Electromechanical coupling coefficient of isotropic sample with a marked electrostriction

Isotropic sample of single ferroelectric crystal (STO) with cubic symmetry or the epitaxial BSTO film does not exhibit a piezoelectric effect, but is characterized by electrostriction. The induced piezoelectric effect occurs under the dc electric field. Using electromechanical equations for this material in presence of both the dc and ac electric field makes possible to describe the transformation of energy of electromagnetic field into acoustic oscillations. The transformation intensity is estimated by the electromechanical coupling coefficient determined as the ratio of the energy of alternating electromechanical interaction to the geometric mean of alternating electrical and mechanical energy. For the induced piezoeffect, the analytical model for the electromechanical coupling coefficient is obtained using phenomenological description of the dc electric field dependence of the dielectric permittivity. There is a maximum on the dependence of the electromechanical coupling coefficient on the dc field. The value of maximum and related dc field depend on the ferroelectric film quality.