Lead-free ferroelectric Na1−xSrx (Sn0.25Ti0.75) xNb1−xO3 system (0.1 ≤ x ≤ 0.4) XRD,dielectric, and Raman properties

Na_1−xSr_x(Sn_0.25Ti_0.75)_xNb_1−xO₃ (SNSTNx) ceramics with composition (0.1 ≤ x ≤ 0.4) were synthesized by the conventional solid-state method. The purity, microstructure, dielectric, ferroelectric and Raman features were sought. This study demonstrated that antiferroelectricity can be stabilized in NaNbO₃-based ceramics by lowering the tolerance factor. This compound exhibited a single phase of perovskite without impurity with tetragonal symmetry (P4mm). Micro structural investigations, using both XRD and SEM, indicated an enhancement of antiferroelectric super lattice peaks with SrSnO₃ and SrTiO₃. The dielectric study showed that our new ceramics had better characteristics than pure NaNbO₃ and the relaxor behavior demonstrated the SNSTNx candidacy for use in several industrial applications. The ferroelectric behavior revealed that only ferroelectric domains existed in the prepared ceramic. In contrast, NaNbO₃ displayed the coexistence of antiferroelectric and ferroelectric domains. The Raman spectra of SNSTNx as a function of composition and temperature were also in favor of our dielectric results.     

NaNbO3-based antiferroelectric multilayer ceramic capacitors for energy storage applications

Antiferroelectric materials feature electric-field-induced phase transitions followed by a large polarization change characterized by double polarization hysteresis loops. Therefore, antiferroelectrics are engaging for high-energy density and high-power density applications, especially in the form of multilayer ceramic capacitors (MLCCs). However, the development of lead-free antiferroelectrics with stable double hysteresis loops is still challenging, especially for compositions based on NaNbO₃. To this end, we have prepared MLCCs with the newly developed antiferroelectric composition 0.90NaNbO₃-0.06SrSnO₃-0.04(Na_0.5Bi_0.5)TiO₃. The double hysteresis loops were determined at 24 kV/mm in the temperature range of 25–150 °C, with resulting recoverable energy storage ranging from 1.16 to 1.42 J/cm³, respectively. Moreover, the energy efficiency is rather constant at 0.4 in the same temperature range. Finally, the MLCCs exhibit resistance to electric field cycling and could withstand up to 1000 cycles. These results verify that NaNbO₃-based antiferroelectrics in the form of MLCCs are promising for use in applications.     

Phase identification and structural evolution in BMT modified NN anti-ferroelectric ceramics

It has been an enormous challenge to obtain double P-E loops and identify structure evolution in NaNbO₃ based ceramics. BiMg_2/3Ta_1/3O₃(BMT) modified NaNbO₃ ceramics were fabricated by solid-state methods, showing slimmer double P-E loops compared with other reported work. Combined with the Raman and refined XRD results, we confirmed that the coexistence of anti-ferroelectric (AFE) P (Pbma) and ferroelectric (FE) Q (P2₁ma) phases gradually transformed into the coexistence of AFE R (Pnma) and AFE P phases by adding BMT, and thus, the relaxor behavior and reversible phase transition can be enhanced. TEM analysis demonstrated the coexistence of the P and Q phases in pure NN, verified by the characteristic ¼ and ½ (010) type reflections in the selected area electron diffraction patterns, respectively. The characteristic antiphase boundaries are observed as well-parallel lines with a 6-fold modulation, interrupting the 4-fold P phase. After BMT modification, the high-temperature R phase was stabilized, evidenced by the 1/6 (001) type reflections. Moreover, the domain morphology changes dramatically, illustrated by the complicated network of APBs and the elongated band morphology comprising orientational nanodomains, which indicate a strong structural heterogeneity in the NN-BMT ceramics.     

Phase Relations in the LaNb3O9 – NaNbO3 System in the Temperature Range of 20 TO 800°C

A portion of the constitutional diagram of the LaNb₃O₉ – NaNbO₃ system in the temperature range from 20 to 800°C is constructed. The existence of a solid-solution region (down to 10 mol.% NaNbO₃ ) involving a low-temperature monoclinic modification of LaNb₃O₉ and of a broad region (from 20 to 75 mol.% NaNbO₃ ) of tetragonal perovskite solid solutions is established. High-temperature perovskite solid solutions based on the LaNb₃O₉ rhombic modification undergo an eutectoid decomposition into two perovskite solid solutions. LaNb₃O₉ and the high-temperature solid solutions are shown to be antiferroelectric phases with high values of dielectric constant.     

Energy storage properties of NaNbO3-CaZrO3 ceramics with coexistence of ferroelectric and antiferroelectric phases

Anti-ferroelectric materials with large saturated polarization, small remnant polarization, and moderate breakdown strength are receiving increasing attention for modern high-power electrical systems. Here we demonstrated that by incorporating CaZrO₃ into NaNbO₃ ceramics, the antiferroelectricity in NaNbO₃-CaZrO₃ solid solutions could be stabilized at room temperature. The effects of phase constitution and microstructure on the dielectric properties, electrical breakdown strength, and energy storage properties of the NaNbO₃-CaZrO₃ ceramics were investigated. Ferroelectric and antiferroelectric phase coexistence in the NaNbO3-CaZrO₃ was confirmed by XRD and TEM analyses. With increasing CaZrO₃ content, the grain size was reduced, and the dielectric breakdown strength was improved. Therefore, a high energy density of 0.55?J/cm³ and efficiency of 63% was obtained in the NaNbO₃-0.04CaZrO₃ ceramics. These lead-free NaNbO₃-CaZrO₃ antiferroelectrics with good electrical energy storage can be exploited for high-power storage devices.     

Modulating the energy storage performance of NaNbO3-based lead-free ceramics for pulsed power capacitors

Nb-containing antiferroelectric materials have recently attracted great research interest as energy storage materials for pulsed power capacitors due to their extraordinary energy storage performances. In this case, the optimization of the energy storage performance is obtained by a compositional modulation of NaNbO₃-Bi(Zn_2/3Nb_1/3)O₃ bulk ceramics. An optimal energy performance can be obtained with a composition of 0.85NaNbO₃-0.15Bi(Zn_2/3Nb_1/3)O₃, which is accompanied by a stable charge energy density in temperatures up to 150 °C owing to its relaxor characteristics and excellent cycling stability after 10⁵ cycles. This work further broadens the scope of NaNbO₃-based ceramic applications in the area of pulsed power sources.     

NaNbO3-based short-range antiferroelectric ceramics with ultrahigh energy storage performance

Lead-free NaNbO₃ (NN) antiferroelectric ceramics provide superior energy storage performance and good temperature/frequency stability, which are solid candidates for dielectric capacitors in high power/pulse electronic power systems. However, their conversion of the antiferroelectric P phase to the ferroelectric Q phase at room temperature is always accompanied with large remnant polarization (P_r), which significantly reduces their effective energy storage density and efficiency. In this study, to optimize the energy storage properties, short-range antiferroelectric (0.95-x)NaNbO₃-xBi(Mg_2/3Nb_1/3)O₃-0.05CaZrO₃ (xBMN) ceramics were designed to stabilize the antiferroelectric phase, in which the local random fields were simultaneously constructed. The results showed that the antiferroelectric orthorhombic P phase was transformed into the R phase, and the local short-range random fields were generated, which effectively inhibited the hysteresis loss and P_r. Of great interest is that the 0.12BMN ceramics displayed a large recoverable energy storage density (W_rec) of 5.9 J/cm³ and high efficiency (η) of 85% at the breakdown strength (E_b) of 640 kV/cm. The material also showed good frequency stability in the frequency range of 2–300 Hz, excellent temperature stability in the temperature range of 20–110 ℃, and a very short discharge time (t_0.9∼4.92 μs). These results indicate that xBMN ceramics have great potential for advanced energy storage capacitor applications.     

Order–disorder behaviour in 0.9Ba([Zn0.60Co0.40]1/3Nb2/3)O3–0.1Ba(Ga0.5Ta0.5)O3 microwave dielectric resonators

0.9Ba([Zn_0.60Co_0.40]_1/3Nb_2/3)O₃–0.1Ba(Ga_0.5Ta_0.5)O₃ (BCZN–BGT) ceramic resonators (quality factor, Q=32,000 at the rate of 3.05 GHz, relative permittivity, ε_r=35 and temperature coefficient of the resonant, τ_f=0) have been fabricated which are suitable in terms of cost and performance for base stations supporting third generation architecture. The new compounds are perovskite structured (a=4.09 Å) but exhibit no superlattice reflections at any heat treatment temperature according to X-ray diffraction (XRD). However, annealing and quenching of samples followed by transmission electron microscopy and Raman spectroscopy revealed an order–disorder phase transition at ∼1200 °C. Annealing below this temperature (1100 °C) gave rise to discrete ±1/3{h k l}_p and diffuse 1/2{h k l}_p superlattice reflections in the same 〈1 1 0〉_p zone axis electron diffraction patterns and the presence of F_2g and A_1g modes in Raman spectra. It is proposed that ±1/3{h k l}_p reflections result from 1:2 long-range ordered domains of BCZN whereas the diffuse 1/2{h k l}_p reflections arise from short range fcc ordered BGT rich regions at the 1:2 domain boundaries. A short-range ordered fcc superlattice was observed in samples quenched from above the order–disorder phase transition (>1200 °C) which was accompanied by the presence of only the A_1g mode in Raman spectra.     

Antiferroelectricity of NaNbO3: Single-crystal experimental study and first-principles calculation

Similar to the canonical antiferroelectric (AFE) compound PbZrO₃ in Pb(Zr,Ti)O₃ solid-solutions, the presence of double hysteresis loops and that of electric field–induced phase transitions are important characteristics of NaNbO₃ AFE materials; yet the phase transition behavior in the latter system is typically irreversible with the related mechanisms not fully understood. Here, we explore the phase transition mechanism of ferroelectric and AFE phases in NaNbO₃ based on measurements of single crystals with different directions in conjunction with density-functional theory (DFT) calculations. The tilting and distortion behaviors of the [NbO₆] octahedra are explained by DFT, and the ion displacement in the lattice is traced. The tilting and distortion behaviors of the [NbO₆] octahedra with different orientations are compared. We confirm that the tilt and distortion of the [NbO₆] octahedra along the [1 1 1] direction is the main reason to improve the stability of AFE phase. This conclusion is verified by the experimental characterizations of large-size NaNbO₃ single crystals successfully obtained in this study.     

Electric field induced phase transition and accompanying giant poling strain in lead-free NaNbO3-BaZrO3 ceramics

A series of phase transitions in (1-x)NaNbO₃-xBaZrO₃ ((1-x)NN-xBZ) ceramics was observed from antiferroelectric orthorhombic phase to ferroelectric orthorhombic phase and finally into ferroelectric rhombohedral phase with increasing x. An electric field induced irreversible phase transition was found in different compositions, irrespective of their virgin phase structures. Particularly, an antiferroelectric orthorhombic phase is irreversibly transformed into a ferroelectric monoclinic phase within 0.02?≤?x?≤?0.05, leading to a giant poling strain of ～0.58%. This is much larger than that observed in ferroelectric orthorhombic (0.06?≤?x?≤?0.07) and rhombohedral phases (0.08?≤?x?≤?0.11) suffering from an irreversible ferroelectric-ferroelectric (monoclinic) phase transition. The synchrotron x-ray diffraction and the measurement of longitudinal and transverse strains suggest that this irreversible phase transition should involve not only a distinct volume expansion, but also an obvious lattice elongation. The present study demonstrates a unique nature of the composition and field dependent phase stability and an underlying mechanism of giant poling strains in NN-BZ ceramics.     

The effect of A-site nonstoichiometry on the microstructure,electric properties,and phase stability of NaNbO3 polycrystalline ceramics

The impact of A-site nonstoichiometry on the microstructure, electric properties, and phase stability of sodium niobate ceramics (Na_1+xNbO₃, x = ?2 to 1 mol %) was investigated. All the components maintained an orthorhombic antiferroelectric (AFE) structure. The grain size increased from 3.9 to 14.3 μm with the variation in x from ?2 to 1. The AFE–FE phase transition electric field dramatically increased from 100 kV cm^?1 at x = 0 to 170 kV cm^?1 at x = ?2, confirming the enlarged energy barrier between AFE Pbma and FE Pmc2₁ phase under external field in A-site deficient components. This is attributed to the lattice compressive stress generated by introducing proper A-site vacancies. Combined results of transmission electron microscopy and Raman spectroscopy indicated that the AFE distortion of Pbma phase was significantly enhanced in A-site deficient components, which jointly contributed to the stability of AFE phase in A-site deficient NaNbO₃ material.     

Potentiality of Bi and Mn co-doped lead-free NaNbO3 ceramics as a pyroelectric material for uncooled infrared thermal detectors

The pyroelectric materials have immense applications in the uncooled infrared thermal detectors. However, owing to increasing environmental concerns due to Pb element, it is required to explore novel, high-performance, environmental-friendly pyroelectric materials. This is the first study to report about the pyroelectric properties of lead-free NaNbO₃ ceramics, which displayed a high pyroelectric coefficient of 1.85 × 10^?8 C cm^-2 K^?1 and figures of merit as F_i = 0.67 × 10^?10 m V^?1, F_v = 3.33 × 10^?2 m² C^?1, and F_d = 5.32 × 10^-5 Pa^?1/2 at room temperature. Also, highly temperature-stable pyroelectric characteristics were also observed in NaNbO₃ ceramics due to the high depolarization temperature of 280 ℃. The high pyroelectric properties and temperature stability were a result of the electric field induced irreversible phase transition from antiferroelectric to ferroelectric. Hence, we can conclude that lead-free NaNbO₃ ceramics are a novel and promising candidate for pyroelectric detectors in a wide temperature range, especially for large area detectors and pyroelectric point detector.     

Ferroelectric Domains and Incommensuration in the Intermediate Phase Region of Lead Zirconate

Electron microscopy of lead zirconate (PbZrO₃) identified the presence of an intermediate region between the high-temperature cubic state and the low-temperature antiferro-electric state. Ferroelectric domains were found to occur between 200° and 230°C on cooling. Convergent-beam electron diffraction studies identified rhombohedral symmetry for the ferroelectric phase. Selected area electron diffraction patterns determined ½{110} superlattice reflections in the ferroelectric state. The reflections were associated with a softening of an M-type oxygen octahedral rotation mode. In addition, 1/x{110} incommensurate superlattice reflections were detected within a narrow temperature range for PbZrO₃ between the intermediate ferroelectric and low-temperature antiferroelectric states.     

Evolving energy-storage performances during temperature-induced antiferroelectric P–R phase transition in NaNbO3-based lead-free ceramics

Temperature-driven antiferroelectric (AFE) P to AFE R phase transition in MnO₂-doped 0.90NaNbO₃-0.10CaTiO₃ ceramics was investigated through polarization-field response, energy-storage and charge-discharge properties as well as ex/in-situ multiscale structure characterization. Both room-temperature AFE P and high-temperature AFE R phases show double polarization-electric field hysteresis loops, indicating a reversible field-driven AFE to ferroelectric (FE) phase transition. An abnormal variation of critical fields for the AFE-FE and FE-AFE phase transition and a faster polarization-field response contribute to the reduced polarization hysteresis for both AFE P and R phases but an obviously expanded linear polarization-field response only for AFE R phase, being responsible for a two-time significant enhancement in energy-storage properties from P phase to P–R phase boundary and then to R phase. The variation of unit cell anisotropy and domain morphology with temperature was found to play crucial roles in the modulated field-driven phase transition behavior and polarization-field response on heating.     

Ultra-fast charge-discharge and high energy storage density realized in NaNbO3–La(Mn0.5Ni0.5)O3 ceramics

Lead-free antiferroelectric (AFE) NaNbO₃ (NN) is one of promising materials for dielectric capacitors, but the recoverable energy-storage density and efficiency get restrained owing to huge remanent polarization and limited dielectric breakdown field strength. In this work, a variety of NN based lead-free bulk (1-x)NaNbO₃-xLa(Mn_0.5Ni_0.5)O₃ (abbreviated as (1-x)NN-xLMN, x = 0, 0.05, 0.10, 0.15, 0.20) ceramics were designed using a solid-state synthesis method. Remarkably, an ultra-fast charge-discharge speed 47 ns and an acceptable recoverable energy-storage density W_rec ~1.77 J/cm³ with a high efficiency η = 77% were obtained under the E_b of 200 kV/cm at x = 0.05. The superior energy storage performance is attributed to the regulation of domain size and voltage resistance by special ions substitution of A and B sites. This work not only proposes an efficient strategy to realize high recoverable energy-storage density and efficiency, but also provide an candidate material for application of advanced pulsed power capacitors.     

Hierarchical domain structures in (Pb,La)(Zr,Sn, Ti)O3 antiferroelectric ceramics

(Pb, La)(Zr, Sn, Ti)O₃ (PLZST) ceramic is one of the most prospective antiferroelectric (AFE) materials for variety of functional applications including energy storage and converter. Systematic structural investigation of domain structures should be of fundamental importance for understanding the structure-property relationship in AFE ceramics. In this study, the hierarchical domain structures and modulated structures correlated to the compositional variation in (Pb_0.97La_0.02) (Zr_0.50Sn_xTi_0.50-x)O₃ (x = 0.375, 0.45 and 0.50) were observed and investigated in details by transmission electron microscopy. The PLZST ceramics show exclusively incommensurate modulated structures (IMS) whose modulation period changed from 9.37 to 6.15 and to 4.04 with increasing of the x value. The hierarchical domain structures include, in decreasing scales, AFE domains, incommensurate domains and nanodomains. The elementary domains in PLZST ceramics are pinstriped nanodomains which were formed based on IMS configuration but by frequent modulation of IMS periodicity and formation of faults. Nanodomains accumulated and then dissociated into incommensurate domains and AFE domains successively. The presently revealed structural characteristics in antiferroelectric PLZST may stimulate future researches on the evolution of IMS-based hierarchical domains under external physical fields, e.g. thermal or electrical, and their correlation to the physical performance.     

Microstructure of Lanthanum Magnesium Niobate at Elevated Temperature

Microstructural studies of the complex perovskite compound La(Mg_2/3Nb_1/3)O₃ (LMN) were conducted using transmission electron microscopy (TEM) and X-ray diffractometry (XRD) at elevated temperatures. 1:1 chemical ordering of B-site cations and tilting of oxygen octahedra were observed in LMN. Three types of superlattice reflections, [1—2]{111}, [1—2]{110}, and [1—2]{100} were observed at room temperature and at 800°C in electron diffraction patterns. In the XRD experiments, the [1—2]{210} and [1—2]{300} extra peaks disappeared at temperatures >1200°C. However, the intensity of the superlattice [1—2]{111} peak did not change with increased temperature up to 1400°C. These results strongly indicated that the origin of superlattice reflection [1—2]{111} was different from that of the other superlattice reflections. It was mainly caused by the 1:1 chemical ordering of magnesium and niobium atoms. The TEM image observed at 800°C showed the ordered domain structures separated by the antiphase boundaries.     

Enhanced piezoelectric polarization by subtle structure distortion to trigger efficient photocatalytic CO2RR

Achieving the goal of carbon neutralization using photocatalytic CO₂ reduction has garnered widespread attention. However, rapid bulk-charge recombination seriously impedes the further improvement of photocatalytic properties. In response, we propose a novel strategy to solve this limitation using enhanced piezoelectric polarized electric fields. Co₃O₄ is introduced into NaNbO₃ by straightforward photo-deposition method, which causes the distortion of NbO₆ octahedron to alter the symmetry and boost the piezoelectricity. Meanwhile, the increased Co sites facilitate the adsorption of CO₂, and reduce the reaction energy barrier. As a result, the Co₃O₄-modified NaNbO₃ nanocubes possess outstanding properties of CO₂ reduction under the synergy of ultrasound and visible light with the yield of CO about 4579.71 μmol g⁻¹. Furthermore, the mechanism of piezo-photocatalytic CO₂ reduction is revealed in detail based on DFT, KPFM, and in-suit DRIFTS characterizations, thus providing guidance for the design of high-performance CO₂ photoreduction systems.     

Triple Hysteresis Loops of PLZST Ceramics Related to Coexistence of Ferroelectricity and Antiferroelectricity

The hysteresis behaviors and phase characteristics of Pb_0.97La_0.02(Zr_0.90Sn_0.025Ti_0.075)O₃ (PLZST) ceramics were investigated in this work. A single mini hysteresis loop at 3 kV/mm with the maximum polarization (P_max) of 8.3 μC/cm² and triple hysteresis loops at 6.6 kV/mm were observed, which indicates the coexistence of rhombohedral ferroelectric phase and tetragonal antiferroelectric phase. The X‐ray Diffraction patterns and dielectric temperature spectra both demonstrate this coexistence. Moreover, the hysteresis loops with increasing temperature indicated that a ferroelectric–antiferroelectric phase transition occurred at about 60°C. These phenomena would be useful for understanding the domain evolution during ferroelectric and antiferroelectric phase transition.     

0.74NaNbO3–0.26Sr(Mg1/3Nb2/3)O3 lead-free dielectric ceramics with high energy storage properties

Sodium niobate (NaNbO₃) ceramics are commonly investigated for use as energy storage ceramics because of their excellent properties. NaNbO₃ ceramics are modified mainly by doping with a Bi-based composite perovskite, that is, by the nonequivalent doping of Bi³⁺ at the A site of the NaNbO₃ ceramic. In addition, the high volatility of Bi at high temperatures increases the defects in the ceramics. This paper provides a new idea of doping modification of sodium NaNbO₃-based energy storage ceramics. Here, (1?x)NaNbO₃–xSr(Mg_1/3Nb_2/3)O₃ (x = 0.17, 0.20, 0.23, 0.26) ceramics were prepared by doping NaNbO₃ with an Sr-based composite perovskite. Compared with Bi-based composite perovskite, Sr-based composite perovskite doping of NaNbO₃ ceramics can also obtained good energy storage properties: a total energy storage density of 4.28 J/cm³ and an energy storage efficiency of 89.3%. In addition, the ceramics exhibited good frequency stability (2–200 Hz) and a high charge/discharge rate (1.06 μs).