THE prospect of making materials so hard they can dent diamond has come a step closer with insight into the structure of a new form of superhard graphite.

In 2003, an experiment suggested that graphite, which is normally soft, could become ultra-hard when compressed. Graphite squeezed between two diamond jaws at pressures of 170,000 atmospheres managed to produce a crack in the diamond. The atomic structure of the material remained elusive, however.

Graphite at 170,000 atmospheres managed to crack diamond, but its structure was elusive

Now computer simulations by Hui-Tian Wang at Nankai University in Tianjin, China, and colleagues have shown that the compressed material could be at least partly made of bct-carbon, which is built up from rings of four carbon atoms. Bct-carbon has attributes of both diamond, which has a cubic structure, and graphite, composed of loosely linked sheets of carbon atoms in a hexagonal lattice. In bct-carbon, layers of carbon rings are linked by strong vertical bonds.


The team modelled various crystal structures that could result when graphite is compressed, and found that bct-carbon requires the least energy to form (Physical Review B, vol 82, p 134126).

Wang’s team reckons that bct-carbon’s shear strength – a measure of how difficult it is to slide the carbon layers over one another – is 17 per cent greater than that of diamond. Their findings raise the prospect of making exceptionally hard materials without extreme heating. Most other materials we suspect to be harder than diamond require both high temperature and pressure to form.

Artem Oganov at the State University of New York in Stony Brook says that, compared with other candidates, there is “a much stronger basis” for bct-carbon to be harder than diamond, but adds that experiments are still needed to test the claims.