First-principles study of lithium-doped carbon clathrates under pressure

We present a theoretical study of the behavior under pressure of the two hypothetical C-46 and Li8C46 type-I carbon clathrates in order to obtain new information concerning their synthesis. Using ab initio calculations, we have explored the energetic and structural properties under pressure of these two carbon based cage-like materials. These low-density metastable phases show large negative pressure transitions compared to diamond, which represent a serious obstacle to their synthesis. However, we show that a minimum energy barrier can be reached close to 40 GPa, suggesting that synthesis of the Li-clathrate under extreme conditions of pressure and temperature may be possible. The electronic band structure with related density of states behavior under pressure, as well as the dependence of the active Raman modes with pressure are also examined.