Phases of silicon at high pressure

X-ray diffraction studies are reported on silicon at pressures up to 250 kbar (25 GPa). A transition to the β-Sn structure (II) initiates at 112 ± 2 kbar and two phases (I + II) coexist to 125 ± 2 kbar. At 132 ± 2 kbar a new phase (V) initiates, and the transition is complete at 164 ± 5 kbar. This phase persists to 250 kbar. Its structure is tentatively assigned as primitive hexagonal with c/ a = 0.941 ± 0.002 at 250 kbar. On release of pressure, the sequence is V → (V + II) (145 - 110 kbar) → II → (II + III) (108 - 85 kbar) → III, the last phase persisting to room pressure.