Crystal Structure of Cold Compressed Graphite

Through a systematic structural search we found an allotrope of carbon with Cmmm symmetry which we predict to be more stable than graphite for pressures above 10 GPa. This material, which we refer to as $Z$-carbon, is formed by pure $s{p}^3$ bonds and it provides an explanation to several features in experimental x-ray diffraction and Raman spectra of graphite under pressure. The transition from graphite to $Z$-carbon can occur through simple sliding and buckling of graphene sheets. Our calculations predict that $Z$-carbon is a transparent wide band-gap semiconductor with a hardness comparable to diamond.

Figure 1

(a) Structure of $Z$-carbon viewed from two different angles revealing planar four-membered and nonplanar eight-membered rings forming chains along the $b$ direction and channels in the $c$ direction. The graphene sheets are in the $a\mathrm{\text{−}}c$ plane. (b) Proposed transition pathway from graphite to $Z$-carbon.

Figure 2

Calculated enthalpy difference per atom with respect to graphite of several carbon allotropes as a function of pressure. Graphite is the horizontal line at zero. $Z$-carbon becomes more stable than graphite at around 10 GPa.

Figure 3

Experimental XRD for cold-compressed graphite at two different pressures from Ref. 7 and simulated XRD pattern for $Z$-carbon (at 23.9 GPa) and graphite (at 13.7 GPa). The main characteristics of the proposed carbon are perfectly in agreement with the experimentally observed changes.

Figure 4

Experimental linewidth of the $G$ band of graphite under pressure. The linewidth stays nearly constant until pressures of the order of 9–10 GPa, above which the linewidth begins to broaden rapidly. This is a strong evidence for a structural transition in graphite. Experiments were conducted using highly oriented pyrophillitic graphite (HOPG) and argon (squares) or paraffin oil (triangles) as pressure transmitting media. Note that the $G$ band broadening is fully reversible under pressure unload. The unload points are however not included for clarity. The black dots are taken from Ref. 3.

Figure 5

Experimental Raman spectra of graphite under pressure. The peak around $1082{\mathrm{cm}}^{-1}$ appearing at around 10 GPa and its evolution can be explained by either $Z$-carbon, $W$-carbon, or nanocrystalline diamond.