We can all only dream of owning rocks made of these. Simulations have revealed three stable forms of pure carbon that would sparkle more than diamond if synthesised.
Carbon atoms can be combined in different configurations with widely varying properties. Graphite and diamond are the most familiar, while more exotic allotropes include graphene, with versatile electrical properties, and M-carbon and Bct-carbon, which rival diamond's legendary hardness.
To explore whether forms of carbon denser than diamond might be possible, Artem Oganov of Stony Brook University in New York and colleagues systematically simulated different configurations of carbon atoms at different temperatures and pressures. Three – named hP3, tI12 and tP12 – seemed stable enough to be made in principle.
The simulations show none would be as hard as diamond, but all three would be between 1.1 and 3.2 per cent denser. "These new allotropes are significantly denser than any of the known allotropes of carbon," says Oganov.
This gives them a higher refractive index – a measure of the extent to which they bend light – leading to greater lustre and sparkle.
Superconducting carbon?
The simulations also suggest that the three materials have band gaps – the amount of energy needed for electrons to jump from one energy level to another – that are very different to one another. One of them, tP12, has the largest band gap of any carbon allotrope. This variability may make the allotropes good candidates for superconductors – exotic substances that conduct electricity without resistance, says Oganov.
According to Boris Yakobson at Rice University in Houston, Texas, a large variation in band gap implies strong interactions between electrons and packets of energy in the lattice called phonons. This in turn could lead to the formation of electron couplings called Cooper pairs, which are necessary for superconductivity.
The presence of widely varying band gaps might also give the materials other unique properties. "The large spread of band gaps gives an excellent playground for engineering new electronic materials," adds Oganov.
Hard to make
Creating the three new materials could be difficult, though. "It is not clear how we can fabricate them," says Vadim Brazhkin of the Institute for High Pressure Physics in Troitsk, Russia, who was not involved in the research.
"Using standard pristine materials, such as graphite or amorphous carbon [an allotrope in which the atoms have no crystal structure], we can possibly obtain a tiny amount of new materials using extreme pressure treatment," he says.
Read more at New Scientist
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