Structural Stability and Phase Transitions inK8Si46Clathrate under High Pressure

Abstract

The structural stability of type-I ${\mathrm{K}}_8{\mathrm{S}\mathrm{i}}_{46}$ clathrate has been investigated at high pressure by synchrotron x-ray diffraction. In contrast to that observed in the Na-doped structure-II analogue [A. San-Miguel et al., Phys. Rev. Lett. 83, 5290 (1999)], no phase separation into the $\beta \mathrm{\text{−}}\mathrm{S}\mathrm{n}$ Si structure was identified at 11 GPa. Instead, ${\mathrm{K}}_8{\mathrm{S}\mathrm{i}}_{46}$ is found to undergo a transition to an isostructural positional disordered phase at around 15 GPa. Ab initio phonon band structure calculations reveal a novel phenomenon of phonon instabilities of K atoms in the large cavities is responsible for this transition. Above 32 GPa, the new structure transforms into an amorphous phase.