Osmium is Forever: Rare metal’s strength humbles mighty diamond’s

In a surprising overturn, the lustrous, blue-white element osmium has beaten diamond in a test of compressibility. Not used much today, osmium has been incorporated into alloys for fountain-pen tips and phonograph needles.

Diamond still reigns as the hardest of materials. However, the finding that osmium is actually the high-pressure champ may open up new routes to materials that may someday surpass diamond in hardness, says Hyunchae Cynn of Lawrence Livermore (Calif.) National Laboratory.

“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.”

Cynn and his colleagues measured just how sturdy osmium is by putting a powder of the metal in a diamond anvil and squeezing it to 600,000 times atmospheric pressure. The researchers observed from X-ray diffraction patterns how much the volume of osmium’s hexagonal atomic structure shrunk with pressure.

From those data, they then calculated the element’s resistance to compression, a property called the bulk modulus. It was a hefty 462 gigapascals (GPa).

Diamond’s bulk modulus, which had been considered supreme, is only 443 GPa. The researchers report their finding, which they say appears to be the first measurement of osmium’s compressibility, in the April 1 Physical Review Letters.

Some theorists had anticipated osmium’s extraordinary sturdiness. For instance, in 1995, one group estimated that the bulk modulus of osmium would be 476 GPa.

However, because that calculated figure was greater than diamond’s measured bulk modulus, the prediction was met with some skepticism, Cynn says.

Although many cutting tools wield diamond or diamond-coated tips, those devices work well for only a fraction of the cutting requirements of industry. For instance, diamond slices through hot steel poorly because the iron in the steel absorbs carbon atoms from the diamond. Moreover, being brittle, diamond fails catastrophically when it breaks down. For such reasons, materials scientists have long been hunting for diamond substitutes.

In some previous attempts to create new, hard materials, researchers embedded noncarbon elements in diamondlike tetrahedral structures (SN: 7/11/98, p. 28).

Although calculations predicted that one such compound, called beta-carbon nitride, would be harder than diamond (SN: 6/5/99, p. 367), no one has been able to make a piece big enough to test.

Although not as hard as diamond, osmium is the hardest of the metals, at least by one common test. Now that it’s been found to be the least compressible of all materials as well, it may serve as a new starting point for creating superhard substances. The recent experiment demonstrated that “you can have high bulk modulus even if you don’t have a diamondlike structure or bonding,” says John E. Klepeis, a theorist on the Livermore team.

Unlike diamond and many other brittle materials, metals are tough. They deform rather than crumble under severe pressure, notes Jeanloz. Consequently, “the experiments on osmium should open the eyes of both theoreticians and experimentalists to new possibilities for very strong, hard, and tough materials,” he says.

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