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Experimental Evidence Shows Why Crystalline Materials Shrink on Heating

As reported in Science Advances, the researchers’ study can have major implications for matching the properties of materials to certain applications in electronics, medicine, and other domains. The study may also offer a better understanding of unusual superconductors- materials that are known to carry electric current without any energy loss.

Accurate measurements of the distances between atoms in scandium fluoride (ScF3) crystals provided the evidence. ScF3 is a material that is known to contract unusually under high temperatures (also referred to as “negative thermal expansion”). The researchers identified a new kind of vibrational motion that renders the sides of the cube-shaped, apparently solid crystals of the ScF3 to buckle on heating, thereby pulling the corners of the crystals closer together.

"Normally as something heats up, it expands. When you heat something up, atomic vibrations increase in magnitude, and the overall material size increases to accommodate the larger vibrations," said Igor Zaliznyak, Project Lead and Physicist, Brookhaven National Laboratory.

However, that association does not work for specific flexible materials, including chainlike polymers such as rubber and plastics. When those materials are subjected to increased heat, vibrations increase perpendicular to the chains’ length (one can imagine the sideways vibrations of a plucked string of a guitar). These are transverse vibrations that pull the chains’ ends closer together, leading to overall shrinkage.

However, what about ScF3? With a solid and cubic crystalline structure, it does not appear like a polymer at all—at least at the initial glance. There is also a widespread assumption that the atoms present in a solid crystal need to preserve their relative orientations, regardless of the size of the crystal. This assumption made it difficult for physicists to elucidate how this material shrinks upon heating.

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