Structural,optical, and multiferroic properties of single phased BiFeO3

A soft chemical coprecipitation method has been proposed for synthesis of nano-sized multiferroic BiFeO₃ (BFO) powders. The X-ray diffraction pattern confirms the perovskite structure of BFO and Rietveld refinement reveals the existence of rhombohedral R3c symmetry. Crystallite size and strain value are studied from Williamson–Hall (W–H) analysis. The transmission electron microscope (TEM) image shows that the particle size of BFO powders lies between 50–100 nm. 4A₁ and 7E Raman modes have been observed in the range 100–650 cm^?1 and a prominent band centered around 1150–1450 cm^?1 have also been observed corresponding to the two-phonon scattering. Differential Thermal Analysis (DTA) shows the existence of two prominent peaks at 330 ^°C and 837 ^°C corresponding to the magnetic and ferroelectric ordering, respectively. From the temperature dependent dielectric studies, an anomaly in the dielectric constant is observed at the vicinity of Neel temperature (T _N ) indicating a magnetic ordering. Also, BFO shows antiferromagnetic behavior measured from the magnetic studies.     

Electrical behavior of high resistivity Ce-doped BiFeO3 multiferroic

Bi_1+xCe_xFeO₃ (Ce–BFO) for x=0, 0.05, 0.1, and 0.15 monophasic ceramic samples were successfully synthesized by conventional solid-state reaction routes. The influences of Ce doping on structural, dielectric, ferroelectric, leakage current and capacitive properties of BiFeO₃ ceramics were investigated intensively. At higher concentrations of x (x=0.1 and 0.15) the samples showed good crystallinity with almost impurity free phases. No structural phase transformation took place after partial doping of Ce ions and all ceramic bulk samples remain in their rhombohedral structure with space group R3c. The dielectric behavior of the samples improved significantly and the ferroelectric hysteresis loops changed their shape from rounded to a strongly nonlinear typical ferroelectric feature mainly originating from the domain switching and became enhanced with increase in doping concentration of cerium (Ce). Experimental results also suggested that partial doping of higher valence, smaller ionic radius Ce ions in BiFeO₃ forces the reduction of oxygen vacancies, resulting in a great suppression of leakage current. It is found that the sharp capacitance peak/discontinuity present in the C–V characteristics of Ce–BFO for different Ce doping concentrations is directly associated with the polarization reversal. Incorporation of excess bismuth in the presence of Ce in BiFeO₃ is expected to compensate Bi loss during high temperature sintering and caused structural distortion which also favors enhancement of ferroelectric properties in Ce-doped BFO.     

Effect of La substitution on conductivity and dielectric properties of Bi1−xLaxFeO3 ceramics: An impedance spectroscopy analysis

Bismuth ferrite (BFO) and La-substituted BFO with composition Bi_1−xLa_xFeO₃ (x=0.05, 0.1 and 0.15) (BLFO_x=0.05-0.15) ceramics were prepared using the solid state reaction route. A structural phase transition from rhombohedral phase to triclinic phase was observed for BLFO_x=0.05-0.15 ceramics. Modulus spectroscopy reveals the deviation of dielectric behavior from ideal Debye characteristics and the dependence of conductivity on ion hopping in BFO and BLFO_x=0.05-0.15 ceramics. The conductivity of the BFO ceramics decreases for La content of 5 mol%, followed by a subsequent increase with 10 and 15 mol% of lanthanum doping. The typical values of the activation energies at high temperature reveal the contribution of short range movement of doubly ionized oxygen vacancies to the conduction process in BFO and BLFO_x=0.05 ceramics. Both short range and long range motion of oxygen vacancies are responsible for large conductivity in BLFO_{x=0.1 and 0.15} ceramics.     

Single or both site doped BiFeO3—Which is better candidate for profound electronic device applications?

Magnesium (Mg) and Zirconium (Zr) doped bismuth ferrite (BiFeO₃; BFO) such as Bi_1?xMg_xFeO₃ (Mg doped BFO; BMO), BiFe_1?xZr_xO₃ (Zr doped BFO; BZO) and Bi_1?xMg_xFe_1?xZr_xO₃ (both Mg and Zr doped BFO; BMZO) were synthesized by solid-state reaction techniques with dopant concentrations x?=?0 and 0.1, respectively. The distorted rhombohedral structures of doped BFO were confirmed by X-ray diffraction analysis. The microstructural analysis revealed that there were uniform dispersions and homogeneous distributions of ceramics in BMZO as compared to BMO, BZO and pure BFO. The presence of both grain and grain boundary in BMZO indicated its good electrical response than others as evidenced from impedance analysis and in agreement with AC conductivity study. The dielectric and ferroelectric measurement signified that BMZO possessed enhanced dielectric constant and high remanent polarization thus could be a better prominent candidate than others to be used in electronic devices.     

单相Bi0.9Ba0.1Fe0.85Mn0.15O3陶瓷中的多铁性

溶胶-凝胶法制备了Bi_0.9Ba_0.1Fe_0.85Mn_0.15O₃陶瓷样品,XRD分析显示其为单相菱方钙钛矿结构,明显的磁滞回线和电滞回线说明其在室温具有弱铁磁性和铁电性,介电常数随外磁场的变化显示双掺样品具有更大的磁容效应. 关键词： 溶胶-凝胶法 多铁材料 磁容效应     

Effect of holmium substitution for the improvement of multiferroic properties of BiFeO3

This paper reports on multiferroic properties of Ho substituted BiFeO₃ (Bi_1−xHo_xFeO₃) ceramics. It is observed that for x=0.15, a prominent ferroelectric loop is seen at 300 K even if the system remains in rhombohedral (R3c) phase without appearance of any observable impurity phases. A well shaped M-H loop is observed at 10 K for x=0.15. However it showed ferromagnetism, confirming the contribution of Ho³⁺ towards enhancement of ferromagnetic properties of BiFeO₃ at 300 K. Suppression of impurity phases of pure BiFeO₃ bulk ceramic favors the reduction of mobile oxygen vacancies and reduces leakage current, due to which ferroelectric properties of BiFeO₃ is enhanced. We argue that Ho substitution at Bi site is likely to suppress the spiral spin modulation and at the same time increase the canting angle, which favors enhanced multiferroic properties. XRD, SEM, magnetization, polarization and chemical bonding analysis measurements were carried out to explain the multiferroic behavior.     

Dielectric,electrical transport and magnetic properties of Er3+substituted nanocrystalline cobalt ferrite

Erbium substituted cobalt ferrite (CoFe_2−xEr_xO₄; x=0.0–0.2, referred to CFEO) materials were synthesized by sol-gel auto-combustion method. The effect of erbium (Er³⁺) substitution on the crystal structure, dielectric, electrical transport and magnetic properties of cobalt ferrite is evaluated. CoFe_2−xEr_xO₄ ceramics exhibit the spinel cubic structure without any impurity phase for x≤0.10 whereas formation of the ErFeO₃ orthoferrite secondary phase was observed for x≥0.15. All the CFEO samples demonstrate the typical hysteresis (M–H) behavior with a decrease in magnetization as a function of Er content due to weak superexchange interaction. The frequency (f) dependent dielectric constant (ε′) revealed the usual dielectric dispersion. The ε′–f dispersion (f=20 Hz to 1 MHz) fits to the modified Debye's function with more than one ion contributing to the relaxation. The relaxation time and spread factor derived are ∼10⁻⁴ s and ∼0.61(±0.04), respectively. Electrical and dielectric studies indicate that ε′ increases and the dc electrical resistivity decreases as a function of Er content (x≤0.15). Complex impedance analyses confirm only the grain interior contribution to the conduction process. Temperature dependent electrical transport and room temperature ac conductivity (σ_ac) analyses indicate the semiconducting nature and small polaron hopping.     

Dipolar and magnetic ordering in Nd-modified BiFeO3 nanoceramics

The polycrystalline samples of Bi_1−xNd_xFeO₃ (BNFO) with x=0, 0.05, 0.1, and 0.15 were prepared by a chemical route (metal ion complex precursor solution method). A systematic change in crystal structure from rhombohedral to tetragonal on increasing Nd substitution was observed. The reduction of low-frequency dispersion in permittivity and loss pattern due to Nd substitution in BiFeO₃ (BFO) was observed in its dielectric response curve. The loss tangent (tan δ) was found to be reduced on increased Nd concentration in BNFO. Dielectric anomaly near the reported Neel temperature of BFO and peaks in all the BNFO samples were observed in the permittivity vs. temperature graph. Significant opening in the room temperature M–H loop was observed on Nd substitution, which was more prominent for higher concentration of Nd.     

(Ba, Ca)(Ti, Zr)O3-BiFeO3 lead-free piezoelectric ceramics

Effects of BiFeO₃ (BFO) content on the microstructure and electrical properties of Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BCTZ) ceramics prepared by normal sintering in air were investigated. A stable solid solution is formed between BCTZ and BFO. The grain size gradually becomes smaller, and the ceramics become denser with increasing the BFO content. The Curie temperature, dielectric constant, and dielectric loss of BCTZ ceramics decrease simultaneously with the introduction of BFO. Moreover, the remanent polarization reaches a maximum at x = 0.2 mol%, and the coercive field continuously increases with increasing the BFO content due to the introduction of BFO with a higher coercive field. Improved piezoelectric properties (d₃₃ ∼ 405 pC/N and k_p ∼ 0.44) are demonstrated for the BCTZ ceramic with x = 0.2 mol%.     

Off-stoichiometry effects on BiFeO3 thin films

The effects of Bi and Fe-excess on the structure, ferroelectric, leakage current and magnetic properties of BiFeO₃ (BFO) thin films are reported. BFO with 5% excess exhibits no change in the structure with an improvement in leakage current properties in comparison to stoichiometric BFO. Raman spectroscopy of 10% Bi excess suggests a structural change from monoclinic to rhombohedral accompanied with an improvement of resistivity and ferroelectric polarization switching. A higher Fe-excess leads to the formation of pyrochlore Bi₂Fe₄O₉ and gamma-Fe₂O₃ that cause an increase in conductivity at the macroscopic scale. The results are discussed in terms of Fe and Bi-excess effects on the defect structure of BFO.     

Dielectric properties of deuterated ammonium Rochelle salt

The dielectric constant of deuterated ammonium Rochelle salt along the three crystallographic axes as well as the pyroelectric charge density developed on the crystal surface perpendicularly to the b-axis have been measured with a high temperature resolution. The para- to ferroelectric transition at T_c = ?159°C shows a thermal hysteresis of ΔT = 0.15°C. This and the discontinuities in the dielectric constant along all three axes show that the phase transition is of the first-order. No phase intermediate between the para- and the ferroelectric phases could be detected.     

Lattice dynamics of BiFeO<Subscript>3</Subscript> under hydrostatic pressure

The vibrational frequencies of the BiFeO₃ crystal lattice in the cubic phase (Pm3m) and the rhombohedral paraelectric phase (R3c) are calculated in terms of the ab initio model of an ionic crystal with the inclusion of the dipole and quadrupole polarizabilities. In the ferroelectric phase with the symmetry R3c, the calculated spontaneous polarization of 136 μC cm^?2 agrees well with the experimental data. The dependences of the unit cell volume, the elastic modulus, and the vibrational frequencies on the pressure are calculated. It is found that the frequency of an unstable ferroelectric mode in both the cubic (Pm3m) and rhombohedral (R3c) phases are almost independent of the applied pressure, in contrast to classical ferroelectrics with a perovskite structure, where the ferroelectric instability is very sensitive to a variation in the pressure.     

La0.1Bi0.9-xEuxFeO3化合物的结构与性能研究

利用固相反应烧结技术制备La_0.1Bi_0.9-xEu_xFeO₃系列化合物. 利用X射线粉末衍射进行物相鉴定和结构分析,确定了材料的相关系：x≤0.05,材料为R3c结构相;0.08≤x≤0.12,材料为赝R3c结构相;x≥0.15是Pbnm相,其中0.15≤x≤0.20区域Pbnm相存在畸变. 磁测量结果表明,材料具有弱铁磁性,对于x≤0.20材料,磁矩在x=0.12成分存在极值. 利用阻抗分析仪测量了室温介电常数随成分的变化关系.讨论了材料的结构与弱铁磁性和室温介电常数间的关系. 关键词： 0.1Bi_0.9-xEu_xFeO₃')" href="#">La_0.1Bi_0.9-xEu_xFeO₃ X射线衍射 磁性 介电常数     

Effect of Dy substitution on structural,magnetic and optical properties of BiFeO3 ceramics

Bi_1−xDy_xFeO₃ (x=0.0, 0.03, 0.05, 0.07, 0.10 and 0.12) ceramics were synthesized by solid state reaction method. Effects of Dy substitution on structural distortion, magnetic and optical properties of BiFeO₃ were examined by X-ray diffraction, Raman and UV–Visible spectroscopy. The samples were found to crystallize in rhombohedral structure of BiFeO₃ with R3c space group. The reduction in lattice parameters and unit cell volume indicate the distortion in FeO₆ octahedra of the rhombohedral structure without any signature of phase transformation up to x=0.12. The predictable weak ferromagnetic hysteresis loops can be observed in the Dy doped samples with maximum remnant magnetization of 0.2103 emu/g for x=0.12. The weak ferromagnetism is ascribed to the suppressed spiral spin structure and magnetically active characteristic of Dy³⁺ ions together with ferromagnetic coupling between Dy³⁺ and Fe³⁺ ions. With optical band gap in visible region, Dy doped BiFeO₃ ceramics are potential material for optoelectronic device and solar cell applications.     

Concentration-dependent structural transition in the La0.70Sr0.30MnO3−δ system

The crystal structure of anion-deficient La_0.70Sr_0.30MnO_3?δ manganites (δ = 0, 0.10, 0.15, and 0.20) has been studied by powder neutron diffraction at room temperature. The crystal structures of the stoichiometric La_0.70Sr_0.30MnO₃ and anion-deficient La_0.70Sr_0.30MnO_2.90 samples are satisfactorily described by the rhombohedral R $\bar 3$ c space group. The anion-deficient La_0.70Sr_0.30MnO_2.85 sample separates into two perovskite phases with the R $\bar 3$ c and I4/mcm space groups. The crystal structure of La_0.70Sr_0.30MnO_2.80 cannot be described by a single perovskite phase in the I4/mcm space group. It is found that the clustering of oxygen vacancies is the cause of structural phase separation at 293 K in anion-deficient La_0.70Sr_0.30MnO_3?δ manganites.     

Investigation of complex impedance and modulus properties of Nd doped 0.5BiFeO3-0.5PbTiO3 multiferroic Composites

0.5BiNd _x Fe_1?x O ₃ ? 0.5PbTiO₃ (BN _x F_1?x ? PT)(x = 0.05, 0.10, 0.15, 0.20) composites were successfully synthesized by a solid state reaction technique. At room temperature, X-ray diffraction shows tetragonal structure for all concentrations of Nd doped 0.5BiFeO₃ ? 0.5PbTiO₃ composites. The nature of Nyquist plot confirms the presence of bulk effects only for BN _x F_1?x ? PT (x = 0.05, 0.10, 0.15, 0.20) composites. The bulk resistance is found to decreases with the increasing temperature as well as Nd concentration and exhibits a typical negative temperature coefficient of resistance (NTCR) behavior. Both the complex impedance and modulus studies have suggested the presence of non-Debye type of relaxation in the composites. Conductivity spectra reveal the presence of hopping mechanism in the electrical transport process of the composites. The activation energy calculated from impedance plot of the composite decreases with increasing Nd _x concentration and found to be 0.89, 0.76, 0.71 and 0.70 eV for x=0.05, 0.10, 0.15 and 0.20 respectively.     

Effect of sintering temperature on the structural, dielectric and ferroelectric properties of tungsten substituted SBT ceramics

Structural, dielectric and ferroelectric properties of tungsten (W) substituted SrBi₂(Ta_1−xW_x)₂O₉ (SBTW) [x=0.0, 0.025, 0.05, 0.075, 0.1 and 0.2] have been studied as a function of sintering temperature (1100-1250 °C). X-ray diffraction patterns confirm the single-phase layered perovskite structure formation up to x=0.05 at all sintering temperatures. The present study reveals an optimum sintering temperature of 1200 °C for the best properties of SBTW samples. Maximum T_c of ∼390 °C is observed for x=0.20 sample sintered at 1200 °C. Peak-dielectric constant (ε_r) increases from ∼270 to ∼700 on increasing x from 0.0 to 0.20 at 1200 °C sintering temperature. DC conductivity of the SBTW samples is nearly two to three orders lower than that of the pristine sample. Remnant polarization (P_r) increases with the W content up to x≤0.075. A maximum 2P_r (∼25 μC/cm²) is obtained with x=0.075 sample sintered at 1200 °C. The observed behavior is explained in terms of improved microstructural features, contribution from the oxygen and cationic vacancies in SBTW. Such tungsten substituted samples sintered at 1200 °C exhibiting enhanced dielectric and ferroelectric properties should be useful for memory applications.     

The non-isotropic character of electric and dielectric properties of ammonium zinc chloride crystal

The dielectric constant, ε, and the d.c. conductivity, σ, were measured along the a-, b- and c-axes of (NH₄)₂ZnCl₄ (AZC) crystal in the 300-450 K temperature range. Crystals of AZC grown from aqueous solutions containing excess of ZnCl₂ were used. The value of the dielectric permittivity of AZC is extremely small compared to other ferroelectric crystals. Pronounced broad or step-like peaks at the phase transition temperatures were detected along the a- and b-axes, while ε along the c-axis is temperature independent up to the end of the measuring range. Reciprocal of the dielectric permittivity in the range of the commensurate to incommensurate phase transition obeys a relation similar to the Curie-Weiss law that is valid for second order ferroelectric/paraelectric phase transitions. The constants of the proposed relationship applied to the cooling run are given. The J-E characteristics along the three crystallographic axes were measured in the normal, incommensurate, commensurate and antiferroelectric phases. Hence, the type of conduction mechanism has been estimated. Parameters of Poole-Frenkel and Richardson-Schottky types of conduction mechanism have been determined. The effect of applied electric field on the conductivity measurement was also tested. Conductivity anomalies with different character were observed at the phase transition temperatures. The lnσ−1000/T dependence revealed thermal activation energy of conduction along the a-, b- and c-axes with different values in different phases of AZC.     

Structure and multiferroic properties of Eu-substituted BiFeO3 ceramics

Polycrystalline Bi_1?x Eu _x FeO₃ (x=0.00–0.25) ceramics were synthesized by the solid state reaction method with the rapid liquid phase sintering process. The effects of Eu substitution on the structure, and ferroelectric and magnetic properties of BiFeO₃ ceramics were investigated. X-ray diffraction measurements reveal that the structure of BiFeO₃ was changed from rhombohedral to orthorhombic and the impurity phases were decreased both due to Eu substitution. Raman spectra results also confirm that a structure transition occurs in the Eu concentration range of 0.15–0.20. The SEM investigation has suggested that the Eu substitution hinders the grain growth. Vibrating sample magnetometer measurements indicate ferromagnetism in Eu-substituted BiFeO₃ ceramics. It is found that the room temperature magnetic moment increases with increasing Eu concentration due to the suppressed or broken cycloid spin structure. Ferroelectric measurements show that Eu substitution enhances the polarization due to the significant decrease of the electric leakage of the samples. Therefore, the Eu-substituted BiFeO₃, or more complicated substituted BiFeO₃ based on Eu substitution, will have great potential for many practical applications.     

Enhanced ferroelectric and piezoelectric properties in La-modified PZT ceramics

The effect of lanthanum (La) doping on ferroelectric and piezoelectric properties of lead zirconate titanate (PZT) sample has been investigated. Pb_1?x La _x Zr_0.52Ti_0.48O₃ ceramics with x = 0.00, 0.02, 0.04, 0.06 and 0.10 were prepared by the sol–gel technique. Raman and Fourier transforms infrared spectroscopy have been employed to understand the structural modification due to ionic size mismatch. Raman spectra show the existence of both rhombohedral and tetragonal crystal symmetries. It also shows the dielectric relaxation with increase in La concentration in the sample. The increase in lattice strain due to La doping increases the remnant polarization and coercive field. The linear piezoelectric coefficient increases with the increase in La concentration. It reveals that La-substituted PZT is a better candidate for piezoelectric sensor applications as compared to that of PZT.