New form of superhard carbon observed

Scientists at Carnegie's Geophysical Laboratory are part of a team that has discovered a new form of carbon, which is capable of withstanding extreme pressure stresses that were previously observed only in diamond. This breakthrough discovery will be published in Physical Review Letters.

The team was led by Stanford's Wendy L. Mao and her graduate student Yu Lin and includes Carnegie's Ho-kwang (Dave) Mao, Li Zhang, Paul Chow, Yuming Xiao, Maria Baldini, and Jinfu Shu. The experiment started with a form of carbon called glassy carbon, which was first synthesized in the 1950s, and was found to combine desirable properties of glasses and ceramics with those of graphite. The team created the new carbon allotrope by compressing glassy carbon to above 400,000 times normal atmospheric pressure.

This new carbon form was capable of withstanding 1.3 million times normal atmospheric pressure in one direction while confined under a pressure of 600,000 times atmospheric levels in other directions. No substance other than diamond has been observed to withstand this type of pressure stress, indicating that the new carbon allotrope must indeed be very strong.

However, unlike diamond and other crystalline forms of carbon, the structure of this new material is not organized in repeating atomic units. It is an amorphous material, meaning that its structure lacks the long-range order of crystals. This amorphous, superhard carbon allotrope would have a potential advantage over diamond if its hardness turns out to be isotropic—that is, having hardness that is equally strong in all directions. In contrast, diamond's hardness is highly dependent upon the direction in which the crystal is oriented.

“These findings open up possibilities for potential applications, including super hard anvils for high-pressure research and could lead to new classes of ultradense and strong materials,” said Russell Hemley, director of Carnegie's Geophysical Laboratory.

This research was funded, in part, by the Department of Energy's Office of Basic Energy Sciences Division of Materials Sciences and Engineering, EFree, HPCAT, where some of the experiments were performed, is funded by DOE-BES, DOE-NNSA, NSF, and the W.M. Keck Foundation. APS, where some of the experiments were performed, is supported by DOE-BES.

The Carnegie Institution for Science (carnegiescience.edu) is a private, nonprofit organization headquartered in Washington, D.C., with six research departments throughout the U.S. Since its founding in 1902, the Carnegie Institution has been a pioneering force in basic scientific research. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.