X-ray-diffraction data have been obtained on Si in a diamond anvil cell to pressures of ∼50 GPa. Crystallographic data are presented in phase I (cubic, diamond), II (tetragonal, β-Sn), V (simple hexagonal), VII (hexagonal close-packed), and the metastable phase III [body-centered-cubic (BC8)] and on the coexistence of the phases. Comparison is made between these data and the predictions of ab initio calculations for these structures and their equations of state.

• Received 31 March 1986

DOI:https://doi.org/10.1103/PhysRevB.34.4679