ANOMALIES IN THE PRESSURE-DEPENDENCE OF THE EFFECTIVE CHARGE IN CUBICSEMICONDUCTORS

We have investigated the effect of hydrostatic pressure on the Born ef fective charge of tetrahedral semiconductors by means of first-princip les pseudopotential calculations. In agreement with recent experiments , we find that the effective charge of IV-IV compounds, such as SiC, i ncreases in magnitude with pressure, whereas that of III-V compounds d ecreases. We explain this behavior in terms of conflicting effects ass ociated with the lattice-constant variation of the long- and short-ran ge components of the crystal potential. We show that an analogous anom aly exists in the pressure dependence of the bond polarity, as measure d by the asymmetry of the static valence charge.