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 powd er diffraction patterns of the phase Si VI are indexed on the basis of an o rthorhombic unit cell containing 16 atoms. The space group is assigned as C mca. 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. T hus, formation of the Cs V type structure is not a unique feature of the pr essure-driven electronic s-d transition in Cs. Instead, the structure type appears to be more common, occurring intermediate between 8- and 12-fold co ordinated structures. [S0031-9007(98)08345-8].