Lattice distortion of holmium doped bismuth ferrite nanofilms

Single-phase multiferroic BiFeO3 and rare-earth metal of holmium （Ho） doped BiFeO3 nanofilms were prepared on Pt （100）/Ti/SiO2/Si wafer via solution-gelation technique. It was suggested that the lattice distortion happened with the lattice parameter of d decreasing after doping rare-earth metal of Ho. Meanwhile, the structure of nanofilms transformed from hexahedron phase to tetragonal phase after doping Ho. The analysis on X-ray photoelectron spectroscopy （XPS） indicated that the ratio of Fe3~ cations to Fe2＋ cations increased with the increase of binding energy between Fe and O and decrease of that between Bi and O after doping Ho. The present work provided an available way on enhancing multiferroic of BiFeO3 nanofilms.

Effect of octahedron tilt on the structure and magnetic properties of bismuth ferrite

Multiferroic BiFeO_(3)-based ceramics were synthesized using the rapid liquid-phase sintering method.The rare-earth ion(Sm^(3+),Gd^(3+),Y^(3+))doping causes structural distortion without changing the intrinsic rhombohedral perovskite structure.Raman analysis shows that the effect of doping on E modes is greater than A1 modes,and the microstructure of FeO_(6) octahedron can be regulated by ion doping.A-site trivalent ion doped ceramics exhibit improved magnetism compared with pure BiFeO_(3) ceramic,which originated from the suppressed spiral spin structure of Fe ions.The tilt of FeO_(6) octahedron as a typical structure instability causes the anomalous change of the imaginary part of permittivity at high frequency,and doped ceramics exhibit natural resonance around 16-17 GHz.

Structural, Dielectric, and Magnetic Properties of Ba-Doped Multiferroic Bismuth Ferrite

The main focus of the research was to correlate the microstructure with dielectric and magnetic properties of Bi1-xBaxFeO3 samples. Bi1-xBaxFeO3 samples（x = 0.1, 0.2 and 0.3） were synthesized by the conventional solid-state reaction method using nano-powders of Bi2O3, Fe2O3, and BaCO3. Thereafter, field emission scanning electron microscope and X-ray diffraction（XRD） techniques were used to examine the structure and phase of the samples. Phase analysis by XRD indicated that the single-phase perovskite structure was formed with possible increment in lattice parameter with increasing Ba doping. Complex permeability（u＇iand u＇＇i） measured using impedance analyzer confirmed the increase in magnetic property with increasing Ba doping. Finally, dielectric constant（k） was analyzed as a function of temperature at different frequencies. Dielectric constant as high as 2900 was attained in this research for Bi0.8Ba0.2FeO3 sample due to reduction in leakage current at this composition.

Magnetic Field Controllable Photocurrent Properties in BiFe_(0.9)Ni_(0.1)O_(3)/La_(0.7)Sr_(0.3)MnO_(3) Laminate Thin Film

This paper reports a multifunctional magnetic-photoelectric laminate device based on the integration of spintronic material(La_(0.7)Sr_(0.3)MnO_(3))and multiferroic(Ni-doped BiFeO_(3)),in which the repeatable modulation effect on the photoelectric properties were achieved by applying external magnetic fields.More obviously,photocurrent density(J)of the laminate was largely enhanced,the change rate of J up to 287.6%is obtained.This sensing function effect should be attributed to the low-field magnetoresistance effect in perovskite manganite and the scattering of spin photoelectron in multiferroic material.The laminate perfectly combines the functions of sensor and controller,which can not only reflect the intensity of environmental magnetic field,but also modulate the photoelectric conversion performance.This work provides an alternative and facile way to realize multi-degree-of-freedom control for photoelectric conversion performances and lastly miniaturize multifunction device.

Influence of lanthanum-doping on photocatalytic properties of BiFeO_3 for phenol degradation

A series of BiFeO3 and lanthanum‐doped BiFeO3 photocatalysts were synthesized by a facile sol‐gel method using citric acid as complexing agent, and used to remove phenol in industrial wastewater under simulated sunlight irradiation. The samples were characterized by X‐ray diffraction, energy dispersive spectroscopy, X‐ray photoelectron spectroscopy, UV‐Vis diffuse reflectance spectroscopy and photoluminescence spectroscopy. The introduction of La effectively suppressed the generation of an impurity phase. All the metals （La, Bi and Fe） are well distributed. Under simulated sunlight irradiation, the La‐doped BiFeO3 photocatalysts exhibited superior photocatalytic activity to pure BiFeO3. The 15%La‐doped BiFeO3 photocatalyst exhibited the best activity, with a degradation rate of 96%and COD removal rate of 81.53%after irradiation for 180 min, and it showed good recycling stability. The enhanced photocatalytic ability of 15% La‐doped BiFeO3 was attributed to the in‐crease of adsorbed surface hydroxyl groups, enhancement of visible light absorption and reduction of electron‐hole recombination. We confirmed that the primary active species was -OH by adding different scavengers during the photodegradation of phenol and proposed a reaction mechanism based on these experiments.

Preparation of a p-n heterojunction BiFeO_3@TiO_2 photocatalyst with a core–shell structure for visible-light photocatalytic degradation

Magnetically separable bismuth ferrite(BiFeO3)nanoparticles were fabricated by a citrate self‐combustion method and coated with titanium dioxide(TiO2)by hydrolysis of titanium butoxide(Ti(OBu)4)to form BiFeO3@TiO2core-shell nanocomposites with different mass ratios of TiO2to BiFeO3.The photocatalytic performance of the catalysts was comprehensively investigated via photocatalytic oxidation of methyl violet(MV)under both ultraviolet and visible‐light irradiation.The BiFeO3@TiO2samples exhibited better photocatalytic performance than either BiFeO3or TiO2alone,and a BiFeO3@TiO2sample with a mass ratio of1:1and TiO2shell thickness of50-100nm showed the highest photo‐oxidation activity of the catalysts.The enhanced photocatalytic activity was ascribed to the formation of a p‐n junction of BiFeO3and TiO2with high charge separation efficiency as well as strong light absorption ability.Photoelectrochemical Mott-Schottky(MS)measurements revealed that both the charge carrier transportation and donor density of BiFeO3were markedly enhanced after introduction of TiO2.The mechanism of MV degradation is mainly attributed to hydroxyl radicals and photogenerated electrons based on energy band theory and the formation of an internal electrostatic field.In addition,the unique core-shell structure of BiFeO3@TiO2also promotes charge transfer at the BiFeO3/TiO2interface by increasing the contact area between BiFeO3and TiO2.Finally,the photocatalytic activity of BiFeO3@TiO2was further confirmed by degradation of other industrial dyes under visible‐light irradiation.

Structural, Magnetic and Optical Properties of BiFe1-xNbxO3

Nb doped multiferroic BiFe1-xNbxO3 （0 〈x 〈0.05） polycrystalline powders have been syn-thesized by using a sol-gel method. The effect of Nb dopant on the structural, magnetic and optical properties is investigated. According to the X-ray di raction data and the result of Rietveld re nement, all the samples maintain the R3c phase, while the lattice parameters a, c, the cell volume V and the Fe-O-Fe bond angle change. The remnant magnetization enhances by appropriate Nb doping due to the decreasing of the grain size. Meanwhile, Nb dopant leads to the narrowing of the band gap of BiFe1-xNbxO3 samples.

Intrinsic photocatalytic water oxidation activity of Mn-doped ferroelectric BiFeO3

The development of stable and efficient visible light-absorbing oxide-based semiconductor photocatalysts is a desirable task for solar water splitting applications.Recently,we proposed that the low photocurrent density in film-based BiFeO_(3)(BFO)is due to charge recombination at the interface of the domain walls,which could be largely reduced in particulate photocatalyst systems.To demonstrate this hypothesis,in this work we synthesized particulate BFO and Mn-doped BiFeO_(3)(Mn-BFO)by the sol-gel method.Photocatalytic water oxidation tests showed that pure BFO had an intrinsic photocatalytic oxygen evolution reaction(OER)activity of 70μmol h^(-1) g^(-1),while BFO-2,with an optimum amount of Mn doping(0.05%),showed an OER activity of 255μmol h^(-1) g^(-1) under visible light(λ≥420 nm)irradiation.The bandgap of Mn-doped BFO could be reduced from 2.1 to 1.36 eV by varying the amount of Mn doping.Density functional theory(DFT)calculations suggested that surface Fe(rather than Mn)species serve as the active sites for water oxidation,because the overpotential for water oxidation on Fe species after Mn doping is 0.51 V,which is the lowest value measured for the different Fe and Mn species examined in this study.The improved photocatalytic water oxidation activity of Mn-BFO is ascribed to the synergistic effect of the bandgap narrowing,which increases the absorption of visible light,reduces the activation energy of water oxidation,and inhibits the recombination of photogenerated charges.This work demonstrates that Mn doping is an effective strategy to enhance the intrinsic photocatalytic water oxidation activity of particulate ferroelectric BFO photocatalysts.

Co-precipitation/Hydrothermal Synthesis of BiFeO_3 Powder

A coprecipitation/hydrothermal route was utilized to fabricate pure phase BiFeO3 powders using FeCl3·6H2O and Bi（NO3）3·5H2O as starting materials, ammonia as precipitant and NaOH as mineralizer. The synthesized powders were characterized by XRD, SEM and DSC-TG analysis. In the process, single-phase BiFeO3 powders could be obtained at a hydrothermal reaction temperature of 180 ℃, with NaOH of 0.15 mol/L, in contrast to 200 ℃ and 4 mol/L for conventional hydrothermal route. Meanwhile, the micro-morphology of synthesized BiFeO3 powders changed with different reaction temperatures and concentrations of NaOH. The N6el temperature, Curie temperature and decomposition temperature of the synthesized BiFeO3 powders were detected to be 301 ℃, 828 ℃ and 964 ℃, respectively. The hydrothermal reactions mechanism to fabricate BiFeO3 powders were discussed based on the in-situ transformation process.

Structural and Dielectric Properties of (Bi2O3Fe2O3)0.4(Nb2O5)0.6 for Different Sintering Temperature

In this research work, (Bi2O3Fe2O3)0.4(Nb2O5)0.6 was made by the solid state reaction method. Samples were sintered at four different temperatures (850°C, 925°C, 1000°C and 1150°C) to study the effect of sintering temperature on the various properties of the samples. X-ray diffraction analysis confirmed that single phase Bi1.721δ0.089Fe1.056Nb1.134O7 was found when sintering temperature increased. At the same time, larger grain size was found when sintering temperature increased. From variation of dielectric loss with respect to frequency, a small peak was found when sample was sintered at higher temperature (1150°C). Dielectric constant of the sample decreases with the increase of frequency for all the samples. With the variation of temperature, DC resistivity of the samples showed that resistivity decreases with the increase of measuring temperature which indicates semiconducting nature.

Large piezoelectric property of Bi(Fe_(0.93)Mn_(0.05)Ti_(0.02))O_(3) film by constructing internal bias electric field

BiFeO_(3)(BFO),Mn-doped-BFO(BFMO),Ti-doped-BFO(BFTO),and(Mn,Ti)-codoped-BFO(BFMTO)thin films are fabricated on the Pt/TiO_(2)/SiO_(2)/Si substrates via a sol–gel method combined with spin-coating and the subsequent layer-by-layer annealing technique.Compared with BFO film,the BFMTO film exhibits the lowest leakage current density(-10^(-4) A/cm^(2)@290 kV/cm).Notably,the polarization–electric field(P–E)loop of BFMTO film exhibits a positive displacement along the x-axis due to the existence of internal bias electric field,which is in agreement with the results of the PFM phase and amplitude curves.Especially,a very prominent inverse piezoelectric constant of d_(33)-160 pm/V was obtained,which overcomes other related thin films.The internal bias electric field of BFMTO film can be caused by the different work functions of the thin film and the bottom electrode,accumulation of oxygen vacancies and the formation of defect dipoles.Besides,the internal bias electric field of BFMTO film has a good stability at the same electric field after experiencing the test cycle from low electric field to high electric field(400–1900 kV/cm).These results indicate that self-polarized BFMTO film can be integrated to devices without additional polarization process,and have a wide range of application in microelectromechanical systems.

Multiferroic and in-plane magnetoelectric coupling properties of BiFeO_3 nano-films with substitution of rare earth ions La^(3+) and Nd^(3+)

Single-phase multiferroic BiFeO3 and Bi（0.9）（La/Nd）（0.1）FeO3（doped with rare earth ions La-（3＋） and Nd-（3＋）） films grown on（111）-Pt/Ti/SiO2/Si substrate were prepared via sol-gel method and a subsequent rapid thermal process. The phase composition, microstructure, ferroelectric, dielectric, ferromagnetic properties were investigated, and meanwhile, the in-plane magnetoelectric（ME） coupling effects of the films were reported and studied for the first time in this work. Structural characterization by X-ray diffraction and scanning electron microscopy showed that both BiFeO3 and Bi（0.9）（La/Nd）（0.1）FeO3 exhabited a rhombohedral structure with（111） preferred orientation. The results of the physical properties indicated that the introduction of rare earth ions improved significantly the polarization, magnetization and dielectric properties than the undoped BiFeO3 crystals, and it enhanced effectively the in-plane ME coupling（the ME coupling coefficient αE increased from 0.13 in the pure BiFeO3 to 0.21 in Bi（0.9）La（0.1）FeO3 and 0.34 V/（Oe·cm） in Bi（0.9）Nd（0.1）FeO3）. The mechanism of these phenomena was investigated systematically.

Enhanced ferroelectricity and magnetism of quenched(1−x)BiFeO3-xBaTiO3 ceramics

Bismuth ferrite(BiFeO3)-based materials are multiferroic materials widely studied.This study reports that strong ferroelectricity and enhanced magnetic performance are simultaneously obtained in the quenched(1−x)BiFeO3−xBaTiO3(BFBT100x,x=0.2 and 0.3)ceramics.Quenching treatment can reduce the amount of defects and Fe2+ions and make the defect dipole in a random state,which is conducive to improving the ferroelectricity and magnetism.Compared with the conventional sintered samples,the quenched ceramics have higher remnant and saturation polarization.As for magnetism,the coercive field(Hc)of the quenched ceramics is smaller and the quenching treatment can increase the maximum magnetization by up to 15%.

Structural phase relations in perovskite-structured BiFeO_(3)-based multiferroic compounds

In this review,the state of the art in understanding the structural phase relations in perovskite-structured BiFeO_(3)-based polycrystalline solid solutions is presented and discussed.Issues about the close relation between the structural phase and overall physical properties of the reviewed systems are pointed out and discussed.It is shown that,by adjusting the structural symmetric arrangement,the ferroelectric and magnetic properties of BiFeO_(3)-based polycrystalline solid solutions can be tuned to find specific multifunctional applications.However,an intrinsic mechanism linking structural arrangement and physical properties cannot be identified,revealing that this subject still deserves further discussion and investigation.

Surface functionalization of BiFeO3： A pathway for the enhancement of dielectric and electrical properties of poly（methyl methacrylate）-BiFe03 composite films

A novel two-phase composite film is prepared by the solvent casting method employing poly（methyl methacrylate） （PMMA） as polymer matrix and bismuth ferrite （BFO） as ceramic filler. The surfaces of BFO are functionalized by proper hydroxylating agents to activate their chemical nature. The structural analysis of the composite films confirms that the composites made up of functionalized BFO （BFO-OH） have a distorted rhombohedral structure. The morphological analysis shows that BFO- OH particles are equally distributed over the polymer matrix. The -OH functionality of BFO-OH is confirmed by FTIR. The dielectric and electrical studies at a frequency range from 100 Hz to I MHz reveal that PMMA-（BFO-OH） composites have enhanced dielectric constant as well as electrical conductivities, much higher than that of unmodified composites. According to the ferroelectric measurement result, the hydroxylated composite film shows a superior ferroelectric behavior than that of the unmodified one, with a remanent polarization （2Pr） of 2.764μC/cm2,

Destructive Interactions between Pore Forming Agents and Matrix Phase during the Fabrication Process of Porous BiFeO_3 Ceramics

Porous bismuth ferrite ceramics were synthesized by sacrificial pore former method. A mixture of BiFeO3 and 20 wt% of various pore formers including high density polyethylene, polyethylene glycol, polyvinyl alcohol, urea and graphite was intensively milled for 10 h in a planetary ball mill, uniaxially cold pressed and then subjected to the multi-stage heat treatment. The results revealed that urea and polyvinyl alcohol are appropriate candidates for maintaining the strength of the final porous structure. Density and porosity measurements showed that by employing 20 wt% of high density polyethylene and graphite, a porous sample with a maximum porosity of nearly 40% could be obtained. Mercury porosimetry results showed that using urea as a pore former gives porous bismuth ferrite with a mean pore diameter of 7μm, uniform pore distribution as well as interconnected pores. Moreover, reactions between BiFeO3 matrix phase and thermal decomposition products of pore formers can lead to degradation of the BiFeO3 phase in the final porous samples. Analysis of X-ray diffraction patterns illustrated that in the samples pro- cessed with graphite, high density polyethylene and polyvinyl alcohol as pore former, BiFeO3 matrix phase partially or completely decomposed to intermediate phases of Bi2Fe4O9 and Bi25FeO40. Using of urea did not damage the matrix phase and porous BiFeO3 within the original perovskite structure could be prepared. Furthermore, thermodynamic investigation was carried out for prediction of possible in- teractions between matrix phase and pore former at elevated temperatures.

Stable self-polarization in lead-free Bi(Fe_(0.93)Mn_(0.05)Ti_(0.02))O_(3)thick films

The BiFeO3-based film is one of the most promising candidates for lead-free piezoelectric film devices.In this work,the 1μm-thick Bi(Fe_(0.93)Mn_(0.05)Ti_(0.02))O_(3)(BFMT)films are grown on the ITO/glass substrate using a sol-gel method combined with spin-coating and layer-by-layer annealing technique.These films display a large saturated polarization of 95μC/cm^(2),and a rema­nent polarization of 70μC/cm^(2).Especially,the films are self-poled caused by an internal bias field,giving rise to asymmetric polarization-electric field(P-E)loops with a positive shift along the x-axis.A stable self-polarization state is maintained during the applied electric field increasing to 1500 kV/cm and then decreasing back.The weak dependence of P-E loops on frequency(1-50 kHz)and temperature(25-125°C)indicate that the internal bias field can be stable within a certain frequency and tempera­ture range.These results demonstrate that the self-polarized BFMT thick films can be integrated into devices without any poling process,with promising applications in micro-electro-mechanical systems.

BiFeO_(3) codoping with Ba,La and Ti:Magnetic and structural studies

Conventional solid state reaction method,from oxides and carbonates,was employed to prepare bismuth(Bi)-based multiferroic systems.The undoped BiFeO_(3)(BFO)and the codoped system with Ba,La and Ti(Bi_(1-x)Ba_(x)Fe_(1-y)Ti_(y)O_(3),Bi_(1-x-z)Ba_(x)LazFe_(1-y)TiyO_(3))with x,y,z=0.1 were prepared stoichiometrically and sintered at two different temperatures.The structural and magnetic properties were investigated at room temperature.XRD measurements confirm the obtaining of the rhombohedral perovskite structure of the BFO family system.For the undoped system,some reflections of undesired phases are present for two different sintering temperatures,while for the doped system only one phase is present for both temperatures.The magnetic characterization at room temperature revealed remarkable differences between the ceramic samples.The results show that for undoped BFO system,spontaneous magnetization is not observed at room temperature.Nevertheless,in doped one,a well-defined ferromagnetic behavior is observed at room temperature,possible,due to the suppression of the spatially modulated spin structure of BFO promoted by the reduction of the rhombohedral distortion and the weakening of the Bi-O bonds.The XPS results confirm the presence of oxygen vacancies and the coexistence of Fe^(3+)and Fe^(2+) in all the studied samples.Calorimetric measurements reveal that the dopant incorporation has not a direct effect in N
eel temperature but possibly yes in ferroelectric-paraelectric transition.

Thermophysical properties of BiFeO_(3)/REE multiferroics in a wide temperature range

The paper presents the results of a comprehensive study of the thermophysical properties(thermal conductivity,thermal diffusivity,heat capacity)of high-temperature multiferroic BiFeO_(3) modified with rare-earth elements(REEs)(La,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Lu).The regularities of the formation of the mentioned characteristics were established.The assumptions about the nature of the observed phenomena were suggested.