Crystal Structure of the High-Pressure Phase Silicon VI

The crystal structure of Si was studied at pressures between 30 and 50 GPa using high-resolution monochromatic synchrotron x-ray diffraction. The powder diffraction patterns of the phase Si VI are indexed on the basis of an orthorhombic unit cell containing 16 atoms. The space group is assigned as Cmca. Full profile refinements reveal that Si VI is isotypic to Cs V; i. e. , axial ratios and atomic coordinates are nearly identical for both phases. Thus, formation of the Cs V type structure is not a unique feature of the pressure-driven electronic s-d transition in Cs. Instead, the structure type appears to be more common, occurring intermediate between 8- and 12-fold coordinated structures.