Structure and Microwave Dielectric Behavior of A‐Site‐Doped Sr(1−1.5x)CexTiO3 Ceramics System

The ion valence state, phase composition, microstructure, and microwave dielectric properties of Sr_(1?1.5x)Ce_xTiO₃ (x = 0.1–0.67, SCT) ceramics were systematically investigated. Sr_(1?1.5x)Ce_xTiO₃ ceramics were produced with gradual structural evolution from a cubic to a tetragonal and turned to an orthorhombic structure in the range of 0.1 ≤ x ≤ 0.67. Above a critical Ce proportion (x = 0.4), microstructural changes and normal grain growth initially occurred. On the basis of chemical analysis results, the reduction of Ti⁴⁺ ions was hastened by tetravalent ions (Ce⁴⁺). By contrast, this reduction was inhibited by trivalent ions (Ce³⁺). The observed dielectric behavior was strongly influenced by phase composition, oxygen vacancies (), and defect dipoles, namely, () and (). Temperature stable ceramics sintered at 1350°C for 3 h in air yielded an intermediate value of dielectric constant (ε_r = 40), with the smallest reported value of temperature coefficient of resonant frequency (τ_f = +0.9 ppm/°C), and quality factor (Q × f = 5699 GHz) at x = 0.6.     

Ferroelectric,piezoelectric and electrostrictive properties of Sn4+‐modified Ba0.7Ca0.3TiO3 lead‐free electroceramics

Lead‐free Ba_0.7Ca_0.3Ti_1?xSn_xO₃ (x=0.00, 0.025, 0.050, 0.075, and 0.1, abbreviated as BCST) electroceramic system was prepared by the solid‐state reaction method and its ferroelectric, piezoelectric, and electrostrictive properties were investigated. X‐ray diffraction shows that the compositions with x≤0.05 exhibit a tetragonal crystal structure having P4mm symmetry; while the compositions x=0.075 and 0.1 exhibit a mixed P4mm+Amm2 phase coexistence of tetragonal and orthorhombic and P4mm+Pmm pseudo‐cubic lattice symmetries, respectively, at room temperature. The dense microstructure having relative density ~90%‐92% and average grain size in the range ~2.36 μm to 8.56 μm was observed for BCST ceramics. Temperature‐dependent dielectric measurements support the presence of phase coexistence and show the decrease in Curie temperature (T_C) with Sn⁴⁺ substitution. The dielectric loss (tan δ) values in the temperature range (?100°C to 150°C) was observed to be <4%, for all BCST ceramics. The BCST compositions exhibit typical polarization‐electric field (P‐E) hysteresis and electric field induced strain (S‐E) butterfly loop, which confirms the ferroelectric and piezoelectric character. The compositions x=0.025, 0.05 and 0.075 show the peaking behavior of displacement current density () to an applied electric field () (J‐E) which implies the saturation state of polarization. The maximum electrostrictive coefficient (Q₃₃) value of 0.0667 m⁴/C² was observed for x=0.075 and it is higher than some of the significant lead‐based electrostrictive materials. The compositions x=0.05 and 0.075 exhibit the notable electrostrictive properties that may be useful for piezoelectric Ac device applications. The observed results are discussed and correlated with the structure‐property‐composition.     

Synthesis and Electrical Properties of New Lead‐free (100−x)(Li0.12Na0.88)NbO3–xBaTiO3 (0 ≤ x ≤ 40) Piezoelectric Ceramics

New lead‐free (100?x)Li_0.12Na_0.88NbO₃‐xBaTiO₃ (0 ≤ x ≤ 40) piezoelectric ceramics have been synthesized using conventional ceramics processing route. Structural analysis revealed an existence of morphotropic phase boundary (MPB), separating orthorhombic and tetragonal phases, between the BaTiO₃ content, x = 10–12.5. A partial phase diagram has been established based on temperature‐dependent permittivity data for this new system and a almost vertical temperature‐independent MPB is observed. Improvement in electrical properties near MPB (e.g., for x = 12.5; ε_r = 8842 at T_m and 795 at room temperature, d₃₃ = 30 pC/N, k_p = 12.0%, Q_m = 162, P_r = 11.2 μC/cm², E_c = 19.2 kV/cm, = 174 pm/V) is observed, and is attributed to the ease of polarization rotation due to coexistence of orthorhombic and tetragonal phases. The results show that these materials could be suitable for piezoelectric vibrators and ultrasonic transducer applications. The sample with x = 25, also exhibited high dielectric permittivity, ε_r = 2400, and low dielectric loss, tanδ = 0.033 at room temperature which could be suitable for capacitor (X7R/Z5U) applications.     

Orthorhombic‐pseudocubic phase transition and piezoelectric properties of (Na0.5K0.5)(Nb1−xSbx)‐SrZrO3 ceramics

0.96(Na_0.5K_0.5)(Nb_1?xSb_x)‐0.04SrZrO₃ ceramics with 0.0≤x≤0.06 were well sintered at 1060°C for 6 hours without a secondary phase. Orthorhombic‐tetragonal transition temperature (T_O‐T) and Curie temperature (T_C) decreased with the addition of Sb₂O₅. The decrease in T_C was considerable compared to that in T_O‐T, and thus the tetragonal phase zone disappeared when x exceeded 0.03. Therefore, a broad peak for orthorhombic‐pseudocubic transition as opposed to that for orthorhombic‐tetragonal transition appeared at 115°C‐78.2°C for specimens with 0.04≤x≤0.06. An orthorhombic structure was observed for specimens with x≤0.03. However, the polymorphic phase boundary structure containing orthorhombic and pseudocubic structures was formed for the specimens 0.04≤x≤0.06. Furthermore, a specimen with x=0.055 exhibited a large piezoelectric strain constant of 325 pC/N, indicating that the coexistence of orthorhombic and pseudocubic structures could improve the piezoelectric properties of (Na_0.5K_0.5)NbO₃‐based lead‐free piezoelectric ceramics.     

Defect‐driven evolution of piezoelectric and ferroelectric properties in CuSb2O6‐doped K0.5Na0.5NbO3 lead‐free ceramics

Defect greatly affects the microscopic structure and electrical properties of perovskite piezoelectric ceramics, but the microscopic mechanism of defect‐driven macroscopic properties in the materials is not still completely comprehended. In this work, K_0.5Na_0.5NbO₃+x mol CuSb₂O₆ lead‐free piezoelectric ceramics were fabricated by a solid‐state reaction method and the defect‐driven evolution of piezoelectric and ferroelectric properties was studied. The addition of CuSb₂O₆ induces the formation of dimeric (DC1) and trimeric (DC2) defect dipoles. At low doping concentration of CuSb₂O₆ (0.5‐1.0 mol%), DC1 and DC2 coexist in the ceramics and harden the ceramics, inducing a constricted double P‐E loop and high Q_m of 895 at x=0.01. However, DC2 becomes more dominant in the ceramics with high concentration of CuSb₂O₆ (≥1.5 mol%) and thus leads to softening behavior of piezoelectricity and ferroelectricity as compared to the ceramic with x=0.01, giving a single slanted P‐E loop and relatively low Q_m of 206 at x=0.025. All ceramics exhibit relatively high d₃₃ of 106‐126 pC/N. Our study shows that the piezoelectricity and ferroelectricity of K_0.5Na_0.5NbO₃ ceramics can be tailored by controlling defect structure of the materials.     

Reduced Dielectric Loss and Strain Hysteresis in Fe and Mn Comodified High‐Temperature BiScO3–PbTiO3 Ceramics

There is a growing requirement for high‐temperature piezoelectric materials in the petrochemical, automotive, and aerospace industries. Here, the piezoelectric materials of Fe and Mn comodified 0.36BiScO₃–0.64PbTiO₃ (BS‐PTFMn) ceramics with high Curie temperature (T_c), large mechanical quality factor (Q_m), and reduced strain hysteresis were presented. XRD results revealed that all the BS‐PTFMn ceramics have a pure perovskite structure with tetragonal symmetry, and the ratio of c/a is insensitive to the contents of Fe. With the modifications of Fe, the dielectric loss tanδ and strain hysteresis decrease clearly, while the mechanical quality factor improves significantly. The Curie temperature, piezoelectric constant, planar electromechanical coupling factor, dielectric loss, and mechanical quality factor of the BS‐PTFMn with 3% Fe content are 492°C, 235 pC/N, 0.38, 0.6%, and 280, respectively. BS‐PTFMn ceramics show 50°C higher T_c than BS‐PT morphotropic phase boundary composition. The figure of merit (product of Q_m, and k_ij) of BS‐PTFMn ceramics is about five times than that of pure BS‐PT ceramics. Furthermore, for the BS‐PTFMn ceramics with Fe content of 3 mol%, the high field strain coefficient value calculated from the electric‐field‐induced strain curves (S_max/E_max) is 320 pm/V, while the strain hysteresis (under 40 kV/cm) is reduced to one fifth that of unmodified BS‐PT ceramics. Moreover, the temperature‐dependent electromechanical coupling coefficient and dielectric constant are very stable in the temperature range from room temperature (RT) to 450°C. These results indicated that BS‐PTFMn ceramics are promising for high‐temperature piezoelectric applications.     

Enhanced O2 Flux of CaTi0.85Fe0.15O3−δ Based Membranes by Mn Doping

Dense symmetric membranes of CaTi_0.85?xFe_0.15Mn_xO_3?δ (x = 0.1, 0.15, 0.25, 0.4) are investigated in order to determine the optimal Mn dopant content with respect to highest O₂ flux. O₂ permeation measurements are performed as function of temperature between 700°C–1000°C and as function of the feed side ranging between 0.01 and 1 bar. X‐ray photoelectron spectroscopy is utilized to elucidate the charge state of Mn, and synchrotron radiation X‐ray powder diffraction (SR‐XPD) is employed to investigate the structure symmetry and cell volume of the perovskite phase at temperatures up to 800°C. The highest O₂ permeability is found for x = 0.25 over the whole temperature and ranges, followed by x = 0.4 above 850°C. The O₂ permeability for x = 0.25 reaches 0.01 mL(STP) min^?1 cm^?1 at 925°C with 0.21 bar feed side and Ar sweep gas. X‐ray photoelectron spectroscopy indicates that the charge state of Mn changes from approx. +3 to +4 when x > 0.1, which implies that Mn mainly improves electronic conductivity for x > 0.1. The cell volume is found to decrease linearly with Mn content, which coincides with an increase in the activation energy of O₂ permeability. These results are consistent with the interpretation of the temperature and dependency of O₂ permeation. The sintering behavior and thermal expansion properties are investigated by dilatometry, which show improved sinterability with increasing Mn content and that the thermal expansion coefficient decreases from 12.4 to 11.9 × 10^?6 K^?1 for x = 0 and x = 0.25, respectively.     

NaNbO3‐BaTiO3‐NaSbO3 lead and potassium‐free ceramics with thermally stable small‐signal piezoelectric properties

A new lead‐potassium‐free ceramic of (0.9‐x)NaNbO₃‐0.1BaTiO₃‐xNaSbO₃ (NN‐BT‐xNS) was successfully prepared via a solid‐state reaction method. The microstructure, phase structure, dielectric, ferroelectric, and piezoelectric properties were investigated as a function of NS content. The substitution of NS for NN was found to dramatically change the grain morphology from cube‐like grains typical for alkaline niobate‐based ceramics to conventional sphere‐like grains especially for Pb‐based perovskite ceramics. A normal to relaxor ferroelectric phase transformation was accompanied by a tetragonal (T) to rhombohedral (R) phase transition. A composition‐temperature phase diagram demonstrated a vertical morphotropic phase boundary between T and R phases in the composition range of x=0.03‐0.04, where optimum electrical properties of d₃₃=252 pC/N, k_p=36%, Q_m=168, =2063, and T_c=109°C were obtained in the x=0.035 ceramic sintered at 1260°C. Particularly, excellent temperature insensitivity of small‐signal piezoelectric properties suggested large application potentials in various actuators and sensors in comparison with other typical lead‐free materials.     

Microstructure and Magnetic Properties of Metastable RFeO3 (R: Rare‐earth element) Formed from Undercooled Melt

Containerless levitation technique, where the undercooling can be treated as one of the major thermodynamic parameters, was used to study the influence of oxygen partial pressure () on the microstructure and physical properties of rare‐earth orthoferrites RFeO₃ (where R = Rare‐earth element) in the ranges from 10⁵ to 10^?1 Pa. The microstructure of the as‐solidified samples changed into orthorhombic RFeO₃ (o‐RFeO₃), metastable hexagonal RFeO₃ (h‐RFeO₃), and Fe²⁺‐containing RFe₂O₄ and a new metastable R₃Fe₂O₇ phases with decreasing . The effect of on the magnetic properties was indicated as that the saturation magnetization gradually increased for R = La to Yb and decreased for R = Lu with decreasing due to the formation of metastable and magnetic phases such as Fe₃O₄ and Fe.     

Improving piezoelectric properties and temperature stability for KNN‐based ceramics sintered in a reducing atmosphere

Lead‐free 0.955K_0.5Na_0.5Nb_1‐zTa_zO₃‐0.045Bi_0.5Na_0.5ZrO₃+0.4%MnO ceramics (abbreviated as KNNTaz‐0.045BNZ+0.4Mn) were prepared by a conventional solid‐state sintering method in a reducing atmosphere (oxygen partial pressure of 1 × 10^?10 atm). All ceramics with a pure perovskite structure show the two‐phase coexistence zone composed of rhombohedral and tetragonal phase. Ta⁵⁺ ions substitute for Nb⁵⁺ ions on the B‐site, which results in a decrease in the R phase fraction in the two‐phase coexistence zone. The R‐T phase transition temperature moves to room temperature due to the substitution of Nb⁵⁺ ions by Ta⁵⁺ ions. A complex domain structure composed of small nano‐domains (~70 nm) formed inside large submicron domains (~200 nm) exists in KNNTa0.02‐0.045BNZ+0.4Mn ceramics, which can induce a strong dielectric‐diffused behavior and improve the piezoelectric properties. The temperature stability for the reverse piezoelectric constant for the KNNTaz‐0.045BNZ+0.4Mn ceramics can be improved at z = 0.02. Excellent piezoelectric properties (d₃₃ = 328 pC/N, and  = 475 pm/V at E_max = 20 kV/cm) were obtained for the KNNTa_0.02‐0.045BNZ+0.4Mn ceramics.     

Oxygen Nonstoichiometry and Thermodynamic Explanation of Large Oxygen‐Deficient Ruddlesden–Popper Oxides LaxSr3−xFe2O7−δ

The oxygen nonstoichiometry of large oxygen‐deficient Ruddlesden–Popper oxides La_xSr_3?xFe₂O_7?δ (LSFO7‐x) (x = 0, 0.25, 0.5) was measured by the high‐temperature gravimetry and the coulometric titration. In the composition series, the P(O₂) dependencies exhibited typical plateaus at δ = (2?[])/2. Meanwhile, La_0.5Sr_2.5Fe₂O_7?δ showed the smallest oxygen nonstoichiometry and was the most thermochemically stable compound against P(O₂), temperature, and the La content. Based on the defect equilibrium model and the statistical thermodynamic calculation derived oxygen nonstoichiometric data, the substitution of La for Sr‐site can promote the forward reaction of oxygen incorporation, the backward reaction of the disproportionation of the charge carriers, and oxygen redistribution between the O1 and O3 sites, resulting in the reduction of oxygen‐deficient and the lower decomposition P(O₂). The obtained thermodynamic quantities of the partial molar enthalpy of oxygen, , and the partial molar entropy of oxygen, , calculated from the statistical thermodynamic calculation are in good agreement with those using the Gibbs–Helmholtz equation.     

Decisive role of mixed‐valence structure in colossal dielectric constant of LaFeO3

The role of mixed‐valence structure in colossal dielectric constant (CDC) behavior has been investigated in LaFeO₃ ceramics by tuning the ratio of Fe²⁺/Fe³⁺ through substituting Al for Fe. The ratio of Fe²⁺/Fe³⁺ is decreased gradually from 1.0 to 0.0 by increasing the concentration of Al³⁺. Two clear‐cut correlations have been found: (i) the relationship between the CDC behavior and the ratio of Fe²⁺/Fe³⁺ follows an exponential function and (ii) the activation energy of the polaron relaxation is proportional to , where is the intrinsic dielectric constant. These findings underscore the role of the mixed‐valence structure in CDC behavior and suggest that adjusting the mixed‐valence structure through doping/alloying can be a promising strategy to achieve superior CDC behavior in transition‐metal oxides.     

Electrical Properties and Relaxor Phase Evolution of Li‐Modified BNT‐BKT‐BT Lead‐Free Ceramics

By conventional ceramics sintering technique, the lead‐free 0.85Bi_0.5Na_0.5(1?x)Li_0.5xTiO₃‐0.11Bi_0.5K_0.5TiO₃‐0.04BaTiO₃ (x =0–0.15) piezoelectric ceramics were obtained and the effects of Li dopant on the piezoelectric, dielectric, and ferroelectric properties were studied. With increasing Li addition, the temperature‐dependent permittivity exhibited the normal ferroelectric‐to‐ergodic relaxor (FE‐to‐ER) transition temperature (T_FE‐ER, abbreviated as T_F‐R) decreasing down to room temperature. The increasing Li content also enhanced the diffuseness of the FE‐to‐ER transition behavior. For composition with x = 0.15, a large unipolar strain of 0.37% ( = S_max/E_max = 570 pm/V) was achieved under 6.5 kV/mm applied electric field at room temperature. Both unipolar and bipolar strain curves related to the temperature closely, and when the temperature reached the T_F‐R, the normalized strain achieved a maximum value (e.g., for x = 0.10, = 755 pm/V) owing to the electric‐field‐induced ER‐to‐FE state transition.     

Temperature stability and electrical properties of MnO‐doped KNN‐based ceramics sintered in reducing atmosphere

Lead‐free MnO‐doped 0.955K_0.5Na_0.5NbO₃‐0.045Bi_0.5Na_0.5ZrO₃ (abbreviate as KNN‐0.045BNZ) ceramics have been prepared by a conventional solid‐state sintering method in reducing atmosphere. The MnO addition can suppress the emergence of the liquid phase and improve the homogenization of grain size. All ceramics sintered in reducing atmosphere show a two‐phase coexistence zone composed of rhombohedral (R) and tetragonal (T) phase. MnO dopant results in the content increase in R phase and slight increase in Curie temperature T_C. For KNN‐0.045BNZ ceramics, Mn²⁺ ions preferentially occupy the cation vacancies in A‐site to decrease oxygen vacancy concentration for 0.2%‐0.4% MnO content, whereas Mn²⁺ ions substitute for Zr⁴⁺ ions in B‐site to form oxygen vacancies at x ≥ 0.5. The defect dipole is formed at the moderate concentration from 0.5 to 0.6, which can provide a preserve force to improve the temperature stability of piezoelectric properties for k_p and . The Mn0.4 ceramics show excellent electrical properties with quasistatic piezoelectric constant d₃₃ = 300 pC/N, electromechanical coupling coefficient k_p = 51.2%, high field piezoelectric constant  = 430 pm/V (at E_max = 25 kV/cm) and T_C = ~345°C, insulation resistivity ρ  =  6.13 × 10¹¹ Ωcm.     

Structural evolution of the R‐T phase boundary in KNN‐based ceramics

Although a rhombohedral‐tetragonal (R‐T) phase boundary is known to substantially enhance the piezoelectric properties of potassium‐sodium niobate ceramics, the structural evolution of the R‐T phase boundary itself is still unclear. In this work, the structural evolution of R‐T phase boundary from ?150°C to 200°C is investigated in (0.99?x)K_0.5Na_0.5Nb_1?ySb_yO₃–0.01CaSnO₃–xBi_0.5K_0.5HfO₃ (where x = 0‐0.05 with y = 0.035, and y = 0‐0.07 with x = 0.03) ceramics. Through temperature‐dependent powder X‐ray diffraction (XRD) patterns and Raman spectra, the structural evolution was determined to be Rhombohedral (R, <?125°C)→Rhombohedral + Orthorhombic (R + O, ?125°C to 0°C)→Rhombohedral + Tetragonal (R + T, 0 °C to 150°C)→dominating Tetragonal (T, 200°C to Curie temperature (T_C)) → Cubic (C, >T_C). In addition, the enhanced electrical properties (e.g., a direct piezoelectric coefficient (d₃₃) of ~450 ± 5 pC/N, a conversion piezoelectric coefficient () of ~580 ± 5 pm/V, an electromechanical coupling factor (k_p) of ~0.50 ± 0.02, and T_C~250°C), fatigue‐free behavior, and good thermal stability were exhibited by the ceramics possessing the R‐T phase boundary. This work improves understanding of the physical mechanism behind the R‐T phase boundary in KNN‐based ceramics and is an important step toward their adoption in practical applications.     

Structure Evolution and Electrical Properties of Y3+‐Doped Ba1−xCaxZr0.07Ti0.93O3 Ceramics

Lead free piezoelectric ceramics of Y³⁺‐doped Ba_1?xCa_xZr_0.07Ti_0.93O₃ with x = 0.05, 0.10, and 0.15 were prepared. Composition and temperature‐dependent structural phase evolution and electrical properties of as‐prepared ceramics were studied systematically by X‐ray diffraction, Raman spectroscopy, impedance analyzer, ferroelectric test system, and unipolar strain measurement. Composition with x = 0.10 performs a good piezoelectric constant d₃₃ of 363 pC/N, coercive field E_c of 257 V/mm, remanent polarization P_r of 14.5 μC/cm², and a Curie temperature T_m of 109°C. High‐resolution X‐ray diffraction was introduced to indicate presence of orthorhombic phase. Converse piezoelectric constant d₃₃* of x = 0.10 composition performed better temperature stability in the range from 50°C to 110°C. That means decreasing orthorhombic–tetragonal phase transition temperature could be an effective way to enlarge its operating temperature range.     

Lead‐Free (Ba0.70Sr0.30)TiO3‐Modified Bi0.5(Na0.80K0.20)0.5TiO3 Ceramics with Large Electric Field–Induced Strains

The dielectric, ferroelectric, and electric field–induced strain behavior of Bi_0.5(Na_0.80K_0.20)_0.5TiO₃ (BNKT) ceramics modified with (Ba_0.70Sr_0.30)O₃ (BST) were investigated as a function of composition and temperature. The ceramic samples were synthesized by a solid‐state mixed oxide method and sintered at 1125°C for 2 h. The XRD and Raman spectra showed coexisting rhombohedral and tetragonal phases throughout the entire compositional range with the tetragonal phase becoming dominant at higher BST concentrations. For all compositions, the temperature dependence of the dielectric spectra revealed a frequency dependence that is characteristic of a relaxor mechanism. This suggests that these ceramics lacked long‐range order and it appears that the maximum disorder was observed for the composition with 5 mol% BST (BNKT–0.05BST sample). This was evidenced by the observation of pinched hysteresis loops, even at room temperature, and a significant decrease in the P_r and E_c values which resulted in large electric field–induced strains (S_max) of 0.40% and a normalized strain coefficient ( = S_max/E_max) of 732 pm/V. This significant strain enhancement at the composition of x = 0.05 may be attributed to both a composition‐induced structural phase transition and a field‐induced relaxor to ferroelectric phase transition.     

Structural,Raman, and Dielectric Studies on Multiferroic Mn‐doped Bi1−xLaxFeO3 Ceramics

Multiferroic Bi_1?xLa_xFeO₃ [BLFO (x)] ceramics with x = 0.10–0.50 and Mn‐doped BLFO (x = 0.30) ceramics with different doping contents (0.1–1.0 mol%) were prepared by solid‐state reaction method. They were crystallized in a perovskite phase with rhombohedral symmetry. In the BLFO (x) system, a composition (x)‐driven structural transformation (R3c→C222) was observed at x = 0.30. The formation of Bi₂Fe₄O₉ impure phase was effectively suppressed with increasing the x value, and the rhombohedral distortion in the BLFO ceramics was decreased, leading to some Raman active modes disappeared. A significant red frequency shift (~13 cm^?1) of the Raman mode of 232 cm^?1 in the BLFO ceramics was observed, which strongly perceived a significant destabilization in the octahedral oxygen chains, and in turn affected the local FeO₆ octahedral environment. In the Mn‐doped BLFO (x = 0.30) ceramics, the intensity of the Raman mode near 628 cm^?1 was increased with increasing the Mn‐doping content, which was resulted from an enhanced local Jahn–Teller distortions of the (Mn,Fe)O₆ octahedra. Electron microscopy images revealed some changes in the ceramic grain sizes and their morphologies in the Mn‐doped samples at different contents. Wedge‐shaped 71° ferroelectric domains with domain walls lying on the {110} planes were observed in the BLFO (x = 0.30) ceramics, whereas in the 1.0 mol% Mn‐doped BLFO (x = 0.30) samples, 71° ferroelectric domains exhibited a parallel band‐shaped morphology with average domain width of 95 nm. Dielectric studies revealed that high dielectric loss of the BLFO (x = 0.30) ceramics was drastically reduced from 0.8 to 0.01 (measured @ 10⁴ Hz) via 1.0 mol% Mn‐doping. The underlying mechanisms can be understood by a charge disproportion between the Mn⁴⁺ and Fe²⁺ in the Mn‐doped samples, where a reaction of Mn⁴⁺ + Fe²⁺→Mn³⁺ + Fe³⁺ is taken place, resulting in the reduction in the oxygen vacancies and a suppression of the electron hopping from Fe³⁺ to Fe²⁺ ions effectively.     

Composition‐induced structural transitions and enhanced strain response in nonstoichiometric NBT‐based ceramics

In this work, the nonstoichiometric 0.99Bi_0.505(Na_0.8K_0.2)_0.5‐xTiO₃‐0.01SrTiO₃ (BNKST(0.5‐x)) ceramics with x=0‐0.03 were synthesized by conventional solid‐state reaction method. The composition‐induced structural transitions were investigated by Raman spectra, dielectric analyses, and electrical measurements. It is found that the relaxor phase can be induced through the modulation of the (Na, K) content. The (Na, K) deficiency in BNKST(0.5‐x) ceramics favors a more disordered local structure and can result in the loss of long‐range ferroelectricity. The x=0.015 critical composition possesses relatively high positive strain S_pos of 0.42% and large signal piezoelectric constant d₃₃* of 479 pm V^?1 at 6 kV mm^?1, along with the good temperature (25‐120°C) and frequency (1‐20 Hz) stability. The recoverable large strain responses in nonstoichiometric ceramics can be attributed to the reversible relaxor‐ferroelectric phase transition, which is closely related to the complex defects (, , and ) and the local random fields. This work may be helpful for the exploration of high‐performance NBT‐based lead‐free materials by means of A‐site compositional modification.     

Crystal structure,defect relaxation,and microwave dielectric properties of Ba[(Mg1/3Nb2/3)1−xHfx]O3 solid solutions

Ba[(Mg_1/3Nb_2/3)_1?xHf_x]O₃ (BMNH, x = 0.05, 0.1, 0.15, 0.2) solid solutions were prepared via the solid‐state reaction method. The effect of BaHfO₃ on the crystal structure, microwave dielectric performance, and defect relaxation behavior of Ba(Mg_1/3Nb_2/3)O₃ (BMN) were studied. BaHfO₃ additions degraded the sintering activity of BMN powder, requiring a high sintering temperature (T_s) ~ 1650°C; but it could be effectively improved by a prolonged sintering process at a lower T_s of 1600°C. The well‐sintered BMNH ceramics (1600°C for 30 h) possessed a high densification >96%, and exhibited cubic perovskite structures without 1:2 cation ordering. Once doped with Hf, the low‐temperature relaxation in dielectric spectroscopy and thermally stimulated depolarization current (TSDC) for pure BMN disappeared, further indicating such relaxation is related to cation‐ordered structure. Oxygen vacancies, namely showing in‐grain and across‐grain‐boundary relaxation of ‐related defects, were the main defect types in BMNH. The concentrations of in‐grain decreased as x increased, which is beneficial to BMNH to maintain high Q × f values of 69 400‐73 000 GHz. Accompanied by a high ε_r of 33.27‐33.59 and a low τ_f of +13.6 to +20.7 ppm/°C, these materials have a good potential for applications in microwave components and devices.