Crystal puts pressure on diamonds

Diamonds have charmed high-pressure scientists. Now, a new suitor is pressing for affection.

A moissanite crystal can squeeze more material than a diamond anvil can. Each pointed crystal (arrow) in this anvil is as big as an 8-carat diamond. Agnes Mao

A synthetic crystal called moissanite has squeezed in on diamond’s role as the jaws of a powerful experimental device known as a diamond anvil cell. With diamond anvils, researchers squeeze materials to extraordinarily high pressures. How substances behave under such conditions can reveal geologic processes deep within planets.

Materials scientists also use these anvils to study samples under extreme conditions. Diamond’s unrivaled strength and rigidity have made it well suited for the intense job. Good as they are, diamond anvils have flaws. They’re small, so they can hold only minute samples. Larger anvils would require larger gems, which are scarce and expensive. Also, diamond’s spectrographic signature—the wavelengths of light it absorbs and emits—interferes with the analysis of many materials.

Now, a team of researchers has replaced an anvil cell’s diamonds with bigger crystals of moissanite, or silicon carbide. The resulting devices can squeeze much larger volumes of material, Ji-an Xu and Ho-kwang Mao of the Carnegie Institution of Washington (D.C.) report in the Oct. 27 Science.

Within one device, the researchers recorded pressures of 52.1 gigapascals, about a half-million times atmospheric pressure. That’s high-enough pressure for many experiments whose samples are too large for current diamond anvils, says Xu. The team’s largest moissanite crystal is about the size of a 300-carat diamond, he says. Most diamonds in anvils are less than 0.3 carat.

Because moissanite’s spectrographic signature differs from that of diamond, the new anvils enable researchers to see, for the first time, clear signatures of certain materials at high pressure, such as diamonds, says Xu. In one early example, the Carnegie researchers have used their new anvils to observe subtle details of hydrogen bonds in ice.

“This anvil allows one to see a lot of new things,” comments Carnegie’s Russell J. Hemley, who has worked closely with Xu and Mao on other projects. Other scientists, however, caution that moissanite’s applications are limited because it’s not as hard as diamond. “Although moissanite is a very, very strong and very rigid material, it is still second place to diamond by a large factor,” says Samuel T. Weir of the Lawrence Livermore (Calif.) National Laboratory, who uses diamond anvils.

He points out that diamond anvils create pressures about 10 times those achieved in the moissanite experiments. Moissanite “is not something that’s going to overtake diamond as the mainstay of high-pressure physics,” he says. Still, he concludes, “for certain classes of experiments, I think it will be an important material.”

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