X-ray-diffraction data have been obtained on Si in a diamond anvil cell to pressures of ,...,50 OPa. Crystallographic data are presented in phase I (cubic, diamond), II (tetragonal, I3-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.

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