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The pressure dependence of the Born effective charge, dielectric constant andzone-center LO and TO phonons have been determined for $3C$-SiC by a linearresponse method based on the linearized augmented plane wave calculationswithin the local density approximation. The Born effective charges are found toincrease nearly linearly with decreasing volume down to the smallest volumestudied, $V/V_0=0.78$, corresponding to a pressure of about 0.8 Mbar. Thisseems to be in contradiction with the conclusion of the turnover behaviorrecently reported by Liu and Vohra [Phys.\ Rev.\ Lett.\ {\bf 72}, 4105 (1994)]for $6H$-SiC. Reanalyzing their procedure to extract the pressure dependence ofthe Born effective charges, we suggest that the turnover behavior they obtainedis due to approximations in the assumed pressure dependence of the dielectricconstant $\varepsilon_\infty$, the use of a singular set of experimental datafor the equation of state, and the uncertainty in measured phonon frequencies,especially at high pressure.Comment: 25 pages, revtex, 5 postscript figures appended, to be published in  Phys. Rev.

Pressure dependence of Born effective charges, dielectric constant, and lattice dynamics in SiC