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The various different forms of carbon include diamond, graphite, graphene (a single sheet of graphite) and the fullerenes, which form when carbon atoms bond together into tube and sphere-like structures. 

But in recent years, materials scientists have been gathering clues that hint at another type of carbon, which forms when graphite is compressed at room temperature to pressures in excess of 10 gigaPascals. 

The clues take the form of changes in various bulk properties of carbon under these conditions, things like its resistivity, optical transmittance and reflectance and so on. All this indicates the existence of some kind of phase change in which a new form of carbon is appearing.

So the race is on to identify this new allotrope, and since carbon atoms can link together in an infinite number of ways, there is no shortage of candidates. 

Today, Maximilian Amsler at the University of Basel in Switzerland and a number of pals put forward a new structure, which they call M10-carbon. These guys have used various computer simulation techniques to model how carbon atoms might bond under these conditions.  

The result, they say, is a structure more stable than graphite at pressures above 14 GPa and, like diamond, consisting entirely of atoms linked together by sp3 bonds. The material is also almost as hard diamond.

What’s more, Amsler and co have simulated the x-ray diffraction pattern this material ought to produce and say it matches that found in experiment.

The problem, however, is that this is just one of half a handful of proposed structures that all attempt to explain the experimental evidence. These also produce x-ray diffraction patterns that match experiment and at least one, an allotrope known as z-carbon, is more thermodynamically favourable than M10-carbon. 

For the moment, the jury is clearly out on which of these theoretical structures actually forms in the real world.

So what to do? Clearly there are a myriads of potential candidates and the only way to distinguish between them is by detailed and careful measurement. 

So the ball is firmly back on the experimenter’s side of the court. These experiments are not easy. however, so it may be some time before the crown for discovering a new allotrope of carbon can be convincingly claimed.

Ref: http://arxiv.org/abs/1202.6030: Prediction Of A Novel Monoclinic Carbon Allotrope

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