NPG Asia Materials Research Highlight (2011)

https://www.nature.com/articles/am2011117

doi:10.1038/asiamat.2011.94

The difference between diamond and graphite is surprisingly modest: the carbon atoms in diamond are bonded into a three-dimensional crystal pattern, while those in graphite are bonded into flat sheets. This variety in possible bonding arrangements, called allotropes, is a distinguishing feature of carbon, and part of the reason why it is the basis for life on earth. Gang Su and colleagues at the Graduate University of the Chinese Academy of Sciences have now predicted that a new form of crystalline carbon should exist.1

The researchers were inspired by a television program about the Egyptian pyramids to consider what would happen if they substituted each atom in a diamond crystal with a tetrahedral pyramid of carbon atoms. By performing calculations from first principles, they found that such a structure would, in fact, be energetically stable at atmospheric pressures, suggesting that it should be possible to experimentally synthesize it.

The calculations also showed that this form of carbon, which the researchers dub ‘T-carbon’, would have some unusual properties. Although it would be a hard substance, about two thirds as hard as diamond, T-carbon would have half the density. “It would be quite fluffy,” says Su. This feature, the result of a relatively large spacing between the material’s constituent atoms, could make T-carbon capable of effectively storing hydrogen or lithium, with applications in energy storage.

The low density could also be relevant to basic scientific questions, such as the observation by astronomers that the amount of interstellar dust that they can measure does not account for the observed distortions in starlight. If T-carbon were present in significant quantities in space, this calculation would need to be remade. On a more prosaic level, the low density may also make an interesting structural material for aerospace or sports equipment.

T-carbon is also predicted to have a direct bandgap, meaning that it will be optically active. The challenge now, says Su, is for experimentalists to try to synthesize this new carbon allotrope. “Synthesis possibilities include chemical vapor deposition under a negative pressure environment, detonation of diamond or graphite starting materials, crystallization of amorphous carbon, and even stretching diamond.”

References
1. Sheng, X.-L., Yan, Q.-B., Ye, F., Zheng, Q.-R. & Su, G. T-Carbon: A Novel Carbon Allotrope. Phys. Rev. Lett. 106 155703 (2011).