The refractive index of each of the four common silicon carbide polytypes has been measured over the visible range. The data were analyzed in an attempt to relate the birefringence to the relative hexagonal character of the polytype. A general relationship exists, namely, that the birefringence increases with increasing hexagonal character of the polytype. This relationship is not sufficiently precise to use for the identification of polytypes.
P. T. B. Shaffer, "Refractive Index, Dispersion, and Birefringence of Silicon Carbide Polytypes," Appl. Opt. 10, 1034-1036 (1971)