Ab initio study of the formation of transparent carbon under pressure

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.

Figure 1

(Color online) Structures of $2\times 2\times 2$ supercell of bct-carbon. (a) along [100]/[001] direction and (b) along [010] direction where the dotted-dashed lines indicate the perpendicular graphenelike structure of bct-carbon.

Figure 2

(Color online) Energy barrier curves of bct-carbon (blue squares), M-carbon (green triangles), and diamond (red circles) at pressure of 20 GPa. The inset shows the fluctuant graphene layers at the intermediate image 4 of the transition path, which is responsible for the activation energy barrier of bct-carbon.

Figure 3

(Color online) Transition paths of bct-carbon and M-carbon under pressure of 20 GPa. The side view of structures at different intermediate images shows the pathways from graphite to bct-carbon and M-carbon. (a)–(d) show the transition path from image 4 to image 7 for bct-carbon and (e)–(h) show the corresponding images for M-carbon for comparison. The detailed transition paths of bct-carbon and M-carbon can also be found in Ref. 37.

Figure 4

(Color online) The calculated stress-strain relations of bct-carbon compared with that of diamond. The filled (hollow) blue square and red circle are used to depict the tensile and shear stress of bct-carbon and diamond, respectively. The arrows indicate the ideal strength (including tensile and shear strength). The inset shows the electron density of bct-carbon in the (010) plane (units ${\text{Å}}^{-3}$) at largest strain, in which the white dotted lines imply that a graphene layer will be reformed in this range as the shear stress or strain increases.