Chaos Key for New Class of Materials

Researchers reveal new class of stable oxides based on five or more elements

By Ken Kingery

Check out the story by North Carolina State University for more details.

Researchers at Duke University and North Carolina State University have discovered a brand-new class of materials.

When students learn about molecular structures, they’re shown crystals like salt, which resembles a 3-D checkerboard. Historically, most non-metal materials that scientists develop and study have similarly ordered and predictable molecular structures.

But now, materials scientists have shown that you can make non-metal materials that rely on chaos rather than organization for their stability. Dubbed entropy-stabilized oxides, the unexplored type of materials could hold the secret to everything from better batteries to better superconductors…or who knows what?

The findings were published on September 29, 2015, in Nature Communications.

“This line of thinking has been used to predict new random metals for several years, but people have never looked outside of metals,” said Stefano Curtarolo, professor of mechanical engineering and materials science and director of the Center for Materials Genomics at Duke. “Now, for the first time, we prove that this can be done for oxides, such as ceramics.”

An oxide is any compound that contains at least one oxygen atom and one atom of another element. Because oxygen atoms are negatively charged, they form oxide compounds with nearly every other element, resulting in ordered, predictable molecular structures. 

In contrast, the new class of materials combines oxygen with at many other elements simultaneously (five in the author article). And rather than relying on organized bonds and configurations for stability, it’s the complete mess of chaotic randomness that make entropy-stabilized oxides possible.

A pile of balls won’t stand on its own, but a pile of balls, shoes, bats, hats and gloves just might.

According to Curtarolo, with so many simpler, more predictable combinations to study, oxides with many ingredients have long been overlooked. After all, calculating the structures and behaviors of an ordered compound is far easier than doing so for a messy, disorganized one.

“People have actually made these types of materials several times before, but they ignored them because of the disorder and difficulty of characterizing them,” said Curtarolo. “This is the first time that we’ve really nailed them down, with calculations, modeling and experiments.”

Although entropy-stabilized oxides are difficult to write down on paper, perhaps counterintuitively, they are easy to make in the laboratory, which is exactly what collaborators at North Carolina State University have done. It’s the difference between trying to make good fudge—which has an ordered, crystalline structure—and throwing together trail mix.

The key, now, is to try every different flavor.

“It’s like you’re driving down the highway and discover an entirely new exit,” said Curtarolo. “So where do you go? These new materials could have revolutionary properties or it could just be a scientific curiosity. The truth is we don’t know yet.”

The research was supported by the U.S. Army Research Office under grant number W911NF-14-0285 and the National Science Foundation under grant number EEC 1156762.

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“Entropy-Stabilized Oxides.” Christina M. Rost, Edward Sachet, Trent Borman, Ali Moballegh, Elizabeth C. Dickey, Dong Hou, Jacob L. Jones, Stefano Curtarolo, Jon-Paul Maria. Nature Communications, September 29, 2015. DOI: 10.1038/NCOMMS9485