Electric field induced charge density variations in partially-ionic compounds

Using a modulation technique and synchrotron radiation, we investigated the charge density variations in cubic ZnSe and trigonal SiO2 induced by an ex ternal high-electric field E applied along the [111] and [11.0] direction, respectively. Selecting particularly weak reflections, we observed an almos t linear variation of the integrated intensity versus E changing by up to 2 % at \E \ = 3 kV/mm. For both crystals, the displacements of the relative i on positions within the unit cell exceed the piezoelectric induced variatio n of the unit cell parameters by at least one order of magnitude. The measu red intensity variations of ZnSe can be explained by the opposite displacem ent of the zinc and selenium ions towards the field direction breaking the stiff tetrahedral coordination of the chemical bond. For quartz, the electr ic field initiates a displacement of those oxygen ions only where the O-Si bond is directed oblique to the field direction. The other oxygen ions and the silicon positions remain nearly unchanged. Thus, the ionic arrangement is described mainly by a change in the O-Si-O bonding angles. (C) 2001 Else vier Science Ltd. All rights reserved.