Phase stability of carbon clathrates at high pressure

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- C136 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- C136, which is more stable than other carbon clathrates such as hex- C40 and sc- C46, and the recently predicted metastable M -carbon up to 26 GPa. Phonon dispersion calculations confirm the dynamic stability of fcc- C136 as well as diamond.