Antiferroelectrics: History,fundamentals, crystal chemistry,crystal structures,size effects,and applications

Antiferroelectric (AFE) materials are of great interest owing to their scientific richness and their utility in high-energy density capacitors. Here, the history of AFEs is reviewed, and the characteristics of antiferroelectricity and the phase transition of an AFE material are described. AFEs are energetically close to ferroelectric (FE) phases, and thus both the electric field strength and applied stress (pressure) influence the nature of the transition. With the comparable energetics between the AFE and FE phases, there can be a competition and frustration of these phases, and either incommensurate and/or a glassy (relaxor) structures may be observed. The phase transition in AFEs can also be influenced by the crystal/grain size, particularly at nanometric dimensions, and may be tuned through the formation of solid solutions. There have been extensive studies on the perovskite family of AFE materials, but many other crystal structures host AFE behavior, such as CuBiP₂Se₆. AFE applications include DC-link capacitors for power electronics, defibrillator capacitors, pulse power devices, and electromechanical actuators. The paper concludes with a perspective on the future needs and opportunities with respect to discovery, science, and applications of AFE.     

Phase transition behavior of Pb(Hf,Sn, Ti,Nb)O3 ceramics at morphotropic phase boundary

The phase transition and dielectric properties of Pb_0.988(Hf_0.945Sn_xTi_0.03-xNb_0.025)O₃ ceramics (0 ≤ x ≤ 0.03, correspondingly abbreviated as H1, H2, H3, and H4) at the morphotropic phase boundary were systematically investigated. X-ray diffraction results and P-E hysteresis loops show that the dominate orthorhombic antiferroelectric phase and a small amount of the tetragonal FE phase coexist in Pb_0.988(Hf_0.945Sn_xTi_0.03-xNb_0.025)O₃ ceramics. As the Sn content increases, the antiferroelectricity is significantly enhanced, accompanied with an increased Curie temperature and sharply reduced peak dielectric constant. H1 and H2 experience an irreversible field-induced AFE-FE phase transition at the ambient temperature, and the transition from a metastable FE phase to the original AFE phase is observed in H2 when heated to 60°C. H3 and H4 experience an invertible AFE-FE phase transition, along with an enhanced forward phase switching field E_F. Moreover a decreased backward phase switching field E_A for H4 is detected as the electric field increases due to the AFE/FE coexistence. These results reveal the unique phase transition characteristics of AFE materials near the phase boundary, which is helpful for better understanding of AFE/FE materials.     

Enhanced energy storage efficiency by modulating field-induced strain in BaTiO3-Bi(Ni2/3Ta1/3)O3 lead-free ceramics

Nowadays, more attention has been paid to nontoxic lead-free ferroelectric (FE) materials as the substitution of lead-based energy storage materials, due to the increasing environmental awareness. BaTiO₃ (BT) has good comprehensive properties, however, its low breakdown electric field leads to the low energy storage density, which is the main challenge in BT-based ceramics. In this work, BT was modified by introducing the Bi(Ni_2/3Ta_1/3)O₃ (BNT) component in order to reduce the field-induced strain and further enhance the breakdown strength. Effects of BNT component on the microstructures, relaxor characteristics, energy storage efficiency and field-induced strain were investigated systematically. The results indicate that the long-range ordered ferroelectric domains of BT ceramics are destroyed by the incorporation of the BNT component, resulting in the distortion of the polar regions. As a result, the BT-BNT ceramics possess slim P-E hysteresis and extremely low remnant polarization, owing to the phase transition from the normal ferroelectrics to relaxor ferroelectrics. The reduced strain promotes to the increase of the breakdown field strength. Ultimately, the energy storage efficiency of BT-BNT ceramics can reach 87.15%, revealing a promising application in energy storage device.     

Structural,optical, and electrical properties of tin iodide-based vacancy-ordered-double perovskites synthesized via mechanochemical reaction

Over the recent past, lead-based halide perovskite materials have drawn significant attention due to their excellent optical and electrical properties for solar cells and optoelectronics applications. However, the toxicity of lead elements and instability under ambient conditions leads to develop alternative compositions. Herein, we report a novel mechanochemical synthesis of tin iodide-based double perovskites (A₂SnI₆; A = Rb⁺, Cs⁺, methylammonium, and formamidinium), and their structural, optical, and electrical properties are investigated. Importantly, we found that the hydrogen iodide (HI) addition during the ball-milling process minimizes secondary phase formation in the synthesized A₂SnI₆ powders. The effects of HI addition and the A-site substitution are investigated with respect to the lattice parameters, optical bandgaps, and electrical properties of the synthesized perovskite materials. Our results demonstrate essential information to improve the understanding of halide perovskite materials and develop efficient lead-free perovskite photoelectric devices.     

Phase transition and piezoelectricity of BaZrO3-modified (K,Na)NbO3 lead-free piezoelectric thin films

(K,Na)NbO₃ (KNN) is a promising lead-free ferroelectric/piezoelectric system, to which incorporating BaZrO₃ can greatly enhance its piezoelectricity, but the mechanism is not clear. This work was conducted to investigate the phase transition in the BaZrO₃-modifed KNN system and its contribution to piezoelectricity enhancement, using thin films with a fixed orientation and high compositional homogeneity fabricated by a sol-gel method. Two ferroelectric-to-ferroelectric phase transitions are revealed, which correspond to monoclinic M_C- M_A phase transition at higher temperature and rhombohedral-monoclinic M_C phase transition at lower temperature. It is difficult to distinguish these phases in KNN-based bulk materials, but their differences are clear when conducting high-resolution X-ray reciprocal space mapping (RSM) on the present thin films. Piezoresponse force microscopy experiments also revealed an interesting finding that local piezoelectricity of monoclinic phases was higher than that of rhombohedral ones in KNN-based thin films. This work could shed insights on the fundamental understandings for the effect of the chemical doping, and offer guidance for property optimization in the KNN-based lead-free piezoelectrics.     

Antiferroelectric‐ferroelectric phase transition in lead‐free AgNbO3 ceramics for energy storage applications

The high‐energy storage density reported in lead‐free AgNbO₃ ceramics makes it a fascinating material for energy storage applications. The phase transition process of AgNbO₃ ceramics plays an important role in its properties and dominates the temperature and electric field dependent behavior. In this work, the phase transition behavior of AgNbO₃ ceramics was investigated by polarization hysteresis and dielectric tunability measurements. It is revealed that the ferrielectric (FIE) phase at room temperature possesses both ferroelectric (FE)‐like and antiferroelectric (AFE)‐like dielectric responses prior to the critical AFE‐FE transition point. A recoverable energy storage density of 2 J/cm³ was achieved at 150 kV/cm due to the AFE‐FE transition. Based on a modified Laudau phenomenological theory, the stabilities among the AFE, FE and FIE phases are discussed, laying a foundation for further optimization of the dielectric properties of AgNbO₃.     

Re-entrant ferroelectric relaxor phenomena in the (1 − x)[Bi1/2(Na1/2K1/2)1/2TiO3]-xPbZrO3 system

The solid solution (1 − x)[Bi_1/2(Na_1/2K_1/2)_1/2TiO₃]-xPbZrO₃, (0.00 ≤ x ≤ 0.12) was investigated to examine the phase equilibria, dielectric and electromechanical properties. The composition corresponding to x = 0.00 exhibits tetragonal symmetry with the expected classical ferroelectric (FE) behavior. The system exhibited FE to relaxor crossover with the addition of lead zirconate at the composition x = 0.05. This is indicated by typical relaxor characteristics such as a transition to the global pseudocubic phase, a constriction in the FE hysteresis loop, and a sudden decrease in the negative strain accompanied by an increase in maximum strain. Most notably, with a further increase in x (>0.05), there is evidence for a return to a FE phase that exhibits classical FE characteristics. The combined results demonstrate that there exists a narrow FE-relaxor boundary near x = 0.05, where FE and relaxor phases coexist. At the critical composition, enhancement in the piezoelectric properties, including an increase in the effective (350 pm/V) was observed. This transition in the electromechanical properties is consistent with changes observed in the phase equilibria for this solid solution. The crystal structure transitions from tetragonal symmetry for x = 0.00, to pseudocubic symmetry for the relaxor compositions (x = 0.05), and finally to a lower symmetry perovskite phase for the re-entrant FE phase (x> 0.05). This composition-induced transition from FE to relaxor to a re-entrant FE state in the (1 − x)[Bi_1/2(Na_1/2K_1/2)_1/2TiO₃]-xPbZrO₃ system is unusual among relaxor FE systems and thus is of great scientific and technological interest.     

Enhanced pyroelectric response from domain-engineered lead-free (K0.5Bi0.5TiO3-BaTiO3)-Na0.5Bi0.5TiO3 ferroelectric ceramics

Enhanced pyroelectric response is achieved via domain engineering from [001] grain-oriented, tetragonal-phase, lead-free 0.2(2/3K_0.5Bi_0.5TiO₃-1/3BaTiO₃)-0.8Na_0.5Bi_0.5TiO₃ (KBT-BT-NBT) ceramics prepared by a templated grain growth method. The [001] crystallographic orientation leads to large polarization in tetragonal symmetry; therefore, texturing along this direction is employed to enhance the pyroelectricity. X-ray diffraction analysis revealed a Lotgering factor (degree of texturing) of 93 % along the [001] crystallographic direction. The textured KBT-BT-NBT lead-free ceramics showed comparable pyroelectric figures of merit to those of lead-based ferroelectric materials at room temperature (RT). In addition to the enhanced pyroelectric response at RT, an enormous enhancement in the pyroelectric response (from 1750 to 90,900 μC m^?2 K^?1) was achieved at the depolarization temperature because of the sharp ferroelectric to antiferroelectric phase transition owing to coherent 180° domain switching. These results will motivate the development of a wide range of lead-free pyroelectric devices, such as thermal sensors and infra-red detectors.     

Phase Stability and Ferroelectric Properties of Lead Strontium Zirconate Titanate Ceramics

The effect of compositional modifications on the field-induced phase-transition behavior and dielectric properties of strontium-doped lead zirconate titanate (PZT) ceramics was studied. PZT compositions with different strontium and titanium contents, within the general formula Pb_{1– x} Sr _x (Zr_{1– y} Ti _y )O₃ and located in the tetragonal antiferroelectric (AFE) and rhombohedral ferroelectric (FE) phase fields were prepared by tape casting and sintering. X-ray diffraction and polarization measurements were used to locate compositions suitable for investigation of the field-induced AFE–FE phase transition. The results indicated that a higher Sr²⁺ content decreased the polarization and hysteresis and increased the switching field; a lower Ti⁴⁺ content decreased the polarization and increased the switching field and hysteresis. A high room-temperature dielectric constant was obtained for compositions near the phase boundary. These results suggest that a combination of both A -site and B -site modifications can be used to tailor ferroelectric properties, such as the switching field and hysteresis, of these strontium-doped PZTs displaying a field-induced AFE–FE phase transition.     

Lead-reduced Bi(Ni2/3Ta1/3)O3-PbTiO3 perovskite ceramics with high Curie temperature and performance

With increasing demand of high-temperature piezoelectric devices and growing concern over environment protection, a feasible reduction in lead from lead-based high Curie temperature piezoelectric materials are desperately needed. Herein, a new system of lead-reduced Bi(Ni_2/3Ta_1/3)O₃-PbTiO₃ (BNT-PT) ferroelectric ceramics is fabricated by a conventional solid-state sintering process. The phase transition behaviors as a function of composition and temperature, electrical properties, as well as the domain configurations from a microscopic level have been investigated in detail. The results indicate that crystal structures, phase transition behaviors, and electric properties of BNT-PT ceramics can be affected significantly by the content of BNT counterpart. Dielectric measurements show that xBNT-(1−x)PT ceramics transfer from the normal ferroelectrics to the relaxor ferroelectrics at compositions of x = 0.3-0.35. The BNT-PT ceramics exhibit high Curie temperature T_C ranging from 474 to 185°C with the variation in BNT content. The relative dielectric tunability n_r also rises from only 0.65% for 0.10BNT-0.90PT to 50.23% for 0.40BNT-0.60PT with increasing BNT content. The tetragonal-rich composition 0.30BNT-0.70PT ceramic possesses the maximum remnant polarization of P_r ~ 34.9 μC/cm². Meanwhile, a highest piezoelectric coefficient of d₃₃ ~ 271 pC/N and a high field piezoelectric strain coefficient of  ~ 560 pm/V are achieved at morphotropic phase boundary (MPB) composition of 0.38BNT-0.62PT. The maximum value of strain ~0.31% is obtained in the 0.36BNT-0.64PT ceramic. The largest electromechanical coupling coefficient k_p is 44.5% for 0.37BNT-0.63PT ceramic. These findings demonstrate that BNT-PT ceramics are a system of high-performance Pb-reduced ferro/piezoelectrics, which will be very promising materials for piezoelectric devices. This study offers an approach to developing and exploring new lead-reduced ferroelectric ceramics with high performances.     

Origin of enhanced depolarization temperature in quenched Na0.5Bi0.5TiO3-BaTiO3 ceramics

Na_0.5Bi_0.5TiO₃-BaTiO₃ (NBT-BT)-based lead-free piezoelectric ceramics have been actively studied in recent years as a potential replacement for lead-based materials in ultrasonic applications. However, its relatively low thermal depolarization temperature (T_d) is still an imperative obstacle hindering implementation in practical application. Recently, it was reported that quenching is an effective way to improve T_d of NBT-based ceramics, but the essential mechanism is still unclear. In this study, 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ (NBT-6BT) ceramics were quenched in air and liquid nitrogen, and then annealed in oxygen and nitrogen atmospheres to explore the origin of enhanced depolarization temperature. The results from this study correlate the enhancement of T_d to the residual stress, which induces a stable rhombohedral ferroelectric phase, thereby increasing the thermal depolarization temperature of NBT-6BT. Our results indicate that the residual stress is also an important factor influencing the electrical properties of quenched piezoelectric ceramics, which should be given more attention in future studies.     

Fabrication,structural and dielectric property of lead-free perovskite silver niobate ceramics

Dielectric capacitors with high recoverable energy density are in high demand for their application in electrical and electronic systems. Among lead-free dielectric materials, silver niobate (AgNbO₃) has attracted growing interest due to its superior energy storage density at room temperature. The field-induced phase transition from antiferroelectric (AFE) phase to ferroelectric (FE) phase contributes to its large energy density. In this work, pure perovskite silver niobate ceramics were fabricated in an oxygen atmosphere by the solid-state reaction technique. The Pbcm orthorhombic phase of AgNbO₃ was closely observed using the Rietveld refinement method to provide explanation for the origin of high spontaneous polarization within a unit cell. Local structural analysis via piezoelectric force microscopy revealed the existence of ferroelectric nano domains, which may contribute to the high energy storage efficiency (η = 99.9926%) in AgNbO₃ at low electric fields. The phase transitions of AgNbO₃ were also investigated via the dependence of the dielectric permittivity (ε′ and ε″) and loss angle tangent (tanδ) on temperatures, providing insights into the further modification of AgNbO₃.     

Enhanced pressure-driven force-electric conversion effect for (Pb,La)(Zr,Ti)O3 ferroelectric ceramics

The pressure-driven force-electric conversion materials with extremely rapid response time have been widely used in mining, defense, and energy areas. The discharge process by the force-electric conversion effect in ferroelectrics is dominated by polar-nonpolar phase transformation. In this work, (Pb_0.985La_0.01)(Zr_xTi_1-x)O₃ (PLZT, x = 0.85–0.94) ceramics is designed by tunning Zr⁴⁺/Ti⁴⁺ ratio and aliovalent La doping to achieve high remnant polarization (P_r) and excellent temperature stability. We focus on the pressure-driven depolarization in PLZT ceramics, and their corresponding phase structure, ferroelectric properties, dielectric properties, and thermal depolarization. In PLZT (x = 0.93) ceramics, the original polarization P₀ increases to 43.42 μC/cm². The pressure-driven depolarization releases 37.66 μC/cm² with the depolarization proportion of 86.73%, which is attributed to irreversible ferroelectric-antiferroelectric phase transition. It also exhibits excellent temperature stability up to 120°C (> 36 μC/cm²). This work provides a high-performance alternative to Pb(Zr_0.95Ti_0.05)O₃ and guidance for the development of pulse power energy conversion devices.     

Enhanced energy storage density in Ca and Ta co-doped AgNbO3 antiferroelectric ceramics

Ca and Ta co-doped AgNbO₃ antiferroelectric lead-free ceramics were fabricated by rolling process technique, and improved energy storage properties were obtained. X-ray diffraction and Raman spectra indicate a single perovskite structure for (Ag_1-2xCa_x)(Nb_1-xTa_x)O₃ ceramics. The dielectric performances were also investigated, showing that increasing the content of Ca and Ta from 0.1 to 5 mol% gradually decreased the temperatures of the phase transition of monoclinic M₁-M₂ and M₂-M₃. This proved the enhanced antiferroelectricity stability associated with the enlarged low temperature phase transition region. The obtained (Ag_0.90Ca_0.05)(Nb_0.95Ta_0.05)O₃ ceramics exhibit an enhanced recoverable energy storage density (3.36 J/cm³) and efficiency (58.3%) with good temperature and frequency stability. The same composition shows excellent charge and discharge properties with a discharge current as high as 91.5 A and fast discharge speed (150 ns discharge period). All these merits demonstrate that AgNbO₃-based antiferroelectric ceramics are competitive with other lead-based and lead-free dielectric capacitors, which are promising candidates for dielectric energy storage applications.     

Enhanced electrical properties and strong red light‐emitting in Eu3+‐doped Sr1.90Ca0.15Na0.9Nb5O15 ceramics

The luminescent‐ferroelectic materials based on Sr_1.90Ca_0.15Na_0.9Nb₅O₁₅ (SCNN) matrix doping with Eu³⁺ were synthesized by the conventional solid‐state reaction method. The crystal structure, photoluminescence, thermal stability, dielectric, ferroelectric, and piezoelectric behaviors were systematically investigated. XRD results revealed that Eu³⁺ introduction could induce the tungsten bronze phase transition from orthorhombic to tetragonal structures. The dielectric spectra of all specimens showed two broad dielectric anomalies: a high‐temperature ferroelectric phase transition (T_c) and a low‐temperature ferroelastic phase transition (T_s), both of which were suppressed at higher Eu³⁺ concentrations. The enhanced electrical properties were obtained in a proper Eu³⁺ concentration range of 0.03‐0.05. For all SCNN:xEu³⁺ samples, the strong red emission peak at 617 nm originating from the electric dipole transition of ⁵D₀ →⁷F₂ was excited by different light excitations of 395 or 463 nm. Our results demonstrated that Eu³⁺‐doped SCNN materials might have promising potential in advanced multifunctional optoelectronic applications.     

Shock-driven depolarization behaviors and electrical output in BiAlO3-doped Bi0.5Na0.5TiO3 ferroelectric ceramics

Ferroelectric materials under shock compression can generate current and power by a drastic change of the remnant polarizations and surface bound charge. This behavior has been employed in applications involving nuclear fusion trigger, energy storage devices, and high pulse power sources. Despite the large power output in lead-containing ferroelectrics, lead-free materials are highly desirable owing to the environmental concerns. Herein, the phase transition behaviors and current outputs of 0.92Bi_0.5Na_0.5TiO₃-0.08BiAlO₃ (BNT-8BA) materials are studied under high pressure. The BNT-8BA ferroelectric ceramics can be completely depolarized from polar to the nonpolar state under shock compression, resulting in a current output in the external circuit. The phase-transition-induced depolarization pressures of BNT-8BA are lower than those of the pure Bi_0.5Na_0.5TiO₃ under both dynamic and static high-pressure loads. These results can allow the understanding of the high-pressure behavior of BNT-8BA for application as ferroelectric pulsed power supply.     

Spontaneous relaxor to ferroelectric transition in lead-free relaxor piezoceramics and the role of point defects

The occupationally disordered structures and associated local polar fluctuations in lead-free relaxors determine their electrical properties that are also sensitive to external stimuli. These stimuli can lead to phase transitions, and the associated enhancement in the electro-mechanical responses necessitate a better understanding of these transitions. Here we report a non-canonical spontaneous phase transition from a relaxor to a ferroelectric phase in (Na_1/2Bi_1/2)TiO₃-BaTiO₃ with temperature. With the help of experiments (dielectric permittivity, diffraction, differential scanning calorimetry, polarization and Raman spectroscopy), a complete picture of the temperature evolution of relaxor behavior leading to this spontaneous phase transition has been reported. Furthermore, it has been shown that internal chemical pressure from oxygen vacancies can be utilized to tailor these phase transitions. Finally, an electric field-temperature phase diagram has been proposed with an emphasis on the influence of the defect chemistry. This work provides new insights into the origin of these spontaneous phase transitions.     

Preparation and Properties of Lead Zirconate Stannate Titanate Sintered by Spark Plasma Sintering

Lanthanum-doped lead zirconate stannate titanate ceramics were successfully compacted to full density by spark plasma sintering (SPS). SPS samples densified at 900° or 950°C exhibit nearly full density and fine grain size (about 300 nm). Compared with samples from conventional sintering (CS), SPS samples show larger permittivity accompanied by a deterioration in dielectric loss and special strain hysteresis loops similar to those of ferroelectric relaxors, with a diffuse AFE–FE phase transition and less field-induced longitudinal strain. The differences in the properties of SPS and CS materials are attributed to the variations of the resultant microstructures, especially the grain size, of the ceramics.     

In situ combined stress- and temperature-dependent Raman spectroscopy of Li-doped (Na,K)NbO3

External thermal, electrical, and mechanical fields can induce structural phase transitions in lead-free Li-modified Na_0.5K_0.5NbO₃ ferroelectrics, which significantly influence the macroscopic electromechanical response. In particular, the relative stability of the polar monoclinic (or orthorhombic) and tetragonal phases under temperature and stress is critical to realize the ferroelectric and piezoelectric response. In this study, the effect of mechanical and thermal fields on the local structure in the vicinity of the monoclinic-tetragonal (M-T) phase boundary was investigated using a novel in situ combined uniaxial compressive stress- and temperature-dependent Raman spectroscopy experimental arrangement. Experiments were performed up to 300°C and −200 MPa, clearly demonstrating stress-induced M-T phase transition in Li-modified Na_0.5K_0.5NbO₃. A stress-temperature phase diagram has been established based on the change in vibrational modes. It was possible to correlate the relative permittivity singularities previously observed to a given stage of the M-T phase transition using ratio between characteristic Raman band areas. In addition, the measurement method reported here can be applied to other functional ceramics to investigate the influence of mechanical fields on local structure.     

Dielectric and ferroelectric properties of lanthanum‐modified lead zirconate stannate titanate (42/40/18) ceramics

The effect of lanthanum (La) content on the phase transformation of Pb_1?3x/2La_x(Zr_0.42Sn_0.40Ti_0.18)O₃ (PLZST 100x/42/40/18, 0 ≤ x ≤ 0.06) ceramics was investigated by the dielectric and ferroelectric properties. The base composition PLZST 0/42/40/18 located in the ferroelectric (FE) rhombohedral phase region. As x increased, the compositions showed successively FE and antiferroelectric (AFE) state at room temperature, and their peak temperatures (T_max) decreased gradually in line as T_max = 162.21‐1507x. Evidence was presented that there were two dielectric anomalies in PLZST 2/42/40/18, which were corresponding to the FE‐AFE and AFE‐paraelectric (PE) phase transformations, respectively. With increasing the dc bias fields, the two phases merged into one. PLZST 3/42/40/18 showed AFE characteristics with the first loop outside of the second loop and there was only one dielectric inflection. The critical lanthanum content occurred at x = 0.03 from the dielectric temperature spectra and hysteresis loops. Furthermore increase in La above 0.03, these compositions showed typical antiferroelectric behaviors with double hysteresis loops. The stored energy properties of the three compositions (PLZST 4/42/40/18, 5/42/40/18 and 6/42/40/18) displayed different temperature dependencies from room temperature to 140°C (over their respective T_max). Comparing the above results with previous investigations on PLZSTs, some questions were discussed.