Relaxor Research Articles

Synergistic enhanced energy storage performance of NBT-KBT ceramics by K0.5Na0.5NbO3 composition design

(1–x)(0.8Na0.5Bi0.5TiO3-0.2K0.5Bi0.5TiO3)-xK0.5Na0.5NbO3 (x = 0, 0.1, 0.2, 0.3, 0.4) (NBT-KBT-KNN) ceramic solid solution was synthesized by solid phase method through designing KNN composition. XRD, Raman and SEM results show that NBT-KBT-KNN ceramics form solid solutions with a stable perovskite structure. The dielectric temperature spectrum and impedance spectrum analysis confirmed that the relaxor ferroelectrics (RFE) properties enhanced with increasing KNN content. The piezoresponse force microscopy (PFM) results reveal that the introduced KNN disrupts the microdomains of NBT-KBT ceramics and promotes the formation of nanodomains, leading to enhanced energy storage properties. The breakdown electric field strength (BDS) was also increased with increasing KNN content, and maximum value was obtained at x = 0.2. The addition of KNN can obviously improve energy storage performance (ESP). At 255 kV cm–1, x = 0.2 produced excellent ESP with recoverable energy storage density (Wrec), amazingly normalized response (ξ), efficiency (η) and maximum polarization (Pmax) are 3.38 J cm–3, 132.55 J kV–1 m–2, 85.4 %, and 45.76 μC cm–2, respectively. ESP is also stable in terms of frequency and temperature at (1–100 Hz) and (20–140 °C). At 120 kV cm–1, the discharge energy density (Wdis), power density (PD), Current density (CD) and time for releasing 90 % of total energy density (t0.9), are 0.202 J cm–3, 23.43 MW cm–3, 390.42 A cm–2, and 56.6 ns. These findings demonstrate that NBT-KBT-KNN ceramics have the ability to be reliable energy storage and pulse power capacitors.

Large Energy Capacitive High-Entropy Lead-Free Ferroelectrics

HighlightsUltrahigh energy storage density of ~ 13.8 J cm−3 and large efficiency of ~ 82.4% are achieved in high-entropy lead-free relaxor ferroelectrics via high-entropy strategy, realizing nearly ten times growth.Outstanding energy storage properties are attributed to the enhanced random field and breakdown field, decreased nanodomain sizes, strong multiple local distortions coexisting in-phase and anti-phase oxygen octahedron tilts.Evolution of energy storage performance and domain structure with the increase in configuration entropy is systematically revealed for the first time.

Mn- and Yb-Doped BaTiO3-(Na0.5Bi0.5)TiO3 Ferroelectric Relaxor with Low Dielectric Loss

In this work, a Mn-and Yb-doped BaTiO3-(Na0.5Bi0.5)TiO3 ferroelectric relaxor was designed and prepared. The effects of Mn on the microstructures, dielectric and electrical properties of the ceramics were investigated. The X-ray structural analysis shows a perovskite structure. The SEM images show the homogeneous microstructure of ceramics with an average grain size of about 1 μm. The temperature-dependent permittivity shows relaxor characteristics as Mn-doped. Mn at a low level (x ≤ 0.005) is beneficial for low dielectric loss and high resistivity. The maximum resistivity of ≥3 × 1012 Ω cm and minimum dielectric loss of ≤0.06 can be achieved at x ≤ 0.005. The resistivity of the ceramics follows the Arrhenius law with activation energy decreasing from ~1.31 to 1.01 eV as x increases. With lower Mn dopant, oxygen vacancies and charge carrier concentration partially decrease with Mn doping, which is helpful to improve the insulation resistance and decrease the dielectric loss.

Multifunctional Triggering by Solid-Phase Molecular Motion: Relaxor Ferroelectricity, Modulation of Magnetic Exchange Interactions, and Enhancement of Negative Thermal Expansion

Although the development of artificial molecular machines has garnered considerable attention in recent years, the construction of multifunctional solid-state molecular machines still faces several challenges. Herein, we report a supramolecular approach as an efficient strategy for building multifunctional trigger systems. In crystals composed of [Ni(dmit)2]− with a spin of S = 1/2 and supramolecular structures consisting of 4-aminopyridinium+ and benzo[18]crown-6, supramolecular cations with dynamic degrees of freedom affect the magnetic and dielectric properties and induce negative thermal expansion (NTE). The supramolecular cations in the crystals form one-dimensional columns. Two adjacent columns form a supramolecular ladder structure via π···π interactions between the phenylene groups of benzo[18]crown-6 and are arranged within a two-dimensional layer. A disorder between the two sites of the phenylene ring was observed in one of the crystallographically independent benzo[18]crown-6. Disordered benzo[18]crown-6 formed polar domains within the crystal, resulting in relaxor ferroelectricity. With increasing temperature, the supramolecular ladders elongated and the translational motion of benzo[18]crown-6 caused the molecular ladder to move closer to each other. Consequently, the crystals shrunk in the direction perpendicular to the ladder, exhibiting uniaxial NTE, and the magnetic exchange interaction between the [Ni(dmit)2]− crystals was disrupted.

Superior energy storage properties in lead-free Na0.5Bi0.5TiO3-based relaxor ferroelectric ceramics via compositional tailoring and bandgap engineering

Currently, lead-free ferroelectrics have been paid much attention due to their versatile applications, environmental friendliness and excellent energy storage properties (ESP). In this work, lead-free relaxor ferroelectrics (0.74-x)Na0.5Bi0.5TiO3-0.06BaTiO3-0.2SrTiO3-xBi(Mg0.5Zr0.5)O3 with superior ESP were successfully designed via compositional tailoring and bandgap engineering. A new peak was observed in the dielectric properties, corresponding to the phase transition from the R3c polar nano regions (PNRs) to the P4bm PNRs, which were consistent with the Rietveld refined XRD and TEM SAED analyses, in which superlattice spots for R3c and P4bm were observed. A recoverable energy storage density (Wrec) of 6.57 J/cm3 and an energy conversion efficiency of 88.6% were procured for ceramics with x = 0.16 at 355 kV/cm. Furthermore, a variation within 5% in Wrec was obtained over a wide temperature and frequency range. Moreover, a power density of 77.39 MW/cm3 and a discharge speed of 52.5 ns were also achieved at 180 kV/cm.

Emergence of high piezoelectricity from competing local polar order-disorder in relaxor ferroelectrics

Relaxor ferroelectrics are known for outstanding piezoelectric properties, finding a broad range of applications in advanced electromechanical devices. Decoding the origins of the enhanced properties, however, have long been complicated by the heterogeneous local structures. Here, we employ the advanced big-box refinement method by fitting neutron-, X-ray-based total scattering, and X-ray absorption spectrum simultaneously, to extract local atomic polar displacements and construct 3D polar configurations in the classical relaxor ferroelectric Pb(Mg1/3Nb2/3)O3–PbTiO3. Our results demonstrate that prevailing order-disorder character accompanied by the continuous rotation of local polar displacements commands the composition-driven global structure evolution. The omnidirectional local polar disordering appears as an indication of macroscopic relaxor characteristics. Combined with phase-field simulations, it demonstrates that the competing local polar order-disorder between different states with balanced local polar length and direction randomness leads to a flattening free-energy profile over a wide polar length, thus giving rise to high piezoelectricity. Our work clarifies that the critical structural feature required for high piezoelectricity is the competition states of local polar rather than relaxor.

Lattice dynamics and dielectric characteristics of c-oriented SBN-50 thin films grown on a Pt(111)/Al2O3(0001) substrate

The lattice dynamics and dielectric properties of Ba0.5Sr0.5Nb2O6 ferroelectric thin films grown by high-frequency RF-deposition in an oxygen atmosphere on a Pt(111)/Al2O3 substrate (c - cut) were studied. It has been established that in the film, in comparison with the bulk material, the transition temperature from the ferroelectric to the paraelectric phase increases. Dielectric spectroscopy data have shown that the Ba0.5Sr0.5Nb2O6/Pt/Al2O3 heterostructure exhibits the behavior of a ferroelectric relaxor. The detected Burns temperature, Tb, is 180-190 °C.

Preparation of Phase-Pure Perovskite 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 Ceramics and Effects of Perovskite Phase Purity

0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMNT) piezoelectric ceramics were prepared by traditional ceramic processing via B-site oxide mixing route and columbite precursor method using oxides as raw materials, where the influences of MgO purity, whether heat treatment of MgO and synthesis techniques were studied. When using MgO with purity of 98%, the amount of pyrochlore phase in the sintered PMNT ceramics is rather high no matter what synthesis techniques used. Phase-pure perovskite PMNT ceramics can be prepared via the columbite precursor method using 99.99% MgO as raw material after pre-heated at 1200 °C for 4 h. All the PMNT ceramics exhibit rhombohedral perovskite structure with composition approaching the morphotropic phase boundary. The ceramics’ density is affected little by pyrochlore impurity, whereas the sintering temperature range is narrow and the 1245 °C sintered phase-pure PMNT ceramics have the largest relative density and compact micro-morphology. The PMNT ceramics present apparent relaxor ferroelectrics characteristic, where the samples with low content of pyrochlore phase or with phase-pure perovskite structure have excellent dielectric property. The samples present easy polarization characteristic, in which the PMNT ceramics with high purity or phase-pure perovskite phase have excellent piezoelectricity.

Tailoring relaxor P4bm+P21ma phases in BNT-based ceramics for enhancing comprehensive energy storage properties

Tailoring relaxor phase is an effective strategy to enhance energy-storage properties (ESP) for Bi0.5Na0.5TiO3 (BNT)-based ceramics. Herein, we incorporate Na0.91Bi0.09Nb0.94Mg0.06O3 (NBNM) into BNT matrix to form a solid solution of BNT-xNBNM with x = 0.1–0.4. Rietveld refinements reveal that the composition with x = 0.3 has 46% relaxor tetragonal P4bm phase and 54% relaxor orthorhombic P21ma one, thus exhibiting a large normalized energy-storage density (Wrec/E = 0.015 J kV−1 cm−2) and a high efficiency (η = 87%) at 370 kV cm−1. The optimized composition also presents superior temperature stability with a variation of <±5% in the range of 20 °C–150 °C, outstanding frequency reliability with a variation of <±4% in the range of 5 Hz–500 Hz and excellent charge-discharge properties (discharge rate of 76 ns, current density of 1032 A cm−2 and power density of 129 MW cm−3 at 250 kV cm−1). The results demonstrate a successful strategy to design high performance Pb-free relaxor ferroelectrics for pulsed power capacitor applications.

Hierarchical compositional ordering in lead-based perovskite relaxors

Lead-based relaxor ferroelectric perovskites with the formula $\mathrm{Pb}({\mathrm{B}}_{1/3}{\mathrm{B}}_{2/3}^{\ensuremath{'}}){\mathrm{O}}_{3}$, such as $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$, can have different degrees of compositional ordering even for the same stoichiometry. Despite intensive investigation, determining the nature of the B and ${\mathrm{B}}^{\ensuremath{'}}$ ordering remains a challenging problem. Here, we reveal a hierarchical ordering in $\mathrm{Pb}({\mathrm{B}}_{1/3}{\mathrm{B}}_{2/3}^{\ensuremath{'}}){\mathrm{O}}_{3}$ on top of the well-known ${\ensuremath{\beta}}^{\text{I}}$ and ${\ensuremath{\beta}}^{\text{II}}$ sublattices, which is the first level of the proposed hierarchy. Such additional ordering, which is found by considering additional nearest neighboring interactions in the model enriches our understanding of the complex nature of the chemical ordering in lead-based ferroelectric relaxors. In addition, we show that $\mathrm{Pb}({\mathrm{Cd}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$ has a higher degree of ordering than that of $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$ or $\mathrm{Pb}({\mathrm{Zn}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$ in which antiphase domains exist, revealing the possible different degree of compositional ordering arising from intralattice ordering.

Achieving ultrahigh energy storage properties with superior stability in novel (Ba(1-x)Bix)(Ti(1-x)Zn0.5xSn0.5x)O3 relaxor ferroelectric ceramics via chemical modification

Relaxor ferroelectrics are receiving widespread attention due to their excellent energy storage properties (ESPs). In this study, (Ba(1-x)Bix)(Ti(1-x)Zn0.5xSn0.5x)O3 (abbreviated as BBTZS-x, x = 0.08, 0.10, 0.12, 0.14, 0.16, 0.18) ceramics were synthesized via a solid-state reaction route, and the effects of chemical modification on their structure and properties were investigated in detail. The introduction of Bi/Zn/Sn (BZS) elements into BaTiO3 (BT) systems induced the change from normal to relaxor ferroelectric behavior, which caused the enhancement of comprehensive ESPs. Significantly, BBTZS-0.12 ceramics acquired the preeminent total energy density (Wtot = 5.8 J/cm3), recoverable energy density (Wrec = 4.99 J/cm3) and efficiency (η = 86.1%) at high field strength of 550 kV/cm because of the existence of polar nanoregions (PNRs) therein. The dynamic response of PNRs to the external field was found to be propitious to the enhancement of energy-storage performance. In particular, excellent current density (CD = 721.5 A/cm2), power density (PD = 151.5 MW/cm3) and the time of release of up to 90% of the discharge energy density value (t0.9 = 53 ns) were simultaneously achieved in BBTZS-0.12 ceramics at 420 kV/cm. Moreover, BBTZS-0.12 ceramics were found to be the most optimal for the long-term applications under different environmental conditions because of their outstanding temperature (30–150 °C), frequency (1–500 Hz), and fatigue cycle (cycle numbers: 1–100000) stabilities. Therefore, exceptional ESPs make BBTZS-x ceramics promising for advanced pulsed power capacitors and energy storage applications.

Improved energy storage performance of BST‒BNT ceramics via composition modification

Lead-free relaxor ferroelectrics are a promising material owing to their excellent energy storage performance. In this work, a new lead-free ceramic system was synthesized by introducing linear dielectric Ca0.85Bi0.1TiO3 as the component modification of 0.5(Ba0.4Sr0.6TiO3)‒0.5(Bi0.5Na0.5TiO3) ceramics. All samples optimized with Ca0.85Bi0.1TiO3 showed strong relaxor behavior. Refined grain sizes and compact microstructures were observed in this pseudoternary system. Moreover, the addition of a linear dielectric decreased the Curie temperature and enhanced the breakdown strength of BST‒BNT ceramics. A high energy storage density of 2.2 J/cm3 with a good storage efficiency of 73.2% was obtained in the 0.72(0.5(Ba0.4Sr0.6TiO3)‒0.5(Bi0.5Na0.5TiO3))‒0.28Ca0.85Bi0.1TiO3 (CBT28) ceramic. Furthermore, the CBT28 ceramic maintained good temperature and frequency stability over a wide range, a feature crucial in practical applications. Results suggest the feasibility of composition modification as an approach for improving the energy storage of ceramics.

Enhanced Energy Storage Properties in Lead-Free (Na0.5Bi0.5)0.7Sr0.3TiO3-Based Relaxor Ferroelectric Ceramics through a Cooperative Optimization Strategy.

Although relaxor ferroelectrics have been widely investigated owing to their various advantages, there are still impediments to boosting their energy-storage density (Wrec) and energy-storage efficiency (η). In this paper, we propose a cooperative optimization strategy for achieving comprehensive outstanding energy-storage performance in (Na0.5Bi0.5)0.7Sr0.3TiO3 (NBST)-based ceramics by triggering a nonergodic-to-ergodic transformation and optimizing the forming process. The first step of substituting NaNbO3 (NN) for NBST generated an ergodic state and induced polar nanoregions under the guidance of a phase-field simulation. The second step was to apply a viscous polymer process (VPP) to the 0.85NBST-0.15NN ceramics, which reduced porosity and increased compactness, resulting in a significant polarization difference and high breakdown strength. Consequently, 0.85NBST-0.15NN-VPP ceramics optimized by this cooperative two-step strategy possessed improved energy-storage characteristics (Wrec = 7.6 J/cm3, η = 90%) under 410 kV/cm as well as reliable temperature adaptability within a range of 20-120 °C, outperforming most reported (Na0.5Bi0.5) TiO3-based ceramics. The improved energy-storage performance validates the developed ceramics' practical applicability as well as the advantages of implementing a cooperative optimization technique to fabricate similar high-performance dielectric ceramics.

Machine Learning Reveals Memory of the Parent Phases in Ferroelectric Relaxors Ba(Ti1−x$_{1-x}$,Zrx)O3

AbstractMachine learning has been establishing its potential in multiple areas of condensed matter physics and materials science. Here an unsupervised machine learning workflow is developed and used within a framework of first‐principles‐based atomistic simulations to investigate phases, phase transitions, and their structural origins in ferroelectric relaxors, Ba(Ti,Zrx)O3. The applicability of the workflow is first demonstrated to identify phases and phase transitions in the parent compound, a prototypical ferroelectric BaTiO3. Then the workflow is applied for Ba(Ti,Zrx)O3 with to reveal i) that some of the compounds bear a subtle memory of BaTiO3 phases beyond the point of the pinched phase transition, which could contribute to their enhanced electromechanical response; ii) the existence of peculiar phases with delocalized precursors of nanodomains—likely candidates for the controversial polar nanoregions; and iii) nanodomain phases for the largest concentrations of x.

Giant piezoelectric response in Ho-doped PbMg1/3Nb2/3TiO3 relaxor ferroelectric single crystals via in-situ neutron scattering

The complex chemical and structural heterogeneity of relaxor ferroelectrics has made the origins of the giant piezoresponse extremely difficult to understand. Here, we successfully grew Ho-doped Pb(Mg1/3Nb2/3)O3–PbTiO3 (Ho-PMNT) single crystals with even higher d33, S33 and dielectric constant ε33/ε0 values of 3900 pC N−1, 0.34% and 15,000. We employed the in situ neutron diffraction to identify that the giant piezoelectric response of the Ho-PMNT originates from the composition near the tricritical triple point of a cubic paraelectric phase (C), ferroelectric monoclinic (M), and tetragonal (T) phases. It leads to a nearly polarization isotropy and thus facilitates polarization rotation between different ferroelectric states. The results of the neutron pair distribution function combined with aberration-corrected scanning transmission electron microscopy qualitatively analyze the angstrom-nanoscaled heterogeneities and their connection to local polarization. The Pb2+ displacement direction of <110> is connected to the A site ordered polarization. There were also observed the different separation distances between the B-site atoms and the neighboring O atoms. It identifies the giant piezoelectric properties arise that the Ho-dopant immensely disrupt long-range polarization and enhance the local structural heterogeneity in the tricritical triple point.

High Piezoelectricity in Eco-Friendly NaNbO3-Based Ferroelectric Relaxor Ceramics via Phase and Domain Engineering

To meet the requirements of environmental friendliness, high-performance lead-free piezoelectric materials have become important materials for next-generation electronic devices. Here, lead-free and potassium-free NaNbO3 (NN)-based ceramics with high piezoelectric (d33 = 361 ± 10 pC/N) and dielectric (εr = 4500) properties were obtained by tolerant preparation techniques. The excellent piezoelectric and dielectric properties can be attributed to the relaxor morphotropic phase boundaries (R-MPB) and coexisting domain regions, which are beneficial in lowering the free energy and greatly improving the dielectric response and domain switching capability. Furthermore, the d33 of NaNbO3-10Ba(Ti0.7Sn0.3)O3-1.5NaSbO3 (NN-10BTS-1.5NS) ceramics can be maintained at 350 pC/N over the range of 25-80 °C with a change rate of less than 10%, exhibiting excellent temperature stability. Based on a series of in situ characterizations, the variations of the phase and domain structures of NN-based relaxor piezoelectric ceramics with temperature are clearly demonstrated. This work not only proposes new materials for sensors and actuators but also provides an excellent strategy for designing high-performance piezoelectric ceramics through phase and domain engineering.

Achieving excellent energy storage performances and eminent charging-discharging capability in donor (1-x)BT-x(BZN-Nb) relaxor ferroelectric ceramics

Prodigious efforts have been paid to explore excellent dielectric energy storage capacitors based on lead-free relaxor ferroelectric (RFE) ceramics in the last decades. However, there are rare candidates among them simultaneously possessing high energy storage density (Wrec), high efficiency (η), wide work temperature range and stable charge–discharge ability, etc. In this work, excellent energy storage performances are achieved in donor doped BaTiO3 (BT)-based RFE ceramics by adopting two effective strategies of domain engineering and defect engineering. By introducing Bi(Zn2/3-0.01xNb1/3+0.01x)O3+δ (BZN-Nb) into BT matrix, firstly, long-range ferroelectric polarization is broken to form polar nanoregions (PNRs) via domain engineering. Subsequently, Nb donor substitution results in a dramatic increase in bulk resistivity (ρb) accompanied by a significant increase in dielectric breakdown electric strength (EDBS) via defect engineering. Finally, a combination of relative high Pmax, much low Pr and giant EDBS ensures that the optimum composition x = 0.3 in (1-x)BT-x(BZN-Nb) system exhibits high Wrec of 5.96 J/cm3 and high η of 89.5 %. Such properties together with good thermal stability (up to 220 °C), good fatigue endurance (for 106 cycles) and eminent charging-discharging capability (e.g., discharge time t0.9 ∼ 50 ns, current density CD ∼ 1.17 kA/cm2 and power density PD ∼ 175.38 MW/cm3 at 300 kV/cm) suggest that the 0.7BT-0.3(BZN-Nb) ceramic is a very promising candidate among lead-free dielectric capacitors for energy storage applications.

Improved Energy Storage Density performance of the (1-x) [0.88BaTiO3–0.12Bi(Li0.5Nb0.5)O3]-x(0.8BaTiO3–0.2SrTiO3) Lead-Free Ceramics

Recently, dielectric ceramic materials have been widely explored for high-power capacitors applications. However, it is still challenged for capacitors to enhance the energy density and efficiency. Here, the (1-x)[0.88BaTiO3–0.12Bi(Li0.5Nb0.5)O3]-x(0.8BaTiO3–0.2SrTiO3) (0.05 ≤ x ≤ 0.20) ceramics were designed to improve both dielectric temperature stability and breakdown strength. Influence of composition and phase structure on dielectric properties were studied in detail. Improvement of both dielectric properties and energy storage performance was achieved in this work. The 0.95[0.88BaTiO3–0.12Bi(Li0.5Nb0.5)O3]-0.05(0.8BaTiO3–0.2SrTiO3) ceramic shows the best performance, with an energy storage density up to 2.88 J/cm3 (We ∼ 2.51 J/cm3) and a high efficiency ∼ 86.81% under 320 kV/cm. This work not only provides key materials for the next generation of high-end energy storage capacitors, but also suggests a different route for optimizing the performance of other relaxor ferroelectrics.

Regional consistency between cation displacement and oxygen octahedral tilt in nanodomain of BNKT-SBT relaxor ferroelectrics revealed by atomic-resolution AC-STEM

Since being discovered, relaxor ferroelectrics have attracted continued interest due to their unusual properties. Relaxor behavior is attributed to nanodomains or PNRs. Therefore, nano-scale even atomic-scale structural analysis is crucial to understanding the relaxor properties. However, the atomic-scale nanodomain structure has not been fully understood. For Bi0.5Na0.5TiO3, cation displacement and oxygen octahedral tilt are two common structural distortions. However, the relationship between them has not been proved experimentally. Here, the cation displacement and octahedral tilt were both investigated based on aberration-corrected STEM images. Cation displacement vectors illustrate a vortex structure. Octahedral tilt is regionally consistent with cation displacement that both of them increase gradually from the domain wall to the interior of the nanodomain. The formation of nanodomains may originate from their combined effect, thereby producing relaxor properties. Moreover, oxygen octahedral displacement stemmed from the complex coordinating environments was observed. These findings provide further insights into the nanodomain structure.

Giant Electrostriction Enabled by Defect-Induced Critical Phenomena in Relaxor Ferroelectric Polymers

Polymers that generate large shape changes under electric stimulation are of great interest for many applications. Recently, it was shown that converting a small amount of chlorofluoroethylene (CFE) in relaxor ferroelectric poly(vinylidene fluoride–trifluoroethylene–CFE) (PVDF-TrFE-CFE) terpolymer into fluorinated alkyne (FA) creates P(VDF-TrFE-CFE-FA) tetrapolymers with giant electromechanical (EM) response at ultralow electric fields (<50 MV/m). We investigate the microscopic origin of this effect and show that converting the bulky CFE into small-size FA defects dramatically weakens the relaxor behavior. Importantly, P(VDF-TrFE-CFE-FA) tetrapolymers with near 2 mol % FA exhibit a diffused critical endpoint transition region at which the energy barriers for switching from nonpolar to polar molecular conformations become small. Consequently, a small change of the electric field induces a large electroactuation, which can enable novel applications. This work opens up a totally new approach to designing ferroelectric polymers that generate large responses at ultralow electric fields.