Pressure dependence of dynamical and dielectric properties of 3C and 4H silicon carbide

We report a first-principle study on the pressure dependence of dynamical and dielectric properties of cubic 3C and hexagonal 4H polytypes of SiC. Results are discussed for the zone-center phonon modes and the tensors of the Born effective charges and the high-frequency dielectric constants. In the case of the hexagonal 4H structure we predict an increasing anisotropy with pressure and a pressure-induced mixing of infra-red active optic modes. The interpretation of recent experiments for 6H SiC is critically examined.