Graphite squeezed between two diamond jaws at pressures of 170,000 atmospheres managed to produce a crack in the diamond. A team modelled various crystal structures that could result when graphite is compressed, and found that bct-carbon requires the least energy to form. 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.
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.
A body-centered tetragonal carbon (bct-carbon) allotrope has been predicted to be a transparent carbon polymorph obtained under pressure. The structural transition pathways from graphite to diamond, M-carbon, and bct-carbon are simulated and the lowest activation barrier is found for the graphite-bct transition. Furthermore, bct-carbon has a higher shear strength than diamond due to its perpendicular graphenelike structure. Our results provide a possible explanation for the formation of a transparent carbon allotrope via the cold compression of graphite. We also verify that this allotrope is hard enough to crack diamond.