BC8 Si has a small Fermi surface and thus electronically can be
regarded as a semimetal as suggested by its band structure. Its
fourfold coordination and brittleness suggest that it is
predominantly held together by directional covalent bonds. To
resolve this apparent contradiction, the valence charge density
has been examined within BC8 and ST12. Figure 4.29 shows an
electron density isosurface in ST12 silicon. It bears an uncanny
resemblance to a ball and stick model of the structure, showing
clearly that the electron density is concentrated into four
`bonds' emanating from each atom - the covalent picture. Figure
4.29: Three dimensional representation of a valence charge
density isosurface in ST12 silicon. This covalency is
illustrated even more clearly in Figures 4.7, 4.8 and 4.15 which
shows the valence charge density in the 110 plane of BC8. Along
the 111 direction are atoms separated by and alternately.
Although these distances are similar, it can very clearly be
seen that the slightly closer pairs are