Ferroelectric and magnetic properties in (1−x)BiFeO3–x(0.5CaTiO3–0.5SmFeO3) ceramics

Multiferroic ceramics were prepared and characterized in (1?x)BiFeO₃–x(0.5CaTiO₃–0.5SmFeO₃) system by a standard solid‐state reaction process. The structure evolution was investigated by X‐ray diffraction and Raman spectrum analyses. The refinement results confirmed the different phase assemblages with varying amounts of polar rhombohedral R3c and nonpolar orthorhombic Pbnm as a function of the substitution content. In the compositions range of 0.2≤x≤0.5, polar R3c and nonpolar Pbnm coexisted, which was referred to polar‐to‐nonpolar morphotropic phase boundary (MPB). According to the dielectric and DSC analysis results, the ceramics with x≤0.2 changed to diffused ferroelectric, and the ferroelectric properties were enhanced significantly. Two dielectric relaxations were detected in the temperature range of 200‐300 K and 500‐700 K, respectively. The high‐temperature dielectric relaxation was attributed to the grain‐boundary effects. While the low temperature dielectric relaxation obtained in the samples with x=0.3‐0.5 was related to the charge transfer between Fe²⁺ and Fe³⁺. The magnetic hysteresis loops measured at different temperature indicated the enhanced magnetic properties in the present ceramics, which could be attributed to the suppressed cycloidal spin magnetic structure by Ti ions. In addition, the rare‐earth Sm spin moments might also affect the magnetic properties at relatively lower temperature.     

Photoluminescence Properties of Er/Pr‐Doped K0.5Na0.5NbO3 Ferroelectric Ceramics

Er/Pr‐doped K_0.5Na_0.5NbO₃ ceramics have been fabricated and the effects of Pr³⁺ on their photoluminescence properties have been investigated systematically. The visible upconversion emissions, near‐infrared and mid‐infrared downconversion emissions of Er³⁺ ions under the excitation of 980 nm have been studied in detail. The effects of Pr³⁺ on PL properties and energy‐transfer processes have also been elucidated. By selecting an appropriate excitation source, simultaneous visible downconversion emissions of Er³⁺ and Pr³⁺ ions can be realized, and the emission colors of the ceramics can be tuned via the concentration of Pr³⁺ ions in a wide range from yellowish green to yellow. Our results also reveal that the photoluminescence emissions of the ceramics can be enhanced by the alignment of polarization of the ferroelectric host.     

Microstructure of Eu3+‐Doped Perovskites‐Type Niobate Ceramic La3Mg2NbO9

Eu³⁺‐doped red‐emitting ceramics of Eu³⁺‐doped La₃Mg₂NbO₉ were prepared via typical solid state. X‐ray diffraction and scanning electron microscope were utilized to characterize the ceramics. The photoluminescence excitation and emission spectra, the fluorescence decay curves, and color coordinates were investigated. The concentration quenching of the samples were discussed as well. The microstructures of the ceramics were discussed according to the spectral properties of probe ions of Eu³⁺, for example, substitution sites for Eu³⁺, inhomogeneous broadening and splitting of the emission bands, nonexponential decay, ⁵D₀→⁷F₀ emission transition, distorted symmetry sites, etc. The crystal structure of La₃Mg₂NbO₉ is heavily distorted due to the mixed occupation of Mg and Nb on B sites. Eu³⁺ ions only substitute La³⁺ sites and Eu³⁺ ions (or rare‐earth ions) are arranged in the heavily disordered environments over the whole structure in La₃Mg₂NbO₉.     

Effect of Yb Codoping on the Phase Transition,and Electrical and Photoluminescence Properties in KNLN:Er/xYb Ceramics

Ceramics 0.94(K_0.5Na_0.5)NbO₃?0.06LiNbO₃:Er/xYb with x = 0, 0.002, 0.004, 0.006, 0.008 were fabricated in this study, and phase structure, dielectric, piezoelectric, ferroelectric, and upconversion photoluminescence properties of the ceramics were systematically studied. Results show that all ceramics are in the polymorphic phase transition region near room temperature. However, a relaxor‐like phase transition was observed as Yb increasing to over x = 0.004. The optimized dielectric, piezoelectric, and ferroelectric properties, and maximized photoluminescence intensity ratio are obtained at x = 0.004. And the electrical and photoluminescence properties of the ceramics were discussed from the point view of the normal to relaxor‐like ferroelectric phase transition. The present study demonstrates that both the electrical and upconversion photoluminescence properties of the ceramics have an intimate correlation with the normal to relaxor‐like ferroelectric transition induced by Yb doping.     

Lattice Dynamics,Dielectric Constants,and Phase Diagram of Bismuth Layered Ferroelectric Bi3Ti1−xWxNbO9+δ Ceramics

Lattice dynamics and phase transitions of Aurivillius ferroelectric Bi₃Ti_1?xW_xNbO_9+δ (BTWN100x, 0 ≤ x ≤ 15%) ceramics have been investigated by temperature‐dependent Raman spectra (80–800 K) and far‐infrared (FIR) reflectance spectra (6–300 K). The frequency, intensity, and line width of phonon modes as well as complex dielectric functions of BTWN100x ceramics have been extracted by fitting Raman and FIR reflectance spectra with the multi‐Lorentzian oscillator model. It was found that the dielectric constants at a certain frequency region become lower with increasing W compositions at room temperature due to the compressive stress on W–O bands. Moreover, the temperature‐dependent behavior of optical phonon modes in BTWN100x ceramics indicates that there are two intermediate phases during the phase transition from paraelectric I4/mmm to ferroelectric A2₁am phase on cooling. The phase diagram of BTWN100x ceramics as a function of W composition has been improved.     

Excellent Transmittance Induced Phase Transition and Grain Size Modulation in Lead‐Free (1–x)(K0.5Na0.5)NbO3–xLaBiO3 Ceramics

A novel lead‐free excellent transmittance electro‐optic ceramics (1–x)(K_0.5Na_0.5)NbO₃–xLaBiO₃ (KNN‐LB, x = 0.000, 0.005, 0.010, 0.015, 0.020, 0.025, 0.040, 0.060) were fabricated by traditional pressureless ceramics processing procedure. The effects of LaBiO₃ dopant concentration x on the microstructure, phase transition, optical property, and electrical properties were studied systematically. The X‐ray diffraction results indicated that the KNN‐LB ceramics with x ≥ 0.025 have the pseudocubic phase. The morphology, density, and microstructure of the KNN‐LB ceramics were characterized by scanning electronic microscopy and optical microscopy. In particular, the KNN‐LB ceramics (0.05 mm thickness) with x = 0.025 exhibited the highest transmittance of 74.00% in the visible spectrum comparable to the 72.00% transmittance of the lead lanthanum zirconate titanate (PLZT 9/65/35 of 0.127 mm thickness). In addition, the related mechanism of transparency variation induced by phase transition and grain size modulation were discussed thoroughly. Finally, the dielectric and ferroelectric properties of as‐prepared KNN‐LB ceramics were also investigated to further clarify the relationship between transparency and relaxor behavior.     

Giant electromechanical strain response in lead‐free SrTiO3‐doped (Bi0.5Na0.5TiO3–BaTiO3)–LiNbO3 piezoelectric ceramics

Lead‐free 0.985[(0.94?x)Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–xSrTiO₃]–0.015LiNbO₃ [(BNT–BT–xST)–LN, x=0‐0.05] piezoelectric ceramics were prepared using a conventional solid‐state reaction method. It was found that the long‐range ferroelectric order in the unmodified (BNT–BT)–LN ceramic was disrupted and transformed into the ergodic relaxor phase with the ST substitution, which was well demonstrated by the dramatic decrease in remnant polarization (P_r), coercive field (E_c), negative strain (S_neg) and piezoelectric coefficient (d₃₃). However, the degradation of the ferroelectric and piezoelectric properties was accompanied by a significant increase in the usable strain response. The critical composition (BNT–BT–0.03ST)–LN exhibited a maximum unipolar strain of ~0.44% and corresponding normalized strain, S_max/E_max of ~880 pm/V under a moderate field of 50 kV/cm at room temperature. This giant strain was associated with the coexistence of the ferroelectric and ergodic relaxor phases, which should be mainly attributed to the reversible electric‐field‐induced transition between the ergodic relaxor and ferroelectric phases. Furthermore, the large field‐induced strain showed relatively good temperature stability; the S_max/E_max was as high as ~490 pm/V even at 120°C. These findings indicated that the (BNT–BT–xST)–LN system would be a suitable environmental‐friendly candidate for actuator applications.     

Lead‐free (K,Na)NbO3‐based ceramics with high optical transparency and large energy storage ability

Highly transparent lead‐free (1‐x)K_0.5Na_0.5NbO₃–xSr(Zn_1/3Nb_2/3)O₃ (KNN–xSZN) ferroelectric ceramics have been synthesized via a conventional pressureless sintering method. All samples are optically clear, showing high transmittance in the visible and near‐infrared regions (~70% and ~80% at 0.5 mm of thickness, respectively). This exceptionally good transmittance is due to the pseudo‐cubic phase structure as well as the dense and fine‐grained microstructure. In addition, a high energy storage density of 3.0 J/cm³ has been achieved for the 0.94K_0.5Na_0.5NbO₃–0.06Sr(Zn_1/3Nb_2/3)O₃ ceramics with submicron‐sized grains (~136 nm). The main reason is likely to be the typical relaxor‐like behavior characterized by diffuse phase transition, in addition to the dense and fine‐grained microstructure. This study demonstrates that the 0.94K_0.5Na_0.5NbO₃–0.06Sr(Zn_1/3Nb_2/3)O₃ ceramic is a promising candidate of lead‐free transparent ferroelectric ceramics for new areas beyond transparent electronic device applications.     

Quantitative phase analyses of Eu3+ -doped (Na1-x,Kx)0.5Bi0.5TiO3ferroelectric ceramics near morphotropic phase boundary by photoluminescence spectra

It is widely reported that rare-earth ions could be used to probe the phase structure by photoluminescence (PL) spectra after doping into ferroelectrics, while most reported results are qualitative. In this work, we show that PL spectra of Eu³⁺ ions could be used to do quantitative phase analyses of (Na_1-x,K_x)_0.5Bi_0.497Eu_0.003TiO₃ (NKBT: Eu) ferroelectric ceramics with compositions near morphotropic phase boundary(MPB). The mechanism was elaborated based on the emission intensity ratios of the “hypersensitive” transition to the magnetic dipole transition with the Judd-Ofelt theory. The results were further confirmed by X-Ray Diffraction Rietveld refinements. This work shows a simple yet practical method for quantitative phase analyses for NKBT: Eu ceramics near MPB, and the method is expected to be applied in more ferroelectric material systems.     

Dielectric Properties and Impedance Analysis of K0.5Na0.5NbO3–Ba2NaNb5O15 Ceramics with Good Dielectric Temperature Stability

(1?x)(K_0.5Na_0.5)NbO₃–xBa₂NaNb₅O₁₅ [(1?x)KNN–xBNN, 0 ≤ x ≤0.1] ceramics were prepared by solid‐state reaction method. X‐ray diffraction analysis of the ceramics revealed that the crystal structure changed from orthorhombic to rhombohedral with increasing BNN content. Dielectric measurement showed that the ceramics exhibited good dielectric temperature stability over a wide temperature range. Basic mechanisms of the conduction and relaxation processes have been investigated using impedance spectroscopy analyses. It was concluded that the conduction and relaxation processes were thermally activated and oxygen vacancies were the possible ionic charge carriers at higher temperatures.     

Bi(Zn2/3Nb1/3)O3–(K0.5Na0.5)NbO3 High‐Temperature Lead‐FreeFerroelectric Ceramics with Low Capacitance Variation in a Broad Temperature Usage Range

The xBi(Zn_2/3Nb_1/3)O₃–(1?x)(K_0.5Na_0.5)NbO₃ (abbreviated as xBZN–(1?x)KNN) ceramics have been synthesized using the conventional solid‐state sintering method. The phase structure, dielectric properties and “relaxorlike” behavior of the ceramics were investigated. The 0.03BZN–0.97KNN ceramics show a broad and stable permittivity maximum near 2000 and lower dielectric loss (≤5%) at a broad temperature usage range (100°C–400°C) and the capacitance variation (ΔC/C_150°C) is maintained smaller than ±15%. The 0.03BZN–0.97KNN ceramics only possess the diffuse phase transition and no frequency dispersion of dielectric permittivity, which indicates that 0.03BZN–0.97KNN ceramics is a high temperature “relaxorlike” ferroelectric ceramics. These results indicate that 0.03BZN–0.97KNN ceramics are excellent promising candidates for preparing high‐temperature multilayer ceramics capacitors.     

Reversible luminescence modulation and temperature-sensing properties of Pr3+/Yb3+ codoped K0.5Na0.5NbO3 ceramics

In this work, we have prepared a novel (K_0.5Na_0.5)_0.99-xPr_xYb_0.01NbO₃ (abbreviated as KNN:xPr³⁺/0.01Yb³⁺, x = 0.0006, 0.0008, 0.001, 0.002, 0.003, and 0.004) ceramics, which possess visible UC emissions, photochromic (PC) and optical thermometric properties. Under the excitation of a 980-nm diode laser, all the samples show the featured emissions of Pr³⁺ ions and the UC emission intensity is greatly dependent on the Pr³⁺ doping content. The optimal UC luminescence intensity is obtained at x = 0.001. All the prepared samples show a strong PC reaction, and a large luminescence quenching degree (ΔR_t) of 74.94% is found. The optical thermometric properties of both the irradiated and unirradiated KNN:0.001Pr³⁺/0.01Yb³⁺ ceramics in the temperature range of 123-573 K have been investigated via measuring the temperature-dependent UC emission spectra of green emissions, which originate from the two ³P₁ and ³P₀ thermally coupled levels. It has been found that the prepared samples have both excellent PC behaviors and temperature-sensing performances. These results suggest that the KNN:xPr³⁺/0.01Yb³⁺ ceramics are promising candidates for the applications in PC reaction and thermometers.     

Ho3+-doped (K,Na)NbO3-based multifunctional transparent ceramics with superior optical temperature sensing performance

Ho³⁺-doped (K_0.5Na_0.5)NbO₃-based transparent ceramics have been prepared via pressureless solid-state method. The ceramics possess moderate optical transparency with the energy band gap of ~2.9 eV and submicron-sized grains (<500 nm). The temperature-dependent dielectric properties and ferroelectric polarization-electric field hysteresis loops demonstrate that the ceramics own relaxor-like characteristics. The up-conversion photoluminescence and optical temperature sensing properties of the ceramics have been investigated. The temperature dependence of photoluminescence provides a fluorescent method to detect phase transitions, which can be expanded to other ferroelectric systems. The outstanding optical temperature sensitivity (~0.0075/K at 430 K) of the ceramic is higher than many other rare-earth-doped ceramics or glasses. These results suggest that the Ho³⁺-doped (K_0.5Na_0.5)NbO₃-based transparent ceramics are promising lead-free transparent materials for multifunctional applications, especially in temperature sensing devices.     

Strong Red Emissions in Pr3+‐Doped (K0.5Na0.5)NbO3–CaTiO3 Diphase Ceramics

The photoluminescence and temperature sensing properties based on down‐shifting emission of Pr³⁺‐doped (K_0.5Na_0.5)NbO₃–yCaTiO₃ (KNN: yCT) diphasic materials were systematically investigated. Under 447‐nm excitation, Pr³⁺‐doped KNN: yCT samples exhibited significantly enhanced red emission at 603 nm assigned to ¹D₂→³H₄ transitions of Pr³⁺ ions. The red emission intensities reached the optimum value with y = 0.05 near the polymorphic phase transition region. The origin of the enhanced red emission is mainly ascribed to the doping‐induced lattice symmetry change. The energy level transitions from the typical f–f transitions to the valence‐to‐conduction transitions were observed as CaTiO₃ concentration increases above a critical concentration of y = 0.05. Furthermore, the sample with y = 0.05 also possessed excellent temperature response properties in a wide temperature range 300–473 K and the maximum sensing sensitivity was 0.016 K^?1. The Pr³⁺‐doped (K_0.5Na_0.5)NbO₃–yCaTiO₃ red emission materials with admirable intrinsic piezoelectric properties may have important technological promise in novel multifunctional devices.     

Large Strain Response in 0.99(Bi0.5Na0.4K0.1)TiO3–0.01(KxNa1−x)NbO3 Lead‐Free Ceramics Induced by the Change of K/Na Ratio in (KxNa1−x)NbO3

Influence of K/Na ratio in (K_xNa_1?x)NbO₃ on the ferroelectric stability and consequent changes in the electrical properties of 0.99(Bi_0.5Na_0.4K_0.1)TiO₃–0.01(K_xNa_1?x)NbO₃ (BNKT–K_xNN) ceramics were investigated. Results showed that change of K/Na ratio in KNN induces a phase transition from ferroelectric to ergodic relaxor phase with a significant disruption of the long‐range ferroelectric order, and correspondingly adjusts the ferroelectric–relaxor transition point T_F?R to room temperature. Accordingly, giant strain of ~0.46% (corresponding to a large signal d₃₃* of ~575 pm/V) which is comparable to that of Pb‐based antiferroelectrics is obtained at a K/Na ratio of ~1, and the emergence of large strain response induced by the change of K/Na ratio of KNN can be well explained by the correlation between the position of ferroelectric–ergodic relaxor phase boundary in the BNKT–K_xNN system and the tolerance factor t of the end number (K_xNN). In situ high‐energy X‐ray scattering experiments with external field reveals that the large strain response in the studied system is likely related to the electric field‐induced distortion from the pseudocubic structure.     

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.     

Room-temperature ferromagnetism in (K0.5Na0.5)NbO3-xBaNi0.5Nb0.5O3-δ ferroelectric ceramics with narrow bandgap

In this paper, a simple, reproducible and cost-effective solid-state reaction sintering process is developed to fabricate (K_0.5Na_0.5)NbO₃-xBaNi_0.5Nb_0.5O_3-δ (KNN-xBNN) ceramics with a narrow bandgap and room-temperature ferromagnetism. Here, we report a systematic investigation of the influence of the BaNi_0.5Nb_0.5O_3-δ (BNN) concentration on the properties of KNN-xBNN ceramics. All ceramics form orthorhombic perovskite structures with a space group Amm2 and a weak peak at the wavelength of 550 cm^?1 that is characteristic of the pillow shoulder of the orthorhombic phase. KNN-xBNN ceramics with x between 0.02 and 0.08 have a narrow bandgap of about 2.5 eV—much smaller than the 3.5 eV of its parent (K_0.5Na_0.5)NbO₃ (KNN) ceramic—which is attributed to Ni²⁺-oxygen vacancy combinations (Ni²⁺-V_O) raising the valence electron energy level of the KNN ceramic. Furthermore, doping BNN into KNN ceramics can significantly convert the magnetism from diamagnetism to ferromagnetism and the component of x = 0.08 achieves both maximum saturation magnetisation intensity (14 memu/g) and minimum coercive magnetic field (80 Oe). Our findings provide a systematic insight into the bandgap tunability and ferromagnetism induction at room temperature in lead-free perovskite KNN-xBNN ceramics, as well as demonstrate their potential applications in perovskite solar cells and multiferroic devices.     

Effects of BiMO3 on dielectric,ferroelectric, and piezoelectric properties of perovskite lead‐free piezoelectric BaTiO3–(Bi0.5Na0.5)TiO3 ceramics

New lead‐free piezoelectric ceramics of 0.9BaTiO₃–(0.1?x)(Bi_0.5Na_0.5)TiO₃–xBiMO₃, M=Al and Ga, where x=0.00‐0.10, were fabricated by the solid‐state reaction technique. The effect of BiMO₃ contents on the perovskite structure, phase transition, and dielectric, ferroelectric, and piezoelectric properties was investigated. X‐ray diffraction patterns showed that the ceramics exhibit a monophasic perovskite phase up to x=0.06, suggesting stabilized perovskite structures with B‐site aliovalent substitutions. Compositional‐dependent phase transitions were observed from tetragonal to pseudo‐cubic phase with increasing BiMO₃ amounts. Al³⁺ ions were found to stabilize the transition temperature of the ceramics, while significantly decreasing transition temperature, and a change in the dielectric peak were found with an increasing amount of Ga³⁺. Regarding Al³⁺ substitution, the remanent polarization (P_r) values were found to decrease slightly with the Al³⁺ amount. With regard to Ga³⁺ substitution, P_r values decreased with the Ga³⁺ amount up to 0.06 and then increased slightly. The ceramics became softer with a higher degree of substitution according to the lower coercive field (E_c), when compared with 0.9BaTiO₃–0.1(Bi_0.5Na_0.5)TiO₃ ceramics. Ceramics with a lower degree of substitution and tetragonal phase showed butterfly strain loops that correlated with normal ferroelectric behavior.     

Noncontact temperature-dependent fluorescence depicting phase transition in Nd3+-doped (K0.5Na0.5)NbO3 ceramics

A noncontact temperature measurement technique based on fluorescence variation was used to depict the temperature-dependent evolution of phase transition of a ferroelectric material, that is, Nd³⁺-doped (K_0.5Na_0.5)NbO₃ ceramics. The slope of the fluorescence intensity curve changes dramatically in the two temperature regions of 450-475 K and 650-675 K, which correspond to orthorhombic-tetragonal and tetragonal-cubic transitions as confirmed by the temperature dependence of dielectric constant. Furthermore, the small deviations in δT_O-T and δT_c indicate the good accuracy of this noncontact method. This work can guide other ergodic ferroelectrics to describe phase experience by the noncontact fluorescence method.     

Optical and Tunable Dielectric Properties of K0.5Na0.5NbO3–SrTiO3 Ceramics

Pure perovskite K_0.5Na_0.5NbO₃–xSrTiO₃ (x = 0.16, 0.17, 0.18, and 0.19) ceramics were prepared by using a solid‐state reaction process. The ceramics were optically transparent for visible and near‐infrared wavelengths. Then, high tunability (24.1%) and low dielectric loss (0.016) for the x = 0.18 sample indicated the transparent ceramics could be used in tunable devices. The Lorentz‐type relation fitting for the temperature dependence of dielectric permittivity showed that these ceramics had a typical relaxor behavior, and the polar nanoregions were related to the tunable dielectric properties. The nonlinear dielectric behavior was further explored by the Johnson model combined with Langevin terms, which revealed that the polar nanoregions contributed to the nonlinear ε(E) dependencies with contributions of 12.3%, 11.6%, 5.9%, and 3.6% for x = 0.16, 0.17, 0.18, and 0.19, respectively.