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-principles pseudopotential calculations to examine the structural, electronic, and vibrational properties of the recently discovered phase XII of silicon (the R8 phase). The R8 phase is synthesized by decompression of the high-pressure beta-Sn phase and exists over a relatively wide pressure range of 2-12 GPa. Although there are structural similarities between BC8 and R8, the latter phase exhibits substantially greater local deviations from ideal tetrahedral bonding and is the most distorted 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 those of other tetrahedral forms.