: Brookhaven National Laboratory

Brookhaven National Laboratory scientists from the US Department of Energy have new experimental evidence and a prediction theory that solves a long-standing science of materials science: why certain crystalline materials shrink when heated. Their work, just published in Science Advances could be widely used to adapt material properties to specific applications in medicine, electronics, and other fields, and even provide new insights into unconventional superconductors (materials that carry electrical current) without energy loss ).

The evidence comes from precision measurements of the distances between atoms in crystals of scandium fluoride (ScF 3 ), a material known for its unusual contraction at elevated temperatures (also known as "negative thermal expansion"). , What the scientists discovered is a new kind of vibrational motion that causes the sides of these cube-shaped, seemingly solid crystals to bend when heated and make the corners closer together.

"Normally, something expands when heated." said the Brookhaven physicist Igor Zaliznyak, who led the project. "When you heat something up, the atomic vibrations increase and the total material size increases to accommodate the larger vibrations."

However, this relationship does not apply to certain flexible materials, including chain-like polymers such as plastics and rubber. In these materials, increasing heat only increases the vibrations perpendicular to the length of the chains (imagine the lateral vibrations of a plucked guitar string). These transverse vibrations pull the chain ends closer together and cause a general shrinkage.

But what about scandium fluoride? With a solid, cubic-crystalline structure, it resembles – at least at first glance – no polymer. The widespread assumption that the atoms in a solid crystal must maintain their relative orientations, regardless of crystal size, irritated the physicists and explained how this material shrinks when heated.

Neutrons and a dedicated disciple to the rescue

A group from the California Institute of Technology (Caltech) investigated this puzzle using a method at the Spallation Neutron Source (SNS), a user facility of the DOE Office of Science at the Oak Ridge National Laboratory. The measurement, such as neutron beams, a type of subatomic particle that scatters atoms in a crystal, can provide valuable information about their arrangement at the atomic level. It is particularly useful for light materials such as fluorine, which are invisible to X-rays, Zaliznyak said.

On this work, Zaliznyak noted that his colleague Emil Bozin, an expert in another neutron scattering analysis technique, could probably make progress understanding the problem. Bozin's method, known as the "pair distribution function," describes the probability of finding two atoms separated by a given distance in a material. Computational algorithms then sort the probabilities to find the structural model that best fits the data.

Zaliznyak and Bozin teamed with the Caltech team to collect data from SNS using Caltech's ScF 3 examples to see how the distances between neighboring atoms changed with increasing numbers Temperature.

David Wendt, a student who began an internship at the Brookhaven Lab High School Research Program after completing his second year of high school (now a freshman at Stanford University), handled much of the data analysis gap. During his school years, he continued to work on the project and received the position of first author on the paper.

"David basically reduced the data to the form we could analyze with our algorithms, fitted the data and created a model to model the positions of the fluorine atoms and perform the statistical analysis to compare our experimental results with the model Zaliznyak said

"I am very grateful for the opportunity the Brookhaven Lab has offered me to contribute to the original research through its High School Research Program," said Wendt.

  The Secret Behind Crystals
Other co-authors The study included (from left) Kate Page, formerly Oak Ridge National Laboratory, Brookhaven Lab physicist Emil Bozin, and ORNL instrument researcher Jörg Neuefeind Photo credit: Genevieve Martin / Oak Ridge National Laboratory

Results: "soft" motion in a solid

The measurements showed that the bonds between scandium and fluorine do not really change on heating. "In fact, they expand slightly," Zaliznyak said, "which is in keeping with the question why most solids expand."

However, the distances between adjacent fluorine atoms have become very different with increasing temperature.

"We searched for evidence that the fluorine atoms remained, as always assumed, in a fixed configuration, and we found exactly the opposite!" Zaliznyak said:

Alexei Tkachenko, an expert on the theory of soft condensed matter at Brookhaven's Lab Center for Functional Nanomaterials (another user facility of the Office of Science) made significant contributions to explaining this unexpected data.

Since the fluorine atoms did not appear to be confined to rigid positions. The explanation could be based on a much older theory, originally developed by Albert Einstein, to explain atomic motions by looking at each atom separately. And surprisingly, the final explanation shows that the shrinkage caused by heat in ScF 3 bears a remarkable similarity to the behavior of soft matter polymers.

"Because every scandium atom has a rigid bond with fluorine, the" chains "of scandium fluoride that form the sides of the crystalline cubes (with scandium at the corners) act much like the rigid parts of a polymer," explained Zaliznyak. However, the fluorine atoms in the middle of each cube face are not inhibited by other bonds. As the temperature increases, the "underconstrained" fluorine atoms can independently oscillate in directions perpendicular to the rigid Sc-F bonds. These transversal thermal vibrations bring the Sc atoms closer together at the corners of the cubic lattice, leading to a shrinkage similar to that seen for polymers.

  The secret behind crystals that shrink when heated.
Igor Zaliznyak, Physicist at Brookhaven Labs The Department of Solid State Physics and Materials Science (right) led a team of scientists, including Alexei Tkachenko from the Center for Functional Nanomaterials (left), to decipher the mechanism behind the ability of scandium fluoride to shrink when heated. Picture credits: Brookhaven National Laboratory

Thermal Adaptation for Applications

This new understanding enhances scientists' ability to predict or strategically design a material's thermal response for applications where temperature changes are expected. For example, materials used in precision machining should ideally have little change in heating and cooling to ensure the same precision under all conditions. Materials used for medical applications, such as dental fillings or bone substitutes, should have thermal expansion properties very close to those of the biological structures in which they are embedded!). And in semiconductors or submarine fiber optic cables, the thermal expansion of insulating materials should match those of functional materials to avoid compromising signal transmission.

Zaliznyak notes that a limited open scaffold architecture such as that in ScF 3 also occurs in copper oxide- and iron-based superconductors – with crystal lattice vibrations believed to play a role in the ability of these materials to provide electrical current without resistance to transport.

"The independent vibration of atoms in this open framework structures can in a way contribute to the properties of these materials, which we can now calculate and understand," said Zaliznyak. "You could actually explain some of our own experimental observations that are still a mystery in these superconductors," he added.

"This work profoundly benefited from the important benefits of the national DOE laboratories – including the unique DOE facilities and our ability to do so. Long-term projects that accumulate important contributions over time to make a discovery." Zaliznyak said. "It is a unique combination of different expertise among the co-authors, including a dedicated student intern, whom we could synergistically integrate for this project." Without the expertise of, it would not have been possible to successfully perform this research for all team members. "


The structural secret of scandium fluoride is clarified


Further information:
"Entropic Elasticity and Negative Thermal Expansion in a Simple Cubic Crystal" Science Advances (2019). advances.sciencemag.org/content/5/11/eaay2748

Provided by
Brookhaven National Laboratory




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The Secret Behind Crystals Shrinking on Heating (2019, Nov. 1)
retrieved on 2 November 2019
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