High Pressure Behavior of Silicon Clathrates: A New Class of Low Compressibility Materials

The high pressure evolution of silicon clathrates is studied at room temperature by x-ray diffraction up to 15 GPa. Remarkably, no transition towards the diamond structure is observed and the clathrate phase transforms directly into the $\beta$-tin metallic phase at 11 GPa. Further, the bulk modulus is found to be $90\pm 5\mathrm{GPa}$, that is, only $(8\pm 5)\%$ smaller than the one of the diamond phase. These results are in good agreement with ab initio calculations which predict further that carbon clathrates, if synthesized, should be less compressible than cubic BN.