BiFeO₃–BaTiO₃ ceramics have been shown to possess considerable promise in the domain of high-temperature lead-free piezoelectric applications, a property attributable to their elevated Curie temperature and superior piezoelectric properties. However, the inherent issue of high leakage current within this system is a consequence of the unavoidable formation of secondary phases within thermodynamically unstable temperature ranges and the volatility of Bi. This issue severely compromises the high-temperature polarization stability and piezoelectric performance. This high conductance hinders the enhancement of the overall performance and obscures the evolution patterns of intrinsic defects and their influence mechanisms on macroscopic properties, necessitating in-depth investigation. The present study successfully prepared a series of BiFe_1+xO₃–BaTiO₃ ceramics using a one-step sintering process. The Fe content was deliberately designed to exhibit both severe excess and deficiency states. It was found that the Fe non-stoichiometry-induced unique defect evolution behavior. A detailed investigation was conducted to ascertain the influence of the defect configuration on the electrical insulation properties across a range of temperatures and strain levels. The respective contributions of the defect-dipole- and space-charge-induced built-in electric fields to the strain response (before and after polarization) were also examined. It is noteworthy that the pure 0.7BiFeO₃–0.3BaTiO₃ system exhibits remarkable comprehensive properties at room temperature (piezoelectric constant d₃₃^* = 1021 pm/V, strain S ≈ 0.38%, and piezoelectric coefficient d₃₃ = 201 pC/N). The material exhibits a high Curie temperature of approximately 501 °C, accompanied by a notable high-temperature piezoelectric activity. The peak d₃₃ and d₃₃^* values are approximately 380 pC/N (measured at 317.9 °C) and 1481 pm/V (measured at 125 °C), respectively.

BiFeO₃—BaTiO₃ lead-free piezoelectric ceramics exhibit superior piezoelectric properties while preserving a high Curie temperature. However, given the inherent Gibbs free energy law of BiFeO₃, the system is difficult to avoid heterogeneous phases such as Bi₂₅FeO₃₉ and/or Bi₂Fe₄O₉, which are accompanied by the volatilization of Bi³⁺ and the change of Fe³⁺, resulting in low insulating properties and high dielectric loss. These factors hinder the enhancement of polarizability and the overall performance at elevated temperatures and electric field conditions. The present study focuses on a highly leaky 0.75BiFeO₃–0.25BaTiO₃ ceramic, in which the Fe content is deliberately designed to be both severely excessive and deficient, and is prepared using a one-step low-temperature sintering process. It is noteworthy that the structural stability and defect suppression, even in this challenging system, are achieved via the one-step low-temperature sintering. This samples exhibit a distinctive self-tuning property and an excellent stability over a wide compositional range. First-principles density functional theory calculations and XPS analysis have for the first time confirmed that suppressing oxygen vacancies and Fe³⁺ valence states can reduce the concentration and mobility of hole carriers, thereby effectively reducing leakage current, with the mechanism shifting from ohmic conduction to space-charge-limited conduction. Even under the extreme compositional conditions of x = ± 5 and a low sintering temperature, the piezoelectric coefficients d₃₃ reach 132 pC/N and 110 pC/N, respectively. These are significantly higher than those of the most stoichiometric 0.75BiFeO₃–0.25BaTiO₃ counterparts, setting a new performance record.

BiFeO₃BaTiO₃ (BF–BT) ceramics exhibit higher piezoelectric coefficients (d₃₃), Curie temperatures (T_C), and temperature stability than other high-temperature lead-free piezoelectric materials. However, despite their crucial role in piezoelectric devices, the mechanical properties of BF–BT ceramics have been underexplored. A thorough evaluation of the mechanical properties of BF–BT is crucial for developing cost-effective and durable lead-free piezoelectric ceramics. Moreover, the specific causes of the high piezoelectric response and excellent temperature stability of BF–BT ceramics remain unclear owing to the instrumental detection threshold, which limits experimental studies to temperatures above 140 °C and below the degradation temperature of d₃₃. To investigate the intrinsic origins of the high piezoelectricity and temperature stability of BF–xBT ceramics and to enhance their mechanical properties, a two-step sintering process is used to fabricate these ceramics (0.25 ≤ x ≤ 0.40). Owing to improvements in grain refinement and reduced Bi³⁺ volatilization, the BF–0.33 BT ceramic exhibits enhanced overall performance, with a modified small punch strength of 155 MPa, Vickers hardness of 5.2 GPa, a d₃₃ of 220 pC/N at room temperature, T_C of 466 °C, and d₃₃ values exceeding 400 pC/N at 260 °C. Phase-field simulations, which address the limitations of device detection thresholds, reveal that with increasing temperature, the domain structure relaxes, and polarization intensity decreases. This indicates that changes in the high-temperature piezoelectric properties can be attributed to domain structure relaxation and the increase in dielectric constant. Overall, BF–BT ceramics exhibit superior piezoelectric performance, mechanical properties, and temperature stability, making them highly suitable for use in high-temperature and demanding environments.

Dielectric capacitors with high energy storage performances are exceedingly desired for the next-generation advanced high/pulsed power devices that demand miniaturization and integration. However, poor energy-storage density (U_rec) and low efficiency (η) resulted from the large remanent polarization (P_r) and low breakdown strength (BDS), have been the major challenge for the application of dielectric capacitors. Herein, a high-entropy strategy with superparaelectric relaxor ferroelectrics (SP-RFE) was adopted to achieve extremely low P_r and high BDS in BaTiO₃ system, simultaneously. Due to the high BDS ~800 kV/cm and low P_r ~0.58 μC/cm², high-entropy SP-RFE (La_0.05Ba_0.18Sr_0.18K_0.115Na_0.115Ca_0.18Bi_0.18)TiO₃ (LBSKNCBT) MLCCs exhibited high U_rec ~6.63 J/cm³ and excellent η ~ 96%. What's more, LBSKNCBT MLCCs with high-entropy and SP-RFE characteristic also possess a good temperature and frequency stability. In a word, this work offers an excellent paradigm for achieving good energy-storage properties of BaTiO₃-based dielectric capacitors to meet the demanding requirements of advanced energy storage applications.

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.

Lots of research on thermoelectric materials (TEs) has focused on improving their thermoelectric (TE) properties to achieve efficient energy conversion. However, the mechanical properties of materials are also the object of concern in practical applications. Nowadays, the field of electronic devices is obviously developing in the direction of flexible electronics, so the research on TEs should also consider the plasticity. Since 2018, it has been discovered that inorganic semiconductor materials have the ability of plastic deformation, giving new possibilities for the development of TEs with plasticity. This paper focuses on the TEs with two-dimensional van der Waals (2D vdW) crystal structures, which have good plasticity but low TE properties. However, these materials have the potential to become excellent materials with TE properties and good plasticity through optimization strategies. In this paper, the latest research progress of 2D vdW TE materials and their applications in electronic devices are reviewed. The plasticity and TE properties of 2D vdW materials with M₂X, MX and MX₂ structure are summarized, and their plasticity mechanisms are discussed. We also introduce the application of high throughput screening in the discovery of novel 2D vdW plastic materials, and outline the future research work of 2D vdW TE materials.

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.