Molecular oscillation may explain why some solids shrink when heated

A mixture of stiffness and flexibility in the movement of atoms can lead to collapse

Like heated objects, most solids expand because higher temperatures cause atoms to vibrate faster and take up more space. However, some solid crystals, like scandium fluoride, shrink when heated – a phenomenon known as negative thermal expansion.

By measuring the distances between atoms in scandium fluoride crystals, scientists now believe they have discovered how this contraction occurs. While the bonds between scandium and fluorine remain strong when heated, the fluorine atoms in the crystal can easily flow. This mixture of stiffness and flexibility is caused by the giving way to crystals, researchers reported online Nov. 1 in Scientific Advances.

“Some of the biggest challenges in our field relate to answering questions about solids,” said Jason Hancock, a physicist at the University of Connecticut at Storrs, who was not involved in the study. Solving the puzzle of negative thermal expansion in scandium fluoride will help physicians understand materials similar to copper-based superconductors, which transmit electricity without resistance but are still at temperatures too low to be used in the field. practice (SN: 12/8/17.

Scandium fluoride “is the simplest material that has all the potential for this phenomenon and allows us to eliminate what is really going on,” said Igor Zaliznyak, a physicist at Brookhaven National Laboratory in Upton, NY. The shrinkage is probably the same as that of other materials, Zaliznyak says.

The team discovered the mechanism using a technique called total neutron diffraction. The researchers pumped out scandium fluoride using a neutron beam and recorded how the subatomic particles swelled into crystals at temperatures as high as 1,100 Kelvin (about 827 ° Celsius). By studying diffuse models, scientists calculate the possible distance between atomic pairs.

The distance between the scandium and fluorine atoms is almost the same when heated, which means that the chemical interaction between the two is close. The same applies to the distance between individual scandium atoms. However, there is a large distance between the fluorine atoms, indicating that their positions in the crystal atomic lattice are more flexible. According to the researchers, the combination of fixed and variable distances between atoms keeps the crystal in its cubic shape while allowing its sides to bend and compress.

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