Breaking of the macroscopic centric symmetry in Ba₁₋xSrxTiO₃ ceramics and single crystals

Beyond the possibility of finding a lead-free piezoceramics as replacement for PZT, the world-wide research has focused on other mechanisms of electro-mechanical coupling alternative to piezoelectricity, such as electrostriction and, more recently, flexoelectricity.The mechanism of the flexoelectric polarization in bulk ceramics is still far from being fully understood. Above all, the controversy concerns the fact that the values of the flexoelectric coefficients measured in the ferroelectric ceramics exceed by several order of magnitude the theoretical estimations.During this thesis work, the measurement of the flexoelectric properties in the paraelectric phase of unpoled ceramics of the Ba0.67Sr0.33TiO3 composition, which was reported previously to exhibit the largest flexoelectric polarization ever measured, lead to the uncovering of an electro-mechanical coupling which cannot be ascribed to the flexoelectric response. During this thesis work, the measurement of the flexoelectric properties in the paraelectric phase of unpoled ceramics of the Ba0.67Sr0.33TiO3 composition, which was reported previously to exhibit the largest flexoelectric polarization ever measured, lead to the uncovering of an electro-mechanical coupling which cannot be ascribed to the flexoelectric response.This electromechanical coupling, which was accompanied by pyroelectric response in as-prepared samples, suggests breaking of the macroscopic centric symmetry of the ceramics. In the effort to understand the mechanism of the symmetry breaking, the solid solutions Ba1-xSrxTiO3 (x=0, 0.025, 0.33, 0.40, 0.45, 0.50, 0.67, 0.90, 0.975, 1) were investigated and compared with properties of corresponding single crystals and other ceramic compositions. The study was conducted mainly by means of piezoelectric, pyroelectric and thermally stimulated current(TSC) characterization.In this work, it is proposed that the appearance of the polar symmetry in ceramics and single crystals above the Curie temperature is due to asymmetries associated to the high temperature processing of crystals and ceramics The symmetry breaking in Ba1-xSrxTiO3 ceramics and associated electromechanical response that is of the same order of magnitude as the flexoelectric response in the same materials, can at least partly help resolving the controversy existing between the theoretical models of flexoelectricity and the experiments.