A new ultra-hard form of carbon may exist between graphite and diamond.
Carbon can exist in a form halfway between graphite and diamond, say researchers in China and the United States. And they believe this stuff is as hard as diamond itself.
Yanming Ma of Jilin University in Changchun, China, and his colleagues think that the new carbon material they have predicted in theoretical calculations1 may have been made already, in 2003.
At that time, Ho-kwang Mao of the Carnegie Institution of Washington in Washington DC, a co-author on the current work, and his collaborators found that when they squeezed graphite in a diamond-toothed press, it formed a new material hard enough to crack the teeth2.
But the researchers couldn't say for sure what form of carbon they had made. Ma and his co-workers now say that their hypothetical material has all the properties needed to explain the earlier results.
Diamond is one of the two common crystal forms of pure carbon, the other being graphite. They could hardly be more different: one brilliantly transparent and ultra-hard, the other silvery black and soft.
That's because the carbon atoms in diamond are linked into a three-dimensional network in which each atom has four neighbours. In graphite, the carbons are joined into sheets of hexagonal rings that are themselves bonded together only weakly. Graphite itself can be turned into diamond by squeezing it at high pressures and temperatures — the industrial method by which synthetic diamond is made for use as an abrasive and in cutting tools.
It has been thought for some time that other, more exotic forms of carbon could offer strong and hard materials. Carbon nanotubes, made from cylinders of graphite-like sheets, are super-strong and very stiff. And in 2005, researchers in Germany and France reported a form of carbon called aggregated diamond nanorods, which seemed to be slightly harder than ordinary diamond3.
Every carbon phase found so far has turned out to have interesting properties. Ho-kwang Mao , Carnegie Institution of Washington
"Every carbon phase found so far has turned out to have interesting properties," says Mao, and he suspects that the same might be true for the material described in the current work. "This material may be a bridge between diamond and graphite, which are both very useful."
But it has been hard to study the new state — which Mao and his co-workers created by squeezing graphite without heating (cold compression) — because, unless it is kept very cold, it reverts to graphite when the pressure is removed.
Mao and his colleagues tried to deduce the atomic structure of this material in 2003 by bouncing X-rays off it while it was held in the diamond press2. It was a tricky experiment to do, however, and the results weren't conclusive.
The researchers suggested that the graphite might form a buckled state in which links formed between the layers. This is similar to what Ma and his colleagues now propose. The team calls the new structure M-carbon because it has a crystal structure known technically as monoclinic.
"So far, our structure matches all experimental observations on the 2003 phase," says Artem Oganov of Stony Brook University in New York, a collaborator on the work.
The calculations indicate that M-carbon is more stable than graphite at pressures of more than about 130,000 atmospheres, and that it has a hardness in the same range as that found experimentally for diamond.
Very recently, researchers in Japan reported a similar new form of carbon, made in tiny patches on the surface of graphite by shining strong laser light onto it4,5. But although this too seemed to contain crosslinks between the graphite's carbon sheets owing to their corrugation, those researchers found that the bonding between carbon atoms differed somewhat from that reported by Mao and his co-workers from cold compression of graphite2.
Neil Ashcroft, a theoretical physicist at Cornell University in Ithaca, New York, advises caution about interpretations of hardness. "Hardness has been a controversial topic, partly because it is not very accurately defined experimentally," he says. Ma and his colleagues use a quantity called bulk modulus — which describes a substance's resistance to being uniformly squeezed — as a measure of hardness, but arguably the 'shear strength', which relates to the resistance to indentation, is more fundamental.
"They don't present any direct information on the shear strength of their proposed new phase of carbon, so far as I can tell," says Ashcroft.
But they do provide other details. M-carbon, they say, should be transparent and electrically insulating — like diamond, but unlike graphite.
However, given that it doesn't stick around when the pressure on it is eased, is it good for anything? Mao says that there might actually be some benefits to the reversible nature of the transformation. A material that becomes ultra-hard only when squashed might be useful in high-pressure gaskets, for example. Besides, he says, it might be possible to stabilize the new phase at lower pressures — for example, by making it as thin films or by adding dopants.
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Ball, P. Carbon that cracks diamond. Nature (2009). https://doi.org/10.1038/news.2009.446