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.     

Hydrothermal synthesis of NaNbO3 with low NaOH concentration

NaNbO₃ fine powders were prepared by reacting niobium pentoxide with low NaOH concentration solution under hydrothermal conditions at 160 °C. The reaction ruptured the corner-sharing of NbO₆ octahedra in the reactant Nb₂O₅, yielding various niobates, and the structure and composition of the niobates depended on the [OH⁻] and reaction time. The fine Nb₂O₅ powder first aggregated to large particles and then turned to metastable intermediates with multifarious morphology. The reaction was fast for the situation of [OH⁻] = 2 M. The [OH⁻] determined the structure of final products, and three types of NaNbO₃ powder with the orthorhombic, tetragonal and cubic symmetries were obtained, respectively, depending on the [OH⁻]. The low [OH⁻] was propitious to yield orthorhombic NaNbO₃. The present work demonstrated that higher [OH⁻] was not favored to synthesize NaNbO₃ powders and the conversion speed in this reaction was not in proportion to the [OH⁻].     

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.     

Hydrothermal synthesis of morphology-controlled KNbO3, NaNbO3, and (K,Na)NbO3 powders

NaNbO₃, (K,Na)NbO₃ and KNbO₃ powders were synthesized using (1− y) NaOH–y KOH solutions ([OH⁻] =7.5–15 M) with y=0, 0.78, and 1 at 200 °C by the hydrothermal method, respectively. Their compositions, structures, and morphologies were analysed. Both of the synthesized NaNbO₃ and KNbO₃ powders had sub-micron- or micron-sized grains. The [OH⁻] drastically influenced the size and morphology of the KNbO₃ particles but did not influence those of the NaNbO₃ particles. In contrast, the morphology of the (K,Na)NbO₃ particles, which were aggregates of nano-grains, was influenced by the hydrothermal-treatment time rather than [OH⁻]. Moreover, their composition and phase were influenced by both annealing and the hydrothermal-treatment time, and their formation mechanism was discussed by comparison with those of KNbO₃ and NaNbO₃ particles. The present synthetic strategy enables tailoring the compositions, morphologies, and structures of the niobate products to different applications by controlling the process parameters.     

Giant strains of 0.70% achieved via a field-induced multiple phase transition in BNT-based relaxor antiferroelectric ceramics

Bismuth sodium titanate (BNT)-based lead-free ceramics have attracted a great deal of attention due to their large electrostrains. In this work, a remarkably symmetric strain of 0.7% together with excellent temperature (0.5–0.7% from 25 to 100 °C)/frequency (ΔS<4% from 1 to 20 Hz) stability was observed in the 0.91(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃-0.03NaNbO₃ (BNT-6BT-3NN) AFE P4bm ceramic through constructing R3c/P4mm/P4bm triple-phase coexistence phase boundary. Compared with other two compositions near double-phase coexistence ferroelectric (FE)-antiferroelectric (AFE) phase boundaries, the BNT-6BT-3NN ceramic exhibits a unique field-induced multiple phase transition from the initial AFE P4bm phase to the metastable FE P4mm phase and finally into the FE R3c phase. In-situ structural analysis evidenced a significantly enhanced lattice strain but a comparable strain value from domain switching in BNT-6BT-3NN compared with other compositions. The present study provides a novel strategy for designing high-performance large-strain ceramics in BNT-based relaxor AFE systems.     

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.     

Thermal stability and local atomic structure of (Na0.5K0.5)NbO3 doped BaTiO3 ceramics

Dielectric properties of x(Na_0.5K_0.5)NbO₃–(1−x)BaTiO₃ (x=0.00 and 0.06) specimens were investigated in terms of changes in local atomic structure, according to the phase transition by elevating the overall temperature. A 0.06(Na_0.5K_0.5)NbO₃–0.94BaTiO₃ (NKN–BT) specimen exhibited enhanced temperature stability along with an increased dielectric constant. The degree of reduction in tetragonality (c/a) at the Curie temperature was smaller in NKN–BT compared to that in pure BaTiO₃, as calculated by Rietveld refinement. From a comparison of the pre-edge region in the Ti K-edge, it was determined that the off-center displacement of the Ti atom was also raised to 13.4% through NKN substitution, with a change in local orientation from the [001] to the [111] directions. The substitution by NKN, which has a different ionic radius and electrical charge compared with BaTiO₃, causes structural distortion of the TiO₆ octahedra in the NKN–BT lattice, resulting in local polarization. These structural changes lead to the temperature stability of the dielectric constant and an overall improvement in the electrical properties of BaTiO₃.     

Realizing room temperature double hysteresis loops in antiferroelectric NaNbO3 based ceramics

Lead-free antiferroelectric (AFE) materials have seen a surge of research activity in environmentally friendly energy storage technologies. Recently, considerable work has been done to improve the stability of AFE in NaNbO₃ (NN) ceramics, but it remains a grand challenge to obtain typical AFE characteristic double P-E loops in NN ceramics at ambient conditions. In a preliminary estimate of tolerance factor versus average electronegativity difference, we reported the stable AFE phase in 0.95NaNbO₃-0.05BiMg_2/3Ta_1/3O₃ sample. The orthorhombic Q to P phase transition was verified by XRD and TEM. Then, the remarkable double P-E loops were obtained in 0.95NaNbO₃-0.05BiMg_2/3Ta_1/3O₃ ceramics. Furthermore, a phenomenological model was proposed to explain the P-E relationships and our results. Compared with other reported compounds, the T_P-R decreased more obviously from 350 °C to 200 °C. Superior temperature stability (variations of maximum current, current density, and power density within 15% over 30–140 °C) and field induced phase transition were also confirmed by the pulse charge testing. Our work develops a new road for achieving room-temperature double P-E loops in NN ceramics by BiM₁M₂O₃ (M₁ might be Mg, Zn, etc; M₂ might be Nb, Ta, etc) additives.     

Low-content barium doping induces two-phase coexistence in NaNbO3 to enhance the piezo–photocatalytic activity

Piezo–photocatalysis induced by ultrasound vibration and light irradiation is an emerging strategy to decelerate the rapid recombination of photoinduced charges in the catalytic process. Element doping-induced structural phase transition is an effective approach to improve catalytic performance. Herein, barium (Ba) doped sodium niobate (abbreviated as Na_1-2xBa_xNbO₃, x = 0, 0.005, 0.010, 0.015, and 0.020) nanomaterials were successfully prepared by a water-based sol-gel method. With the increase of Ba content, the morphology changes from sphere to cubic, and the orthorhombic NaNbO₃ transforms to an orthorhombic–cubic mixed phase. Compared with the single orthorhombic phase NaNbO₃, the piezo–photocatalytic degradation ratio of mixed-phase Na_0.97Ba_0.015NbO₃ is enhanced by 16% in the decomposing of Rhodamine B (RhB), and the kinetic rate constant is up to 0.0114 min^?1, which is 1.65 times higher than that of the orthorhombic NaNbO₃. The improved piezo–photocatalytic activity is attributed to the two-phase coexistence and optimum amount of oxygen vacancies. In addition, the photocatalytic, piezocatalytic, and piezo?photocatalytic performances and mechanisms of the Na_0.97Ba_0.015NbO₃ sample were also investigated. This work provides a reference for the construction of structural phase and the practical application of the piezo–photocatalysis in the environmental field.     

Orientation dependence of electric field induced phase transitions in lead-free (Na0.5Bi0.5)TiO3-based single crystals

The orientation dependence of the electric field induced strain and phase transitions in 0.92(Na_0.5Bi_0.5)TiO₃–0.06BaTiO₃–0.02(K_0.5Na_0.5)NbO₃ (NBT–6BT–2KNN) single crystals has been investigated. The evolution of Raman spectra with electric field reveals that a tetragonal ferroelectric phase is initially induced at E = 14 kV/cm and completed above E = 25 kV/cm for [001] oriented single crystals. When the electric field is applied along [111] direction, a partial phase transition from pseudocubic to rhombohedral structure is triggered at E = 19 kV/cm, which is higher than that for inducing tetragonal ferroelectric phase along [001] direction. Both field-induced phase structures and stability of NBT–6BT–2KNN single crystal are strongly associated with the crystallographic orientations. These results provide a better understanding to the field-induced macroscopic strain in lead-free NBT-based ferroelectrics.     

Excellent energy-storage properties of NaNbO3-based lead-free antiferroelectric orthorhombic P-phase (Pbma) ceramics with repeatable double polarization-field loops

The energy-storage performance of stable NaNbO₃-based antiferroelectric (AFE) ceramics was for the first time reported in (0.94-x)NaNbO₃-0.06BaZrO₃-xCaZrO₃ lead-free ceramics. A gradual evolution from an instable AFE phase (x≤0.01) to an orthorhombic AFE P phase (Pbma) (0.01<x≤0.05) was found to accompany the appearance of repeatable double-like polarization versus electric field loops although poled samples (x<0.01) own an AFE monoclinic phase (P2₁). Interestingly, compared with x≤0.01 samples with instable antiferroelectricity, a relatively high recoverable energy storage density W_rec ? 1.59 J/cm³ (@ 0.1 Hz) and a storage efficiency η of ?30% were achieved in the x = 0.04 ceramic. Moreover, a high W_rec of > 1.16 J/cm³ and an outstanding charge-discharge performance with fast discharge rate (t_0.9 < 100 ns) were generated in the temperature range from room temperature to 180 °C in the x = 0.04 ceramic. These results suggest that NaNbO₃-based AFE P-phase ceramics could be new potential dielectric materials for high-energy storage capacitors.     

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.     

Structure evolution and electric-field-induced reversible transition in perovskite Na(Nb1−xTax)O3 ceramics

Na(Nb_1−xTa_x)O₃ binary solid-solution ceramics with high quality were fabricated by conventional solid-state sintering routes for improving the electric(E)-field-induced irreversible polarization and transition behaviors of NaNbO₃. The studied results confirm that this binary solid-solution ceramics exhibit orthorhombic Pbcm space group companying with reduced unit-cell volume at x ≤ 0.4, and orthorhombic Pbnm space group at x = 0.5. As the Ta⁵⁺ content increases in the binary solid-solutions, the E-field-induced irreversible antiferroelectric → ferroelectric (AFE → FE) transition becomes reversible at x ≥ 0.2, giving rise to double-polarization hysteresis; the key E-fields triggering both irreversible and reversible transitions (E_F) increase in general. In particular, the E-field-induced FE phase at x = 0.15 is unstable upon unloading E-field to zero, which can return to AFE phase with time lapse. At x = 0.5, the Curie temperature (T_C) of AFE shifts to below room temperature, but E-field-induced reversible transition is still observed, which results in a nonlinear polarization with the lowest hysteresis and contributes to the largest energy-storage density. This transition is not due to the AFE ↔ FE transition but rather to the order ↔ disorder behavior of polar clusters or/and nanoregions within nonpolar Pbnm structure matrix.     

Evolving antiferroelectric stability and phase transition behavior in NaNbO3-BaZrO3-CaZrO3 lead-free ceramics

The stability of antiferroelectricity in NaNbO₃ ceramics was found to evolve with co-doping x mol% CaZrO₃ and 6 mol% BaZrO₃, from a dominant ferroelectric (FE) orthorhombic Q phase (x = 0) to a gradually stabilized antiferroelectric (AFE) orthorhombic P phase owing to different ionic radii of Ba and Ca ions. Although a complete AFE P phase appears at x = 0.5, the field induced AFE-FE phase transformation is irreversible at first, and then becomes partially reversible at x = 1 and finally completely reversible at x = 3. The above-mentioned change process proves to be associated with the enhancing stability of antiferroelectricity with x, as evidenced by means of dielectric, polarization and strain properties as well as in/ex-situ synchrotron x-ray diffraction and Raman spectra. A composition-field phase diagram for the NN-based lead-free AFE ceramic was constructed on basis of the phase structural change, which would provide a clear understanding of how ion doping influences its antiferroelectricity.     

Phase transition and morphology evolution of superfine NaNbO3 particles synthesized by the hydrothermal method with the assistance of K+

NaNbO₃ (NN) is a well-known perovskite-type dielectric material. However, its phase transition is a complex process and the phase transition mechanism is insufficiently investigated yet. Therefore, in the work, the NN superfine particles were synthesized by a hydrothermal method with the assistance of K⁺ and the microstructures of NN were carefully investigated to understand the strain-driven phase transition and morphology evolution of NN. The results suggest that K⁺ leads to strong planar bending of NbO₆ by influencing the Na ⁺ vibration, which accounts for the O-R phase transition as the K⁺ increases. Once the R phase is triggered, the O-R phase transition proceeds spontaneously based on phonon calculations of orthorhombic (O) NN and R NN. The antiferroelectric nature of rhombohedral (R) NN with the space group R–3H is confirmed through charge density distribution. Additionally, the morphology evolution is deduced on the TEM analysis basis.     

Significantly enhanced energy storage performance in Sm-doped 0.88NaNbO3-0.12Sr0·7Bi0·2TiO3 lead-free ceramics

Lead-free antiferroelectric (AFE) ceramic materials have attached increasing attention in application of high-power capacitors for the past few years, due to their high energy storage density and environmental protection. However, the related applications are seriously restricted because of the limited number of environment friendly AFE candidate materials, high cost and low energy storage efficiency. In this work, the A-site ion Sm³⁺ doped 0.88NaNbO₃-0.12Sr_0·7Bi_0·2TiO₃ lead-free AFE P phase ceramics (0.88Na_1-3xSm_xNbO₃-0.12Sr_0·7Bi_0·2TiO₃, abbreviated as NN-SBT-100xSm) were prepared and characterized. With the increase of Sm doping amount, a relaxor-like behavior was found in the dielectric-temperature curves of NN-SBT-100xSm, indicating the AFE orthorhombic P phase is gradually replaced by an AFE orthorhombic R phase. As a result, double-like and slim P-E curve with near-zero residual polarization and suppressed hysteresis loss was obtained at x > 0.01. More encouragingly, a good discharge energy storage density (W_rec = 3.58 J/cm³) and a high efficiency (η = 82%) at a low electric field (E = 200 kV/cm) has been recorded simultaneously for NN-SBT-2Sm relaxor AFE ceramic, which are better than the other lead-free energy storage ceramics under the same E. In addition, the energy storage properties of NN-SBT-2Sm ceramics exhibit outstanding temperature and frequency stability. These results indicate that NN-SBT-2Sm relaxor AFE ceramic has a great practical value in pulse power capacitors.     

Anisotropic field induced phase transitions and negative electrocaloric effect in rhombohedral Mn doped Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals

The electrocaloric effect (ECE) of Mn doped Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT:Mn) single crystals with particular emphasis on the impact of crystallographic orientations and phase transitions were investigated systematically. Orientation-dependent phase transitions have been demonstrated by the dielectric and strain behaviors. Intriguingly, the negative ECE of –0.02?°C and –0.002?°C were obtained firstly in [001]-oriented PIN-PMN-PT:Mn crystals near the rhombohedral→tetragonal phase transformation and in [011]-oriented crystals near the rhombohedral→orthorhombic phase transformation, respectively. However, only the positive ECE was found in [111]-oriented crystals near the tetragonal→rhombohedral phase transition. Additionally, the maximum ECE temperature changes calculated in [001]-, [011]- and [111]-oriented crystals were 0.33?°C, 0.46?°C and 0.38?°C, respectively. Our results suggest that the negative ECE is attributed to electric field-induced phase transitions, whose critical field decreases with the increase of temperature. The phase transition-mediated coexistence of positive and negative effects in the relaxor-ferroelectric single crystals is beneficial to enhance the efficiency of the solid-state cooling devices.     

High performance lead-free piezoelectric 0.96(K0.48Na0.52)NbO3-0.03[Bi0.5(Na0.7K0.2Li0.1)0.5]ZrO3-0.01(Bi0.5Na0.5)TiO3 single crystals grown by solid state crystal growth and their phase relations

0.96(K_0.48Na_0.52)NbO₃-0.03[Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5]ZrO₃-0.01(Bi_0.5Na_0.5)TiO₃ single crystals were grown for the first time by the solid state crystal growth method, using [001] or [110]-oriented KTaO₃ seed crystals. The grown single crystal shows a dielectric constant of 2720 and polarization-electric field loops of a lossy normal ferroelectric, with P_r = 45 μC/cm² and E_c = 14.9 kV/cm, while the polycrystalline samples with a dielectric constant of 828 were too leaky for P-E measurement due to humidity effects. The single crystal has orthorhombic symmetry at room temperature. Dielectric permittivity peaks at 26 °C and 311 °C, respectively, are attributed to rhombohedral-orthorhombic and tetragonal–cubic phase transitions. Additionally, Raman scattering shows the presence of an orthorhombic-tetragonal phase transition at ∼150 °C, which is not indicated in the permittivity curves but by the loss tangent anomalies. A transition around 700 °C in the high temperature dc conductivity is suggested to be a ferroelastic-paraelastic transition.     

High-performance antiferroelectric ceramics via multistage phase transition

Lead-based antiferroelectric (AFE) ceramics have attracted increasing interest in pulse power systems owing to their high-energy storage and power densities. However, the single AFE–ferroelectric (FE) phase transition in conventional AFE materials usually leads to premature polarization saturation and low breakdown strength, which are disadvantageous to energy storage performance. In this study, high energy storage performance was achieved in Pb_0.94−xLa_0.04Ca_x[Nb_0.02(Zr_0.99Ti_0.01)_0.975]O₃ (PLCNZT) AFE ceramics by constructing electric-field-induced multiple phase transitions. A maximum recoverable energy storage density of 12.15 J/cm³ and a high energy efficiency of 85.4% were obtained for the PLCNZT ceramic with x = 0.03 at 420 kV/cm. These excellent properties are attributed to the AFE–FE Ⅰ-FE Ⅱ multiple phase transitions induced by Ca²⁺ doping, which effectively enhances the breakdown strength. This result indicates that field-induced multiple phase transitions significantly improve the energy storage of AFE materials.     

Structure and energy storage performance of Ba-modified AgNbO3 lead-free antiferroelectric ceramics

The AgNbO₃ antiferroelectric (AFE) ceramics have attracted increasing attention for their high energy storage performance and environmentally friendly characters. In this work, Ag_1–2xBa_xNbO₃ ceramics were successfully prepared by the conventional solid-state reaction method. The effect of Ba-modification on phase structure, microstructure, and electric properties was systematically investigated. The introduction of Ba²⁺ ion led to complex cell parameter evolution and significant refinement of grain size. Room temperature dielectric permittivity increased obviously from ~260 for the pure AgNbO₃ counterpart to ~350 for those after adding a small amount of Ba element. Slim P-E hysteresis loops with improved AFE phase were achieved after Ba modification, due to the decrease of tolerance factor. A high recoverable energy density up to 2.3?J/cm³ with energy efficiency of 46% can be obtained for the composition of Ag_0.96Ba_0.02NbO₃, in correlation with the enhanced AFE stability, reduced P_r, increased P_m and decreased ΔE. Moreover, the Ag_0.96Ba_0.02NbO₃ ceramics also exhibited excellent temperature stability in both energy density and efficiency with small variation of <?5% over 20–120?℃. The results suggest that the electric properties of AgNbO₃ system can be largely tuned after Ba modification, making it a promising candidate for energy storage applications.