STRUCTURE AND PROPERTIES OF SILICON-XII - A COMPLEX TETRAHEDRALLY BONDED PHASE

Angle-dispersive powder diffraction using an image-plate area detector and synchrotron radiation have been used in conjunction with first-pr inciples pseudopotential calculations to examine the structural, elect ronic, and vibrational properties of the recently discovered phase XII of silicon (the R8 phase). The R8 phase is synthesized by decompressi on of the high-pressure beta-Sn phase and exists over a relatively wid e pressure range of 2-12 GPa. Although there are structural similariti es between BC8 and R8, the latter phase exhibits substantially greater local deviations from ideal tetrahedral bonding and is the most disto rted crystalline structure containing fourfold-coordinated silicon. We present a detailed investigation of the pressure response of the BC8 structure, suggest plausible atomic trajectories for the beta-Sn to R8 transition, and we investigate the energy of R8 silicon relative to t hose of other tetrahedral forms.