Domain Wall Displacement is the Origin of Superior Permittivity and Piezoelectricity in BaTiO3 at Intermediate Grain Sizes

The dielectric and piezoelectric properties of ferroelectric polycrystalline materials have long been known to be strong functions of grain size and extrinsic effects such as domain wall motion. In BaTiO₃, for example, it has been observed for several decades that the piezoelectric and dielectric properties are maximized at intermediate grain sizes (≈1 μm) and different theoretical models have been introduced to describe the physical origin of this effect. Here, using in situ, high‐energy X‐ray diffraction during application of electric fields, it is shown that 90° domain wall motion during both strong (above coercive) and weak (below coercive) electric fields is greatest at these intermediate grain sizes, correlating with the enhanced permittivity and piezoelectric properties observed in BaTiO₃. This result validates the long‐standing theory in attributing the size effects in polycrystalline BaTiO₃ to domain wall displacement. It is now empirically established that a doubling or more in the piezoelectric and dielectric properties of polycrystalline ferroelectric materials can be achieved through domain wall displacement effects; such mechanisms are suggested for use in the design of new ferroelectric materials with enhanced properties.     

Mobile Domain Walls as a Bridge between Nanoscale Conductivity and Macroscopic Electromechanical Response

The interfaces in complex oxides present unique properties exploitable in nanoscale devices. Recent studies on ferroelectric BiFeO₃, BaTiO₃, and Pb(Zr,Ti)O₃ have revealed an unusually high electric conductivity of the domain walls (DWs), adding another degree of freedom for controlling the local properties of these materials. While most of the investigations are focused on thin films for nanoscale applications, many practical devices, including piezoelectric sensors, actuators, and transducers, rely on the macroscopic properties of bulk polycrystalline materials where the average effect of local properties should be small. It is shown that in polycrystalline BiFeO₃ the local domain‐wall conductivity interferes with the dynamics of the DWs within the grains, resulting in an unexpectedly large effect on the macroscopic piezoelectric response. The results thus bridge the local conductivity and the macroscopic piezoelectricity via domain‐wall dynamics, revealing that the domain‐wall conductivity must be considered when interpreting and controlling macroscopic electromechanical properties.     

Millimeter-range Induced Flexo-Pyrophotronic Effect in Centrosymmetric Heterojunction for Ultrafast Night-Photomonitoring

Unlike the structure-specific piezoelectric effect, flexoelectricity is a universal phenomenon that can offer a wide range of energy-efficient, cost-effective, mechano-opto-electro-coupled applications. Even though the flexoelectric effect has been extensively studied at nanoscale, a fundamental, yet unresolved, the issue is how it can be exploited at larger scales for potential applications. Herein, the long-range (>millimeter) stimulated and regulated impact of the localized inhomogeneous strain-induced flexoelectric potential on centrosymmetric metal/titanium oxide heterojunction with nanoscale precision (≈5.8 nm) is demonstrated. The noticed phenomenon is attributed to the long-range interaction between flexoelectric and build-in potentials, which is further utilized to develop mechanically regulated (enhancement > 10⁴%), self-powered (i.e., 0 V), ultrafast (>10 million bits per second), and broadband (λ = 365–1720 nm) pyro-photosensors having high responsivity (≈1.18 mA W⁻¹). As prospective applications, proof-of-concept ultrafast night movement monitors (>720 km h⁻¹), high-performing stationery, and dynamic obstacle sensors with possible impact alerts are developed. These findings lay the groundwork for the micro-to-millimeter-range flexo-opto-electrical coupling in centrosymmetric materials, which can have a wide variety of practical applications.     

Graphene Oxide‐Based Lamella Network for Enhanced Sound Absorption

Noise is an environmental pollutant with recognized impacts on the psychological and physiological health of humans. Many porous materials are often limited by low sound absorption over a broad frequency range, delicacy, excessive weight and thickness, poor moisture insulation, high temperature instability, and lack of readiness for high volume commercialization. Herein, an efficient and robust lamella‐structure is reported as an acoustic absorber based on self‐assembled interconnected graphene oxide (GO) sheets supported by a grill‐shaped melamine skeleton. The fabricated lamella structure exhibits ≈60.3% enhancement over a broad absorption band between 128 and 4000 Hz (≈100% at lower frequencies) compared to the melamine foam. The enhanced acoustic absorption is identified to be structure dependent regardless of the density. The sound dissipation in the open‐celled structure is due to the viscous and thermal losses, whereas it is predominantly tortuosity in wave propagation and enhanced surface area for the GO‐based lamella. In addition to the enhanced acoustic absorption and mechanical robustness, the lamella provides superior structural functionality over many conventional sound absorbers including, moisture/mist insulation and fire retardancy. The fabrication of this new sound absorber is inexpensive, scalable and can be adapted for extensive applications in commercial, residential, and industrial building structures.     

Electric Actuation of Nanostructured Thermoplastic Elastomer Gels with Ultralarge Electrostriction Coefficients

Electrostriction facilitates the electric field‐stimulated mechanical actuation of dielectric materials. This work demonstrates that introduction of dielectric mismatched nanodomains to a dielectric elastomer results in an unexpected ultralarge electrostriction coefficient, enabling a large electromechanical strain response at a low electric field. This strong electrostrictive effect is attributed to the development of an inhomogeneous electric field across the film thickness due to the high density of interfaces between dielectric mismatched periodic nanoscale domains. The periodic nanostructure of the nanostructured gel also makes it possible to measure the true electromechanical strain from the dimensional change monitored via in situ synchrotron small angle X‐ray scattering. The work offers a promising pathway to design novel high performance dielectric elastomers as well as to understand the underlying operational mechanism of nanostructured multiphase electrostrictive systems.     

Large Nonclassical Electrostriction in (Y,Nb)‐Stabilized δ‐Bi2O3

Classical electrostriction, describing a second‐order electromechanical response of insulating solids, scales with elastic compliance, S, and inversely with dielectric susceptibility, ε. This behavior, first noted 20 years ago by Robert Newnham, is shown to apply to a wide range of electrostrictors including polymers, glasses, crystalline linear dielectrics, and relaxor ferroelectrics. Electrostriction in fluorite ceramics of (Y, Nb)‐stabilized δ‐Bi₂O₃ is examined with 16%–23% vacant oxygen sites. Given the values of compliance and dielectric susceptibility, the electrostriction coefficients are orders of magnitude larger than those expected from Newnham's scaling law. In ambient temperature nanoindentation measurements, (Y, Nb)‐stabilized δ‐Bi₂O₃ displays primary creep. These findings, which are strikingly similar to those reported for Gd‐doped ceria, support the suggestion that ion conducting ceramics with the fluorite structure, a large concentration of anion vacancies and anelastic behavior, may constitute a previously unknown class of electrostrictors.     

MgSiAs: An Unexplored System with Promising Nonlinear Optical Properties

Two new non‐centrosymmetric ternary compounds, MgSiAs₂ and Mg₃Si₆As₈, are discovered via metal flux and solid‐state synthetic methods. MgSiAs₂ belongs to the well‐known II‐IV‐V₂ family, which is extensively studied experimentally and computationally for their optical properties. MgSiAs₂ is computationally predicted but not experimentally known prior to this work. Mg₃Si₆As₈ crystallizes in a new non‐centrosymmetric cubic chiral structure type with the Pearson symbol cP68. The syntheses, crystal structure, thermal and chemical stabilities, electronic structures, and optical properties of these two new compounds are investigated in this work. Optical absorption measurements and electronic structure calculations reveal the two compounds to be direct or pseudo‐direct bandgap semiconductors (1.8–2 eV). The crystal structures of both compounds are non‐centrosymmetric, though Mg₃Si₆As₈ belongs to the 432 chiral crystal class, which is optically active but does not exhibit second harmonic generation (SHG) behavior. The SHG response of MgSiAs₂ is 60% of that for AgGaS₂, but MgSiAs₂ exhibits a higher laser damage threshold than AgGaS₂ at 33.2 MW cm^?2.     

Mg?Si?As: An Unexplored System with Promising Nonlinear Optical Properties

Two new non‐centrosymmetric ternary compounds, MgSiAs₂ and Mg₃Si₆As₈, are discovered via metal flux and solid‐state synthetic methods. MgSiAs₂ belongs to the well‐known II‐IV‐V₂ family, which is extensively studied experimentally and computationally for their optical properties. MgSiAs₂ is computationally predicted but not experimentally known prior to this work. Mg₃Si₆As₈ crystallizes in a new non‐centrosymmetric cubic chiral structure type with the Pearson symbol cP68. The syntheses, crystal structure, thermal and chemical stabilities, electronic structures, and optical properties of these two new compounds are investigated in this work. Optical absorption measurements and electronic structure calculations reveal the two compounds to be direct or pseudo‐direct bandgap semiconductors (1.8–2 eV). The crystal structures of both compounds are non‐centrosymmetric, though Mg₃Si₆As₈ belongs to the 432 chiral crystal class, which is optically active but does not exhibit second harmonic generation (SHG) behavior. The SHG response of MgSiAs₂ is 60% of that for AgGaS₂, but MgSiAs₂ exhibits a higher laser damage threshold than AgGaS₂ at 33.2 MW cm^?2.     

Half Cycle Terahertz Pulse Generation by Prism-Coupled Cherenkov Phase-Matching Method

Nonlinear optical terahertz wave generation is a promising method for realizing a practical source with wide frequency range and high peak power. Unfortunately, many nonlinear crystals have a strong absorption in the terahertz frequency region. This limits efficient and widely tunable terahertz wave generation. The Cherenkov phase-matching method is one of the most promising techniques for overcoming these problems. We propose a prism-coupled Cherenkov phase-matching method, in which a prism with a suitable refractive index at terahertz frequencies is coupled to a nonlinear crystal. We demonstrate prism-coupled Cherenkov phase-matching terahertz generation using the DAST and LiNbO₃ crystals. With a DAST crystal, we obtain a spectral flat tunability up to 10 THz by difference frequency generation. With a LiNbO₃ crystal, we observe a spectral flat broadband terahertz pulse generation up to 5 THz pumped by a femto second fiber laser. The obtained temporal waveform is an ideal half cycle pulse suitable for reflection terahertz tomography.     

A Novel Room‐Temperature Multiferroic System of Hexagonal Lu1−xInxFeO3

In the present work, h‐RFeO₃ multiferroic ceramics are designed and created by introducing chemical pressure (In‐substitution for Lu) in LuFeO₃. Lu_1?xIn_xFeO₃ (x = 0‐0.75) ceramics are prepared by the standard solid‐state reaction process. The crystal structure of the present ceramics is tuned from centrosymmetric Pbnm (x = 0) to non‐centrosymmetric P6₃cm (x = 0.4–0.6), and subsequently to centrosymmetric P6₃/mmc (x = 0.75), while the Pbnm and P6₃cm biphase structure is detected for x = 0.25. The Curie temperature for the polar P6₃cm (x = 0.4–0.6) phase decreases from >1000 to ≈550 K with increasing x. Cloverleaf ferroelectric domain structures are determined in polar Lu_0.5In_0.5FeO₃ samples, and the ferroelectric domain walls at atomic scale are evaluated by the aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF STEM), where the spontaneous polarization of 1.73 µC cm^?2 is determined for x = 0.5. The spontaneous polarization is also confirmed by calculating the site displacement from the centrosymmetric phase based on the X‐ray diffraction (XRD) data. Meanwhile, two magnetic transitions are determined for all compositions, that is, paramagnetic to antiferromagnetic transition at Néel temperature T_N (≈350 K for x = 0.4–0.6), and antiferromagnetic to weak‐ferromagnetic transition at spin‐reorientation temperature T_SR. The co‐presence of ferroelectric and antiferromagnetic orders confirms the present ceramics as promising room‐temperature multiferroic materials.     

Structural Transformations during Formation of Quasi‐Amorphous BaTiO3

A model of structural transformations of amorphous into quasi‐amorphous BaTiO₃ is suggested. The model is based on previously published data and on X‐ray photoelectron spectroscopy data presented in the current report. Both amorphous and quasi‐amorphous phases of BaTiO₃ are made up of a network of slightly distorted TiO₆ octahedra connected in three different ways: by apices (akin to perovskite), edges, and faces. Ba ions in these phases are located in the voids between the octahedra, which is a nonperovskite environment. These data also suggest that Ba ions compensate electrical‐charge imbalance incurred by randomly connected octahedra and, thereby, stabilize the TiO₆ network. Upon heating, the edge‐to‐edge and face‐to‐face connections between TiO₆ octahedra are severed and then reconnected via apices. Severing the connections between TiO₆ octahedra requires a volume increase, suppression of which keeps some of the edge‐to‐edge and face‐to‐face connections intact. Transformation of the amorphous thin films into the quasi‐amorphous phase occurs during pulling through a steep temperature gradient. During this process, the volume increase is inhomogeneous and causes both highly anisotropic strain and a strain gradient. The strain gradient favors breaking those connections, which aligns the distorted TiO₆ octahedra along the direction of the gradient. As a result, the structure becomes not only anisotropic and non‐centrosymmetric, but also acquires macroscopic polarization. Other compounds may also form a quasi‐amorphous phase, providing that they satisfy the set of conditions derived from the suggested model.     

Piezotronic Effect Enhanced Plasmonic Photocatalysis by AuNPs/BaTiO3 Heterostructures

Piezopotential‐assisted catalysis is of great significance for low cost and efficient catalysis processes. Here, Au_x/BaTiO₃ plasmonic photocatalysts are fabricated by precipitating Au nanoparticles on piezoelectric BaTiO₃ nanocubes through a chemical approach. The Au nanoparticles (<8 nm) are decorated uniformly on the surface of BaTiO₃, which endows the heterostructure with a wide light absorption from 300 to 600 nm. The photocatalytic properties of the heterostructures are investigated in detail toward methyl orange (MO) degradation. The Au content, piezoelectric potential of the BaTiO₃ substrate, and surface plasmon resonance (SPR) are confirmed to be vital to the photocatalytic activity. The Au₄/BaTiO₃ shows an optimum photocatalytic performance for a complete degradation of MO in 75 min under full spectrum light irradiation with auxiliary ultrasonic excitation. The piezoelectric field originating from the deformation of BaTiO₃ further enhances the separation of photon‐generated carriers induced by SPR and promotes the formation of hydroxyl radicals, which results in a strong oxidizing ability of organic dyes. This work introduces the piezotronic effect to enhance plasmonic photocatalysis with Au_x/BaTiO₃ heterostructures, which is ready to extend to other catalytic systems and offers a new option to design high‐performance catalysts for pollutant treatment.     

Strong Piezoelectricity and Improved Rectifier Properties in Mono- and Multilayered CuInP2S6

Most atomically thin piezoelectrics suffer from weak piezoelectric response or current rectification along the thickness direction, which largely hinders their applications in a vertical crossbar architecture. Therefore, exploring new types of ultrathin materials with strong longitudinal piezoelectric coefficient and rectification is highly desired. In this study, the monolayer of van der Waals CuInP2S6 (CIPS) is successfully exfoliated and its strong piezoelectricity in the out-of-plane direction with an effective coefficient d33eff of ≈5.12 pm V⁻¹, which is one or two orders of magnitude higher than that of most existing monolayer materials with intrinsic d33, is confirmed. A prototype vertical device is further constructed and the current rectification is achieved through the flexoelectricity induced by the scanning tip force. The switching between low and high rectification states can be readily controlled by tuning the mechanical loads. These findings manifest that CIPS possesses promising application in vertical nanoscale piezoelectric devices and provides a novel strategy for achieving a good current rectification in ultrathin piezoelectrics.     

Oxygen Stoichiometry Effect on Polar Properties of LaAlO3/SrTiO3

Discovery of a ferroelectric‐like behavior of the LaAlO₃/SrTiO₃ (LAO/STO) interfaces provides an attractive platform for the development of nanoelectronic devices with functionality that can be tuned by electrical or mechanical means. However, further progress in this direction critically depends on deeper understanding of the physicochemical mechanism of this phenomenon. In this report, this problem by testing the electronic properties of the LAO/STO heterostructures with oxygen stoichiometry used as a variable is addressed. Local probe measurements in conjunction with interface electrical characterization allow to establish the field‐driven reversible migration of oxygen vacancies as the origin of the ferroelectric‐like behavior in LAO/STO. In addition, it is shown that oxygen deficiency gives rise to the formation of micrometer‐long atomically sharp boundaries with robust piezoelectricity stemming from a significant strain gradient across the boundary region. These boundaries are not ferroelectric but they can modulate the local electronic characteristics at the interface. The obtained results open a possibility to design and engineer electromechanical functionality in a wide variety of nominally nonpolar and non‐piezoelectric complex oxide heterostructures and thin films.     

Achieving the Upper Bound of Piezoelectric Response in Tunable,Wearable 3D Printed Nanocomposites

The trade‐off between processability and functional responses presents significant challenges for incorporating piezoelectric materials as potential 3D printable feedstock. Structural compliance and electromechanical coupling sensitivity have been tightly coupled: high piezoelectric responsiveness comes at the cost of low compliance. Here, the formulation and design strategy are presented for a class of a 3D printable, wearable piezoelectric nanocomposite that approaches the upper bound of piezoelectric charge constants while maintaining high compliance. An effective electromechanical interphase model is introduced to elucidate the effects of interfacial functionalization between the highly concentrated perovskite nanoparticulate inclusions (exceeding 74 wt%) and light‐sensitive monomer matrix, shedding light on the significant enhancement of piezoelectric coefficients. It is shown that, through theoretical calculation and experimental validations, maximizing the functionalization level approaches the theoretical upper bound of the piezoelectric constant d₃₃ at any given loading concentration. Based on these findings, their applicability is demonstrated by designing and 3D printing piezoelectric materials that simultaneously achieve high electromechanical sensitivity and structural functionality, as highly sensitive wearables that detect low pressure air (<50 Pa) coming from different directions, as well as wireless, self‐sensing sporting gloves for simultaneous impact absorption and punching force mapping.     

One‐Step Synthesis of W2C@N,P‐C Nanocatalysts for Efficient Hydrogen Electrooxidation across the Whole pH Range

Highly active and low‐cost non‐noble metal electrocatalysts for hydrogen oxidation reaction (HOR) are crucial for the large‐scale applications of fuel cells, which, unfortunately, are rarely documented up to now. Here, a facile one‐step strategy to fabricate W₂C nanoparticles (≈3 nm) encased in N, P‐doped few layer carbon materials (W₂C@N,P‐C, WNPC) as an efficient non‐noble metal HOR electrocatalyst simply by calcining the mixture of recrystallized phosphotungstic acid and dicyandiamide is reported. The obtained WNPC catalyst shows extraordinarily high HOR activities (1.03/0.91/0.84 mA cm^?2 at 0.05 V vs reversible hydrogen electrode in 0.1 m HClO₄/0.1 m KOH/0.1 m neutral phosphate buffered saline electrolytes, respectively), excellent durability during accelerated degradation tests for 10 000 cycles, and outstanding CO tolerance. These high performances are attributed to the uniform structure of WNPC, and more essentially, the synergistic effect among N, P, and C species which elevates the reducibility of WNPC, favoring the generation of abundant HOR active sites.     

Influence of polarization electric field on the dielectric properties of BaTiO3-based ceramics

The titanate of barium (BaTiO₃) is a ferroelectric material with perovskite structure. The electric properties (ferroelectricity, positive temperature coefficient, piezoelectricity, etc.) of BaTiO₃ were largely studied. Usually, addition of small quantities of impurity can modify the dielectric properties of BaTiO₃ and widen its number of applications. In this investigation different additions of sodium niobate (NaNbO₃), according to the composition, (1−x) BaTiO₃+x NaNbO₃ with x=1, 3, 5, 7 and 10 mol%, were considered. Additionally, the effect of simultaneous addition of 3 mol% NaNbO₃ and 3 mol% SiO₂ on the dielectric properties we studied as function of temperature, frequency and the applied bias. The obtained results reveal a net evolution of permittivity with addition. Furthermore, the obtained results show that the permittivity is independent of the applied electric field at temperature lower than Curie point (T_c), but presents relatively high values at low frequencies. Beyond this temperature, the permittivity considerably increases (about 25×10³) and largely depends both on frequency and applied voltage.     

New Organic Nonlinear Optical Polyene Crystals and Their Unusual Phase Transitions

A series of new nonlinear optical chromophores based on configurationally locked polyenes (CLPs) with chiral pyrrolidine donors are synthesized. All CLP derivatives exhibit high thermal stability with decomposition temperatures T_d at least > 270 °C. Acentric single crystals of enantiopure D ‐ and L ‐prolinol‐based chromophores with a monoclinic space group P2₁ exhibit a macroscopic second‐order nonlinearity that is twice as large than that of analogous dimethylamino‐based crystal. This is attributed to a strong hydrogen‐bonded polar polymer‐like chain built by these molecules, which is aligned along the polar crystallographic b‐axis. Five α‐phase CLP crystals with different donors grown from solution exhibit a reversible or irreversible thermally induced structural phase transition to a β‐phase. These phase transitions are unusual, changing the crystal symmetry from higher to lower at increasing temperatures, for example, from centrosymmetric to non‐centrosymmetric, enhancing their macroscopic second‐order nonlinear optical properties.     

Diffused Phase Transition Boosts Thermal Stability of High‐Performance Lead‐Free Piezoelectrics

High piezoelectricity of (K,Na)NbO₃ (KNN) lead‐free materials benefits from a polymorphic phase transition (PPT) around room temperature, but its temperature sensitivity has been a bottleneck impeding their applications. It is found that good thermal stability can be achieved in CaZrO₃‐modified KNN lead‐free piezoceramics, in which the normalized strain d ₃₃* almost keeps constant from room temperature up to 140 °C. In situ synchrotron X‐ray diffraction experiments combined with permitivity measurements disclose the occurrence of a new phase transformation under an electrical field, which extends the transition range between tetragonal and orthorhombic phases. It is revealed that such an electrically enhanced diffused PPT contributed to the boosted thermal stability of KNN‐based lead‐free piezoceramics with high piezoelectricity. The present approach based on phase engineering should also be effective in endowing other lead‐free piezoelectrics with high piezoelectricity and good temperature stability.     

Safe,Stable Cycling of Lithium Metal Batteries with Low‐Viscosity,Fire‐Retardant Locally Concentrated Ionic Liquid Electrolytes

Ionic liquid (IL) electrolytes with concentrated Li salt can ensure safe, high‐performance Li metal batteries (LMBs) but suffer from high viscosity and poor ionic transport. A locally concentrated IL (LCIL) electrolyte with a non‐solvating, fire‐retardant hydrofluoroether (HFE) is presented. This rationally designed electrolyte employs lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1‐methyl‐1‐propyl pyrrolidinium bis(fluorosulfonyl)imide (P13FSI) and 1,1,2,2‐tetrafluoroethyl 2,2,3,3‐tetrafluoropropyl ether (TTE) as the IL and HFE, respectively (1:2:2 by mol). Adding TTE enables a Li‐concentrated IL electrolyte with low viscosity and good separator wettability, facilitating Li‐ion transport to the Li metal anode. The non‐flammability of TTE contributes to excellent thermal stability. Furthermore, synergy between the dual (FSI/TFSI) anions in the LCIL electrolyte can help modify the solid electrolyte interphase, increasing Li Coulombic efficiency and decreasing dendritic Li deposition. LMBs (Li||LiCoO₂) employing the LCIL electrolyte exhibit good rate capability (≈89 mAh g^?1 at 1.8 mA cm^?2, room temperature) and long‐term cycling (≈80% retention after 400 cycles).