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Liquid metal inter-atomic distance shown to contract on heating, surprisingly

Structural changes in liquid Al with increasing temperature reproduced by MD simulation. (D) Schematically showing the evolution from high-coordinated to low-coordinated polyhedra with temperature increase. The increased weight factor of low-coordinated polyhedra leads to the reduced bond length between centre atom and atoms in the first shell.

Structural changes in liquid Al with increasing temperature reproduced by MD simulation. (D) Schematically showing the evolution from high-coordinated to low-coordinated polyhedra with temperature increase. The increased weight factor of low-coordinated polyhedra leads to the reduced bond
length between center atom and atoms in the first shell.

When matter heats, it generally expands. Now scientists reveal that when several metallic liquids get heated, the distance between atoms can actually contract, findings detailed in this week’s Proceedings of the National Academy of Sciences.

Using synchrotron x-ray diffraction and molecular dynamics simulations, materials scientist Jian-Zhong Jiang, director of the International Center for New-Structured Materials at Zhejiang University in Hangzhou, China, and his colleagues investigated a variety of metallic liquids. These included molten aluminum, zinc, tin, indium, copper, nickel, silver, and gold.

Metallic liquids are composed of polyhedral clusters of atoms. These atomic clusters often have high coordination numbers—that is, many atoms surround a central atom.

As the heat rises, the scientists found the coordination numbers of these clusters drops—the bonds between atoms break, resulting in more close-packed smaller clusters. As such, the average distance between a center atom and its neighbors contracts, Jiang explains.

“This is not an expected result—very exciting!” Jiang says. “We believe that our finding is a universal feature for liquids that are composed of various-sized polyhedra.”

The distance between atomic clusters still continues to grow as metallic liquids get heated, meaning they

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expand overall. In that sense, they differ from materials that demonstrate negative thermal expansion, contracting when heated, such as cubic zirconium tungstate and, to a certain extent, water.

These findings could shed light on how liquids behave and how glasses and crystals form, leading to better control of the mechanical and thermal properties of matter for novel materials, Jiang says.

Categories: Physics
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