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Under pressure. Osmium holds up to the extreme pressure of a diamond anvil.

Osmium Resists the Squeeze

Look to the tip of your fountain pen, not your jewelry, for the most incompressible material known. The exotic metal osmium, used in alloy form in some pen nibs and surgical needles, is even harder to squash than diamond, says a team of high-pressure physicists. The finding could open up a new path in the hunt for superhard materials, the researchers say.

Osmium's résumé already singles it out as something special. One of the platinum metals, osmium is the densest naturally occurring element, almost twice as dense as lead, and has a stratospheric melting point in the same league as that of the tungsten used for lightbulb filaments. It's also very hard. But Hyunchae Cynn and his colleagues at Lawrence Livermore National Laboratory in California noticed that theoretical estimates of osmium's compressibility were unaccountably low. Because no one had ever bothered to actually measure osmium's compressibility, Cynn and his team decided to take things into their own hands.

The team crushed a sample containing tiny crystals of osmium and liquid argon between the jeweled jaws of a diamond anvil, which exerts several hundred tons per square centimeter of pressure. As the researchers racked up the pressure, they used x-ray diffraction to watch how the spacing between osmium atoms narrowed. "We were quite surprised," says Cynn. Osmium turned out to be less compressible than diamond, the existing record holder, the team reports in the 1 April issue of Physical Review Letters.

"It's stunning that such an incompressible element is only now recognized," says Raymond Jeanloz of the University of California, Berkeley. "The additional surprise is that a metal could be so incompressible." Diamond had always been assumed to be a special case because of its particular bonding pattern, which is much more rigid than the way atoms of a metal are held together, Jeanloz explains. Although diamond retains its title as hardest substance because of its additional ability to resist any attempt to slide, or shear, one face parallel to another, Cynn says osmium's low compressibility hints at a new class of superhard materials.

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Lawrence Livermore High-Pressure Physics Group