Origin of the high piezoelectric response in PbZr1-xTixO3

Revisiting Structural and Electromechanical Properties of the Lead-free (K,Na)NbO3 High-Piezoelectric Material

Having lead-free systems with excellent piezoelectric responses is crucial to the development of environmentally friendly electromechanical applications. In this work, we build an effective Hamiltonian model to explore the promising (KxNa1-x)NbO3 system, whose rich phase diagram near x = 50% remains poorly understood meanwhile exhibiting a colossal effective piezoelectric response. Thanks to the numerical implementation of this effective Hamiltonian scheme into a Monte Carlo Metropolis algorithm, we reveal striking features. First, a long-period state can be the ground state at low temperatures for some concentrations while only a short-period conventional polar ground state exists for larger x. Second, the electric field-driven transformation, via a first-order transition, of this long-period state into a short-period polar state creates large electromechanical strains (on the order of the percent) and is likely the origin of the colossal piezoelectric response reported in KNN, for which we evaluate an effective piezoelectric coefficient of several thousands of pC/N.

Ultrahigh-power-density flexible piezoelectric energy harvester based on freestanding ferroelectric oxide thin films

Flexible piezoelectric nanogenerators are emerging as a promising solution for powering next-generation flexible electronics by converting mechanical energy into electrical energy. However, traditional ferroelectric ceramics, despite their excellent piezoelectric properties, lack flexibility; while piezoelectric polymers, although highly flexible, have low piezoelectricity. The quest to develop materials that combine high piezoelectricity with exceptional flexibility has thus become a research focus. Herein, we present a breakthrough in this field with the fabrication of freestanding (111)-oriented PbZr0.52Ti0.48O3 single crystalline thin films, which exhibit remarkable flexibility and a high converse piezoelectric coefficient (~585 pm/V). This is achieved through water-soluble sacrificial layer to relieve substrate clamping and controlling the crystal orientation to further enhance the piezoelectric response. Our nanogenerators, constructed using these freestanding nanoscale membranes, demonstrate a record-high output power density (~63.5 mW/cm3), excellent flexibility (with a strain tolerance >3.4%), and superior mechanical stability in cycling tests (>60,000 cycles). These advancements pave the way for high-performance, flexible electronic devices utilizing ferroelectric oxide thin films.

Colossal Electromechanical Response in Antiferroelectric-based Nanoscale Multilayers

The pursuit of smaller, energy-efficient devices drives the exploration of electromechanically active thin films (<1 µm) to enable micro- and nano-electromechanical systems. While the electromechanical response of such films is limited by substrate-induced mechanical clamping, large electromechanical responses in antiferroelectric and multilayer thin-film heterostructures have garnered interest. Here, multilayer thin-film heterostructures based on antiferroelectric PbHfO3 and ferroelectric PbHf1-xTixO3 overcome substrate clamping to produce electromechanical strains >4.5%. By varying the chemistry of the PbHf1-xTixO3 layer (x = 0.3-0.6) it is possible to alter the threshold field for the antiferroelectric-to-ferroelectric phase transition, reducing the field required to induce the onset of large electromechanical response. Furthermore, varying the interface density (from 0.008 to 3.1 nm-1) enhances the electrical-breakdown field by >450%. Attaining the electromechanical strains does not necessitate creating a new material with unprecedented piezoelectric coefficients, but developing heterostructures capable of withstanding large fields, thus addressing traditional limitations of thin-film piezoelectrics.

Relationship between Pb ion off-centering and lone pair electrons

High-pressure synthesized Pb-based perovskites exhibit diverse functional properties. PbCrO3, for instance, displays distinctive diffuse scattering and valence skipping, forming Pb2+ and Pb4+ ions. However, the spatial distribution of Pb ions in the crystal remains largely unexplored. Here, we elucidate the role of Pb ions with different valences through Sr substitution, using high-resolution transmission electron microscopy combined with elemental mapping. This approach allows us to accurately examine the distribution of Sr and Pb ions in the same atomic columns. The results reveal that in Pb0.8Sr0.2CrO3, Sr ions occupy a squared lattice, while Pb ions exhibit positional distortions. Simulations based on the experimentally determined Pb distribution reproduce the diffuse scattering observed in PbCrO3. These findings suggest that the lone pair electrons of s orbitals are responsible for the local lattice distortion. Our study provides atomistic insights into the local structures of materials exhibiting valence skipping.

Electrically Active Biomaterials for Stimulation and Regeneration in Tissue Engineering

In the human body, bioelectric cues are crucial for tissue stimulation and regeneration. Electrical stimulation (ES) significantly enhances the regeneration of nerves, bones, cardiovascular tissues, and wounds. However, the use of conventional devices with stimulating metal electrodes is invasive and requires external batteries. Consequently, electrically active materials with excellent biocompatibility have attracted attention for their applications in stimulation and regeneration in tissue engineering. To fully exploit the potential of these materials, biocompatibility, operating mechanisms, electrical properties, and even biodegradability should be carefully considered. In this review, we categorize various electrically active biomaterials based on their mechanisms for generating electrical cues, such as piezoelectric effect, triboelectric effect, and others. We also summarize the key material properties, including electrical characteristics and biodegradability, and describe their applications in tissue stimulation and regeneration for nerves, musculoskeletal tissues, and cardiovascular tissues. The electrically active biomaterials hold great potential for advancing the field of tissue engineering and their demonstrated success underscores the importance of continued research in this field.

Recent Progress in the Auxiliary Phase Enhanced Flexible Piezocomposites

Piezocomposites with both flexibility and electromechanical conversion characteristics have been widely applied in various fields, including sensors, energy harvesting, catalysis, and biomedical treatment. In the composition of piezocomposites or their preparation process, a category of materials is commonly employed that do not possess piezoelectric properties themselves but play a crucial role in performance enhancement. In this review, the concept of auxiliary phase is first proposed to define these materials, aiming to provide a new perspective for designing high‐performance piezocomposites. Three different categories of modulation forms of auxiliary phase in piezocomposites are systematically summarized, including the modification of piezo‐matrix, the modification of piezo‐fillers, and the construction of special structures. Each category emphasizes the role of the auxiliary phase and systematically discusses the latest advancements and the physical mechanisms of the auxiliary phase enhanced flexible piezocomposites. Finally, a summary and future outlook of piezocomposites based on the auxiliary phase are provided.

Deciphering the atomistic mechanism underlying highly tunable piezoelectric properties in perovskite ferroelectrics via transition metal doping

Piezoelectricity, a fundamental property of perovskite ferroelectrics, endows the materials at the heart of electromechanical systems spanning from macro to micro/nano scales. Defect engineering strategies, particularly involving heterovalent trace impurities and derived vacancies, hold great potential for adjusting piezoelectric performance. Despite the prevalent use of defect engineering for modification, a comprehensive understanding of the specific features that positively impact material properties is still lacking, this knowledge gap impedes the advancement of a universally applicable defect selection and design strategy. In this work, we select perovskite KTa1-xNbxO3 single crystals with orthorhombic phase as the matrix and introduce Fe and Mn elements, which are commonly used in "hard" ferroelectrics as dopants. We investigate how transition-metal doping modifies piezoelectric properties from the perspective of intrinsic polarization behaviors. Interestingly, despite both being doped into the B-site as an acceptor, Mn doping enhances the local structural heterogeneity, greatly bolstering the piezoelectric coefficient beyond 1000 pC/N, whereas Fe doping tends to stabilize the polarization, leading to a substantial improvement in the mechanical quality factor up to 700. This work deciphers the diverse impacts of transition metal impurities on regulating polarization structures and modifying piezoelectric properties, providing a good paradigm for strategically designing perovskite ferroelectrics.

Highly Responsive Polar Vortices in All-Ferroelectric Heterostructures

The discovery of polar vortices and skyrmions in ferroelectric-dielectric superlattices [such as (PbTiO3)n/(SrTiO3)n] has ushered in an era of novel dipolar topologies and corresponding emergent phenomena. The key to creating such emergent features has generally been considered to be related to counterpoising strongly polar and non-polar materials thus creating the appropriate boundary conditions. This limits the utility these materials can have, however, by rendering (effectively) half of the structure unresponsive to applied stimuli. Here, using advanced thin-film deposition and an array of characterization and simulation approaches, polar vortices are realized in all-ferroelectric trilayers, multilayers, and superlattices built from the fundamental building block of (PbTiO3)n/(PbxSr1- xTiO3)n wherein in-plane ferroelectric polarization in the PbxSr1- xTiO3 provides the appropriate boundary conditions. These superlattices exhibit substantially enhanced electromechanical and ferroelectric responses in the out-of-plane direction that arise from the ability of the polarization in both layers to rotate to the out-of-plane direction under field. In the in-plane direction, the layers are found to be strongly coupled during switching and when heterostructured with ferroelectric-dielectric building blocks, it is possible to produce multistate switching. This approach expands the realm of systems supporting emergent dipolar texture formation and does so with entirely ferroelectric materials thus greatly improving their responses.

Well-balanced performance achieved in PZT piezoceramics via a multiscale regulation strategy

Emerging high-power piezoelectric applications demand the development of piezoelectric materials featuring both a high mechanical quality factor (Qm) and a large piezoelectric coefficient (d33). However, it is widely accepted that an increase in d33 is usually accompanied with a decrease in Qm, and vice versa. Herein, a multiscale regulation strategy is proposed to improve Qm and d33 simultaneously from the perspectives of phase structure, ferroelectric domains, and lattice defects. A well-balanced combination of electromechanical performances with Qm = 726, d33 = 502 pC N-1, kp = 0.69, tan δ = 0.0024, and TC = 267 °C was obtained. Through structural characterization, it was observed that the morphotropic phase boundary and enhanced dispersion behavior lead to a lowered energy barrier, which contributes to polarization rotation and enhances piezoelectric performance. At the same time, the excellent piezoelectric performances also benefit from the highly oriented domain structure and small domain size after high-temperature poling. Furthermore, the segregation of Ba2+ causes A-site defects in the crystal lattice, accompanied with an increase in oxygen vacancies, which maintains the hardening effect of the ceramics. This study proposes a multiscale regulation strategy, providing insights for the design of high-power piezoelectric ceramics with high d33 and Qm.

Giant Piezoelectric Effects of Topological Structures in Stretched Ferroelectric Membranes

Freestanding ferroelectric oxide membranes emerge as a promising platform for exploring the interplay between topological polar ordering and dipolar interactions that are continuously tunable by strain. Our investigations combining density functional theory (DFT) and deep-learning-assisted molecular dynamics simulations demonstrate that DFT-predicted strain-driven morphotropic phase boundary involving monoclinic phases manifest as diverse domain structures at room temperatures, featuring continuous distributions of dipole orientations and mobile domain walls. Detailed analysis of dynamic structures reveals that the enhanced piezoelectric response observed in stretched PbTiO_{3} membranes results from small-angle rotations of dipoles at domain walls, distinct from conventional polarization rotation mechanism and adaptive phase theory inferred from static structures. We identify a ferroelectric topological structure, termed "dipole spiral," which exhibits a giant intrinsic piezoelectric response (>320  pC/N). This helical structure, possessing a rotational zero-energy mode, unlocks new possibilities for exploring chiral phonon dynamics and dipolar Dzyaloshinskii-Moriya-like interactions.

Unconventional Polarization Response in Titanite-Type Oxides due to Hashed Antiferroelectric Domains

Domains in a crystal, which have crystallographic uniformity and are geometrically segmented, typically arise from various phase transitions. The physical properties within individual domains are inherently the same as those in the homogeneous bulk. As a result, sufficiently large domains have little influence on the bulk properties. However, as the domains decrease in size to the nanoscale, for instance, due to multiple phase instabilities or spatial inhomogeneities, then the materials often acquire exceptional functionalities that are unattainable without these domains. This effect is exemplified by the ultrahigh dielectric and piezoelectric responses observed in ferroelectric oxides with nanoscale polar domains as well as in ferroelectric relaxors with polar nanoclusters. Here, we demonstrate that hashed nanoscale domains in an antiferroelectric material are also capable of boosting dielectric permittivity in an unconventional way. This discovery has been made in an antiferroelectric titanite-type oxide, CaTi(Si1-xGex)O5, in which the permittivity significantly increases when the antiferroelectric order becomes short-range. Our transmission electron microscopy observations have clarified that polar regions simultaneously appear around antiphase boundaries in the antiferroelectric phase of CaTi(Si1-xGex)O5. As the concentration of the antiphase boundary increases, the polar regions become denser and play a crucial role in boosting the permittivity. At the composition of x = 0.5, the value of the permittivity finally reaches double that in the bulk and shows excellent linearity, at least until an electric field of 500 kV/cm is applied. The present findings highlight the promise of domain engineering for boosting the permittivity in antiferroelectrics as a way to develop materials with excellent dielectric properties.

Ultrahigh energy storage in high-entropy ceramic capacitors with polymorphic relaxor phase

Ultrahigh-power-density multilayer ceramic capacitors (MLCCs) are critical components in electrical and electronic systems. However, the realization of a high energy density combined with a high efficiency is a major challenge for practical applications. We propose a high-entropy design in barium titanate (BaTiO3)-based lead-free MLCCs with polymorphic relaxor phase. This strategy effectively minimizes hysteresis loss by lowering the domain-switching barriers and enhances the breakdown strength by the high atomic disorder with lattice distortion and grain refining. Benefiting from the synergistic effects, we achieved a high energy density of 20.8 joules per cubic centimeter with an ultrahigh efficiency of 97.5% in the MLCCs. This approach should be universally applicable to designing high-performance dielectrics for energy storage and other related functionalities.

Centrosymmetry-Breaking Morphotropic Phase Boundary: A Pathway to Highly Sensitive and Strong Pressure-Responsive Nonlinear Optical Switches

The quest for high-performance piezoelectric materials has been synonymous with the pursuit of the morphotropic phase boundary (MPB), yet the full potential of MPBs remains largely untapped outside of the realm of ferroelectrics. In this study, we reveal a new class of MPB by creating continuous molecular-based solid solutions between centro- and noncentrosymmetric compounds, exemplified by (tert-butylammonium)1-x(tert-amylammonium)xFeCl4 (0 ≤ x ≤ 1), where the MPB is formed due to disorder of molecular cations. Near the MPB, we discovered an exceptionally sensitive nonlinear optical material in the centrosymmetric phase, capable of activation at pressures as low as 0.12-0.27 GPa, and producing tunable second-harmonic generation (SHG) signals from zero to 18.8 times that of KH2PO4 (KDP). Meanwhile, synchrotron diffraction experiments have unveiled a third competing phase (P212121) appearing at low pressure, forming a triple-phase point near the MPB, thereby providing insight into the mechanism underpinning the nonlinear optical (NLO) switch behavior. These findings highlight the opportunity to harness exceptional physical properties in symmetry-breaking solid solution systems by strategically designing novel MPBs.

Reversible Optical Control of Polarization in Epitaxial Ferroelectric Thin Films

Light is an effective tool to probe the polarization and domain distribution in ferroelectric materials passively, that is, non-invasively, for example, via optical second harmonic generation (SHG). With the emergence of oxide electronics, there is now a strong demand to expand the role of light toward active control of the polarization. In this work, optical control of the ferroelectric polarization is demonstrated in prototypical epitaxial PbZrx Ti1-x O3 (PZT)-based heterostructures. This is accomplished in three steps, using above-bandgap UV light, while tracking the response of the polarization with optical SHG. First, it is found that UV-light exposure induces a transient enhancement or suppression of the ferroelectric polarization in films with an upward- or downward-oriented polarization, respectively. This behavior is attributed to a modified charge screening driven by the separation of photoexcited charge carriers at the Schottky interface of the ferroelectric thin film. Second, by taking advantage of this optical handle on electrostatics, remanent optical poling from a pristine multi-domain into a single-domain configuration is accomplished. Third, via thermal annealing or engineered electrostatic boundary conditions, a complete reversibility of the optical poling is further achieved. Hence, this work paves the way for the all-optical control of the spontaneous polarization in ferroelectric thin films.

Pseudopolymorphic Phase Engineering for Improved Thermoelectric Performance in Copper Sulfides

Polymorphism (and its extended form - pseudopolymorphism) in solids is ubiquitous in mineralogy, crystallography, chemistry/biochemistry, materials science, and the pharmaceutical industries. Despite the difficulty of controlling (pseudo-)polymorphism, the realization of specific (pseudo-)polymorphic phases and associated boundary structures is an efficient route to enhance material performance for energy conversion and electromechanical applications. Here, this work applies the pseudopolymorphic phase (PP) concept to a thermoelectric copper sulfide, Cu2- x S (x ≤ 0.25), via CuBr2 doping. A peak ZT value of 1.25 is obtained at 773 K in Cu1.8 S + 3 wt% CuBr2 , which is 2.3 times higher than that of a pristine Cu1.8 S sample. Atomic-resolution scanning transmission electron microscopy confirms the transformation of pristine Cu1.8 S low digenite into PP-engineered high digenite, as well as the formation of (semi-)coherent interfaces between different PPs, which is expected to enhance phonon scattering. The results demonstrate that PP engineering is an effective approach for achieving improved thermoelectric performance in Cu-S compounds. It is also expected to be useful in other materials.

Permissible domain walls in monoclinic MAB ferroelectric phases

The concept of monoclinic ferroelectric phases has been extensively used over recent decades for the understanding of crystallographic structures of ferroelectric materials. Monoclinic phases have been actively invoked to describe the phase boundaries such as the so-called morphotropic phase boundary in functional perovskite oxides. These phases are believed to play a major role in the enhancement of such functional properties as dielectricity and electromechanical coupling through rotation of spontaneous polarization and/or modification of the rich domain microstructures. Unfortunately, such microstructures remain poorly understood due to the complexity of the subject. The goal of this work is to formulate the geometrical laws behind the monoclinic domain microstructures. Specifically, the result of previous work [Gorfman et al. (2022). Acta Cryst. A78, 158-171] is implemented to catalog and outline some properties of permissible domain walls that connect `strain' domains with monoclinic (MA/MB type) symmetry, occurring in ferroelectric perovskite oxides. The term `permissible' [Fousek & Janovec (1969). J. Appl. Phys. 40, 135-142] pertains to the domain walls connecting a pair of `strain' domains without a lattice mismatch. It was found that 12 monoclinic domains may form pairs connected along 84 types of permissible domain walls. These contain 48 domain walls with fixed Miller indices (known as W-walls) and 36 domain walls whose Miller indices may change when free lattice parameters change as well (known as S-walls). Simple and intuitive analytical expressions are provided that describe the orientation of these domain walls, the matrices of transformation between crystallographic basis vectors and, most importantly, the separation between Bragg peaks, diffracted from each of the 84 pairs of domains, connected along a permissible domain wall. It is shown that the orientation of a domain wall may be described by the specific combination of the monoclinic distortion parameters r = [2/(γ - α)][(c/a) - 1], f = (π - 2γ)/(π - 2α) and p = [2/(π - α - γ)] [(c/a) - 1]. The results of this work will enhance understanding and facilitate investigation (e.g. using single-crystal X-ray diffraction) of complex monoclinic domain microstructures in both crystals and thin films.

Polarization-tunable interfacial properties in monolayer-MoS2 transistors integrated with ferroelectric BiAlO3(0001) polar surfaces

With the explosion of data-centric applications, new in-memory computing technologies, based on nonvolatile memory devices, have become competitive due to their merged logic-memory functionalities. Herein, employing first-principles quantum transport simulation, we theoretically investigate for the first time the electronic and contact properties of two types of monolayer (ML)-MoS2 ferroelectric field-effect transistors (FeFETs) integrated with ferroelectric BiAlO3(0001) (BAO(0001)) polar surfaces. Our study finds that the interfacial properties of the investigated partial FeFET devices are highly tunable by switching the electric polarization of the ferroelectric BAO(0001) dielectric. Specifically, the transition from quasi-Ohmic to the Schottky contact, as well as opposite contact polarity of respective n-type and p-type Schottky contact under two polarization states can be obtained, suggesting their superior performance metrics in terms of nonvolatile information storage. In addition, due to the feature of (quasi-)Ohmic contact in some polarization states, the explored FeFET devices, even when operating in the regular field-effect transistor (FET) mode, can be extremely significant in realizing a desirable low threshold voltage and interfacial contact resistance. In conjunction with the formed van der Waals (vdW) interfaces in ML-MoS2/ferroelectric systems with an interlayer, the proposed FeFETs are expected to provide excellent device performance with regard to cycling endurance and memory density.

Piezoelectric and Dielectric Properties in Bi0.5(Na,K)0.5TiO3-x Ag2O Lead-Free Piezoceramics

Lead-free piezoceramics of Bi0.5(Na0.825K0.175)0.5TiO3 with varying concentrations of x mol% Ag2O (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, denoted as BNKT-xA) were fabricated using the solid-state technique. An extensive investigation was undertaken to analyze the structural, piezoelectric, and dielectric properties of these piezoceramics in the presence of Ag ions. There is no evidence of any secondary solid solution in the BNKT-xA piezoceramics. The ceramics with x mol% Ag2O in BNKT still demonstrate the presence of both rhombohedral (R) and tetragonal (T) phases. The addition of Ag+ is helpful to increase the relative density of the BNKT-xA piezoceramics. It is noteworthy that the BNKT-0.3 mol% A piezoceramics show remarkable improvements in their properties (d33 = 147 pC/N, kp = 29.6%, ε = 1199, tanδ = 0.063). These improvements may be ascribed to the denser microstructure and the preservation of morphotropic phase boundaries between R and T phases caused by the appropriate addition of Ag cations. The addition of Ag+ results in the relaxor behavior of the BNKT-xA ceramics, characterized by disorder among A-site cations. With the increase in temperature, the d33 value of BNKT-xA ceramics does not vary significantly in the range of 25 to 125 °C, ranging from 127 to 147 pC/N (with a change of d33 ≤ 13.6%). This finding shows that piezoelectric ceramics have a reliable performance over a certain operating temperature range.

New High-Performance Piezoelectric: Ferroelectric Carbon-Boron Clathrate

High-performance piezoelectrics have been extensively reported with a typical perovskite structure, in which a huge breakthrough in piezoelectric constants is found to be more and more difficult. Hence, the development of materials beyond perovskite is a potential means of achieving lead-free and high piezoelectricity in next-generation piezoelectrics. Here, we demonstrate the possibility of developing high piezoelectricity in the nonperovskite carbon-boron clathrate with the composition of ScB_{3}C_{3} using first-principles calculations. The robust and highly symmetric B-C cage with mobilizable Sc atom constructs a flat potential valley to connect the ferroelectric orthorhombic and rhombohedral structures, which allows an easy, continuous, and strong polarization rotation. By manipulating the cell parameter b, the potential energy surface can be further flattened to produce an extra-high shear piezoelectric constant d_{15} of 9424  pC/N. Our calculations also confirm the effectiveness of the partial chemical replacement of Sc by Y to form a morphotropic phase boundary in the clathrate. The significance of large polarization and high symmetric polyhedron structure is demonstrated for realizing strong polarization rotation, offering the universal physical principles to aid the search for new high-performance piezoelectrics. This work takes ScB_{3}C_{3} as an example to exhibit the great potential for realizing high piezoelectricity in clathrate structure, which opens the door to developing next-generation lead-free piezoelectric applications.

Electro-thermal actuation in percolative ferroelectric polymer nanocomposites

The interconversion between electrical and mechanical energies is pivotal to ferroelectrics to enable their applications in transducers, actuators and sensors. Ferroelectric polymers exhibit a giant electric-field-induced strain (>4.0%), markedly exceeding the actuation strain (≤1.7%) of piezoelectric ceramics and crystals. However, their normalized elastic energy densities remain orders of magnitude smaller than those of piezoelectric ceramics and crystals, severely limiting their practical applications in soft actuators. Here we report the use of electro-thermally induced ferroelectric phase transition in percolative ferroelectric polymer nanocomposites to achieve high strain performance in electric-field-driven actuation materials. We demonstrate a strain of over 8% and an output mechanical energy density of 11.3 J cm^-3 at an electric field of 40 MV m^-1 in the composite, outperforming the benchmark relaxor single-crystal ferroelectrics. This approach overcomes the trade-off between mechanical modulus and electro-strains in conventional piezoelectric polymer composites and opens up an avenue for high-performance ferroelectric actuators.

Mitigation of Thermal Instability for Electrical Properties in CaZrO3-Modified (Na, K, Li) NbO3 Lead-Free Piezoceramics

Lead-free ceramics 0.96(Na_0.52K_0.48)_0.95Li_0.05NbO₃-0.04CaZrO₃ (NKLN-CZ) are prepared by using the solid-state procedure and two-step synthesis technique. The crystal structure and thermal stability of NKLN-CZ ceramics sintered at 1140-1180 °C are investigated. All the NKLN-CZ ceramics are ABO₃-type perovskite phases without impure phases. With the increase in sintering temperature, a phase transition occurs in NKLN-CZ ceramics from the orthorhombic (O) phase to the concomitance of O-tetragonal (T) phases. Meanwhile, ceramics become dense because of the presence of liquid phases. In the vicinity of ambient temperature, an O-T phase boundary is obtained above 1160 °C, which triggers the improvement of electrical properties for the samples. The NKLN-CZ ceramics sintered at 1180 °C exhibit optimum electrical performances (d₃₃ = 180 pC/N, k_p = 0.31, dS/dE = 299 pm/V, ε_r = 920.03, tanδ = 0.0452, P_r = 18 μC/cm², T_c = 384 °C, E_c = 14 kV/cm). The relaxor behavior of NKLN-CZ ceramics was induced by the introduction of CaZrO3, which may lead to A-site cation disorder and show diffuse phase transition characteristics. Hence, it broadens the temperature range of phase transformation and mitigates thermal instability for piezoelectric properties in NKLN-CZ ceramics. The value of kp for NKLN-CZ ceramics is held at 27.7-31% (variance of k_p < 9%) in the range from -25 to 125 °C. The results indicate that lead-free ceramics NKLN-CZ is one of the hopeful temperature-stable piezoceramics for practical application in electronic devices.

Nib-Assisted Coaxial Electrohydrodynamic Jet Printing for Nanowires Deposition

This paper presents the concrete design of nanowires under the precise size and morphology that play a crucial role in the practical operation of the micro/nano devices. A straightforward and operative method termed as nib-assistance coaxial electrohydrodynamic (CEHD) printing technology was proposed. It extracts the essence of a nib-assistance electric field intensity to enhance and lessen the internal fluid reflux of the CEHD jet. The experiments were performed to add microparticles into the inner liquid to indicate the liquid flow consistency within the coaxial jet. The reflux in the coaxial jet was observed for the first time in experiments. The nanowires with a minimum size of 70 nm were printed under optimum experimental conditions. The nanopatterns contained aligned nanowires structures with diameters much smaller than the inner diameter of nozzle, relying on the coaxial nib-assisted technique. The printed results revealed that the nib-assisted CEHD printing technique offers a certain level high quality for application of NEMS system.

Tuning Piezoelectricity via Thermal Annealing at a Freestanding Ferroelectric Membrane

Tuning the ferroelectric domain structure by a combination of elastic and electrostatic engineering provides an effective route for enhanced piezoelectricity. However, for epitaxial thin films, the clamping effect imposed by the substrate does not allow aftergrowth tuning and also limits the electromechanical response. In contrast, freestanding membranes, which are free of substrate constraints, enable the tuning of a subtle balance between elastic and electrostatic energies, giving new platforms for enhanced and tunable functionalities. Here, highly tunable piezoelectricity is demonstrated in freestanding PbTiO₃ membranes, by varying the ferroelectric domain structures from c-dominated to c/a and a domains via aftergrowth thermal treatment. Significantly, the piezoelectric coefficient of the c/a domain structure is enhanced by a factor of 2.5 compared with typical c domain PbTiO₃. This work presents a new strategy to manipulate the piezoelectricity in ferroelectric membranes, highlighting their great potential for nano actuators, transducers, sensors and other NEMS device applications.

Ferroelectric hybrid organic-inorganic perovskites and their structural and functional diversity

Molecular ferroelectrics have gradually aroused great interest in both fundamental scientific research and technological applications because of their easy processing, light weight and mechanical flexibility. Hybrid organic-inorganic perovskite ferroelectrics (HOIPFs), as a class of molecule-based ferroelectrics, have diverse functionalities owing to their unique structure and have become a hot spot in molecular ferroelectrics research. Therefore, they are extremely attractive in the field of ferroelectrics. However, there seems to be a lack of systematic review of their design, performance and potential applications. Herein, we review the recent development of HOIPFs from lead-based, lead-free and metal-free perovskites, and outline the versatility of these ferroelectrics, including piezoelectricity for mechanical energy-harvesting and optoelectronic properties for photovoltaics and light detection. Furthermore, a perspective view of the challenges and future directions of HOIPFs is also highlighted.

Simulating Terahertz Field-Induced Ferroelectricity in Quantum Paraelectric SrTiO_{3}

Recent experiments have demonstrated that light can induce a transition from the quantum paraelectric to the ferroelectric phase of SrTiO_{3}. Here, we investigate this terahertz field-induced ferroelectric phase transition by solving the time-dependent lattice Schrödinger equation based on first-principles calculations. We find that ferroelectricity originates from a light-induced mixing between ground and first excited lattice states in the quantum paraelectric phase. In agreement with the experimental findings, our study shows that the nonoscillatory second harmonic generation signal can be evidence of ferroelectricity in SrTiO_{3}. We reveal the microscopic details of this exotic phase transition and highlight that this phenomenon is a unique behavior of the quantum paraelectric phase.

Annealing-Dependent Morphotropic Phase Boundary in the BiMg0.5Ti0.5O3-BiZn0.5Ti0.5O3 Perovskite System

The annealing behavior of (1-x)BiMg0.5Ti0.5O3−xBiZn0.5Ti0.5O3 [(1-x)BMT−xBZT] perovskite solid solutions synthesized under high pressure was studied in situ via X-ray diffraction and piezoresponse force microscopy. The as prepared ceramics show a morphotropic phase boundary (MPB) between the non-polar orthorhombic and ferroelectric tetragonal states at 75 mol. % BZT. It is shown that annealing above 573 K results in irreversible changes in the phase diagram. Namely, for compositions with 0.2 < x < 0.6, the initial orthorhombic phase transforms into a ferroelectric rhombohedral phase. The new MPB between the rhombohedral and tetragonal phases lies at a lower BZT content of 60 mol. %. The phase diagram of the BMT−BZT annealed ceramics is formally analogous to that of the commercial piezoelectric material lead zirconate titanate. This makes the BMT−BZT system promising for the development of environmentally friendly piezoelectric ceramics.

Data-Driven Materials Innovation and Applications

Owing to the rapid developments to improve the accuracy and efficiency of both experimental and computational investigative methodologies, the massive amounts of data generated have led the field of materials science into the fourth paradigm of data-driven scientific research. This transition requires the development of authoritative and up-to-date frameworks for data-driven approaches for material innovation. A critical discussion on the current advances in the data-driven discovery of materials with a focus on frameworks, machine-learning algorithms, material-specific databases, descriptors, and targeted applications in the field of inorganic materials is presented. Frameworks for rationalizing data-driven material innovation are described, and a critical review of essential subdisciplines is presented, including: i) advanced data-intensive strategies and machine-learning algorithms; ii) material databases and related tools and platforms for data generation and management; iii) commonly used molecular descriptors used in data-driven processes. Furthermore, an in-depth discussion on the broad applications of material innovation, such as energy conversion and storage, environmental decontamination, flexible electronics, optoelectronics, superconductors, metallic glasses, and magnetic materials, is provided. Finally, how these subdisciplines (with insights into the synergy of materials science, computational tools, and mathematics) support data-driven paradigms is outlined, and the opportunities and challenges in data-driven material innovation are highlighted.

Influence of the Template Layer on the Structure and Ferroelectric Properties of PbZr0.52Ti0.48O3 Films

The microstructure of the PbZr_0.52Ti_0.48O₃ (PZT) films is known to influence the ferroelectric properties, but so far mainly the effect of the deposition conditions of the PZT has been investigated. To our knowledge, the influence of the underlying electrode layer and the mechanisms leading to changes in the PZT microstructure have not been explored. Using LaNiO₃ (LNO) as the bottom electrode material, we investigated the evolution of the PZT microstructure and ferroelectric properties for changing LNO pulsed-laser deposition conditions. The explored deposition conditions were the O₂ pressure, total pressure, and thickness of the electrode layer. Increasing both the O₂ pressure and the thickness of the electrode layer changes the growth of PZT from a smooth, dense film to a rough, columnar film. We explain the origin of the change in PZT microstructure as the increased roughness of the electrode layer in relaxing the misfit strain. The strain relaxation mechanism is evidenced by the increase in the crystal phase with bulk LNO unit cell dimensions in comparison to the crystal phase with substrate-clamped unit cell dimensions. We explain the change from a dense to a columnar microstructure as a result of the change in the growth mode from Frank-van der Merwe to Stranski-Krastanov. The ferroelectric properties of the columnar films are improved compared to those of the smooth, dense films. The ability to tune the ferroelectric properties with the microstructure is primarily relevant for ferroelectric applications such as actuators and systems for energy harvesting and storage.

Deciphering the phase transition-induced ultrahigh piezoresponse in (K,Na)NbO3-based piezoceramics

Here, we introduce phase change mechanisms in lead-free piezoceramics as a strategy to utilize attendant volume change for harvesting large electrostrain. In the newly developed (K,Na)NbO₃ solid-solution at the polymorphic phase boundary we combine atomic mapping of the local polar vector with in situ synchrotron X-ray diffraction and density functional theory to uncover the phase change and interpret its underlying nature. We demonstrate that an electric field-induced phase transition between orthorhombic and tetragonal phases triggers a dramatic volume change and contributes to a huge effective piezoelectric coefficient of 1250 pm V⁻¹ along specific crystallographic directions. The existence of the phase transition is validated by a significant volume change evidenced by the simultaneous recording of macroscopic longitudinal and transverse strain. The principle of using phase transition to promote electrostrain provides broader design flexibility in the development of high-performance piezoelectric materials and opens the door for the discovery of high-performance future functional oxides.

Functional oxides with coexisting states of comparable energy typically exhibit extraordinary responses to external stimuli. Here, the authors demonstrate that coexisting phase structures provide large electric field-triggered volume change.

Facile Glycothermal Synthesis of KxNa(1−x)NbO3 Particles

KxNa(1−x)NbO3 particles (KNN, 0 < x < 1) were successfully synthesized through a facile glycothermal method by using KOH, NaOH and Nb2O5 as precursors and 1,4-butanediol as solvent at 200 °C for 12 h. The effects of varying the 1,4-butanediol/deionized water (B/W) volume ratio as solvent on the growth behavior, the morphological evolution, and the particle size of the synthesized KNN particles were investigated. In order to obtain K0.5Na0.5NbO3 with the morphotropic phase boundary (MPB) at the potassium content of x ≈ 0.5, the effect of varying K+/Na+ molar ratio on the composition of the obtained KNN particles was investigated. The crystal phase structure, morphology, particle size, chemical composition, and thermal behavior of the obtained particle samples were characterized using XRD, FE-SEM, EDS, TG, FT-IR, PSA, and TEM. The pure orthorhombic KNN particle close to NaNbO3 phase was obtained at the same concentration K+/Na+ of 1.0/1.0 and [K++Na+]/Nb molar ratio of 2.0/0.1. The synthesized K0.01Na0.99NbO3 particle exhibited a hexahedron shape with an average crystallite size of approximately 400 nm by glycothermal treated at 200 °C for 12 h. It is also demonstrated that the size of Na-rich KNN particles was decreased from 15 µm to 400 nm with increasing 1,4-butanediol content at various reaction conditions such as the volume ratio of B/W and can be controlled by 1,4-butanediol with an additive of water. Until the molar ratio of K+/Na+ reaches 1.6/0.4, the obtained particles have produced a Na-rich KNN phase, whereas when the molar ratio of K+/Na+ is 1.8/0.2, the particles could obtain a K-rich KNN phase. The results revealed that single-phase K0.5Na0.5NbO3 particles could be obtained at a relatively narrow molar ratio of K+/Na+ to 1.7/0.3. The particles with weakened agglomerate could obtain the average particle size of approximately 400 nm and a hexahedron shape. In comparison with the traditional hydrothermal method, the glycothermal method has been confirmed to be a more efficient method in controlling the particle size of KNN particles from micro- to sub-micron.

Evolution from Lead-Based to Lead-Free Piezoelectrics: Engineering of Lattices, Domains, Boundaries, and Defects Leading to Giant Response

Piezoelectric materials are known to mankind for more than a century, with numerous advancements made in both scientific understandings and practical applications. In the last two decades, in particular, the research on piezoelectrics has largely been driven by the constantly changing technological demand, and the drive toward a sustainable society. Hence, environmental-friendly "lead-free piezoelectrics" have emerged in the anticipation of replacing lead-based counterparts with at least comparable performance. However, there are still obstacles to be overcome for realizing this objective, while the efforts in this direction already seem to culminate. Therefore, novel structural strategies need to be designed to address these issues and for further breakthrough in this field. Here, various strategies to enhance piezoelectric properties in lead-free systems with fundamental and historical context, and from atomic to macroscopic scale, are explored. The main challenges currently faced in the transition from lead-based to lead-free piezoelectrics are identified and key milestones for future research in this field are suggested. These include: i) decoding the fundamental mechanisms; ii) large temperature-stable piezoresponse; and iii) fabrication-friendly and tailorable composition. Strategic insights and general guidelines for the synergistic design of new piezoelectric materials for obtaining a large piezoelectric response are also provided.

Hafnium Oxide (HfO2 ) - A Multifunctional Oxide: A Review on the Prospect and Challenges of Hafnium Oxide in Resistive Switching and Ferroelectric Memories

Hafnium oxide (HfO₂ ) is one of the mature high-k dielectrics that has been standing strong in the memory arena over the last two decades. Its dielectric properties have been researched rigorously for the development of flash memory devices. In this review, the application of HfO₂ in two main emerging nonvolatile memory technologies is surveyed, namely resistive random access memory and ferroelectric memory. How the properties of HfO₂ equip the former to achieve superlative performance with high-speed reliable switching, excellent endurance, and retention is discussed. The parameters to control HfO₂ domains are further discussed, which can unleash the ferroelectric properties in memory applications. Finally, the prospect of HfO₂ materials in emerging applications, such as high-density memory and neuromorphic devices are examined, and the various challenges of HfO₂ -based resistive random access memory and ferroelectric memory devices are addressed with a future outlook.

Piezoelectric Materials: Properties, Advancements, and Design Strategies for High-Temperature Applications

Piezoelectronics, as an efficient approach for energy conversion and sensing, have a far-reaching influence on energy harvesting, precise instruments, sensing, health monitoring and so on. A majority of the previous works on piezoelectronics concentrated on the materials that are applied at close to room temperatures. However, there is inadequate research on the materials for high-temperature piezoelectric applications, yet they also have important applications in the critical equipment of aeroengines and nuclear reactors in harsh and high-temperature conditions. In this review, we briefly introduce fundamental knowledge about the piezoelectric effect, and emphatically elucidate high-temperature piezoelectrics, involving: the typical piezoelectric materials operated in high temperatures, and the applications, limiting factors, prospects and challenges of piezoelectricity at high temperatures.

Atomic reconfiguration among tri-state transition at ferroelectric/antiferroelectric phase boundaries in Pb(Zr,Ti)O3

Phase boundary provides a fertile ground for exploring emergent phenomena and understanding order parameters couplings in condensed-matter physics. In Pb(Zr_1-xTi_x)O₃, there are two types of composition-dependent phase boundary with both technological and scientific importance, i.e. morphotropic phase boundary (MPB) separating polar regimes into different symmetry and ferroelectric/antiferroelectric (FE/AFE) phase boundary dividing polar and antipolar dipole configurations. In contrast with extensive studies on MPB, FE/AFE phase boundary is far less explored. Here, we apply atomic-scale imaging and Rietveld refinement to directly demonstrate the intermediate phase at FE/AFE phase boundary exhibits a rare multipolar Pb-cations ordering, i.e. coexistence of antipolar or polar displacement, which manifests itself in both periodically gradient lattice spacing and anomalous initial hysteresis loop. In-situ electron/neutron diffraction reveals that the same parent intermediate phase can transform into either FE or AFE state depending on suppression of antipolar or polar displacement, coupling with the evolution of long-/short-range oxygen octahedra tilts. First-principle calculations further show that the transition between AFE and FE phase can occur in a low-energy pathway via the intermediate phase. These findings enrich the structural understanding of FE/AFE phase boundary in perovskite oxides.

Ferroelectric/antiferroelectric phase boundary is both technologically and scientifically important. Here, the authors reveal the structure of intermediate phase involved in classical Pb(Zr_1-xTi_x)O₃.

Numerical modeling and analysis of coaxial electrohydrodynamic jet printing

Coaxial electrohydrodynamic jet (CE-Jet) printing is an encouraging method for fabrication of high-resolution micro and nanostructures in MEMS systems. This paper presents a novel simulation work based on phase field method which is considered as a precise technique in fluid dynamics. The study explores influence of various parameters such as applied voltage, needle-substrate distance, dynamic viscosity, relative permittivity, needle size and flow rate on stability and resolution of CE-Jet morphologies. The morphology of CE-Jet exhibits that width of cone-jet profile and printed structures on substrate were directly proportional to relative permittivity and flow rate. In addition, it was inversely proportional to dynamic viscosity and applied voltage. The study examine that CE-Jet length of inner liquid is inversely proportional to needle-substrate distance in same time. It was later verified in experimental study by producing stable CE-Jet morphology with 300 μm diameter using optimized parameters (i.e., DC voltage 7.0 kV and inner liquid flow rate 400 nl/min) as compared to other validation studies such as 400 μm and 500 μm. The CE-Jet printing technique investigates significant changes in consistency and stability of CE-Jet morphologies and makes Jet unique and comparable when adjustment accuracy reaches 0.01 mm. PZT sol line structures with a diameter of 1 µm were printed directly on substrate using inner needle (diameter of 120 µm). Therefore, it is considered as a powerful tool for nano constructs production in M/NEMS devices.

Boosting Piezoelectricity under Illumination via the Bulk Photovoltaic Effect and the Schottky Barrier Effect in BiFeO3

Piezoelectricity is a key functionality induced by conversion between mechanical and electrical energy. Enhancement of piezoelectricity in ferroelectrics often has been realized by complicated synthetical approaches to host unique structural boundaries, so-called morphotropic phase boundaries. While structural approaches are well-known, enhancing piezoelectricity by external stimuli has yet to be clearly explored, despite their advantages of offering not only simple and in situ control without any prior processing requirement, but compatibility with other functionalities. Here, it is shown that light is a powerful control parameter to enhance the piezoelectric property of BiFeO₃ single crystals. A series of measurements based on piezoresponse force microscopy and conductive atomic force microscopy, under illumination, reveal a locally enhanced effective piezoelectric coefficient, d_zz , eventually showing almost a sevenfold increase. This phenomenon is explained with theoretical models by introducing the two main underlying mechanisms attributed to the bulk photovoltaic effect and Schottky barrier effect, involving the role of open-circuit voltage and photocharge carrier density. These results provide key insights to light-induced piezoelectricity enhancement, offering its potential for multifunctional optoelectronic devices.

Direct Observation of Polarization Rotation in the Monoclinic MB Phase under Electrical Loading

The monoclinic phase has received a lot of research because of its importance in explaining the origin of high piezoelectric and ferroelectric performances around the morphotropic phase boundary. In the present study, we have investigated the detailed structural evolution in monoclinic PbZr_0.535Ti_0.465O₃ ferroelectric ceramics induced by an electric field with in situ high-energy synchrotron diffraction combined with two-dimensional (2D) geometry scattering technology. It has been discovered that an electric-field-induced single monoclinic M_B phase persists indefinitely. The lattice, unit cell volume, and spontaneous polarization of the monoclinic M_B structure exhibit significant and flexible responses to the external electric field, i.e., the spontaneous polarization rotates continuously and the lattice and unit cell volume present a butterfly form under the influence of the bipolar electric field. Particularly, direct experimental evidence demonstrates that the macropolarization of PbZr_0.535Ti_0.465O₃ is derived from the spontaneous polarization rotation rather than domain switching, and its volume expansion plays a vital role in the piezoelectric response.

Monitoring Electrical Biasing of Pb(Zr0.2Ti0.8)O3 Ferroelectric Thin Films In Situ by DPC-STEM Imaging

Increased data storage densities are required for the next generation of nonvolatile random access memories and data storage devices based on ferroelectric materials. Yet, with intensified miniaturization, these devices face a loss of their ferroelectric properties. Therefore, a full microscopic understanding of the impact of the nanoscale defects on the ferroelectric switching dynamics is crucial. However, collecting real-time data at the atomic and nanoscale remains very challenging. In this work, we explore the ferroelectric response of a Pb(Zr0.2Ti0.8)O3 thin film ferroelectric capacitor to electrical biasing in situ in the transmission electron microscope. Using a combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and differential phase contrast (DPC)-STEM imaging we unveil the structural and polarization state of the ferroelectric thin film, integrated into a capacitor architecture, before and during biasing. Thus, we can correlate real-time changes in the DPC signal with the presence of misfit dislocations and ferroelastic domains. A reduction in the domain wall velocity of 24% is measured in defective regions of the film when compared to predominantly defect-free regions.

Influence of Phase Transitions on Electrostrictive and Piezoelectric Characteristics in PMN-30PT Single Crystals

The ultrahigh electrostrain and piezoelectric constant (d₃₃) in relaxor piezoelectric PMN-30PT single crystals are closely related to the coexistence and transition of multiple phases at the morphotropic phase boundary (MPB). However, the key mechanisms underlying the stability of the phases and their transitions are yet to be fully understood. In this work, we undertake a systematic study of the influences of phase transitions on the electrostrictive and piezoelectric behaviors in ⟨001⟩-, ⟨011⟩-, and ⟨111⟩-oriented PMN-30PT single crystals. We first classify the various phase transitions within the quasi-MPB in electric field-temperature phase diagrams as either dominated by the electric field or by temperature. We find that the electrostrain reaches a maximum at each phase transition, especially in the electric-field-dominated transitions, whereas d₃₃ only peaks at specific phase transitions. In particular, the electrostrain in the ⟨001⟩ crystal reaches a maximum of S = 0.52% at 55 °C under an external electric field with E = 15 kV/cm, primarily due to a joint contribution of the electric field-dominated rhombohedral-monoclinic and monoclinic-tetragonal phase transitions at the quasi-MPB. An ultrahigh d₃₃ (∼2460 pC/N) only occurs at the rhombohedral-monoclinic phase transition in the ⟨001⟩ crystal and at the rhombohedral-orthorhombic transition in the ⟨011⟩ crystal (d₃₃ ∼ 1500 pC/N) due to the lower energy barriers. The temperature-dominated phase transitions also contribute toward minor peaks in electrostrain and/or d₃₃. This work provides a deeper and quantitative understanding of the microscopic mechanisms underlying electrostrictive and piezoelectric behaviors relevant for the design of high-performance materials.

A CMOS-compatible morphotropic phase boundary

Morphotropic phase boundaries (MPBs) show substantial piezoelectric and dielectric responses, which have practical applications. The predicted existence of MPB in HfO₂-ZrO₂solid solution thin film has provided a new way to increase the dielectric properties of a silicon-compatible device. Here, we present a new fabrication design by which the density of MPBρMPBand consequently the dielectric constantϵ_rof HfO₂-ZrO₂thin film was considerably increased. TheρMPBwas controlled by fabrication of a 10 nm [1 nm Hf_0.5Zr_0.5O₂(ferroelectric)/1 nm ZrO₂(antiferroelectric)] nanolaminate followed by an appropriate annealing process. The coexistence of orthorhombic and tetragonal structures, which are the origins of ferroelectric (FE) and antiferroelectric (AFE) behaviors, respectively, was structurally confirmed, and a double hysteresis loop that originates from AFE ordering, with some remnant polarization that originates from FE ordering, was observed inP-Ecurve. A remarkable increase inϵ_rcompared to the conventional HfO₂-ZrO₂thin film was achieved by controlling the FE-AFE ratio. The fabrication process was performed at low temperature (250 °C) and the device is compatible with silicon technology, so the new design yields a device that has possible applications in near-future electronics.

Effect of Hydrothermal Treatment and Doping on the Microstructural Features of Sol-Gel Derived BaTiO3 Nanoparticles

Barium Titanate (BaTiO₃) is one of the most promising lead-free ferroelectric materials for the development of piezoelectric nanocomposites for nanogenerators and sensors. The miniaturization of electronic devices is pushing researchers to produce nanometric-sized particles to be embedded into flexible polymeric matrices. Here, we present the sol-gel preparation of crystalline BaTiO₃ nanoparticles (NPs) obtained by reacting barium acetate (Ba(CH₃COO)₂) and titanium (IV) isopropoxide (Ti(OⁱPr)₄). The reaction was performed both at ambient conditions and by a hydrothermal process carried on at 200 °C for times ranging from 2 to 8 h. Doped BaTiO₃ nanoparticles were also produced by addition of Na, Ca, and Bi cations. The powders were annealed at 900 °C in order to improve NPs crystallinity and promote the cubic-to-tetragonal (c⟶t) phase transformation. The microstructural features of nanoparticles were investigated in dependence of both the hydrothermal reaction time and the presence of dopants. It is found that short hydrothermal treatment (2 h) can produce BaTiO₃ spherical and more homogeneous nanoparticles with respect to longer hydrothermal treatments (4 h, 6 h, 8 h). These particles (2 h) are characterized by decreased dimension (approx. 120 nm), narrower size distribution and higher tetragonality (1.007) in comparison with particles prepared at ambient pressure (1.003). In addition, the short hydrothermal treatment (2 h) produces particles with tetragonality comparable to the one obtained after the longest process (8 h). Finally, dopants were found to affect to different extents both the c⟶t phase transformation and the crystallite sizes.

PFM (piezoresponse force microscopy)-aided design for molecular ferroelectrics

With prosperity, decay, and another spring, molecular ferroelectrics have passed a hundred years since Valasek first discovered ferroelectricity in the molecular compound Rochelle salt. Recently, the proposal of ferroelectrochemistry has injected new vigor into this century-old research field. It should be highlighted that piezoresponse force microscopy (PFM) technique, as a non-destructive imaging and manipulation method for ferroelectric domains at the nanoscale, can significantly speed up the design rate of molecular ferroelectrics as well as enhance the ferroelectric and piezoelectric performances relying on domain engineering. Herein, we provide a brief review of the contribution of the PFM technique toward assisting the design and performance optimization of molecular ferroelectrics. Relying on the relationship between ferroelectric domains and crystallography, together with other physical characteristics such as domain switching and piezoelectricity, we believe that the PFM technique can be effectively applied to assist the design of high-performance molecular ferroelectrics equipped with multifunctionality, and thereby facilitate their practical utilization in optics, electronics, magnetics, thermotics, and mechanics among others.

Evolution of electromechanical properties in Fe-doped (Pb,Sr)(Zr,Ti)O3 piezoceramics

Defects in acceptor-doped perovskite piezoelectric materials have a significant impact on their electrical properties. Herein, the defect mediated evolution of piezoelectric and ferroelectric properties of Fe-doped (Pb,Sr)(Zr,Ti)O3 (PSZT-Fe) piezoceramics with different treatments, including quenching, aging, de-aging, and poling, was investigated systematically. Oxygen vacancies with a cubic symmetry are preserved in the quenched PSZT-Fe ceramics, rendering them robust ferroelectric behaviors. In the aged PSZT-Fe polycrystals, defect dipole between Fe dopant and oxygen vacancy has the same orientation with spontaneous polarization PS, which enables the reversible domain switching and hence leads to the emergence of pinched polarization hysteresis and recoverable strain effect. And the defect dipoles can be gradually disrupted by bipolar electric field cycling, once again endowing the aged materials with representative ferroelectric properties. For the poled PSZT-Fe polycrystals, the defect dipoles are reoriented to be parallel to the applied poling field, and an internal bias field aligning along the same direction emerges simultaneously, being responsible for asymmetric hysteresis loops.

Control of polarization in bulk ferroelectrics by mechanical dislocation imprint

Defects are essential to engineering the properties of functional materials ranging from semiconductors and superconductors to ferroics. Whereas point defects have been widely exploited, dislocations are commonly viewed as problematic for functional materials and not as a microstructural tool. We developed a method for mechanically imprinting dislocation networks that favorably skew the domain structure in bulk ferroelectrics and thereby tame the large switching polarization and make it available for functional harvesting. The resulting microstructure yields a strong mechanical restoring force to revert electric field-induced domain wall displacement on the macroscopic level and high pinning force on the local level. This induces a giant increase of the dielectric and electromechanical response at intermediate electric fields in barium titanate [electric field-dependent permittivity (ε₃₃) ≈ 5800 and large-signal piezoelectric coefficient (d ₃₃*) ≈ 1890 picometers/volt]. Dislocation-based anisotropy delivers a different suite of tools with which to tailor functional materials.

Polarization Rotation at Morphotropic Phase Boundary in New Lead-Free Na1/2Bi1/2V1-xTixO3 Piezoceramics

In this work, we show that polarization rotation enhances the piezoresponse in a high-performance lead-free piezoelectric material, Na_1/2Bi_1/2V_1-xTi_xO₃, a solid solution between tetragonal Na_1/2Bi_1/2VO₃ and rhombohedral Na_1/2Bi_1/2TiO₃, obtained by high-pressure synthesis. The system forms a pure perovskite structure with a favorable morphotropic phase boundary (MPB) located around x = 0.90, which separates the tetragonal and rhombohedral phases. In addition, a distinct monoclinic phase with polarization rotation as functions of composition and temperature is observed. XRD measurements revealed the moderately high Curie temperature of 523 K at x = 0.95 in the MPB. The piezoelectric coefficient d₃₃ of the monoclinic x = 0.95 sample, 42 pC/N, is higher than those of the tetragonal and rhombohedral phases. Even though the present lead-free Na_1/2Bi_1/2V_1-xTi_xO₃ ceramics feature smaller d₃₃ values compared to many currently available lead-free piezoelectric materials as a result of insufficient poling and low density, we expect our findings open up opportunities for exploring promising lead-free piezoelectric materials in Na_1/2Bi_1/2VO₃-based perovskites.

Knowledge-Based Descriptor for the Compositional Dependence of the Phase Transition in BaTiO3-Based Ferroelectrics

Descriptors play a central role in constructing composition-structure-property relationships to guide materials design. We propose a material descriptor, δτ, for the composition dependence of the Curie temperature (T_c) on single doping elements in BaTiO₃ ferroelectrics, which is then generalized to a linear combination of multiple dopants in the solid solutions. The descriptor δτ depends linearly on the Curie temperature and also serves to separate the ferroelectric phase from the relaxor phase. We compare δτ to other commonly used descriptors such as the tolerance factor, electronegativity, and ionic displacement. By using regression analysis on our assembled experimental data, we show how it outperforms other descriptors. We use the trained machine-learned models to predict compositions in our search space with the largest ferroelectric, dielectric, and piezoelectric properties, namely, d₃₃, electrostrain, and recoverable energy storage density. We experimentally verify our predictions for T_c and classification into ferroelectrics and relaxors by synthesizing and characterizing six solid solutions in BaTiO₃ ferroelectrics. Our definition of δτ can shed light on the design of knowledge-based descriptors in other systems such as Pb-based and Bi-based solid solutions.

Polarization Spinodal at Ferroelectric Morphotropic Phase Boundary

Here, we report a new phenomenon of uniform and continuous transformation of a single polarization domain into alternating nanodomains of two polarization vectors with the same magnitude but different directions at ferroelectric morphotropic phase Boundary (MPB). The transformation is fully reversible and could enhance the piezoelectric coefficient d_{33}. Further free energy calculations illustrate that such a polarization "decomposition" process occurs within the region on the Landau free energy curve with respect to the polarization direction where the second derivative becomes negative, which is similar to spinodal instability in phase transformations such as spinodal ordering or isostructural phase separation (e.g., spinodal decomposition). This "polarization spinodal" uncovers a new mechanism of polarization switching that may account for the ultrahigh ahysterestic piezoelectric strain at the MPB. This work could shed light on the development of phase transition theory and the design of novel ferroelectric memory materials.