Scandium trifluoride: Negative expansion X-rayed

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  • Published: Oct 15, 2015
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Scandium trifluoride: Negative expansion X-rayed

Accentuate the negatives

Negative thermal expansion is a rare but intriguing phenomenon that is known to occur in scandium trifluoride. A recent X-ray study offers hope of an explanation that could lead to the design of other materials that behave this way. 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]

Negative thermal expansion is a rare but intriguing phenomenon that is known to occur in scandium trifluoride. A recent X-ray study offers hope of an explanation that could lead to the design of other materials that behave this way.

The majority of substances expand when they are heated, density decreases as they warm and when they cool they shrink. The cracking and warping that often result from this expansion, are an everyday occurrence in buildings, railway track, bridges, electronics, and almost anything else exposed to wide temperature swings. Physicists have some trouble explaining why solids behave that way. The well-known exception being the anomalous behaviour of water close its freezing point as hydrogen bonds come into play.

There is another anomalous material, scandium(III) fluoride, commonly known as scandium trifluoride. At ambient pressures this compound has familiar cubic crystals with the perovskite structure wherein one metal position is vacant. Perovskites usually have many tuneable ordered phases with interesting electronic and magnetic properties of proven and putative technological importance in energy conversion and other areas. For scandium trifluoride it only really adopts other phases when under pressure taking on the rhombohedral structure, above 3 gigapascals it goes tetrahedral. Very few materials have such stability of crystal structure, indeed not only does this material sustain its cubic structure of a truly wide temperature range, it essentially undergoes no phase change until it melts. But, it is its negative thermal expansion, which occurs over the wide temperature range of 10 to 1100 Kelvin that intrigued physicist Jason Hancock of the University of Connecticut in Mansfield and his colleagues. They have been investigating the shrinkage of scandium trifluoride as it is heated. Understanding this process is far tougher than explaining conventional positive thermal expansion although that is difficult too.

Swell shrinkage

Writing in the journal Physical Review B, Hancock, Sahan Handunkanda, Erin Curry, Vladimir Voronov, Ayman Said, Gian Guzmán-Verri, Richard Brierley, and Peter Littlewood, hope to provide a clearer understanding of why materials change volume with temperature at all. Understanding conventional thermal expansion might lead to new approaches to engineering and electronics but understanding negative thermal expansion might lead to entirely novel applications and the development of a new class of materials that do not break under thermal stress. Scandium trifluoride itself is transparent so there are putative applications for its use in see-through components in electronic devices one might imagine.

In the classical sense, solids such as glass, metal, and rock comprise atoms hooked together by bonds that act like springs. These springs stretch and flex in response to heat. But because they are all interconnected, a rise in temperature should just mean more agitation as each spring affects its neighbouring springs symmetrically throughout the solid, a material should neither expand nor contract with a rise or fall in temperature.

Even more odd

“In many ways, the model is good,” explains Hancock. “It explains inelastic scattering of neutrons and X-rays, lots of other optical effects, the speed of sound waves, aspects of elasticity and heat conduction, even the transition temperature of some superconductors.” But it doesn’t do a good job of explaining thermal expansion.

Hancock and graduate student Sahan Handunkanda decided to look at scandium trifluoride because its odd behaviour might give them a new insight into what to look for in conventional materials. To explain the behaviour of scandium trifluoride, the team obtained low-temperature crystal structures at close to absolute zero using a near-perfect crystal provided by researchers at the Kerensky Institute of Physics in Krasnoyarsk, Siberia, led by Vladimir Voronov.

The data from energy- and momentum-resolved inelastic X-ray scattering as well as X-ray diffraction suggested that the octahedral units within the scandium trifluoride lattice are rotating almost freely in place even close to absolute zero. This is odd and unlike the behaviour of other materials, it is as if the structure is on the verge of undergoing a quantum phase transition but does not quite reach the tipping point to do so. Hancock and his colleagues suspect this anomaly underpins the negative thermal expansion of scandium trifluoride, the quantum forces somehow shifting in such a way as to lead to giant shrinkage as the material warms.

"We are next going to look at the behaviour of ScF3 on the approach to the quantum phase transition using other probes," Hancock told SpectroscopyNOW. "It seems the elasticity tensor may have strong changes and we would like to know more. We are also planning to investigate other materials with extremely strong isotropic NTE properties."

Related Links

Phys Rev B 2015, 92, 134101: "Large isotropic negative thermal expansion above a structural quantum phase transition"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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