The response of the electron densities of GaAs and ZnSe to an external electric field was probed by measuring the variation of the integral intensity of several weak X-ray structure factors due to a field parallel to [1 1 1]. In the range −3 kV/mm < E < +3 kV/mm we found a nearly linear dependence of the relative intensity variation ∆R/R on E for low indexed reflections (2 2 2, 0 6 0) and an additional E 2 -dependence for higher-indexed ones (4 4 2, 6 6 6). The results are interpreted in terms of a bond-charge model. The linear effect can be interpreted by the redistribution of valence charges from bonds parallel to E into bonds lying oblique (bond-charge transfer) whereas the origin of the non-linear effect is still unclear. After the exclusion of lattice effects (piezo-electric effect, internal strain) it ought to be allocated to a charge density response of the core region.
The parameters of the interatomic one-particle potentials of cubic ZnSe were determined experimentally unsing X-ray diffraction methods. The harmonic force constants
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