Abstract:
Silicon carbide, which has many polytypic modifications of a very simple and
very symmetric structure, is an excellent model system for exploring, the
relationship between chemical shift, long-range dipolar shielding, and crystal
structure in network solids. A simple McConnell equation treatment of bond
anisotropy effects in a poly type predicts chemical shifts for silicon and carbon sites
which agree well with the experiment, provided that contributions from bonds up
to 100 A are included in the calculation. The calculated chemical shifts depend on
three factors: the layer stacking sequence, electrical centre of gravity, and the
spacings between silicon and carbon layers. The assignment of peaks to lattice sites
is proved possible for three polytypes (6H, 15R, and 3C).
The fact that the calculated chemical shifts are very sensitive to layer spacings
provides us a potential way to detennine and refine a crystal structure. In this work,
the layer spacings of 6H SiC have been calculated and are within X-ray standard
deviations. Under this premise, the layer spacings of 15R have been detennined.
29Si and 13C single crystal nmr studies of 6H SiC polytype indicate that all
silicons and carbons are magnetically anisotropic. The relationship between a
magnetic shielding tensor component and layer spacings has been derived. The
comparisons between experimental and semi-empirical chemical shielding tensor
components indicate that the paramagnetic shielding of silicon should be included in
the single crystal chemical shift calculation.
