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.

The 0.93(Na_0.5Bi_0.5)_1-xSm_xTiO₃-0.07BaTiO₃ multifunctional ceramics were prepared by solid-phase reaction method. The phase structure, microstructure, electrical and photoluminescent properties were systematically studied. With increasing x, the ceramics undergoes the phase transition from rhombohedral to tetragonal with some rhombohedral distortion, along with a reduced grain size and increased relative density. On the other hand, the Sm³⁺ doping enhances the electric-field driven reversible phase transition and domain size, and reduces the domain walls, thereby contributing to improved piezoelectricity and decreased depolarization temperature (T_d) from 91 ℃ to 40 ℃. Excellent piezoelectric properties of d₃₃ = 213 pC/N and k_p = 29.9% are achieved in the x = 0.010 ceramic. Under excitation (407 nm), the Sm³⁺-doped ceramic exhibits bright reddish-orange fluorescence at 564, 599, 646 nm and 710 nm. A polarization-induced enhancement of photoluminescence is obtained in BNBT-xSm ceramics with an improved relative intensity of emission band at 646 nm. These results indicate that Sm³⁺-doped BNBT ceramics show great potential in electro-optic integration and coupling device applications.

BiFeO₃–BaTiO₃ (BF–BT) based piezoelectric ceramics are a kind of high-temperature lead-free piezoelectric ceramics with great development prospects due to their high Curie temperature (T_C) and excellent electrical properties. However, large leakage current limits their performance improvement and practical applications. In this work, direct current (DC) test, alternating current (AC) impedance, and Hall tests were used to investigate conduction mechanisms of 0.75BiFeO₃–0.25BaTiO₃ ceramics over a wide temperature range. In the range of room temperature (RT)−150 ℃, ohmic conduction plays a predominant effect, and the main carriers are p-type holes with the activation energy (E_a) of 0.51 eV. When T > 200 ℃, the E_a value calculated from the AC impedance and Hall data is 1.03 eV with oxygen vacancies as a cause of high conductivity. The diffusion behavior of thermally activated oxygen vacancies is affected by crystal symmetry, oxygen vacancy concentration, and distribution, dominating internal conduction mechanism. Deciphering the conduction mechanisms over the three temperature ranges would pave the way for further improving the insulation and electrical properties of BiFeO₃–BaTiO₃ ceramics.

BiFeO₃-BaTiO₃ based ceramics are considered to be the most promising lead-free piezoelectric ceramics due to their large piezoelectric response and high Curie temperature. Since the piezoelectric response of piezoelectric ceramics just appears after poling engineering, in this work, the domain evolution and microscopic piezoresponse were observed in-situ using piezoresponse force microscopy (PFM) and switching spectroscopy piezoresponse force microscopy (SS-PFM), which can effectively study the local switching characteristics of ferroelectric materials especially at the nanoscale. The new domain nucleation preferentially forms at the boundary of the relative polarization region and expands laterally with the increase of bias voltage and temperature. The maximum piezoresponse (R_s), remnant piezoresponse (R_rem), maximum displacement (D_max) and negative displacement (D_neg) at 45 V and 120 ℃ reach 122, 69, 127 pm and 75 pm, respectively. Due to the distinct effect of poling engineering in full domain switching, the corresponding d₃₃ at 50 kV/cm and 120 ℃ reaches a maximum of 205 pC/N, which is nearly twice as high as that at room temperature. Studying the evolution of ferroelectric domains in the poling engineering of BiFeO₃-BaTiO₃ ceramics provides an insight into the relationship between domain structure and piezoelectric response, which has implications for other piezoelectric ceramics as well.

Morphotropic phase boundary (MPB) plays a key role in tuning piezoelectric responses of ferroelectric ceramics. Here, Bi_0·5Na_0·5TiO₃ modified BiFeO₃–BaTiO₃ ternary solid solutions of 0.7BiFeO₃-(0.3-x)BaTiO₃-xBi_0.5Na_0·5TiO₃ (referred to as BF-BT-xBNT, 0.00 ≤ x ≤ 0.04) were prepared for lead-free piezoelectrics. All the ceramics exhibit an MPB with coexisting rhombohedral (R) and tetragonal (T) phases, and the R/T phase ratio decreases upon increasing x. The increment of BNT promotes the grain growth, lowers the leakage current and Curie temperature (T_C), and gradually drives the ferroelectric to relaxor transition. Because of the MPB with appropriate R/T phase ratio, increased grain size and density, and decreased leakage current, the well-balanced performance between d₃₃ = 206 pC/N and T_C = 488 ℃ is obtained in x = 0.01 case. In addition, the further enhanced in-situ d₃₃ = 286–347 pC/N is obtained in BF-BT-xBNT ceramics along with the improved depolarization temperature T_d from 280 to 312 ℃, showing a potential application for lead-free piezoceramics at high temperature.

It is well-known that the performance of BiFeO₃BaTiO₃ (BF-BT) ceramics is sensitive to composition, calcining and sintering temperature (T_cal and T_sint) due to the formation of Bi₂₅FeO₃₉ and/or Bi₂Fe₄O₉ impurities and/or the volatilization of Bi₂O₃. We report remarkably stable electrical properties over the range of −0.03 ≤ x ≤ 0.05 and 930 ℃ ≤ T_sint ≤ 970 ℃ in 0.7Bi_(1+x)FeO₃-0.3BaTiO₃ ceramics prepared by one-step process. This method avoids the thermodynamically unstable region of BiFeO₃ and prevents the formation of Bi₂₅FeO₃₉ and/or Bi₂Fe₄O₉ impurities even when the addition of α-Bi₂O₃ raw material is intentionally deficient or rich to make off-stoichiometric BF-BT, thus greatly improving the robustness of compositional and processing. Rhombohedral-pseudocubic phase coexists in all ceramics, and their C_R/C_PC fraction are 48.0/52.0–50.6/49.4 and 55.9/44.1–56.6/43.4 when x increases from −0.05 ≤ x ≤ 0 to 0.01 ≤ x ≤ 0.05. The stable electrical properties of d₃₃ = 180–205 pC/N, P_r = 17.9–23.8 μC/cm², and T_C = 485–518 ℃ are achieved. The maximum d_33T/d_33RT of BF-BT is twice that of soft PZT, superior to most the-state-of-art lead-free ceramics. Our results provide a synthesis strategy for designing high performance piezoelectric materials with good stability and easy industrialization.

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.

Cu_1.8S has been considered as a potential thermoelectric (TE) material for its stable electrical and thermal properties, environmental benignity, and low cost. Herein, the TE properties of nanostructured Cu_1.8S_1-xTe_x (0 ≤ x ≤ 0.2) bulks fabricated by a facile process combining mechanical alloying (MA) and room-temperature high-pressure sintering (RT-HPS) technique were optimized via eliminating the volatilization of S element and suppressing grain growth. Experimentally, a single phase of Cu_1.8S was obtained at x = 0, and a second Cu_1.96S phase formed in all Cu_1.8S_1-xTe_x samples when 0.05 ≤ x ≤ 0.125. With further increasing x to 0.15 ≤ x ≤ 0.2, the Cu_2-zTe phase was detected and the samples consisted of Cu_1.8S, Cu_1.96S, and Cu_2-zTe phases. Benefiting from a modified band structure and the coexisted phases of Cu_1.96S and Cu_2-zTe, the power factor is enhanced in all Cu_1.8S_1-xTe_x (0.05 ≤ x ≤ 0.2) alloys. Combining with a drastic decrease in the thermal conductivity due to the strengthened phonon scatterings from multiscale defects introduced by Te doping and nano-grain boundaries, a maximum figure of merit (ZT) of 0.352 is reached at 623 K for Cu_1.8S_0.875Te_0.125, which is 171% higher than that of Cu_1.8S (0.130). The study demonstrates that doping Te is an effective strategy to improve the TE performance of Cu_1.8S based materials and the proposed facile method combing MA and RT-HPS is a potential way to fabricate nanostructured bulks.

(1–x)K_0.48Na_0.56NbO₃–xBi_0.5Li_0.5ZrO₃ (KNN–xBLZ, x = 0–0.06) lead-free piezoelectric ceramics were prepared by the conventional solid-state sintering method, and their phase structures and electric properties as well as $T_{\text{C}}$ were systematically investigated. The orthorhombic–tetragonal (O–T) two phases were detected in all (1–x)K_0.48Na_0.56NbO₃–xBi_0.5Li_0.5ZrO₃ ceramics at 0.01 ≤ x ≤ 0.05. Due to the appropriate ratio between O phase and T phase ( $C_{\text{O}}/C_{\text{T}}$ = 45/55), high piezoelectric properties of $d_{33}$ = 239 pC/N, $k_{\text{p}}$ = 34%, and $P_r$ = 25.23 μC/cm² were obtained at x = 0.04. Moreover, a high $T_{\text{C}}$ = 348 ℃ was also achieved in KNN–xBLZ ceramic at x = 0.04. These results indicate that (1–x)K_0.48Na_0.56NbO₃–xBi_0.5Li_0.5ZrO₃ system is a promising candidate for high-temperature piezoelectric devices.