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Anomalies in the pressure dependence of the effective charge in cubic semiconductors.

We have investigated the effect of hydrostatic pressure on the Born effective charge of tetrahedral semiconductors by means of first-principles pseudopotential calculations. In agreement with recent experiments, we find that the effective charge of IV-IV compounds, such as SiC, increases in magnitude with pressure, whereas that of III-V compounds decreases. We explain this behavior in terms of conflicting effects associated with the lattice-constant variation of the long- and short-range components of the crystal potential. We show that an analogous anomaly exists in the pressure dependence of the bond polarity, as measured by the asymmetry of the static valence charge.