PRESSURE-DEPENDENCE OF BORN EFFECTIVE CHARGES, DIELECTRIC-CONSTANT, AND LATTICE-DYNAMICS IN SIC

The pressure dependence of the Born effective charge, dielectric const ant and zone-center LO and TO phonons have been determined for 3C-SiC by a linear response method based on the linearized augmented plane wa ve calculations within the local density approximation. The Born effec tive charges are found to increase nearly linearly with decreasing vol ume down to the smallest volume studied, V/V-0 = 0.78, corresponding t o a pressure of about 0.8 Mbar. This seems to be in contradiction with the conclusion of the turnover behavior recently reported by Liu and Vohra [Phys. Rev. Lett. 72, 4105 (1994)] for 6H-SiC. Reanalyzing their procedure to extract the pressure dependence of the Born effective ch arges, we suggest that the turnover behavior they obtained is due to a pproximations in the assumed pressure dependence of the dielectric con stant epsilon(infinity), the use of a singular set of experimental dat a for the equation of state, and the uncertainty in measured phonon fr equencies, especially at high pressure.