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 $\mathrm{Cmca}$. 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.

• Received 1 October 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.1197