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.     

Can the ferroelectric soft mode trigger an antiferromagnetic phase transition?

Type-II multiferroics, where spin interactions induce a ferroelectric polarization, are interesting for new device functionalities due to large magnetoelectric coupling. We report on a new type of multiferroicity in the quadruple-perovskite BiMn₃Cr₄O₁₂, where an antiferromagnetic phase is induced by the structural change at the ferroelectric phase transition. The displacive nature of the ferroelectric phase transition at 125 K, with a crossover to an order-disorder mechanism, is evidenced by a polar soft phonon in the THz range and a central mode. Dielectric and pyroelectric studies show that the ferroelectric critical temperature corresponds to the previously reported Néel temperature of the Cr³⁺ spins. An increase in ferroelectric polarization is observed below 48 K, coinciding with the Néel temperature of the Mn³⁺ spins. This increase in polarization is attributed to an enhanced magnetoelectric coupling, as no change in the crystal symmetry below 48 K is detected from infrared and Raman spectra.     

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.     

The influence of phase transition on electrocaloric effect in lead‐free (Ba0.9Ca0.1)(Ti1−xZrx)O3 ceramics

In this paper, the influence of phase evolution on polarization change and electrocaloric response in lead‐free (Ba_0.9Ca_0.1)(Ti_1?xZr_x)O₃ ceramics (BCTZ) was systematically investigated. With increasing Zr/Ti ratio, the phase structure and phase transition behavior were greatly changed, resulting in various temperature and electric field dependence of electrocaloric responses. For x=0.05, a peak electrocaloric temperature change 1.64 K (at 130°C) and corresponding entropy change 1.78 J·kg^?1·K^?1 were obtained for 0‐7 kV·mm^?1 electric field. Negative electrocaloric temperature change in ?0.1 K was obtained below Curie temperature (T_c), which may be induced by the orthorhombic‐tetragonal ferroelectric phase transition. With the increase in x, the peak value of the electrocaloric response decreased but much better temperature stability was observed. Simultaneously the negative electrocaloric response gradually disappeared with the disappearance of the low temperature ferroelectric‐ferroelectric phase transition. For x=0.2, electrocaloric response showed good temperature stability ranging from room temperature to 130°C, attributing to the relaxor ferroelectric feature.     

Revealing the solid-state processing mechanisms of antiferroelectric AgNbO3 for energy storage

AgNbO₃ is one of the prominent lead-free antiferroelectric (AFE) oxides, which readily exhibits a field-induced AFE to ferroelectric phase transition and thus a high energy storage density. The solid-state synthesis of AgNbO₃ is considered difficult and an oxidizing atmosphere is typically employed during AgNbO₃ processing, on the premise that oxygen can prevent possible decomposition of the silver oxide at high temperatures. However, details about the influence of processing parameters on the functional properties of AFE AgNbO₃ are insufficiently understood. In this work, the solid-state reaction of a stoichiometric AgO and Nb₂O₅ mixture was investigated. We found that ball milling can convert AgO into metallic Ag, which is beneficial for lowering the reaction temperature for the formation of the perovskite phase to 500‒600℃. Moreover, the influence of the processing atmosphere (air, O₂, and N₂) was investigated by thermal analysis and in situ X-ray diffraction. Since the reaction between Ag and Nb₂O₅ to form AgNbO₃ requires oxygen uptake, AgNbO₃ was only found to form in air and O₂, whereby the kinetics were faster in the latter case. All the sintered AgNbO₃ samples exhibited a similar crystallographic structure, although the samples processed in O₂ had a lower oxygen vacancy concentration. Despite this, well-defined AFE double polarization loops were obtained in all cases. Our results indicate that decomposition of sliver oxide during ball milling is beneficial for the solid-state reaction, while a pure O₂ atmosphere is not essential for the synthesis of high-quality AgNbO₃. These findings may simplify the processing and facilitate further research of AgNbO₃-based antiferroelectrics.     

B-site acceptor doped AgNbO3 lead-free antiferroelectric ceramics: The role of dopant on microstructure and breakdown strength

B-site aliovalent modification of AgNbO₃ with a nominal composition of Ag(Nb_1-xM_x)O_3-x/2 (x = 0.01, M = Ti, Zr and Hf) was prepared. The effects of dopants on microstructure, dielectric, ferroelectric and conduction properties were investigated. The results indicate that the introduction of acceptor dopant does not lead to grain coarsening. Zr⁴⁺ and Hf⁴⁺ doping are beneficial to stabilize the antiferroelectric phase of AgNbO₃. Among all the samples, Ti⁴⁺ doped AgNbO₃ has the minimum resistivity while Hf⁴⁺ doped AgNbO₃ has the maximum resistivity, therefore, Hf⁴⁺ doped AgNbO₃ has high BDS. The XPS results indicate that the conduction behaviour is associated with the concentration of oxygen vacancies. This work hints that acceptor dopant is also effective on the microstructure control and chemical modification of AgNbO₃-based ceramics.     

Enhanced hybrid improper ferroelectricity in Fe/Nb cosubstituted Ca3Mn2O7 ceramics

The prototypical Ruddlesden-Popper compound Ca₃Mn₂O₇ has been predicted to possess hybrid improper ferroelectricity, where the polarization is induced by the condensation of two oxygen octahedral distortion modes. Nevertheless, it is a big challenge to switch the polarization at room temperature in Ca₃Mn₂O₇ since the presence of intermediate nonpolar Acaa phase generally leads to the complex domain morphology. Here, the effects of Fe/Nb cosubstitution on hybrid improper ferroelectricity in Ca₃Mn₂O₇ are reported, and easy polarization switching at room temperature is achieved in Ca₃[Mn_0.5(Fe_0.5Nb_0.5)_0.5]₂O₇. The ferroelectric phase transition occurs directly from I4/mmm to A2₁am at a temperature far above room temperature without intermediate nonpolar Acaa phase. The distinct transition pathway forms the alternating 180°-type ferroelectric domains rather than the irregular 90°-type ferroelastic domains stacked along [001], resulting in easy polarization switching at room temperature. Moreover the enhanced ferroelectric polarization (P_r ~2.0 μC/cm²) is obtained due to the increased anti-ferrodistortive displacements of Ca cations at A-site, arising from the larger amplitudes of oxygen octahedral distortions. Chemical pressure is emphasized here for the tunability of phase transition, domain morphology, and ferroelectric characteristics, and it provides a useful approach for designing and creating high-performance improper ferroelectrics.     

The dependence of magnetic properties on temperature for rare earth ErCrO3 chromites

Multiferroic ErCrO₃ was synthesized and the detailed magnetic as well as ferroelectric properties were investigated. The dc magnetization shows that ErCrO₃ undergoes a antiferromagnetic ordering at T_N = 133 K due to the Cr³⁺–Cr³⁺ followed by weak ferromagnetic ordering. Around T_SR ≈ 22 K, ErCrO₃ exhibits a spin reorientation from Γ₄ to Γ₁. And the stability of the ferromagnetic Γ₄ phase increases with the applied magnetic field increasing. Furthermore, at lower temperature, it shows weak antiferromagnetic ordering of Er³⁺. We also present the low temperature polarization data for ErCrO₃ and find a remarkable decreasing of polarization around T_N = 133 K on increasing temperature, this effect might be due to the coupling between magnetic and ferroelectric order parameters, and the magnetic field suppresses the polarization which demonstrates convincingly the strong magnetoelectric (ME) coupling in ErCrO₃.     

Dielectric and ferroelectric properties of Ba0.8Sr0.2Ti1−5x/4NbxO3 ceramics

Ba_0.8Sr_0.2Ti_1−5x/4Nb_xO₃ ceramics, x = 0, 0.01, 0.05, 0.10, were fabricated by conventional solid-state reaction. With increasing niobium content the ferroelectric phase transition temperature decreases linearly, and the dispersivity of the transition increases. Niobium B-site decreases transition temperature more pronounced than Sr²⁺ at A-site. The heterovalent substitution of Nb⁵⁺ in low content causes local defect dipole, while more substitutions introduce disorder to disturb the long-range dipole correlation. Ba_0.8Sr_0.2Ti_1−0.5/4Nb_0.1O₃ ceramic shows weak ferroelectric loop at room temperature far from its transition temperature, 153 K.     

Improved energy storage density and energy efficiency of Samarium modified PMNT electroceramic

We report the improved energy storage density and efficiency after 2.5% of Samarium substitution in ferroelectric Pb[(Mg_1/3Nb_2/3)_0.80Ti_0.20]O₃ (PMNT) electroceramic. The microstructure and surface morphology were analyzed and correlated with various functional properties. The energy storage density, leakage current density, ferroelectric and dielectric properties were investigated thoroughly, indicating that Samarium's substitution significantly modified the microstructure, the dielectric strength, breakdown electric field, and turned ferroelectric PMNT to relaxor ferroelectrics. Due to the relaxor nature, the gap between remanent polarization and maximum polarization increases with the substitution of Samarium in PMNT matrix, which further increases the recoverable energy storage density and energy efficiency. A nearly 100% increase in recoverable energy density and efficiency was obtained at an electric field strength of 35 kV/cm at room temperature (～296 K). The electroceramic shows maximum energy density near the ferroelectric phase transition temperature (325 K–345 K) region and provides a moderate energy storage density for possible applications in power microelectronics.     

Preparation and optimization of silver niobate-based lead-free ceramic energy storage materials

AgNbO₃ has broad research prospects in dielectric energy storage due to its unique antiferroelectric properties. The enhancement of ferroelectric/antiferroelectric phase stability can be achieved by tuning the tolerance factor t of AgNbO₃-based ceramics. On the other hand, the stability of the antiferroelectric and ferroelectric phases can be improved by adjusting the phase transition temperature of AgNbO₃-based ceramics. This is of great significance and value to the research and development of lead-free energy storage materials. Although there have been many studies on the energy storage performance of AgNbO₃-based ceramic materials, there are still challenges in achieving high energy storage density and energy efficiency at the same time. This review discusses the optimization strategy, preparation technology, and sintering technology of AgNbO3-based ceramics, summarizes the current research progress and obstacles, and puts forward the development direction for the application of AgNbO₃-based ceramics.     

Giant strain response and structure evolution in (Bi0.5Na0.5)0.945−x(Bi0.2Sr0.7□0.1)xBa0.055TiO3 ceramics

The microstructure, electric-field-induced strain, polarization, and dielectric permittivity in (Bi_0.5Na_0.5)_0.945−x(Bi_0.2Sr_0.7□_0.1)_xBa_0.055TiO₃ (BNBT–xBST) (0 ≤ x ≤ 0.08) electroceramics are investigated. An irreversible transition from rhombohedral and monoclinic coexistence phase to single rhombohedral phase is indicated with the remnant strain S_r = 0.330% at x = 0. As the BST content increases, the ferroelectric order is disrupted resulting in a degradation of the remnant polarization, coercive field, and the ferroelectric-to-relaxor transition temperature (T_F–R). The coexistence of ferroelectric relaxor and ferroelectric phase is observed for the optimum composition x = 0.02 at ambient temperature with a large strain of 0.428% at 60 kV/cm (normalized strain S_max/E_max = 713 pm/V). The large strain is contributed by both ferroelectric domain reorientation behavior and the reversible relaxor to ferroelectric phase transition.     

Room-temperature multiferroic characteristics and unique vortex domain structures of h-Yb1−xInxFeO3 solid solutions

Hexagonal rare-earth ferrites (h-RFeO₃) have attracted much scientific attention due to their room-temperature multiferroicity. However, it is still a hard job to obtain h-RFeO₃ bulk materials because of the meta-stability of such hexagonal phase, and the evaluation of room-temperature ferroelectric and magnetoelectric characteristics in such materials is also a challengeable issue. In the present work, Yb_1−xIn_xFeO₃ ceramics with the stable hexagonal structure were obtained by introducing chemical pressure, where the unique ferroelectric domain structures of sixfold vortex combined with tenfold vortex with a typical size of ~400 nm were determined. Symmetry of the present system evolved from centrosymmetric orthorhombic Pbnm (x = 0–0.4) to non-centrosymmetric hexagonal P6₃cm (x = 0.5 and 0.6) with a ferroelectric polarization up to 3.2 μC/cm², and finally to centrosymmetric hexagonal P6₃/mmc (x = 0.7 and 0.8). The Curie point decreased monotonically from 723 K to a temperature below room temperature with increasing x, and the antiferromagnetic phase transition above room temperature was determined for all compositions. Meanwhile, a large linear magnetoelectric coefficient (α_ME) up to 0.96 mV/cm Oe was obtained at room temperature, and this indicated the great application potential for magnetoelectric devices.     

Electrical performance and ferroelectric phase transition characteristic in different oriented 0.73PMN-0.27PT single crystals

The electrical and optical properties of (001)- and (110)-oriented 0.73 Pb(Mg_1/3Nb_2/3)O₃-0.27PbTiO₃ single crystals are systematically investigated at various temperatures, both of which present a series of ferroelectric phase transition processes. Dielectric performance measurements reveal that the ferroelectric phase transition occurs over a temperature range, rather than at one temperature point. By testing the ferroelectric hysteresis P–E curves as well as bipolar and unipolar electric field-induced strain S–E curves, the values of remnant polarization, coercive field, maximum strain, and converse piezoelectric constant d₃₃^* change considerably near the phase transition temperatures. Simultaneously, the 0.73PMN-0.27PT single crystals with (001)- and (110)-orientations under a low electric field show ultrahigh d₃₃^* values of 3540 and 2817 pm/V, respectively, which can be attributed to the electric field-induced monoclinic and orthorhombic phases, respectively. The series of ferroelectric phase transitions upon heating, that is, from rhombohedral ferroelectric to monoclinic/orthorhombic, followed by from monoclinic/orthorhombic to tetragonal, and finally from tetragonal to cubic paraelectric, are further investigated via polarized light microscopy and Raman spectroscopy.     

Electric-field-temperature phase diagram and electromechanical properties in lead-free (Na0.5Bi0.5)TiO3-based incipient piezoelectric ceramics

In this work, the (1-x)(0.8Na_0.5Bi_0.5TiO₃-0.2K_0.5Bi_0.5TiO₃)-xSrTiO₃ (NKBT-xST) incipient piezoelectric ceramics with x = 0–0.07 (0ST-7ST) were prepared by the solid-state reaction method and their structural transformation and electromechanical properties were investigated as a function of ST content. As the ST content increases, the long-range ferroelectric order is disrupted, and the ferroelectric-relaxor phase transition temperature (T_FR) shifts to around room temperature for NKBT-5ST ceramics, accompanied by a relatively high electrostrain of 0.3% at 6 kV/mm. The large strain response associated with the vanished ferroelectric properties around T_FR can be attributed to the reversible relaxor-ferroelectric phase transition. The electric-field-temperature (E-T) phase diagrams were established, and the transition between the two field-induced long-range ferroelectric states were found to take place via a two-step switching process through an intermediate relaxor state. The threshold electric field to trigger the conversion between ferroelectric state and relaxor state depends strongly on the dynamics of polarization relaxation, which is influenced by temperature and composition.     

Li+ and Sm3+ co-doped AgNbO3-based antiferroelectric ceramics for high-power energy storage

Ag_1-x-3yLi_xSm_yNbO₃ (x≤0.05, y≤0.05) (ALSN) antiferroelectric ceramics were successfully prepared via conventional solid-state reaction and sinter routes in oxygen atmosphere for improving the energy storage characteristic of pure AgNbO₃. The results indicate that all of the studied compositions display a pure orthorhombic antiferroelectric (AFE) perovskite structure, while their key parameters of electric-field-induced antiferroelectric-ferroelectric transition can be affected by Li⁺ or/and Sm³⁺ doping contents. The Sm³⁺ doping can enhance the stability of antiferroelectric state, giving rise to higher antiferroelectric-ferroelectric transition electric-field (E_F and E_B), while Li⁺ doping can reduce E_F and E_B for Sm³⁺ doped AgNbO₃ with low Sm³⁺ content (y≤0.03). When co-doping the same amounts of Li⁺ and Sm³⁺ at x=y≤0.03, both E_F and E_B almost remain unchanged. At x=y=0.05, the diffuse phase transition (DPT) behavior of antiferroelectric-paraelectric (AFE-PE) phase transition occurred, resulting in a “slim-like” double-polarization hysteresis with significantly enhanced E_F. Due to these features, both W_rec and η are improved compared with pure AgNbO₃. The W_rec and η with composition at x=y=0.05 is 2.33 J/cm³ and 58% under applying electric field of 240 kV/cm, respectively. The results suggest that building DPT behavior of AFE-PE phase transition could be an alternative strategy to improve the energy storage characteristic of AgNbO₃.     

Effect of sintering temperature on the ferroelectric property and electrocaloric effect of Pb0.8Ba0.2ZrO3 ceramics

This work presents the effects of sintering temperature ranging from 1200 °C to 1300 °C at intervals of 20 °C on the crystal structure, ferroelectric properties, and electrocaloric effect (ECE) of Pb_0.8Ba_0.2ZrO₃. Samples sintered at 1240 °C, 1260 °C, and 1280 °C have large remanent polarization and small coercive field. Meanwhile, samples sintered at 1260 °C, 1280 °C, and 1300 °C possess large breakdown field strength. Samples sintered at 1260 °C for 4 h exhibit the optimal ferroelectric properties. Antiferroelectricity-ferroelectricity (AFE-FE) phase transition occurs at room temperature T₁ (279 K). Directly examining ECE at this temperature is meaningful, and the temperature change is 0.068 K at approximately 60 °C and 30 kV/cm. Results laid the foundation for studying the performance of ferroelectric and ECE within this phase-transition temperature range and provide a reference for new solid-state refrigeration technology.     

Ferroelectricity driven by soft phonon and spin order in multiferroic BiMn3Cr4O12

BiMn₃Cr₄O₁₂ shows an unusual joint multiferroicity, which facilitates the coexistence of considerable ferroelectric polarization and remarkable magnetoelectric coupling in a single-phase multiferroic material. Based on first-principles calculations, we investigate the two different types of ferroelectric phase transitions in the BiMn₃Cr₄O₁₂ material. Our results show that the first ferroelectric phase transition is driven by soft mode and leads BiMn₃Cr₄O₁₂ into the Cm space group. The predicted ferroelectric polarization in single crystal is about ~9.8 μC/cm². With the emergence of spin order on both Mn and Cr sublattices, it is the polar Cm structure that triggers the exchange striction mechanism and therefore results in a large type-II multiferroicity (~1.1 μC/cm²). In addition, the intrinsic direction of the spin-driven ferroelectric polarization is always opposite to that of the existing Cm phase structure. Our results imply a feasible strategy in searching/designing novel type-II multiferroics with large ferroelectric polarization.     

Novel NaNbO3–Sr0.7Bi0·2TiO3 lead-free dielectric ceramics with excellent energy storage properties

NaNbO₃ (NN) is considered to be one of the most prospective lead-free antiferroelectric energy storage materials due to the merits of low cost, nontoxicity, and low density. Nevertheless, the electric field-induced ferroelectric phase remains dominant after the removal of the electric field, resulting in large residual polarization, which prevents NN ceramics from obtaining superior energy storage performance. In this work, the relaxor ferroelectric Sr_0·7Bi_0·2TiO₃ (SBT) was chosen to partially replace the NN ceramics, and the introduction of the nanodomain of the relaxor ferroelectric hinders the generation of field-induced ferroelectric phases, allowing the material to combine the large polarization strength of the relaxor ferroelectric with the near-zero residual polarization of the antiferroelectric. Large recoverable energy storage density (4.5 J cm^?3) and ultra-high energy storage efficiency (90.3%) were gained in NN-20SBT under an electric field of 288 kV cm^?1. Furthermore, superior temperature (25–120 °C) and frequency (1–500 Hz) stabilities were acquired. These performances demonstrate that NN-20SBT ceramics are potential candidates as dielectric materials for high energy storage density pulsed power capacitors.     

Effect of CuO addition on magnetic and electrical properties of Sr2Bi4Ti5O18 lead-free ferroelectric ceramics

The effect of CuO addition on magnetic and electrical properties of Sr₂Bi₄Ti₅O₁₈ (SBT) lead-free bismuth layered structure ferroelectric ceramics have been studied and reported. Interestingly, the prepared samples exhibit multiferroic behavior with the coexistence of magnetic and ferroelectric phase transition temperature. Magnetic phase transition with Neel׳s temperature (T_N) of 657 K is observed at 0.75 mol% of CuO added SBT ceramics, which is higher than the well known multiferroic BiFeO₃ (643 K) and the ferroelectric phase transition with Curie temperature (T_C) of 587 K is observed at 1 mol% of CuO added SBT ceramics, which is relatively higher than the reported pure SBT ceramics (558 K). Further, the electrical properties such as dielectric, ferroelectric, piezoelectric, leakage current density characteristics and optical properties were investigated as a function of x (x=0, 0.25, 0.5, 0.75 and 1 mol%). Presence of strong magnetic super-exchange interactions in CuO and the creation of oxygen vacancies play a vital role in the enhancement of magnetic and electrical properties of CuO doped SBT ceramics. Moreover, the present results indicate that, small amount (0.25 mol%) of CuO addition in SBT ceramics enhances the electrical properties significantly and vice versa, large amount (0.75 mol%) of CuO addition enhances the magnetic properties. Thus, the presence of magneto-electric coupling effect was observed in CuO doped Sr₂Bi₄Ti₅O₁₈ ferroelectric ceramics.