• PDF / 73,793 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 24 Downloads / 244 Views

DOWNLOAD

REPORT

---

Response of the Electric Field Gradient in Ion implanted BaTiO3 to an External Electric Field Marc Dietrich, Jörn Bartels1, Manfred Deicher, Kristian Freitag1, Vyacheslav Samokhvalov2 and Sepp Unterricker2 Fachbereich Physik, Universität Konstanz, D-78457 Konstanz, Germany 1 Institut für Strahlen- und Kernphysik, Universität Bonn, D-53115 Bonn, Germany 2 Institut für Angewandte Physik, TU Bergakademie Freiberg, D-09596 Freiberg, Germany

ABSTRACT Single crystalline, ferroelectric BaTiO3 as material with the highest piezoelectric constants among the perovskites with ordered sublattices was implanted with 111In(111Cd). The electric field gradient at the Ti position was measured with perturbed γγ-angular correlation spectroscopy (PAC) while the crystal was exposed to an external electric field. A quadratic dependence could be observed: νQ(E) = (34.8(1) + 0.16(4) E/kV/mm + 0.080(2) E2/kV2/mm2) MHz. Point charge model calculations reproduce the linear change of Vzz, but not the quadratic term. The polarizability of the host ions of BaTiO3 is known to be nonlinear with respect to an electric field. The resulting quadratic shift of the electron density is reflected in the strength of the EFG.

INTRODUCTION In solids, the electric field gradient (EFG) at a certain lattice site is determined mainly by the atoms in its nearest neighborhood, i.e. their electronic properties and the distances to each other. Therefore, variations of lattice constants lead to changes in EFG. Lattice constants change when the temperature is varied. Such studies have been performed for numerous materials with perturbed γγ-angular correlation spectroscopy (PAC) [1]. A few papers report on the response of the EFG on hydrostatic pressure [2] or on the bending angle of a single crystal [3]. Another way of applying uniaxial stress can be achieved by the inverse piezoelectric effect. In polar crystals lattice constants change or atoms are shifted when an electric field is applied externally resulting in an influence on the EFG at lattice sites. We have carried out experiments in order to observe these slight changes in LiNbO3 with PAC recently. The measurements revealed only a tiny change of the EFG in dependence on the external electric field. In this report we present further studies using a different material. The primary requirement for the success of such an experiment is a high piezoelectric constant of the material. Solids used as piezoelectric actuators with very high piezoelectric constants like SrxBa1-xNbO3 are not useful for PAC investigations because of the random occupation of the Ba sublattice by Ba and Sr atoms. This would result in a strongly damped spectrum since the probes are not exposed to an unique EFG. Therefore we have chosen BaTiO3 as an ordered material with piezoelectric constants larger than those of LiNbO3. Calculations of EFG at lattice sites in BaTiO3 with WIEN97 [4] using the FP-LAPW method will be presented. Since these first principles calculations are rather complex, we have chosen a CC10.11.1

much more easy way of nai