Polarization origin and iron positions in indium doped barium hexaferrites

M-type hexaferrite BaFe_12?xIn_xO₁₉ (x = 0.1, 1.2) samples were investigated by high resolution neutron powder diffraction and vibration sample magnetometry in a wide temperature range of 4–730 K. Structural and magnetic parameters were determined including the unit cell parameters, ionic coordinates, thermal isotropic factors, occupation positions, bond lengths and bond angles, microstrain values and magnetic moments. In³⁺ cations may be located only in the Fe1 - 2a and Fe2 - 2b crystallographic positions with equal probability for the x = 0.1 sample. At x = 1.2 about half of In³⁺ cations occupy the Fe5 - 12k positions whilst the other half are equiprobably located in the Fe1 – 2a and Fe2 – 2b positions. The spontaneous polarization was observed for these compositions at 300 K. The influence of structural parameters on the temperature behavior of Fe³⁺(i) - O^2- - Fe³⁺(j) (i, j = 1, 2, 3, 4, 5) indirect superexchange interactions was established. With the substitution level increase the superexchange interactions between the magnetic positions inside and outside the sublattices are broken which leads to a decrease in the value of the corresponding magnetic moments.     

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.     

X‐ray Absorption Spectroscopy Studies of the Room‐Temperature Ferromagnetic Fe‐Doped 6H–BaTiO3

We investigated the effect of annealing temperature on magnetic properties of 2% and 10% Fe‐doped BaTiO₃. To understand the possible structural differences between samples treated at different annealing temperatures, and to correlate them with the magnetic properties, several characterization techniques, such as X‐ray diffraction and X‐ray absorption spectroscopic methods (XANES and EXAFS) were employed. We found that the 2% Fe‐doped BaTiO₃ pseudocubic perovskite is paramagnetic regardless of the heat‐treatment conditions. Initially paramagnetic 10% Fe‐doped 6H–BaTiO₃, treated at 1250°C, became ferromagnetic after additional annealing at higher temperature. We have crystalographically characterized the cation ordering processes in the 6H–BaTiO₃ that occurred during the high‐temperature annealing. The ferromagnetism that is induced in this stage is most probably associated with the observed diffusion processes but it extrinsic character still cannot be fully disregarded.     

Co-substitution tailored dielectric relaxation and electrical conduction in lanthanum orthoferrite

Effect of co-substitution on structural, dielectric and electrical conduction property of LaFeO₃ is reported. Partial co-substitution of La by Na and Fe by Mn has been observed to cause substantial modification in structural property including lattice distortion in LaFeO_3. Evidence of Jahn teller distortion has been noticed in Raman analysis due to presence of Mn³⁺ Jahn teller active ion at Fe lattice site. Raman analysis also indicated Fe–O–Fe bond weakening in LaFeO₃ on co-substitution. Dielectric response provides evidence of temperature dependent polydispersive relaxations contributed by combined effect of dipolar and space charge relaxations. Temperature response of dielectric parameter (ε_r and tan δ) provided indication of phase transition that is similar to found in ferroelectric materials. The dielectric relaxations are studied in the framework of complex impedance and electrical modulus spectrum. The electrical response comprises both grain and grain boundary contribution in the reported polycrystalline sample and it seems consistent with brick layer model of electrical equivalent circuit. It is evident from the activation energy estimation from modulus and conductivity plot that the small polarons hoping are the key factor in both dielectric ordering as well as electrical conduction mechanism in co-substituted LaFeO₃.     

The Abnormal Optical Property and Room‐Temperature Exchange Bias Behavior in Na‐ and Ru‐Codoped BiFeO3 Nanoparticles

Bi_0.98Na_0.02Fe_1?xRu_xO₃ (x = 0, 0.01, 0.02) samples were prepared via a sol–gel method. The formation of the desired materials was confirmed using X‐ray diffraction. The tetravalent Ru doping could effectively reduce the number of oxygen vacancy, and subsequently reduce the leakage current. Interestingly, the optical band gap was narrowed gradually in our samples, which seems contradictory with the variation in oxygen vacancy. To better understand this abnormal optical property, the effect of bandwidth was analyzed based on the change in Fe–O bond length and Fe–O–Fe bond angle, and the corresponding phenomenological qualitative model was proposed. The magnetization was increased with Ru substitution. In addition, an exchange bias (EB) phenomenon without field cooled process was observed at room temperature for all the samples, which was explained by introducing a core–shell structure model. Moreover, the EB behavior becomes more pronounced at 5 K for 1% Ru‐doped sample.     

Thermal Expansion of Alkaline‐Doped Lanthanum Ferrite Near the Néel Temperature

The thermal expansion and magnetic behaviors of divalent, alkaline‐doped lanthanum ferrites (La_0.9M_0.1FeO₃, M=Ca, Sr, Ba) were assessed using a combination of dilatometry, magnetometry, time‐of‐flight neutron diffraction, and high‐temperature X‐ray diffraction. Néel temperatures were determined through vibrating sample magnetometry and correlated well with changes in thermal expansion behavior observed during both dilatometry and X‐ray diffraction. The Néel temperatures observed for pure, Ca‐doped, Sr‐doped, and Ba‐doped lanthanum ferrites were 471°C, 351°C, 465°C, and 466°C, respectively. The effect of divalent substitutions on the magnetic behavior are attributed to charge compensation mechanisms and structural changes in the material.     

Probing the Local Structure and Phase Transitions of Bi4V2O11‐Based Fast Ionic Conductors by Combined Raman and XRD Studies

In this article, we report the structural phase transitions in Bi₄V₂O₁₁ as observed from temperature‐dependent Raman scattering and X‐ray diffraction measurements. Four different types of highly disordered coordination polyhedra around the vanadium atoms with large dispersion of V–O bond lengths are observed in Bi₄V₂O₁₁ at ambient temperature. The observed V–O bond lengths could be grouped into two categories, viz. shorter <1.7 Å and longer >1.7 Å. The Raman modes of Bi₄V₂O₁₁ could be assigned to vibration of these bonds and V–O–V linkages. We could correlate the difference in degree of anharmonicity of the phonon modes with temperature to differences in V–O bond strength. The local structure of vanadium–oxygen network in Bi₄V_1.8Cu_0.2O_10.7 was also obtained by similar studies. The effect of highly disordered anion sublattice in the doped compound is reflected in the broadening of the Raman modes.     

Structural,optical, and magnetic evaluation of Co-, Ni-, and Mn-modified multiferroic BiFeO3 ceramics

Co-, Ni-, and Mn-doped BiFeO₃ (BFO) ceramics were synthesized herein through a solid-state reaction. All doped BFO samples exhibit visible-light response, and the Co- and Ni-doped BFO samples present enhanced ferromagnetic order at room temperature. All doped samples show secondary phases in minor quantities. Optical spectra reveal two absorptions bands, indicating multiple electron transitions for BFO and its secondary phases. M ? H hysteresis loops suggest enhanced ferromagnetism in the Co- and Ni-doped BFO samples because of magnetic spinel CFP and NFO phases, respectively, whereas changes in oxygen vacancies and Fe–O–Fe bond angle play minor roles in the ferromagnetic behavior.     

The effects of Mg2+ and Ba2+ dopants on the microstructure and magnetic properties of doubly-doped LaFeO3 perovskite catalytic nanocrystals

Using citrate sol-gel method, we prepared La_0.85Mg_0.15-xBa_xFeO₃(x?=?0.02–0.12) samples. X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM) were used to investigate whether the microstructure, composition, and magnetic properties of LaFeO₃ were affected by the following parameters: calcination temperature, calcination time, and the doping concentration of Mg²⁺ ion and Ba²⁺ ions. The XRD spectra showed that a trace of impurity phase (MgFe₂O₄) is visible when the content of Mg²⁺ ions was higher; however, there was no change in the orthorhombic perovskite structure of LaFeO₃ even at higher doping concentrations. The space group was still Pnma. No other phase was generated in the sample subjected to low-temperature calcination. Furthermore, FT-IR spectra confirmed the presence of some functional groups in the sample. Then, SEM showed that the size distribution of the particles is uniform in the sample, and the grain boundary is also clear. Finally, VSM measurements proved that the significant changes were produced in the magnetic properties of samples when they were doubly doped with Mg²⁺ and Ba²⁺ ions. Moreover, calcination temperature has a great influence on the magnetic properties of the samples.     

Evolution of magnetic order in multiferroic Pb(Fe2/3W1/3)O3-BiFeO3 solid solution

Multiferroic materials have attracted much interest in the last decade due to both the intriguing fundamental science and the potential applications in spintronics and magnetoelectric data storage devices. In this work, we have investigated and discussed the evolution of the magnetic properties of the multiferroic (1-x)Pb(Fe_2/3 W_1/3)O₃-xBiFeO₃ solid solution ((1-x)PFW-xBFO, x = 0, 0.025, 0.05, 0.075, 0.1 and 0.15). The magnetic phase diagram is established based on the magnetic measurement results, which reveals six magnetically ordered states on the PFW-rich side of the solid solution. The origins of the complex evolution of magnetic order in the PFW-BFO solid solution are discussed from the point view of the variations in both the –Fe–O–Fe– and –Fe–O–W–O–Fe– superexchange routes, which are intimately related to the ratio of magnetic Fe³⁺ ion concentration on the B-site and the changes in the local structural order/disorder and chemical homogeneities. Combining the magnetic phase diagram with the relaxor characteristic phase diagram of the (1-x)PFW-xBFO system, a striking feature is found that the ergodic relaxor (ER) state and the weakly ferromagnetic phase coexist in the composition range of 0.025 ≤ x ≤ 0.1 between the freezing temperature T_f and the Burns temperature T_B.     

Dependence on sintering temperature of structure,optical and magnetic properties of La0.625Ca0.315Sr0.06MnO3 perovskite nanoparticles

In this study, La_0.625Ca_0.315Sr_0.06MnO₃ (LCSMO) nanoparticles were prepared by facile sol-gel method at low crystallization temperatures. Various test methods were used to characterize structure, optical and magnetic properties of LCSMO nanoparticles. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) suggested complete crystallization of LCSMO nanoparticles sintered at 700 °C. In addition, unit cell volume and grain size increased with sintering temperature. Besides, X-ray photoemission spectroscopy (XPS) fitting results of Mn2p core level peaks confirmed the increase in Mn³⁺ ion concentration with sintering temperature, mainly attributed to formation of more oxygen vacancies. Raman microscopy and Fourier transform infrared spectrometry (FTIR) jointly depicted the existence of Mn–O bond, indicating that sintering temperature definitely impacted vibration mode of Mn–O and affected both crystal structure and performance. UV–vis optical band gap width of LCSMO nanoparticles sintered at 700 °C, 1000 °C, and 1500 °C decreased from 1.2 to 0.75 eV as sintering temperature increased, suggesting the semiconducting properties of nanoscale LCSMO particles. Magnetization dependent temperature (M-T) and magnetic field (M-H) measurements revealed degradation in magnetic properties of the specimens with temperature. Overall, LCSMO nanoparticles sintered at different sintering temperatures provided novel insights into properties of rare earth doped perovskite manganites.     

Molecular structure of CaO–FeOx–SiO2 glassy slags and resultant inorganic polymer binders

The molecular structures of CaO–FeO_x–SiO₂ slags and their inorganic polymer counterparts were determined using neutron and X‐ray scattering with subsequent pair distribution function (PDF) analysis. The slags were synthesized with approximate molar compositions: 0.17CaO–0.83FeO–SiO₂ and 0.33CaO–0.67FeO–SiO₂ (referred to as low‐Ca and high‐Ca, respectively)_. The PDF data on the slags reasserted the predominantly glassy nature of this iron‐rich industrial byproduct. The dominant metal‐metal correlation was Fe–Si (3.20‐3.25 Å), with smaller contributions from Fe–Ca (3.45‐3.50 Å) and Fe–Fe (2.95‐3.00 Å). After inorganic polymer synthesis, a rise in the amount of Fe³⁺ was observed via the shift of the Fe–O bond length to shorter distances. This shortening of the Fe–O distance in the binder is also evidenced by the apparent rise of the Fe–Fe correlation at 2.95‐3.00 Å, although this feature may also suggest a potential aggregation of FeO_x clusters. In general, the atomic arrangements of the reaction product was shown to be very similar to the precursor structure and the dominance of the Fe–Si correlation suggests the participation of Fe in the silicate network. The binder was shown to be glassy, as no distinct atom‐atom correlations were observed beyond 8 Å.     

High temperature crystallographic and low temperature magnetic phase transitions in Fe doped Sr2RuMnO6

Fe doped Sr₂RuMnO₆ (SRMO) double perovskites (Sr₂RuMn_1-xFe_xO₆, x = 0, 0.1, 0.2 and 0.3) were prepared by solid-state route. Both x-ray diffraction and Raman spectroscopy were performed to investigate the crystal structure of the synthesized double perovskites. Rietveld refinement of the x-ray diffraction patterns confirmed a phase transition from tetragonal to cubic space group as a function of doping concentration of iron. Raman spectroscopy at room temperature and group theory analysis revealed the phonon modes associated with the space group of the samples. The temperature dependent Raman spectroscopy showed an anharmonic behaviour of the phonon modes of the Fe doped SRMO samples. The temperature evolution of the phononic modes in the range of 300 K–620 K is predominantly influenced by the lattice degrees of freedom. The presence of several oxidation states Mn (2⁺, 3⁺ and 4⁺) and Fe (3⁺ and 4⁺) was confirmed by an X-ray photoemission spectroscopy analysis of the highest doped sample (x = 0.3). The magnetic properties measurements showed that the samples were completely paramagnetic at room temperature. The samples exhibit antiferromagnetism at very low temperatures and we conclude that they exhibit ferrimagnetic ground state in the mid temperature region.     

Formation of Tetravalent Fe Ions in LaFeO3 Perovskite Through Mechanochemical Modification by Ball Milling

Mechanochemical modification of previously synthesized LaFeO₃ perovskite‐type oxide by a high‐energy ball milling was investigated to introduce Fe⁴⁺ ions or transform some Fe³⁺ into Fe⁴⁺ in LaFeO₃. X‐ray absorption fine structure studies revealed that the formation of Fe⁴⁺ ions into LaFeO₃ perovskite has been achieved at first time by ball milling at room temperature without any additives or replacement of La³⁺ ions by some divalent cations. The structural model of Fe⁴⁺ containing LaFeO₃ could be described as with a modified perovskite having equal amounts of La and Fe vacancies, which is supported by a good correlation between the results of Fe K‐edge XANES spectra and O₂‐TPD. The synthesis of Fe⁴⁺‐containing LaFeO₃ perovskite by ball milling was able to produce the O₂ adsorption capacity of nonsubstituted perovskite‐type oxide.     

Cu/LaFeO3 as an efficient and stable catalyst for CO2 reduction: Exploring synergistic effect between Cu and LaFeO3

Cu-based catalysts, which are regarded as the most promising catalysts for CO₂ conversion, suffer dramatic deactivation at high temperatures. In this work, LaFeO₃, a typical perovskite-type oxide, is employed to disperse and stabilize Cu particles for the reverse water gas shift reaction. Compared to traditional Cu-based catalysts, Cu/LaFeO₃ exhibits a higher conversion with 100% CO selectivity and better stability at 873 K. Structural and spectroscopic characterization including N₂O chemisorption, high-resolution transmission electron microscopy, in situ x-ray diffraction, and x-ray absorption fine structure show that metallic Cu is well dispersed on LaFeO₃, forming more Cu-LaFeO₃ interface. CO₂ temperature-programmed surface reaction (CO₂-TPSR), two-step transient surface reaction (two-step TSR), and transient in situ diffuse reflectance infrared Fourier transformed spectroscopy experiments demonstrate that the superior activity is attributed to the synergistic effect between the highly dispersed Cu particles for H₂ dissociation and the abundant oxygen vacancies in LaFeO₃ support for CO₂ activation. The synergistic effect between metal and perovskite-type oxide increases metal-support interfaces and enhances CO₂ activation, leading to a potential application in a variety of chemical reactions.     

Magnetic and phonon transitions in B-site Co doped BiFeO3 ceramics

Magnetic susceptibility and phonons have been characterized in multiferroic Bi(Fe_1−xCo_x)O_3−δ ceramics for x=0.0, 0.05, and 0.10 (BFO100xCo) as functions of temperature. A preferred (100) crystallographic orientation and increasing average oxygen vacancies were observed in BFO5Co and BFO10Co. The Fe and Co K-edge synchrotron X-ray absorptions revealed mixed valences of Fe³⁺, Fe⁴⁺, Co²⁺, and Co³⁺ ions in BFO5Co and BFO10Co, which exhibit a ferromagnetic (or ferrimagnetic) phase below room temperature due to appearance of ferromagnetic B–O–B (B=Fe and Co) superexchange interactions. Field–cooled (FC) and zero–field–cooled (ZFC) magnetic susceptibilities exhibit a significant spin-glass splitting below room temperature in BFO5Co and BFO10Co. Two Raman-active phonon anomalies at ~170 K (or 200 K) and ~260 K were attributed to the Fe³⁺–O–Co³⁺ and Co³⁺–O–Co³⁺ magnetic orderings, respectively. This work suggests that the low-spin Co²⁺–O–Co²⁺, Fe³⁺–O–Fe³⁺ (or Fe⁴⁺), and high-spin Co²⁺–O–Co²⁺ superexchange interactions are responsible for phonon anomalies at ~290 (or ~300 K), ~400, and ~470 K (or ~520 K) in BFO5Co and BFO10Co.     

LaFeO3 ceramics as selective oxygen sensors at mild temperature

In this study, an investigation about the oxygen sensing properties of lanthanum orthoferrite (LaFeO₃) ceramics is reported. LaFeO₃ nanoparticles were synthesized by using tartaric sol-gel route and annealed in air at different temperatures (500, 700 and 900 °C). The samples have been characterized by using thermal analysis (TA), BET surface area and porosity, Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Results of sensing tests indicate that LaFeO₃ nanoparticles exhibit good response to oxygen at mild temperatures (300–450 °C). The effect of annealing temperature on gas sensing performance was investigated, demonstrating that LaFeO₃ ceramics obtained after annealing at 500 °C display better characteristics with respect to others. The oxygen sensor developed shows also high stability in humid environment and excellent selectivity to oxygen over other interfering gases such as CO, NO₂, CO₂, H₂ and ethanol.     

Two intra-molecular inter-ligand C(aromatic)–H⋯O(carboxyl) interactions reinforce the formation of a single Cu(II)–N4(pza) bond in the molecular recognition between pyrazine-2-carboxamide (pza) and the (iminodiacetato)copper(II) chelate. Synthesis,molecular and crystal structure and properties of [Cu(IDA)(pza)(H2O)]·H2O

Aqua(pyrazine-2-carboxamide)(iminodiacetato)copper(II) monohydrate, [Cu(IDA)(pza)(H₂O)]·H₂O, was synthesised and characterised by thermal, spectral, magnetic and X-ray diffraction methods. Its crystal structure was solved to a final R1=0.058. The Cu(II) atom exhibits a square base pyramidal coordination (type 4+1) with IDA ligand in mer-tridentate configuration [Cu–N(aliphatic) 1.986(7), Cu–O(carboxyl) 1.933(6) and 1.938(5) Å], the Cu–N4(pza) bond [1.984(7) Å] and Cu–O(apical aqua) bond [2.347(8) Å]. The N4-monodentate ligand role of pza is in contrast with that of the N,O-bidentate pza-Cu(II) chelation in [Cu(pza)₂(ClO₄)₂] or [Cu(acac)(pza)(ClO₄)]·H₂O. In the molecular recognition between Cu(IDA) chelate and pza the Cu–N4(pyridine-like) coordination mode is preferred because it enables the additional contribution of two weak intra-molecular inter-ligand C(aromatic)–H⋯O (IDA) interactions.     

The effect of the calcination temperature of LaFeO3 precursors on the properties and catalytic activity of perovskite in methane oxidation

In the synthesis of perovskite-type LaFeO₃ oxides iron and lanthanum nitrates were used as a precursors. The nitrates were dissolved in water, evaporated, crushed and calcined in temperature range of 650–850?°C. The obtained perovskites were applied as an active layer on monolithic catalysts for the oxidation of methane. The increase in the calcination temperature of the perovskite precursors from 650° to 850°C results in a reduction in the surface area of the powders from 10.1 to 4.2?m²/g. XRD studies revealed that calcination at 800–850?°C caused the formation of an almost homogeneous LaFeO₃ perovskite phase. A decrease in the La/Fe surface ratio from 12 to 5.2 with the rise in calcination temperature from 650° to 800°C was detected by XPS. EDX results confirmed that at 750–850?°C, the La/Fe ratio in the perovskite layer is close to the stoichiometric and amount to 1.01–1.03. The highest activity in methane oxidation was achieved when the LaFeO₃ perovskite was calcined at 700?°C. A further slight increase in the activity was noticed after H₂ treatment. As the calcination temperature of the perovskites is increased, the catalyst activity decreases due to a reduction in the specific surface area, despite the more complete LaFeO₃ perovskite phase formation.     

Investigation on Resonant Vibration Performances of Fe‐Doped BiScO3–PbTiO3 Ceramics in High‐Temperature Environment

The high‐temperature performance of a series of Fe‐doped BiScO₃‐PbTiO₃ (BSPT) piezoelectric ceramics at the morphotropic phase boundary was investigated. The effects of different Fe contents on the piezoceramics were assessed with regard to variations in structure, morphology, dielectric properties, piezoelectric properties, and high‐temperature resonant vibration. X‐ray diffraction (XRD) results indicated that the Fe‐doped BSPT ceramics show a single perovskite structure and that the c/a ratio undergoes a slight increase with increasing Fe concentrations. It was also found that, as the proportion of Fe in the ceramics was increased, the grain size was enlarged somewhat, the dielectric loss (tan δ) decreased, the mechanical quality factor (Q_m) was gradually improved, and the Curie temperature (T_C) was increased from 426°C to approximately 460°C. Despite these complex effects, it was evident that Fe doping can improve the high‐temperature resonant vibration performance of BSPT ceramics, and that these materials exhibit stable resonant vibration velocities at temperatures as high as 225°C. Our results indicate that Fe‐doped BSPT ceramics have the potential to be used as piezoelectric power devices intended for high‐temperature environments.