Enhancement in electrical and magnetodielectric properties of Ca‐ and Ba‐doped BiFeO3 polycrystalline ceramics

We carried out a comparative study on the electrical and magnetodielectric properties of polycrystalline BiFeO₃, Bi_0.9Ca_0.1FeO_2.95, Bi_0.9Ba_0.05Ca_0.05FeO_2.95, and Bi_0.9Ba_0.1FeO_2.95 ceramics. The two dielectric anomalies, near 25 K and 281 K, are observed for BiFeO₃. Interestingly, the anomaly near 25 K shifts towards a higher temperature above 60 K with Ca and/or Ba doping, attributed to the doping induced chemical pressure. In addition, the room temperature switchable magnetodielectric effect is witnessed for the doped BiFeO₃ compounds, due to the quadratic magnetoelectric coupling. This indicates the improved magnetoelectric coupling in BiFeO₃ with the Ca and Ba doping. This is essentially due to the enhanced magnetic ordering and reduced leakage current in BiFeO₃ after the doping.     

Structural,dielectric, vibrational and magnetic properties of Sm doped BiFeO3 multiferroic ceramics prepared by a rapid liquid phase sintering method

Rare earth Sm substituted Bi_1−xSm_xFeO₃ with x=0, 0.025, 0.05, 0.075 and 0.10 polycrystalline ceramics were synthesized by a rapid liquid phase sintering method. The effect of varying composition of Sm substitution on the structural, dielectric, vibrational, optical and magnetic properties of doped BiFeO₃ (BFO) ceramics have been investigated. X-ray diffraction patterns of the synthesized rare earth substituted multiferroic ceramics showed the pure phase formation with distorted rhombohedral structure with space group R3c. Good agreement between the observed and calculated diffraction patterns of Sm doped BFO ceramics in Rietveld refinement analysis of the X-ray diffraction patterns and Raman spectroscopy also confirmed the distorted rhombohedral perovskite structure with R3c symmetry. Dielectric measurements showed improved dielectric properties and magnetoelectric coupling around Néel temperature in all the doped samples. FTIR analysis establishes O–Fe–O and Fe–O stretching vibrations in BiFeO₃ and Sm-doped BiFeO₃. Photoluminescence (PL) spectra showed visible range emissions in modified BiFeO₃ ceramics. The magnetic hysteresis measurements at room temperature and 5 K showed the increase in the magnetization with the increase in doping concentration of Sm which is due to the structural distortion and partial destruction of spin cycloid caused by Sm doping in BFO ceramics.     

Enhancement of photoelectrochemical performance of BiFeO3 by Sm3+ doping

Bismuth ferrite (BiFeO₃) holds great potentials in the photoelectrocatalysis due to the advantages of low cost, narrow band gap and good chemical stability. However, the photoelectrochemical performance of BiFeO₃ is usually inhibited by the poor charge carrier transport. Here, we report the flame annealing synthesized Sm³⁺ doped BiFeO₃ photocathodes for efficient hydrogen production. Greatly enhanced water reduction activity was found in doped samples. The Bi_0.95Sm_0.05FeO₃ composition exhibited largest photocurrent density of 0.1061 mA cm^?2 at 0 V vs. RHE in 0.5 M Na₂SO₄, which was 5.6 times higher than that of the pristine BiFeO₃. Mott-Schottky analysis and electrochemical impedance spectra proved that the Bi³⁺substitution increased the charge carrier concentration and facilitated the charge migration. Incident photon-to-electron conversion efficiency (IPCE) value for Bi_0.95Sm_0.05FeO₃ film (～6.39% at 325 nm) was approximately ten times higher than that of undoped sample. The high performance can be ascribed to the rational Sm³⁺ doping, which can improve visible light absorption ability, facilitate the charge carrier transport kinetics and hinder the recombination of photogenerated carriers. Our work provides a facile cation doping with rare earth ions to improve the photoelectrochemical performances.     

Dielectric,ferroelectric, and photoluminescent properties of Sm-doped Bi4Ti3O12 thin films synthesized by sol-gel method

We report on the structure, dielectric, ferroelectric, and photoluminescent properties of Sm³⁺-doped Bi₄Ti₃O₁₂ thin films which were prepared on fused silica and Pt/Ti/SiO₂/Si substrates by sol-gel method. The X-ray diffraction analysis confirmed that the Bi_4-xSm_xTi₃O₁₂ (BSmT) thin films were well crystallized in layered perovskite structure without any secondary phase. Raman spectra indicated that the structure of BSmT thin films was significantly distorted because of the Sm³⁺ doping. An appropriate doping amount of Sm³⁺ ions leads to obvious enhancement in ferroelectric and dielectric properties of BSmT thin films due to structure distortion and reduction in defects. In addition, the BSmT thin films also show orange-red color emission at 601?nm and long florescence lifetime (> 0.6?ms). This study indicated that lead-free BSmT thin films, which are featuring good electrical and photoluminescent properties, may have potential applications in integrated optoelectronic devices.     

Microstructural,ferroelectric, and photoluminescent properties of (100)‐oriented Sm3+‐doped Na0.5Bi0.5TiO3 thin films

(100)_C‐oriented Na_0.5Bi_0.5‐xSm_xTiO₃ (NBST) lead‐free ferroelectric thin films were prepared on Pt/Ti/SiO₂/Si substrates by chemical solution deposition method, and their microstructural, dielectric, ferroelectric, and photoluminescent properties were studied. X‐ray diffraction and scanning electron microscopy analysis indicated that both the grain size and (100)_C orientation degree of NBST thin films were decreased by doping Sm³⁺ ions. Raman spectra showed that structural symmetry of NBST thin films decreased at low Sm³⁺ doping concentration and then increased at high doping concentration of Sm³⁺ ions. An appropriate amount of Sm³⁺ dopants was confirmed to enhance dielectric and ferroelectric properties of the NBST thin films. Among all the compositions, the Na_0.5Bi_0.492Sm_0.008TiO₃ thin film exhibited the largest remnant polarization (2P_r) of 27.3 μC/cm² and high dielectric constant of 1068, as well as a low dielectric loss of 0.04. Temperature‐ and frequency‐dependent dielectric characteristics illustrated the relaxor ferroelectric behavior of Na_0.5Bi_0.492Sm_0.008TiO₃ thin film. Meanwhile, the Na_0.5Bi_0.492Sm_0.008TiO₃ thin film also showed optimal orange‐red emission at 600 nm, which is originating from the ⁴G_5/2 → ⁴H_7/2 transition of Sm³⁺ ions.     

Structural transformation and multiferroic properties of Sm and Ti co-doped BiFeO3 ceramics with Fe vacancies

Sm and Ti co-doped BiFeO₃ (BFO) ceramics with Fe vacancies—Bi_0.86Sm_0.14FeO₃, Bi_0.86Sm_0.14Fe_0.99Ti_0.01O₃, and Bi_0.86Sm_0.14Fe_0.9Ti_0.05O₃—were prepared by a solid-state method using a rapid liquid process. X-ray diffraction indicated that all samples exhibited a rhombohedral structure with a minor secondary phase. The structural transformation from a rhombohedral (space group: R3c) to orthorhombic structure (space group: Pnma) was observed in the sample of Bi_0.86Sm_0.14Fe_0.9Ti_0.05O₃, which was also confirmed by Raman scattering spectra. Microstructural investigations with scanning electron microscopy showed a reduction in grain size with doping of BFO. The dielectric loss of Bi_0.86Sm_0.14Fe_0.9Ti_0.05O₃ reaches 0.05 (at 100 Hz) owing to the introduction of Ti and Fe vacancies. Ferroelectromagnetic measurements revealed the existence of ferroelectricity with a remanent polarization of 0.24 µC/cm² in Bi_0.86Sm_0.14FeO₃, paraelectricity in Bi_0.86Sm_0.14Fe_0.9Ti_0.05O₃, and weak ferromagnetism with a remanent magnetization of 0.2 emu/g in Bi_0.86Sm_0.14Fe_0.99Ti_0.01O₃. The two composition-driven phases exist simultaneously and the different coercive field might be related to the jumps in the ferromagnetic hysteresis loops. Both the ferroelectric and magnetic properties were shown to correlate with the composition-driven structural evolution.     

Crystal structure,leakage conduction mechanism evolution and enhanced multiferroic properties in Y-doped BiFeO3 ceramics

Ceramics of pure phase Yttrium (Y) doped BiFeO₃ prepared by a solid-state sintering route were characterized by X-ray diffraction, Raman spectroscopy, magnetic and electrical measurements. The results and analysis show that Y substitution greatly reduces the leakage current and enhances the multiferroic properties of BiFeO₃. Leakage conduction mechanism is shown to change from space-charge-limited conduction type in pure BiFeO₃ to a Poole–Frenkel emission behavior in Bi_0.90Y_0.10FeO₃. A Fowler-Nordheim tunneling mechanism in Bi_0.95Y_0.05FeO₃ ceramic is also evidenced under high electric fields. At the same time, enhanced magnetic properties due to Y-doping are confirmed by temperature dependent magnetometry and supported by Raman spectroscopy. An unexpected and sharp switching behavior in the magnetization under low magnetic fields observed in Bi_0.90Y_0.10FeO₃ ceramic, together with its improved ferroelectric property, may trigger such system for promising magneto-electric applications.     

Structural,capacitive and resistive characteristics of (Pb0.6Bi0.2Sm0.2)(Fe0.4Ti0.6)O3

Samarium substituted BiFeO₃–PbTiO₃ ceramic compound of (Pb_0.6 Sm_0.2 Bi_0.2)(Fe_0.4Ti_0.6)O₃ has been fabricated by mixed oxide solid state reaction method. The crystallographic structure from XRD study, distribution of grains from SEM micrograph, dielectric behavior, conductivity spectrum, impedance along with electric modulus spectroscopy have been illustrated. The experimental results corroborate the impact of samarium substitution in BiFeO₃–PbTiO₃ entailing reduced grain size, higher dielectric response with reasonably dielectric loss and enrichment in capacitive behavior with negative temperature coefficient of resistance. The temperature dependent conductivity spectra exhibit Arrhenius behavior, whereas the frequency dependent conductivity spectra follow the Jonscher universal power law. The basic correlated barrier hopping (CBH) model governs the charge transport mechanism in the fabricated compound. The exploration reveals the enriched dielectric and electrical behavior that endorse the samarium substituted material as a potential ceramic entity for designing electronic devices such as capacitors and ferroelectric accessories.     

Synthesis and characterization of electrodeposited samaria and samaria-doped ceria thin films

Samaria (Sm₂O₃) and samaria-doped ceria (SDC) films are electrochemically deposited on stainless steel in view of a potential use in solid oxide fuel cells. As it is possible to deposit separately pure ceria (CeO₂) and pure samaria (Sm₂O₃) in similar conditions, SDC films were successfully obtained in one electrochemical conditions set. Thin films have been fabricated at low-temperature (30 °C) by applying a cathodic potential of −0.8 V/SCE, for 2 h. Structural and morphological properties of electrodeposited films have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), techniques and Raman spectroscopy. Special attention has been focused on the Raman spectroscopy study to emphasize the effect of heat treatment and samarium doping. Despite cracks, single SDC phase was obtained crystallizing in a cubic symmetry.     

Dispersion studies of La substitution on dielectric and ferroelectric properties of multiferroic BiFeO3 ceramic

Lanthanum La-substituted multiferroic Bi_1−xLa_xFeO₃ ceramics with x = 0.0, 0.05, 0.10, 0.15, 0.20 and 0.25 have been prepared by solution combustion method. The effect of La substitution for the dispersion studies on dielectric and ferroelectric properties of Bi_1−xLa_xFeO₃ samples have been studied by performing x-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), density, dc resistivity and dielectric measurements as well as characterizing the polarization-field hysteresis loop. The results of prepared samples are compared with those of bismuth ferrite (BiFeO₃). In the measuring frequency of 10 KHz to 1 MHz, the dielectric constants and dielectric losses for samples x = 0.20, 0.25 are almost stable and exhibited lowest dielectric loss close to 0.1. The resistivity of Bi_1−xLa_xFeO₃ samples reaches a maximum value of 10⁹ ohm-cm, which is about three times higher than that for pure BiFeO₃. The results also show that stabilization of crystal structure and nonuniformity in spin cycloid structure by La substitution enhances the resistivity, dielectric and ferroelectric properties. Furthermore, the substitution of rare earth La for Bi helps to eliminate the impurity phase in BiFeO₃ ceramic.     

Structural,electric and multiferroic properties of Sm-doped BiFeO3 thin films prepared by the sol–gelprocess

Pure polycrystalline Bi_1−xSm_xFeO₃ (BSFO) (x=0–0.12) thin films were successfully prepared on FTO/glass substrates by the sol–gel method. The influence of Sm doping on the structure, dielectric, leakage current, ferroelectric and ferromagnetic properties of the BSFO films was investigated. X-ray diffraction analysis and FE-SEM images both reveal a gradual rhombohedra to pseudo-tetragonal phase transition with the increase of Sm dopant content. On one hand, a proper amount of Sm doping can decrease the leakage current densities of the BSFO thin films. On the other hand, excess Sm substitution for Bi will lead to multiphase coexistence in the film, the lattice inhomogeneity results in more defects in the film, which can increase the leakage current density. The result shows that defects in the complexes lead to electric domain back-switching in the BSFO_x=0.06 thin film, resulting in a decreased dielectric constant, leakage current and remanent polarization. The BSFO_x=0.09 thin film is promising in practical application because of its highest dielectric constant, remanent polarization and remanent magnetization of 203–185, 70 μC/cm² and 1.31 emu/cm³, respectively.     

Impact of texturing on the phase transitions in sol–gel-processed Bi(Sm)FeO3 thin films on LaNiO3-buffered silicon

The orientation modulation of ferroelectric materials is a suitable method to optimize material performance. Textured Bi_1-xSm_xFeO₃ thin films (near the rhombohedral-orthorhombic (R-O) phase boundary, that is, x = 0, 0.1, 0.12, 0.14, and 0.16) were fabricated using the sol-gel process by introducing a LaNiO₃ (LNO) seed layer. Structural and ferroelectric characterizations were used to investigate the effect of texturing on the Sm doping-induced R-O phase transition of the BiFeO₃ thin films. It was found that a phase transition occurred from the rhombohedral to the orthorhombic structure with increasing Sm content in the nontextured polycrystalline films, resulting in an R-O phase boundary at x = 0.12. In contrast, the R-O phase boundary in the textured films was more diffuse, indicating a two-phase coexistence in a boarder range of Sm doping levels (x = 0.12-0.16). This discrepancy was attributed to the complexity of the stress status in thin films. The dielectric and electrical properties of the nontextured and textured samples were investigated. The current study shows that the phase boundary in ferroelectric thin films can be altered by diverse means, thus providing insights into potential applications.     

The near room temperature electrocaloric cycling refrigeration in Bi5Ti3FeO15/BiFeO3 mesoscopic composites: Experiment and simulation

Recently, the high efficient and environment–friendly electrocaloric cooling technology has become a hot topic. Unfortunately, the single process and extreme conditions have greatly limited its application. Due to the similar perovskite structure, Bi₅Ti₃FeO₁₅ and BiFeO₃ were selected to fabricate mesoscopic composites in this work to realize the double consecutive cooling effect. We propose that the non–collinear interfacial polarization is the origin of near room temperature electrocaloric cycling refrigeration. The electrons are trapped at the Bi₅Ti₃FeO₁₅/BiFeO₃ interfacial potential well, where the electrons possess discontinuous energy and jump by thermal activity. The structure, dielectric capacitance and ferroelectric polarization of the composite films were further analyzed. Finally, the performance of the mesoscale cooling device is simulated, which shows a promising avenue to high efficient double cycling refrigeration. More importantly, the results are helpful to understand the cycling principle of electrocaloric cooling.     

Effects of Ho and Ti Doping on Structural and Electrical Properties of BiFeO3 Thin Films

Effects of Ho and Ti ions individual doping and co‐doping on the structural, electrical, and ferroelectric properties of the BiFeO₃ thin films are reported. Pure BiFeO₃, (Bi_0.9Ho_0.1)FeO₃, Bi(Fe_0.98Ti_0.02)O_3+δ, and (Bi_0.9Ho_0.1)(Fe_0.98Ti_0.02)O_3+δ thin films were prepared on Pt(111)/Ti/SiO₂/Si(100) substrates by using a chemical solution deposition method. All thin films were crystallized in distorted rhombohedral structure containing no secondary or impurity phases confirmed by using an X‐ray diffraction study. Changes in microstructural features, such as grain morphology and grain size distribution, for the doped samples were analyzed by a scanning electron microscopy. From the experimental results, a low electrical leakage (1.2 × 10^?5 A/cm² at 100 kV) and improved ferroelectric properties, such as a large remnant polarization (2P_r) of 52 μC/cm² and a low coercive field (2E_c) of 886 kV/cm, were observed for the (Bi_0.9Ho_0.1)(Fe_0.98Ti_0.02)O_3+δ thin film. Fast current relaxation and stabilization observed in the (Bi_0.9Ho_0.1)(Fe_0.98Ti_0.02)O_3+δ imply effective reduction and neutralization of charged free carriers.     

Dielectric and ferroelectric properties of Ho-doped BiFeO3 nanopowders across the structural phase transition

We have studied Ho-doped BiFeO₃ nanopowders (Bi_1−xHo_xFeO₃, x = 0–0.15), prepared via sol-gel method, in order to analyse the effect of substitution-driven structural transition on dielectric and ferroelectric properties of bismuth ferrite. X-ray diffraction and Raman study demonstrated that an increased Ho concentration (x ≥ 0.1) has induced gradual phase transition from rhombohedral to orthorhombic phase. The frequency dependent permittivity of Bi_1−xHo_xFeO₃ nanopowders was analysed within a model which incorporates Debye-like dielectric response and dc and ac conductivity contributions based on universal dielectric response. It was shown that influence of leakage current and grain boundary/interface effects on dielectric and ferroelectric properties was substantially reduced in biphasic Bi_1−xHo_xFeO₃ (x > 0.1) samples. The electrical performance of Bi_0.85Ho_0.15FeO₃ sample, for which orthorhombic phase prevailed, was significantly improved and Bi_0.85Ho_0.15FeO₃ has sustained strong applied electric fields (up to 100 kV/cm) without breakdown. Under strong external fields, the polarization exhibited strong frequency dependence. The low-frequency remnant polarization and coercive field of Bi_0.85Ho_0.15FeO₃ were significantly enhanced. It was proposed that defect dipolar polarization substantially contributed to the intrinsic polarization of Bi_0.85Ho_0.15FeO₃ under strong electric fields at low frequencies.     

Structural,magnetic and photocatalytic properties of Sr2+ doped BiFeO3 nanofibres fabricated by electrospinning

Sr²⁺ doped BiFeO₃ (Bi_1-xSr_xFeO₃, 0?≤x?≤?0.35) nanofibres were fabricated by a sol-gel based electrospinning method. The as-spun BiFeO₃ (BFO) nanofibres consist of fine grained particles with high crystallinity. With Sr²⁺ doping, both the magnetic and photocatalytic properties of BFO are effectively improved. The best photocatalytic property for degradation of the methylene blue (MB) is obtained in Bi_0.75Sr_0.25FeO₃ nanofibres due to their weakest photoluminescence (PL) intensity. Meanwhile, the photocatalytic property of Bi_0.75Sr_0.25FeO₃ nanofibres is much higher than that of nanoparticles with the same constituent, which is attributed to the unique one-dimension fibrous structure benefiting the separation and decreased recombination of e^-/h⁺ pairs. This work proposes an effective approach for the degradation of organic pollutes.     

Investigation of transport behavior in Ba doped BiFeO3

Bi_1?xBa_xFeO₃ (x = 0.00–0.25) samples were prepared by conventional solid state reaction method. X-ray diffraction revealed the rhombohedrally distorted perovskite structure for undoped BiFeO₃ with a phase transition from rhombohedral to pseudo cubic on Ba substitution. The leakage current density of 10% Ba substituted sample is found to be four orders of magnitude less than that of the pure BiFeO₃. Grain boundary limited conduction and space charge limited conduction mechanisms are involved in low and high electric field regions respectively for all the samples except 10% Ba doped BFeO₃ which obeys grain boundary limited conduction mechanism in whole of the electric field range. Dielectric measurements showed that the dielectric constant and dielectric loss attained their minimum values at 10% Ba substitution. Thus 10% Ba is found to be optimum concentration to have better multiferroic properties. Undoped BiFeO₃ and 5% Ba doped samples have very large values of dielectric constants and leakage current densities which can be attributed to a large number of oxygen vacancies in these samples, indicating an extrinsic response of these compositions.     

Investigation of Tb-doping on structural transition and multiferroic properties of BiFeO3 thin films

Pure BiFeO₃ (BFO) and Bi_1−xTb_xFeO₃ (BTFO) thin films were successfully prepared on FTO (fluorine doped tin oxide) substrates by the sol–gel spin-coating method. The effects of Tb-doping on the structural transition, leakage current, and dielectric and multiferroic properties of the BTFO thin films have been investigated systematically. XRD, Rietveld refinement and Raman spectroscopy results clearly reveal that a structural transition occurs from the rhombohedral (R3c:H) to the biphasic structure (R3c:H+R-3m:R) with Tb-doping. The leakage current density of BTFO_x=0.10 thin film is two orders lower than that of the pure BFO, i.e. 5.1×10⁻⁷ A/cm² at 100 kV/cm. Furthermore, the electrical conduction mechanism of the BTFO thin films is dominated by space-charge-limited conduction. The two-phase coexistence of BTFO_x=0.10 gives rise to the superior ferroelectric (2P_r=135.1 μC/cm²) and the enhanced ferromagnetic properties (M_s=6.3 emu/cm³). The optimal performance of the BTFO thin films is mainly attributed to the biphasic structure and the distorted deformation of FeO₆ octahedra.     

Dielectric properties of Mn doped Bismuth Barium Titanate based ceramic thin films prepared by PLD technique

In this article, the effect of Mn doping on the permittivity and dielectric loss in 0.67BiFeO₃-0.33BaTiO₃ (BF-BT) based film bulk acoustic resonator test structures has been investigated. BF-BT thin films were deposited on the fused silica substrates with Pt/TiO₂/Ti as bottom electrode. During the study of the BF-BT based parallel-plate structures, it has been revealed that BF-BT is in the ferroelectric state at room temperature. Higher permittivity (ԑ) is observed at a growth temperature of 600 °C and lower dielectric loss is achieved at 0.3 wt% Mn doping contents. These results show that the proposed BF-BT based FBAR test structure has a great potential for applications in tunable thin Film Bulk Acoustic Resonator (FBAR) devices. Comparison of the measured and simulation results has been made by utilizing the Mason equivalent circuit.     

Electrical properties of bismuth ferrites: Bi2Fe4O9 and Bi25FeO39

Bi₂Fe₄O₉ was prepared by solid-state reaction and the electrical properties measured by impedance spectroscopy. After annealing in O₂ at 900 °C, Bi₂Fe₄O₉ is an electrically-homogeneous insulator. Its high frequency permittivity is constant (∼14.1) over the temperature range 300–400 °C and shows no evidence of incipient ferroelectricity at lower temperatures. On annealing in N₂ at 900 °C, the pellets gradually decompose.Bi₂₅FeO₃₉ was prepared by both solid-state reaction and mechanosynthesis. It showed a modest amount of mixed conduction of both oxide ions and holes. Impedance analysis showed a complex response that best fitted an equivalent circuit consisting of a parallel combination of long-range conduction and short range dielectric relaxation elements.The electrical conductivity of both Bi₂Fe₄O₉ and Bi₂₅FeO₃₉ is less than that of BiFeO₃ prepared by solid-state reaction, which indicates that any leakage conductivity of BiFeO₃ is not due to the possible presence of small amounts of these secondary phases.