Cold comfort: Graphite under pressure

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  • Published: Aug 1, 2012
  • Author: David Bradley
  • Channels: Raman
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Cold compress

High-pressure experiments have finally yielded the structure of cold-compressed graphite for the first time, according to US researchers using Raman spectroscopy, X-ray crystallography and optical techniques. 

High-pressure experiments have finally yielded the structure of cold-compressed graphite for the first time, according to US researchers using Raman spectroscopy, X-ray crystallography and optical techniques. This allotrope of carbon is comparable in hardness to its more familiar chemical cousin, diamond, but can be made under pressure without the need for extreme temperatures.

Methods for inducing changes in materials from one polymorphic form that do not require extreme temperatures might open the way to alternative superhard materials to replace synthetic diamond or for applications where diamond degrades too rapidly. Writing in the Nature journal Scientific Reports, the team explains how under normal conditions, pure carbon can exist in a wide range of forms from soft graphite to hard diamond by way of various amorphous forms, fullerenes, nanotubes and the wondrous graphene. Each form is singular in structure as well as having a range of intriguing properties that have found applications in industry, engineering, electronics and elsewhere and in which these and other previously unseen forms might have applications yet unimagined.

The team, based at Yale University, report a novel polymorph, which they refer to as M-carbon. This form of carbon was predicted in theoretical work back in 2006. It can now be prepared by compressing graphite to 200,000 times atmospheric pressure but at room temperature. By contrast there are several ways to make synthetic diamond using high pressures of just 50,000 times atmospheric pressure, but at 1500 Celsius. Previous researchers had, in fact, seen changes in graphite under high pressure and room temperature half a century ago, but this is the first proof, based on X-ray and Raman data to show that M-carbon exists experimentally and not just theoretically.

"Besides the unique mechanical properties discovered in M-carbon, we find that the transformation of graphite to M-carbon is extremely sluggish and requires a long time to reach equilibrium, which may be the additional reason why this puzzle remained unsolved for the past half century," explains Yuejian Wang, formerly a post-doctoral researcher at Yale who has since taken up a professorship at Oakland University.

M-carbon has much lower symmetry than diamond but is nevertheless just as hard. Indeed, it seems that M-carbon is so extremely incompressible that it can rival diamond and even cause an indentation in a diamond, Yale's Kanani Lee, who led the research explains. "Over the past few years, many theoretical computations have suggested at least a dozen different crystal structures for this new phase, but our experiments showed that only one crystal structure fits the data: M-carbon," Lee adds.

Raman reflections

The team explains that, "As Raman spectra reflect bonding rather than the atom arrangement in lattice planes as measured by X-rays, the change in Raman bands also suggests that at approximately 20 GPa, graphite transforms into a new phase, consistent with our X-ray diffraction measurements." Unfortunately, their current experiments cannot discern sp3 carbon-carbon bonds because of the overlapping bands due to the diamond anvil apparatus used to compress the graphite.

Lee and Wang worked with Yale's Joseph Panzik and Boris Kiefer of New Mexico State University with financial support from the Carnegie/Department of Energy (DOE) Alliance Center and by national synchrotron facilities supported by the DOE, National Science Foundation, and the W.M. Keck Foundation.

Related Links

  • Sci Rep, 2012, online: "Crystal structure of graphite under room-temperature compression and decompression"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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