Investigation on Electrical and Magnetic Properties of Gd-doped BiFeO<Subscript>3</Subscript>

Multiferroic ceramic samples of Bi_1−x Gd _x FeO₃ (x=0, 0.05, 0.1 and 0.15) have been prepared by rapid liquid-phase sintering technique. The effect of Gd substitution on ferroelectric and magnetic properties of Bi_1−x Gd _x FeO₃ ceramics has been investigated. The results of X-ray diffraction (XRD) patterns show that the single-phase BiFeO₃ sample has a rhombohedral structure and Gd³⁺ substitution for Bi³⁺ has not affected its structure. Experimental results suggest that for Bi_1−x Gd _x FeO₃ system, the ferroelectric and magnetic properties of BiFeO₃ are improved by Gd doping and the loop area increases with the Gd content. When x=0.15, saturated ferroelectric hysteresis loop is observed at room temperature with the maximal 2Pr=1.62 μC/cm², which is about 578.6% higher than that of BiFeO₃.     

Properties of neodymium-doped Ca<Subscript>0.15</Subscript>Sr<Subscript>1.85</Subscript>Bi<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>18</Subscript> ferroelectric ceramics

Bismuth-layered compound Ca_0.15Sr_1.85Bi_4−xNd_xTi₅O₁₈ (CSBNT, x = 0–0.25) ferroelectric ceramics samples were prepared by solid-state reaction method. The effects of Nd³⁺ doping on their ferroelectric and dielectric properties were investigated. The remnant polarization Pr of CSBNT ceramics increases at beginning then decreases with increasing of Nd³⁺ doping level, and a maximum Pr value of 9.6 μC/cm² at x = 0.05 was detected with a coercive field Ec = 80.2 kV/cm. Nd³⁺ dopant not only decreases the Curie temperature linearly, but also the dielectric constant (ε_r) and dielectric loss tangent (tan δ). The magnitudes of ε_r and tan δ at the frequency of 100 kHz are estimated to be 164 and 0.0083 at room temperature, respectively.     

Influence of Dy-doping on ferroelectric and dielectric properties in Bi<Subscript>1.05−x</Subscript>Dy<Subscript>x</Subscript>FeO<Subscript>3</Subscript> ceramics

Bi_1.05−xDy_xFeO₃ (BDFO) (x = 0−0.2) ceramics were synthesized by solid-state reaction method. The influence of Dy dopant on crystal structural, dielectric and ferroelectric properties was investigated. The lattice parameter and the Curie temperature of BDFO were degraded continuously with increasing contents of Dy³⁺ cations. Leakage current density, ferroelectric polarization and dielectric loss were improved by appropriate Dy doping. When x = 0.1, BDFO showed the best electric properties. At applied electric field of 53 kV/cm, the remnant polarization (2P _r ) was 12.2 μC/cm².These improvements in electric properties in BDFO ceramics could have resulted from the relatively low oxygen vacancy concentration and structural distortion.     

Rietveld analysis, magnetic, vibrational and impedance properties of (Bi1−xPrx)(Fe1−xZrx)O3 ceramics

(Bi_1?xPr_x)(Fe_1?xZr_x)O₃ ceramics with x = 0.03, 0.06 and 0.10 were prepared via solid state reaction route. X-ray diffraction and Raman spectra of the ceramics were recorded for calculating its lattice parameters and structural analysis. Magnetic studies indicated a significant enhancement in magnetization of (Bi_1?xPr_x)(Fe_1?xZr_x)O₃ ceramics with maximum remnant magnetization of 0.12 emu g^?1 for x = 0.06 sample. Fourier transform infrared spectra and Rietveld analysis confirmed the change in bond length arising due to Pr and Zr codoping. Dielectric measurements showed dielectric anomaly around Neel temperature, indicating magnetoelectric coupling in the samples as well as the dielectric relaxation for higher doping. The effect of Pr and Zr codoping on the impedance and modulus behavior of BiFeO₃ lattice is discussed. The frequency dependence of electric modulus and impedance of the material show the presence of non-Debye relaxation in the samples. All samples showed energy band gap in the range 2.16–2.0 eV, indicating possibility of optical activity in visible range and making it suitable for photocatalytic applications.     

Structural studies of Fe2O3-Bi2O3-CdO glass system

xFe₂O₃·(100 − x)[Bi₂O₃·CdO] system with 0 ≤ x ≤ 50 mol% was prepared and investigated by X-ray diffraction, density, FT-IR and Raman spectroscopies. The XRD patterns confirm the formation of a vitreous structure for x < 35 mol% Fe₂O₃. The evolution of density and molar volume with the addition and increasing of iron content indicates structural changes in the structure of Bi₂O₃·CdO glass matrix. The FT-IR spectrum of the glass matrix reveals a structure realized from BiO₃ pyramidal and BiO₆ octahedral units. With the addition of iron the structure proposed by the glass matrix is changing by the appearance of FeO₄ units. Also the existence of FeO₆ units cannot be excluded. The Raman spectra suggest a structure build from BiO₆ octahedral units. By Raman scattering the presence of structural units characteristic to Fe₂O₃ was not directly observed but the evolution of the spectra is dependent of the iron content.     

Enhancement of ferromagnetic and dielectric properties of lanthanum doped bismuth ferrite nanostructures

Cylindrical-shaped multiferroic Bi_1?xLa_xFeO₃ (x = 0.0, 0.05, 0.1 and 0.15) were synthesized successfully by hydrothermal method. All samples were found to be rhombohedrally distorted perovskite structure. Diameter of the cylindrical particles reduces from ～450 nm for x = 0.0 to ～100 nm for x = 0.1 prepared under the same conditions. The Neél temperature as well as the dielectric constant was also found to increase with the increase in lanthanum content. Lanthanum doping also enhanced the magnetic properties. Magnetization measurements above room temperature show a significant increase in magnetization at around 400 °C. Enhanced magnetic properties due to lanthanum doping are caused by the breakage of spin cycloid as observed by electron spin resonance study.     

Effects of Nb doping on microstructure and properties of Bi4Ti3?xNbxO12 thin films prepared by magnetron sputtering

Bi₄Ti_3−xNb_xO₁₂ ferroelectric thin films were fabricated on p-Si substrates by magnetron sputtering. The effects of Nb doping on microstructure and properties of Bi₄Ti_3−xNb_xO₁₂ films were investigated. Bi₄Ti_3−xNb_xO₁₂ films had the same structure as Bi₄Ti₃O₁₂ with smaller and more uniform grains. The dielectric and ferroelectric properties of Bi₄Ti_3−xNb_xO₁₂ films were improved by Nb doping. Bi₄Ti_3−xNb_xO₁₂ films have better dielectric and ferroelectric properties with P _r = 16.5 μC/cm², E _C < 100 kV/cm, ε _r > 290, low dielectric loss (<0.9%) and clockwise C–V curves with a memory window of 0.9 V when x = 0.03–0.045, while an excessive Nb doping would lead to bad dielectric and ferroelectric properties.     

Dielectric properties of Ba(Ti1 − xZrx)O3 solid solutions

This paper reports the structural and dielectric properties of Ba(Ti_1 − xZr_x)O₃ (x = 0-0.3) ceramics. Single-phase solid solutions of the samples were determined by X-ray diffraction. Microscopic observation by scanning electron microscope revealed dense, single-phase microstructure with large grains (20-60 μm). The evolution of dielectric behavior from a sharp ferroelectric peak (for x ≤ 0.08) to a round dielectric peak (for 0.15 ≤ x ≤ 0.25) with pinched phase transitions and successively to a ferroelectric relaxor (for x = 0.3) was observed with increasing Zr concentration. Compared with pure BaTiO₃, broaden dielectric peaks with high dielectric constant of 25,000-40,000 and reasonably low loss (tanδ: 0.01-0.06) in the Ba(Ti_1 − xZr_x)O₃ ceramics have been observed, indicating great application potential as a dielectric material.     

Hydrothermal synthesis and mechanism and property study of La-doped BiFeO3 crystallites

Lanthanum-doped bismuth ferrite (Bi_1?xLa_xFeO₃, wherein x is equal to 0, 0.15, 0.3, and 0.4) crystallites were synthesized by hydrothermal method. In the synthesis, precursor composition, potassium hydroxide concentration, and hydrothermal temperature and time played important roles in the crystallinity and morphology of Bi_1?xLa_xFeO₃ crystallites. Pure Bi_1?xLa_xFeO₃ crystallites could be obtained when x < 0.3, and ferroelectric transition temperature decreased from 834.2 to 828.7?°C with increasing La doping. The growth mechanism of the Bi_1?xLa_xFeO₃ crystallines was also discussed. Furthermore, our results showed that La doping greatly enhanced the remnant polarizations.     

Phase coexistence in Bi1−x Pr x FeO3 ceramics

Bi_1?x Pr _x FeO₃ ceramics across the rhombohedral–orthorhombic phase boundary have been studied by X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. The structural phase transitions in Bi_1?x Pr _x FeO₃ driven by doping concentration and temperature are significantly different from those in BiFeO₃ compounds doped with other rare-earth elements. The features of the structural transformations have been discussed based on the specific character of the chemical bonds associated with praseodymium ions. The detailed study of the crystal structure evolution clarified the ranges of both single-phase and phase coexistence regions at different temperatures and dopant concentrations. For x = 0.125, compound extraordinary three-phase coexistence state has been observed in a narrow temperature range at about 400 °C. The results explicate driving forces of the structural transitions and elucidate the origin of the remarkable physical properties of BiFeO₃-based compounds near the morphotropic phase boundary.     

Application of secondary neutral mass spectrometry in the investigation of doped perovskites

Electromagnetic properties of doped perovskites depend sensitively on the doping level. Both the superconducting transition temperature of Bi₂Sr₂Ca(Pr)Cu₂O_8+δ compounds and the magnetic and electronic transport properties of La(Sr)Co(Fe)O₃ perovskites change dramatically with the doping level. Apart from doping, oxygen deficiency is influenced by the details of preparation processes such as calcination and sintering. Simultaneous determination of constituents is of crucial importance from sample characterization point of view. Quantitative analysis of perovskite oxides can be performed by Secondary Neutral Mass Spectrometry (SNMS) which is a suitable technique to measure the chemical composition of almost any sample because the flux of atoms sputtered from the sample is representative of the stoichiometry of the top-most layers. The composition and oxygen content of Bi₂Sr₂Ca_0.86Pr_0.14Cu₂O_8.4 and La_1−xSr_xCo_0.975Fe_0.025O_3−δ, where 0 ≤ x ≤ 0.25, were determined by SNMS. The results show that the method is equally applicable for insulating and conducting compounds. The observed electromagnetic properties reflect well the compositions obtained experimentally.     

Effects of Europium Substitution on the Microstructure and Electric Properties of Bismuth Ferrite Ceramics

Bi_1−x Eu _x FeO₃ (x=0.00–0.15) ceramics were synthesized by solid state reaction method with rapid liquid phase sintering process at different europium content and the microstructure and electrical properties of the samples were investigated. X-ray diffraction (XRD) studies show that europium substitution has changed the structure of BiFeO₃ from rhombohedral R3c to orthorhombic Pnma and decreased the impurity phase. Raman spectra results also conform that a structure transition occurs at about x=0.20, and indicate that the Eu substitution at Bi site can obviously affect the Bi–O bond. Impedance analyzer measurements show that both dielectric constant and dielectric loss are strongly dependent on the Eu content. The dielectric constant of the Bi_1−x Eu _x FeO₃ increases with increasing Eu content from 0.00 to 0.20, then decreases with increasing Eu content from 0.20 to 0.30. The dielectric constant measured at 100 Hz is 542.0 for the x=0.20 sample, which is about 8.5 times as big as that for unsubsitituted BiFeO₃. The dielectric loss can be effectively decreased by the substitution of Eu for Bi. In addition, the leakage current measurements show that the substitution of Eu can effectively reduce the leakage current density of BiFeO₃.     

New substitutions and novel derivatives of the Aurivillius phases, Bi5TiNbWO15 and Bi4Ti3O12

We investigated isomorphous substitution of several metal atoms in the Aurivillius structures, Bi₅TiNbWO₁₅ and Bi₄Ti₃O₁₂, in an effort to understand structure-property correlations. Our investigations have led to the synthesis of new derivatives, Bi₄LnTiMWO₁₅ (Ln = La, Pr; M = Nb, Ta), as well as Bi₄PbNb₂WO₁₅ and Bi₃LaPbNb₂WO₁₅, that largely retain the Aurivillius (n = 1) + (n = 2) intergrowth structure of the parent oxide Bi₅TiNbWO₁₅, but characteristically tend toward a centrosymmetric/tetragonal structure for the Ln-substituted derivatives. On the other hand, coupled substitution, 2Ti^IV → M^V + Fe^III in Bi₄Ti₃O₁₂, yields new Aurivillius phases, Bi₄Ti_3−2xNb_xFe_xO₁₂ (x = 0.25, 0.50) and Bi₄Ti_3−2xTa_xFe_xO₁₂ (x = 0.25) that retain the orthorhombic noncentrosymmetric structure of the parent Bi₄Ti₃O₁₂. Two new members of this family, Bi₂Sr₂Nb₂RuO₁₂ and Bi₂SrNaNb₂RuO₁₂ that are analogous to Bi₂Sr₂Nb₂TiO₁₂, possessing tetragonal (I4/mmm) Aurivillius structure have also been synthesized.     

Ferroelectric properties of lithia-doped (Bi0.95Na0.75K0.20)0.5Ba0.05TiO3 ceramics

Lead-free piezoelectric (Bi_0.95Na_0.75K_0.20−xLi_x)_0.5Ba_0.05TiO₃ ceramics have been prepared by conventional process for different lithium substitutions. The SEM images show that the ceramics are well sintered at 1428 K. Dielectric and ferroelectric measurements have been performed. With the increasing of lithium substitution, the Curie temperature of the (Bi_0.95Na_0.75K_0.20−xLi_x)_0.5Ba_0.05TiO₃ ceramics shifts from 570 K to 620 K, but the maximum value of the dielectric constant decreases from 6700 to 4700 correspondingly. A relatively larger remanent polarization of 36.8 μC/cm² has been found in the x = 0.05 sample. The coercive field decreases as the lithium substitution amount increases. An optimized d₃₃ = 194 × 10^− 12 C/N and a relative dielectric constant ε_r = 1510 have been obtained in (Bi_0.95Na_0.75K_0.15Li_0.05)_0.5Ba_0.05TiO₃.     

Effect of BaTiO3 additive on the electrical properties of Na0.50Bi0.50TiO3 lead free ceramics

The near morphotropic phase boundary (MPB) compositions of lead-free piezoelectric ceramics based on sodium bismuth titanate (Na_0.50Bi_0.50TiO₃: NBT) and barium titanate (BaTiO₃: BT) were carefully investigated by conventional high temperature mixed-oxide method. All the ceramics exhibit single phase rhombohedral symmetry. The frequency (100 Hz to 1 MHz) and temperature (Room temperature–500 °C) dependence of impedance spectroscopy of (1 − x)Na_0.50Bi_0.50TiO3–xBaTiO₃ (x = 0.0, 0.06, 0.07 and 0.08) ceramics were investigated by impedance analyzer. The frequency explicit plots of Z″ versus frequency at various temperatures show peaks in the higher temperature range (>400 °C). The compounds show dielectric relaxation, which is found to be of non-Debye type and the relaxation frequency shifted to higher side with increase in temperature. The activation energy values obtained for different BT content suggest that the electrical conduction in NBT is mainly due to the mobility of the ionized oxygen defects.     

Characterization of Bi4−xRxTi3O12 (Rx = Pr, Nd, Gd, Dy, x = 0.8) layered electroceramics by a photoacoustic method

We have carried out photoacoustic experiments to study the layered electroceramics Bi_4−xR_xTi₃O₁₂ (R_x=Pr, Nd, Gd, Dy), with x=0.8. Results are shown in terms of successive analyses performed on functions, PA(t, T_i), which result from the interaction of the laser beam with the crystalline lattice. Previous permittivity experiments performed in the materials suggested anomalous ferroelectric behavior. Using a pulsed Nd:YAG laser (10 Hz, 5 ns pulse width), photoacoustic experiments were run from room temperature up to 800 °C. Perturbations in the resultant correlation curves can be interpreted as the existence of a wide set of different transition temperatures in the material, such as are believed to occur in relaxors. From these experiments, we conclude that the temperature dependence of non-classical ferroelectrics can be more closely monitored.     

Phase evolution and magnetic property of Bi1 − xDyxFeO3 ceramics

Rare-earth ion Dy was substituted at the bismuth site in BiFeO₃ to produce Bi_1 − xDy_xFeO₃ (x = 0, 0.1, 0.2 and 0.3) polycrystalline ceramics. A two-stage solid-state reaction method was adopted in synthesizing the materials. The effects of varying the Dy doping concentration on the crystalline structures, morphologies and magnetic properties of the final products have been investigated. It is found that Dy doping resulted in a sequence of structural phase transitions and led to small grain sizes in the materials. As a result, the magnetic property of the doped samples was much enhanced. Therefore, it seems a promising way to improve the weak ferromagnetic property of BiFeO₃ based materials by Dy doping.     

Synthesis, dielectric behavior and impedance measurement studies of Cr-substituted Zn-Mn ferrites

Nanocrystalline ZnMn_1−xCr_xFeO₄ (1.0 ≥ x ≥ 0) ferrites were synthesized by sol-gel technique. X-ray diffraction (XRD) confirmed the formation of single phasic cubic spinel lattice for all the compositions studied. Lattice parameter shows a decreasing trend with an increase in Cr content in the compositions. Formation of spherical nanoparticles was revealed by scanning electron microscopy (SEM) analysis. Infrared spectroscopic studies revealed two main absorption bands in the range 400-800 cm⁻¹ arising due to tetrahedral (A) and octahedral (B) site vibrations. Dielectric constant, dielectric loss tangent, ac conductivity and complex impedance were measured as a function of frequency in the range 20 Hz to 1 MHz. Frequency dependence of dielectric constant shows dielectric dispersion due to the Maxwell-Wagner type of interfacial polarization. The role of chromium in modifying structural and dielectric properties of these ferrites has been explained.     

The effect of (Li,Ce) and (K,Ce) doping in Aurivillius phase material CaBi4Ti4O15

The effect of (Li,Ce) or (K,Ce) doping on the structures and properties of bismuth layer-structured ferroelectric (BLSF) CaBi₄Ti₄O₁₅-based ceramics was studied. In Ca_1−x(K,Ce)_x/2Bi₄Ti₄O₁₅ and Ca_1−x(Li,Ce)_x/2Bi₄Ti₄O₁₅ (0 ≤ x ≤ 0.9) compounds, the effect of the substitution on the lattice parameters was given. The XRD patterns of Ca_1−x(K,Ce)_x/2Bi₄Ti₄O₁₅ and Ca_1−x(Li,Ce)_x/2Bi₄Ti₄O₁₅ ceramics showed that the compounds were Aurivillius phases. SEM micrographs show that the grains of CBT-based ceramics were plate-like. Auger electron spectroscopy (AES) results showed that Li⁺ was present in bismuth layer-structured ferroelectrics. Based on the AES analysis and the comparisons with A-site (K,Ce) substitution, Li substitution of A-site cations in BLSF is possible. For x = 0.15 compositions, (Li,Ce) or (K,Ce) dopants can improve the high temperature resisitivity and piezoelectric constant d₃₃ of the doped ceramics.     

Dielectric properties of bismuth doped Ba1−xCaxTiO3 ceramics

The dielectric properties of (Ba_1−xCa_x)_1−1.5yBi_yTiO₃ (x = 0.10, 0.20 and 0.30, y = 0.05) ceramics were investigated. XRD analysis shows that 5 at.% of Bi doping can be fully incorporated into the perovskite lattice of (Ba_1−xCa_x)TiO₃. The maximal dielectric constant K_m of (Ba_1−xCa_x)_1−1.5yBi_yTiO₃ ceramics decreases significantly with increasing x for all the compositions. Compared with undoped Ba_1−xCa_xTiO₃ ceramics [Mater. Chem. Phys. 77 (2002)], Bi doping remarkably shifts the temperature of the peak dielectric constants T_m to lower temperature and a broad dielectric peak exhibits strong frequency dispersion. With increasing frequency, K_m decreases and T_m shifts to higher temperatures in (Ba_1−xCa_x)_1−1.5yBi_yTiO₃ ceramics. A typical behavior to well-known relaxor ferroelectric is observed. The relaxation behavior observation is suggested due to a random electric field induced domain state.