  The pressure dependence of the Born effective charge, dielectric constant and
zone-center LO and TO phonons have been determined for $3C$-SiC by a linear
response method based on the linearized augmented plane wave calculations
within the local density approximation. The Born effective charges are found to
increase nearly linearly with decreasing volume down to the smallest volume
studied, $V/V_0=0.78$, corresponding to 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.\ {\bf 72}, 4105 (1994)]
for $6H$-SiC. Reanalyzing their procedure to extract the pressure dependence of
the Born effective charges, we suggest that the turnover behavior they obtained
is due to approximations in the assumed pressure dependence of the dielectric
constant $\varepsilon_\infty$, the use of a singular set of experimental data
for the equation of state, and the uncertainty in measured phonon frequencies,
especially at high pressure.
