Over the past two decades, (K_0.5Na_0.5)NbO₃ (KNN)-based lead-free piezoelectric ceramics have made significant progress. However, attaining a high electrostrain with remarkable temperature stability and minimal hysteresis under low electric fields has remained a significant challenge. To address this long-standing issue, we have employed a collaborative approach that combines defect engineering, phase engineering, and relaxation engineering. The LKNNS-6BZH ceramic, when sintered at T_sint = 1170 ℃, demonstrates an impressive electrostrain with a $d_{33}^{\ast }$ value of 0.276% and 1379 pm·V^–1 under 20 kV·cm^–1, which is comparable to or even surpasses that of other lead-free and Pb(Zr,Ti)O₃ ceramics. Importantly, the electrostrain performance of this ceramic remains stable up to a temperature of 125 ℃, with the lowest hysteresis observed at 9.73% under 40 kV·cm^–1. These excellent overall performances are attributed to the presence of defect dipoles involving ${\mathrm{V}}_{\mathrm{A}}^{{}^{\prime }}\text{–}{\mathrm{V}}_{\mathrm{O}}^{\text{∙∙}}$ and ${\mathrm{B}}_{\mathrm{N}\mathrm{b}}^{{}^{\prime }}\text{–}{\mathrm{V}}_{\mathrm{O}}^{\text{∙∙}}$, the coexistence of R–O–T multiphase, and the tuning of the trade-off between long-range ordering and local heterogeneity. This work provides a lead-free alternative for piezoelectric actuators and a paradigm for designing piezoelectric materials with outstanding comprehensive performance under low electric fields.

Understanding the mechanisms and spatial correlations of crystallographic symmetry breaking in ferroelectric materials is essential to tuning their functional properties. While optical second harmonic generation (SHG) has long been utilized in ferroelectric studies, its capability for probing complex polar materials has yet to be fully realized. Here, we develop a SHG spectral imaging method implemented on a home-designed laser-scanning SHG microscope, and demonstrate its application for a model system of (K, Na)NbO₃ single crystals. Supervised model fitting analysis produces comprehensive information about the polarization vector orientations and relative fractions of constituent domain variants as well as their thermal evolution across the polymorphic phase transitions. Multiple domains and phases are clearly delineated at different temperatures, suggesting the phase competitions in (K, Na)NbO₃. Besides, we show that unsupervised matrix decomposition analysis can quickly and faithfully reveal domain configurations without a priori knowledge about specific material systems. The SHG spectral imaging method can be readily extended to other ferroelectric materials with potentials to be further enhanced.

NaNbO₃-based ceramics usually show ferroelectric-like P-E loops at room temperature due to the irreversible transformation of the antiferroelectric orthorhombic phase to ferroelectric orthorhombic phase, which is not conducive to energy storage applications. Our previous work found that incorporating CaHfO₃ into NaNbO₃ can stabilize its antiferroelectric phase by reducing the tolerance factor (t), as indicated by the appearance of characteristic double P-E loops. Furthermore, a small amount of MnO₂ addition effectively regulate the phase structure and tolerance factor of 0.94NaNbO₃-0.06CaHfO₃ (0.94NN-0.06CH), which can further improve the stability of antiferroelectricity. The XRD and XPS results reveal that the Mn ions preferentially replace A-sites and then B-sites as increasing MnO₂. The antiferroelectric orthorhombic phase first increases and then decreases, while the t shows the reversed trend, thus an enhanced antiferroelectricity and the energy storage density W_rec of 1.69 J/cm³ at 240 kV/cm are obtained for 0.94NN-0.06CH-0.5%MnO₂(in mass fraction). With the increase of Mn content to 1.0 % from 0.5 %, the efficiency increases to 81 % from 45 %, although the energy storage density decreases to 1.31 J/cm³ due to both increased tolerance factor and non-polar phase.

Room-temperature thermoelectric materials provide promising solutions for energy harvesting from the environment, and deliver a maintenance-free power supply for the internet-of-things (IoTs). The currently available Bi₂Te₃ family discovered in the 1950s, still dominates industrial applications, however, it has serious disadvantages of brittleness and the resource shortage of tellurium (1 × 10⁻³ ppm in the earth's crust). The novel Mg₃Sb₂ family has received increasing attention as a promising alternative for room-temperature thermoelectric materials. In this review, the development timeline and fabrication strategies of the Mg₃Sb₂ family are depicted. Moreover, an insightful comparison between the crystallinity and band structures of Mg₃Sb₂ and Bi₂Te₃ is drawn. An outlook is presented to discuss challenges and new paradigms in designing room-temperature thermoelectric materials.

Most widely used piezoelectric ceramics are based on Pb(Zr,Ti)O₃ (PZT) composition which has adverse environmental and health effects due to its high lead content. Environmental and safety concerns with respect to the utilization, recycling, and disposal of lead-based piezoelectric ceramics have induced a new surge in developing lead-free piezoelectric ceramics. Among all the lead-free ceramics, (K,Na)NbO₃ (KNN) has drawn increasing attention because of its well-balanced piezoelectric properties and better environmental compatibility. On basis of the author's work, this review summarizes the progress that has been made in recent years on development of KNN-based piezoelectric ceramics, including crystallographic structure and phase transition analysis, pressurized solid-state sintering as well as liquid-phase-assisted sintering process, and poling treatment for property enhancement. All in all, KNN is a promising lead-free system, but more research is still required both from academic and industrial interests.