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New Alloy Forms Under Pressure

A research team has created a material that was supposed to be impossible: an alloy of cerium and aluminum.

The new alloy was coaxed into being at high pressures—200,000 times the Earth's atmospheric pressure at sea level. The process may be useful for creating novel materials in the future. "It opens exciting possibilities for making new alloys," said SLAC physicist Wendy Mao, who collaborated on the data analysis for the paper. The work, led by Charles Zeng, a visiting graduate student from Zhejiang University in China, was published in the February 24 issue of the Proceedings of the National Academy of Sciences.

An alloy is a combination of two metal elements in which an atom of one element can swap places with an atom of the other without changing the properties of the material. Alloys generally have properties that make them different from, and often more useful than, either metal alone. Some common examples include steel and brass. In order for two elements to naturally form an alloy, however, their atoms need to be similar in characteristics such as size.

Cerium, a magnetic rare earth metal, is about twice as large as aluminum by volume, preventing the two metals from forming an alloy. At room temperature and pressure, the two have limited options for combining. They can be tightly ordered, with each atom having a specific position in relation to each other atom; or they can form an orderless glass. But high pressure, the new study found, can coax the two metals into the lattice-like configuration of an alloy.

To reach high pressure, Mao's collaborators at Argonne National Lab and the Carnegie Institute of Washington placed a sliver of a cerium-aluminum mixture in a gasket full of gas, and compressed the gasket between the tips of two diamonds. Diamonds are ideal for this work because they are not only the hardest known material, they also make good windows for viewing changes in the sample during compression. Because cerium has a looser electron cloud than aluminum, it shrank more under pressure. At a pressure of 20 gigapascals, about the pressure needed to make synthetic diamonds, cerium was small enough to form an alloy with aluminum. Surprisingly, when the team returned the pressure to normal, the alloy structure remained.

"We were able to bring what we got at high pressure back to ambient conditions," Mao said. "We made the alloy, and we got to keep it."

Not all of the properties of the alloy made it back to normal pressure. Cerium is magnetic because its outer electron usually stays with it. At high pressures, however, the electron left home. When the alloy returned to normal pressures, the outer electron returned to a single cerium atom. This may mean that the alloy is also magnetic at normal pressure, Mao said.

Although the alloy structure's persistence at normal pressure is interesting, it's not unique. Another familiar material that forms only at extreme pressures or temperatures is diamond, the basis for the billion dollar abrasives and jewelry industry, and fittingly the material used to create the new cerium-aluminum alloy.

Mao noted that this method of creating the alloy is not practical on a large scale, but that compression between diamonds is a valuable research tool. "They're useful for exploration of a wide range of pressure-temperature compositions," she said. "Then if there's something interesting, people can think about a practical way of making it." Practical synthesis and applications are still down the road, she said.

In the coming months, the team plans to see what cerium aluminum can do by measuring more of its properties at room pressure. They will also explore other alloy possibilities. "Cerium aluminum is a fascinating material, but it also opens up many other ideas," Mao said. "We can look all around the Periodic Table. If you just squeeze something, maybe you'll find something new."

Read about Wendy Mao's use of diamonds to study other materials in "Under Pressure."

—Lisa Grossman
SLAC Today, March 17, 2009