Group-IV element clathrates have attracted considerable interest in recent years. Here, we report an ab initio study on the structural stability of carbon clathrates at high pressure and identify fcc-C(136) clathrate as the third most stable carbon phase after cubic diamond and hexagonal graphite. A pressure-induced phase transition is predicted to occur around 17 GPa from hexagonal graphite to fcc-C(136), which is more stable than other carbon clathrates such as hex-C(40) and sc-C(46), and the recently predicted metastable M-carbon up to 26 GPa. Phonon dispersion calculations confirm the dynamic stability of fcc-C(136) as well as diamond.

http://dx.doi.org/10.1063/1.3359682