Cationic disorder, microstructure and dielectric response of ferroelectric SBT ceramics

Polycrystalline samples of SrBi₂Ta₂O₉ (acronym SBT) have been prepared by means of solid-state reaction either using a classical route or by mechano­chemical activation. For each compound, a structural analysis of the ferroelectric orthorhombic phase (space group A2₁am) has been performed from the fitting of neutron and/or X-ray powder diffraction data using the Rietveld method. A cationic disorder on Bi³⁺ and Sr²⁺ crystallographic sites has been revealed, the Sr atoms occupying the Bi sites and vice versa. From diffraction peak broadening analyses of high-resolution synchrotron X-ray diffraction data, it has been shown that the two grinding methods (manual or mechanical) induce local strains, the average apparent strain being three times larger for the mechanically ground sample. In order to link microstructure and ferroelectric properties, the dielectric constant has been measured as a function of the temperature. It appears that the position and the shape of the dielectric anomaly strongly depend upon the composition and the route used to elaborate the powders. More exactly, for the mechanically ground powder, the large apparent strain, probably correlated to the strong cation exchange revealed by the structural refinement, leads to a significantly enhanced dielectric response.