Explore the vast range of titles available.

The liquid-like feature of thermoelectric superionic conductors is a double-edged sword: the long-range migration of ions hinders the phonon transport, but their directional segregation greatly impairs the service stability. We report the synergetic enhancement in figure of merit (ZT) and stability in Cu 1.99 Se-based superionic conductors enabled by ion confinement effects. Guided by density functional theory and nudged elastic band simulations, we elevated the activation energy to restrict ion migrations through a cation–anion co-doping strategy. We reduced the carrier concentration without sacrificing the low thermal conductivity, obtaining a ZT of ∼3.0 at 1,050 K. Notably, the fabricated device module maintained a high conversion efficiency of up to ∼13.4% for a temperature difference of 518 K without obvious degradation after 120 cycles. Our work could be generalized to develop electrically and thermally robust functional materials with ionic migration characteristics. Cu 2 Se is of interest for thermoelectrics as it is environmentally sustainable and has a high figure of merit ZT; however, copper ion migration impacts device stability. Here a co-doping strategy that combines steric and electrostatic effects is shown to improve device stability as well as improving ZT to 3.

Domain engineering in ferroelectrics endows flexibility for different functional applications. Whereas the domain engineering strategy for single crystals and thin films is diverse, there is only a limited number of strategies for bulk ceramics. Here, a domain engineering strategy for achieving a compact domain architecture with increased domain‐wall density in (K,Na)NbO3 (KNN)‐based ferroelectric ceramics via mesoscopic chemical inhomogeneity (MCI) is developed. The MCI‐induced interfaces can effectively hinder domain continuity and modify the domain configuration. Besides, the MCI effect also results in diffused phase transitions, which is beneficial for achieving enhanced thermal stability. Modulation of chemical inhomogeneity demonstrates great potential for engineering desirable domain configuration and properties in ferroelectric ceramics. Additionally, the MCI can be easily controlled by regulating the processing condition during solid‐state synthesis, which is advantageous to industrial production. A novel domain engineering strategy is developed for ferroelectric ceramics based on the control of mesoscopic chemical inhomogeneity. Since the strategy can be easily realized via processing control, it is highly accessible to the community. This method is expected to be applicable to various ferroelectric ceramics, offering a valuable opportunity to engineer their macroscopic properties in an unexplored way.

Despite the pivotal role of stannum doping in achieving ultrahigh piezoelectric performance in barium titanate-based ceramics, the fundamental mechanisms underlying this enhancement remain elusive. Here, we introduce a single variable nonstoichiometric stannum strategy in lead-free barium titanate-based ceramics with giant piezoelectricity, revealing that stannum doping contributes intrinsically and extrinsically to enhance piezoelectricity. Density functional theory calculations elucidate the intrinsic enhancement of polarization arising from lattice distortion and increased space for titanium-oxygen bonds induced by optimal stannum doping, which is corroborated by Rayleigh analysis. A phase transition from ferroelectric multiphase coexistence to paraelectric phase is observed, alongside a rapid miniaturized and eventually disappeared domains with increasing stannum doping. This evolution in phase structure and domain configuration induces a nearly vanishing polarization anisotropy and low domain wall energy, facilitating easy polarization rotation and domain wall motion, thereby significantly contributing to the extrinsic piezoelectric response. Consequently, the origins of ultrahigh performance can be attributed to the synergistic effect of stannum-induced intrinsic and extrinsic contributions in barium titanate-based ceramics. This study provides fundamental insights into the role of doping elements and offers guidance for the design of high-performance piezoelectrics. The mechanisms of the enhancement of stannum doping in achieving high piezoelectric performance in barium titanate-based ferroelectric ceramics remain elusive. Here, the authors introduce a single variable nonstoichiometric stannum strategy to reveal the intrinsic and extrinsic contributions for enhancing piezoelectricity after stannum doping.

Ecologically sustainable development of piezoelectric ceramics has been the primary target of the community over the past 20 years. While the development of “soft” lead-free piezoelectric ceramics has been of high maturity, the understanding of “hard” lead-free piezoelectric ceramics is still far from satisfactory, leading to a limited chance for high-power applications. The review starts with an introduction of loss mechanisms and the hardening effect in piezoelectric ceramics, including three different models mainly developed based on the lead zirconate titanate system. Then, studies on the hardening behavior of BaTiO 3 -based, (Bi 0.5 Na 0.5 )TiO 3 -based, and (K 0.5 Na 0.5 )NbO 3 -based lead-free piezoelectric ceramics are summarized with emphasis on the approaches to enhance mechanical quality factor. Meanwhile, three different characterization methods of high-power performances are introduced: the constant-voltage method, constant-current method, and transient (or burst) method. Finally, the state-of-the-art lead-free ultrasonic transducer applications are highlighted. This paper concludes with the remaining challenges for the development of “hard” lead-free piezoelectric ceramics for high-power piezoelectric applications. Graphic Abstract

The electrocaloric effect of ferroelectrics holds great promise for solid-state cooling, potentially replacing traditional vapor-compression refrigeration systems. However, achieving adequate electrocaloric cooling capacity at room temperature remains a formidable challenge due to the need for a high intrinsic electrocaloric effect. While barium titanate ceramic exhibits a pronounced electrocaloric effect near its Curie temperature, typical chemical modifications to enhance electrocaloric properties at room temperature often reduce this intrinsic electrocaloric effect. Herein, a structural design is introduced for barium titanate-based ceramics by incorporating isovalent cations. This leads to a well-ordered local structure that decreases the Curie temperature to room temperature while preserving a sharp phase transition, enabling a large dielectric constant and tunable polarization. This design achieves a remarkable electrocaloric strength of ~1.0 K·mm/kV, surpassing previous reports. Atomic-resolution structural analyses reveal that the presence of multiscale nanodomains (from ~10 nm to >100 nm), and the dipole polarization distribution with gradual dipole rotation enable rapid phase transition and facile polarization rotation, accounting for the giant electrocaloric response. This work provides a strategy for achieving a strong intrinsic electrocaloric effect in ferroelectrics near room temperature and offers key insights into the microstructure landscapes driving this enhanced electrocaloric effect. The authors introduce a structural design with a well-ordered local structure for barium titanate-based ceramics, which decreases Curie temperature while preserves a sharp phase transition, enabling tunable polarization, large dielectric constant and intrinsic electrocaloric effect near room temperature.

Atomistic control of phase boundaries is crucial for optimizing the functional properties of solid-solution ferroelectrics, yet their microstructural mechanisms remain elusive. Here, we harness machine-learning-driven molecular dynamics to resolve the phase boundary behavior in the KNbO 3 –KTaO 3 (KNTO) system. Our simulations reveal that chemical composition and ordering enable precise modulation of polymorphic phase boundaries (PPBs), offering a versatile pathway for materials engineering. Diffused PPBs and polar nano regions, predicted by our model, highly match with experiments, underscoring the fidelity of the machine-learning atomistic simulation. Crucially, we identify elastic and electrostatic mismatches between ferroelectric KNbO 3 and paraelectric KTaO 3 as the driving forces behind complex microstructural evolution. This work not only resolves the longstanding microstructural debate but also establishes a generalizable framework for phase boundary engineering toward next-generation high-performance ferroelectrics.

The functionality of ferroelectrics is often constrained by their Curie temperature, above which depolarization occurs. Lithium (Li) is the only experimentally known substitute that can increase the Curie temperature in ferroelectric niobate-based perovskites, yet the mechanism remains unresolved. Here, the unique phenomenon in Li-substituted KNbO 3 is investigated using first-principles density functional theory. Theoretical calculations show that Li substitution at the A-site of perovskite introduces compressive chemical pressure, reducing Nb–O hybridization and associated ferroelectric instability. However, the large off-center displacement of the Li cation compensates for this reduction and further enhances the soft polar mode, thereby raising the Curie temperature. In addition, the stability of the tetragonal phase over the orthorhombic phase is predicted upon Li substitution, which reasonably explains the experimental observation of a decreased orthorhombic-to-tetragonal phase transition temperature. Finally, a metastable anti-phase polar state in which the Li cation displaces oppositely to the Nb cation is revealed, which could also contribute to the variation of phase transition temperatures. These findings provide critical insights into the atomic-scale mechanisms governing Curie temperature enhancement in ferroelectrics and pave the way for designing advanced ferroelectric materials with improved thermal stability and functional performance.

Polarization switching in lead-free (K 0.40 Na 0.60 )NbO 3 (KNN) single crystals was studied by switching spectroscopy piezoresponse force microscopy (SS-PFM). Acquisition of multiple hysteresis loops on a closely spaced square grid enables polarization switching parameters to be mapped in real space. Piezoresponse amplitude and phase hysteresis loops show collective symmetric/asymmetric characteristics, affording information regarding the switching behavior of different domains. As such, the out-of-plane polarization states of the domains, including amplitudes and phases can be determined. Our results could contribute to a further understanding of the relationships between polarization switching and polarization vectors at the nanoscale, and provide a feasible method to correlate the polarization hysteresis loops in a domain under an electric field with the polarization vector states.

Being capable of interconverting electrical and mechanical energy, piezoelectric materials are essential for electromechanical devices. In this study, high-performance 0.95Pb(Zr0.52Ti0.48)O3–0.05Pb(Mn1/3Nb2/3)O3 (PZT-PMnN) piezoelectric ceramics were prepared by spark plasma sintering (SPS). We systematically investigated temperature-dependent electrical properties and phase structure of ceramics. The SPS temperature was found to be optimized at 950 °C. PZT-PMnN ceramic sintered at 950 °C possessed high piezoelectric performances, i.e. d33 of 411 pC/N and d33* of 556 pm/V. Furthermore, a high mechanical quality factor Qm (480) and low dielectric loss (0.19%) were obtained. Therefore, the spark-plasma-sintered PZT-PMnN ceramic is a promising candidate for applications.

Piezoelectric materials are indispensable in electromechanical actuators, which require a large electrostrain with a fast and precise response. By designing a chemopiezoelectric effect, we developed an approach to achieve a high electrostrain of 1.9% under −3 kV mm −1 , at 1 Hz, corresponding to an effective piezoelectric coefficient of >6,300 pm V −1 at room temperature in lead-free potassium sodium niobate piezoceramics. This electrostrain has satisfactory fatigue resistance and thermal stability, and low hysteresis, far outperforming existing lead-based and lead-free perovskite counterparts. From tracer diffusion, atomic optical emission spectrometry experiments, combined with machine-learning molecular dynamics and phase-field simulations, we attribute the high electrostrain to short-range hopping of oxygen vacancies near ceramic surfaces under an alternating electric field, which is supported by strain levels reaching 3.0% under the same applied field when the sample was annealed at a low oxygen partial pressure. These findings provide an additional degree of freedom for designing materials on the basis of defect engineering, which will favour not only the electrostrain of piezoelectrics but also the functional properties of a broader range of oxide-based materials. The authors demonstrate a chemopiezoelectric effect in which the displacive migration of oxygen vacancies driven by an electric field induces a large strain in the surface layer of thin (K,Na)NbO 3 ceramics. They achieve an electrostrain of 1.9% under a field of −3 kV mm −1 , with thermal stability up to 200 °C.