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

Abstract

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 Eʈ͓111͔ is estimated to be 1.5ϫ10 Ϫ8 Å/V mm Ϫ1 for GaAs and 13.5ϫ10 Ϫ8 Å/V mm Ϫ1 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.

Investigations of the bond-selective response in a piezoelectric Li2SO4·H2O crystal to an applied external electric field

“…The types of perturbations which are usually applied to a crystal in an X-ray diffraction experiment are high pressure, high or low temperatures, laser irradiation and external high voltage (Oganov et al, 2006;Istomin et al, 2007;Coppens & Novozhilova, 2002;Stahn et al, 2001). The specific response of the crystal to such perturbations defines its intrinsic physical properties, e.g.…”

“…Although many technical applications are essentially based on both of these phenomena, their microscopic nature is not yet well understood. Starting with the pioneering work by Fujimoto (1978) which was further continued by Paturle et al (1991), Graafsma et al (1993), van Reeuwijk et al (2001), Stahn et al (2001), Guillot et al (2002), Davaasambuu et al (2003), Guillot et al (2004), Gorfman et al (2006) and Schmidt et al (2008), the atomistic origin of the piezoelectric effect has so far been investigated for only a very narrow class of compounds. Moreover, the fundamental relationship between the atomic arrangement, the electron-density distribution and piezoelectric properties can not be explained in full detail for any single structure .…”

Studies of electric field induced structural and electron-density modifications by X-ray diffraction

“…One attempt to circumvent this is to use a supercell approach in which thè applied' electrostatic potential takes a saw-tooth shape. This has been employed by Stahn et al (2001) on GaAs and by Kochin et al (2004) on low quartz, both in collaboration with K. Schwarz and P. Blaha from Vienna. This theoretical approach indicates that the major effect of the ®eld is a modi®cation of the atomic coordinates.…”

X-ray diffraction by a crystal in a permanent external electric field: general considerations

“…This type of study of electric-field-induced structural changes was initiated by Graafsma et al (1993Graafsma et al ( , 1998 and Reeuwijk et al (2000aReeuwijk et al ( ,b, 2001, who investigated KD 2 PO 4 at the European Synchrotron Radiation Facility. Stahn et al (1998Stahn et al ( , 2001 have investigated the change in a structure of GaAs/ZnSe compounds at HASYLAB. Davaasambuu et al (2003) (HASYLAB) and Guillot et al (2004) (LURE) have measured the changes in X-ray diffraction intensities of selected reflections for -quartz in an E-field to analyze the reorganization of atoms within a unit cell.…”