Electric-field-induced charge-density variations in covalently bonded binary compounds

Extended data sets for the relative intensity variation of x-ray Bragg reflections of GaAs and ZnSe caused by an external electric field were collected using a modulation demodulation technique. In particular, the functional dependence of the intensity variation on field strength and wavelength of the synchrotron radiation was determined. The wavelength dependence allows to determine the phases of the difference-structure factors. An interpretation of all data using a semiempirical model leads to the following conclusions: The main contribution to the measured effects comes from a displacement of the anion sublattice relative to the cation sublattice. The response of the electron density to an external electric field is negligibly small according to density functional theory calculations with both, clamped and “free” ions. This effect is much smaller than the redistribution of the electron density in the bond region associated with the displacements of the atoms causing a change in overlap. The dependence of the sublattice displacement on the strength of the external electric field $\mathbf{E}\parallel \left[111\right]$ is estimated to be $1.5\times {10}^{-8}\AA /{\mathrm{V}\mathrm{}\mathrm{mm}}^{\mathrm{-}1}$ for GaAs and $13.5\times {10}^{-8}\AA /{\mathrm{V}\mathrm{}\mathrm{mm}}^{\mathrm{-}1}$ for ZnSe. For the mainly covalent GaAs the inverse piezoelectric effect can be explained by this bond-length variation, in contrast to the much more ionic compound ZnSe for which this is not the case.