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Reversible antiferroelectric-ferroelectric phase transitions were recently observed in a series of SrSnO3-modified NaNbO3 lead-free antiferroelectric materials, exhibiting well-defined double polarization hysteresis loops at ambient conditions. Here, transmission electron microscopy was employed to investigate the crystallographyand domain configuration of this newly designed system via electron diffraction and centered dark-field imaging. It was confirmed that antiferroelectricity is maintained in all compositions, manifested by the characteristic ¼ superlattice reflections in the electron-diffraction patterns. By investigating the antiferroelectric domains and domain boundaries in NaNbO3, we demonstrate that antiphase boundaries are present and their irregular periodicity is responsible for the streaking features along the ¼ superlattice reflections in the electron-diffraction patterns. The signature domain blocks observed in pure NaNbO3 are maintained in the SrSnO3-modified ceramics, but disappear when the amount of SrSnO3 reaches 7 mol.%. In particular, a well-defined and distinct domain configuration is observed in the NaNbO3 sample modified with 5 mol.% SrSnO3, which presents a parallelogram domain morphology.

In this study lead-free 2-2 and 0-3 ceramic/ceramic composites comprised of the non-ergodic relaxor 0.93(Bi1/2Na1/2)TiO3–0.07BaTiO3 and ergodic relaxor 0.94Bi0.5(Na0.75K0.25)0.5TiO3–0.06BiAlO3 were investigated. The macroscopic electromechanical behavior was characterized as a function of continuent content,revealing an enhancement in the unipolar strain from the multilayer composite structure. Systematic evaluation of the effects of co-sintering on microstructural properties, such as grain size and porosity, revealed potential mechanisms by which the increase in unipolar strain was achieved. In addition, interdiffusion between the constituents was observed, providing evidence for the formation of a functionally graded ceramic by co-sintering. These data are contrasted with highresolution energy dispersive X-ray microanalysis for measurement of chemical composition across the interface of 2-2 ceramics. These findings provide insight into how synthesis routes can be optimized for tailoring the enhancement of electromechanical properties of lead-free electroceramic composite systems.

The design of core–shell materials affords additional degrees of freedom to tailor functional properties as compared to solid solution counterparts. Although to date most of the work in core–shell materials has focused on dielectrics, piezoelectric core–shell ceramics may gain similar interest. Generalities of core–shell functional ceramics features are addressed in this work. A model system, Bi1/2Na1/2TiO3–SrTiO3, is introduced to discuss structure–property relationships. We demonstrate that this system features a core–shell microstructure for the composition corresponding to 25 at.% Sr. The material is studied by means of macroscopic functional properties and in situ structural characterization techniques at different length scales, such as X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The evolution of the core–shell with field and temperature determines its functional properties. The high strain of the system, ~0.3% at 4 kV/mm, is due to an electric-field-induced phase transition of the core and shell. Upon field removal the core remains in a poled state, whereas the shell is characterized by a reversible transformation. The reversibility of the phase transition of shells and associated switching are key features in the observed giant strain. Dielectric anomalies are found to be related to changes in oxygen octahedral tilting angles within the core and shell.