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In this work, the relationship between the structural mechanisms and macroscopic electrical properties of the Nb-modified 0.96(Bi0.5Na0.84K0.16TiO3)–0.04SrTiO3 (BNKT–ST) system were elucidated by using temperature dependent and in situ synchrotron X-ray diffraction (XRD) techniques. For the composition x = 0.0175, a large-signal piezoelectric coefficient (Smax/Emax = d33*) of 735 pm V−1 at 6 kV mm−1 was observed at room temperature. Interestingly, at a higher temperature of 110 °C, the sample still showed a large d33* of 570 pm V−1. Furthermore, the temperature-invariant electrostrictive coefficient for this sample was found to be 0.0285 m4 C−2 over the temperature range of 25–170 °C. Moreover, the energy density for x = 0.030 sample was ∼1.0 J cm−3 with an energy storage efficiency of ˃70% in the temperature range of 25–135 °C. These results suggest that the synthesized Nb-modified BNKT–ST system is promising for the design of ceramic actuators as well as capacitor applications.

The Bi1/2Na1/2TiO3-based materials exhibit the largest electric-field-induced strains among lead-free piezoceramics and are considered as promising candidates for actuation applications. A typical representative of this group is (1-x)Bi1/2Na1/2TiO3-xSrTiO3, where its excellent electromechanical properties were recently related to the existence of a core–shell microstructure. Although the latter was also reported in other Bi1/2Na1/2TiO3-based ceramics, the formation mechanism remains unknown. In the present work we therefore first investigated the solid-state reaction occurring during calcination using simultaneous thermogravimetric analysis, X-ray diffraction, scanning and transmission electron microscopy. The reaction occurred in two steps, whereby the cores and shells had different formation reaction temperatures, which resulted in a metastable heterogeneous microstructure. Furthermore, a series of sintered samples with different relative densities, grain sizes, and core densities was prepared. Modifications of these microstructural parameters resulted in variation of the maximal strain by 17% and in the electric-field required to trigger the phase transitions by 38%.

The effect of 1 at% Fe- and FeNb-doping on the temperature and stress stability of the electromechanical properties of (Ba 1− x Ca x )(Zr y Ti 1− y )O 3 (BCZT) was investigated. For the composition (Ba 0.89 Ca 0.11 )(Zr 0.135 Ti 0.865 )O 3 with rhombohedral symmetry, doping reduces the Curie point and the temperature stability of the large-signal electromechanical properties significantly. The large-signal piezoelectric coefficient d 33 * at room temperature was reduced to 500 pm/V compared to 700 pm/V in the undoped composition at 1 kV/mm. The electrostrain, however, was found to be less sensitive to mechanical prestresses, showing a plateau up to stresses of 80 MPa in both doped compositions. These effects were attributed to a reduction of the domain wall mobility due to a smaller grain size, charged defect dipoles and the proximity of the room-temperature measurements to the reduced ferroelectric-paraelectric phase transition temperature. The study reveals that the exceptionally large strains observed in BCZT rely on the instabilities around the polymorphic phase transition in the system. Aliovalent doping changes this sensitive system and reduces the electrostrain considerably.

Materials in two composition regimes, Bi1/2Na1/2TiO3–BaTiO3–CaZrO3 (BNT–BT–CZ) and Bi1/2Na1/2TiO3–BaTiO3–K0.5Na0.5NbO3–CaZrO3 (BNT–BT–KNN–CZ), were synthesized via the mixed oxide route and their structural, dielectric and electrical properties were investigated. CZ was identified to render the two local maxima in permittivity more diffused. This resulted in temperature-insensitive relative permittivity spectra with average values from ∼470 up to ∼2300 and operational windows of at least ∼400 °C with less than 15% of variation in the temperature range from −100 up to above 500 °C. Moreover, loss factors are below ∼10% and RC constants range from ∼0.03 s up to ∼4 s at 300 °C. The materials of current investigation are highly attractive for developing capacitors of wide temperature usage.