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Osmium Is Stiffer than Diamond, Scientists Discover

Whether it will compete for the title of a girl's best friend remains to be seen, but the element osmium can already challenge diamond in at least one respect: stiffness. According to a report published in the current issue of Physical Review Letters, osmium can withstand compression better than any known material. The results provide a potentially new lead in the search for superhard materials.

Diamond's ability to resist scratches, dents and chipping--in short, its hardness--makes it an ideal choice for tips in industrial-strength machines. A related quality that is easier to calculate than hardness is an element's resistance to compression, known as its bulk modulus. The properties are interrelated because the stiffest materials also tend to be the hardest ones. But even though osmium is much softer than diamond, initial estimates of its bulk modulus indicated a similar value to that of diamond.

Hyunchae Cynn and colleagues at Lawrence Livermore National Laboratory thus set out to test the property experimentally. They squeezed osmium powder under 600,000 atmospheres of pressure and calculated changes in the spacing between atoms in the sample using x-ray diffraction patterns. The team reports that osmium's bulk modulus is 462 gigapascals (GPa), as compared with diamond's 443 GPa. "It is intriguing that a light, covalently bonded element such as diamond and a heavy, metallic element such as osmium, with very different chemical bonding, would both have large values of the bulk modulus," the authors note. They conclude that related compounds such as transition metal carbides, nitrides and oxides could be sources of new superhard materials.

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