Structural aspects of bonding in silicon carbide polytypes: ab initio MO calculations

The series of polytype structures of silicon carbide was modelled using Si(CH₃)₄(SiH₃)₄ molecules with C_3v symmetry. The Si-C bond lengths and angles were calculated using ab initio molecular orbital methods where the external dimensions of the molecules were constrained to values equivalent to the lattice parameters of the polytypes. Second order coordinations of hexagonal and cubic stacking sequences in the crystal structures were simulated by the relative positions of hydrogen atoms. The 6-31G basis level bond lengths and angles are shown to conform to linear relationships with respect to the lattice parameters but differ between the two second order coordinations. Except for the perfect tetrahedral structure of 3C-SiC all vertical bonds in the [001] direction are longer than the lateral bonds inclined to the basal plane. It is shown that the charge on the central atom is independent of the c a ratio of lattice parameters but is slightly different between hexagonal and cubic configurations.