Perovskite-type Ferrites Research Articles

Praseodymium dysprosium co-doped M-type strontium ferrite: Intentionally manufacturing heterophase growth to improve microwave absorption performance

When excessive praseodymium and dysprosium ions are doped with M-type strontium ferrite, heterophases with different magnetic and electrical properties will be formed. We use this to intentionally induce different heterogeneous growth in the Pr–Dy co-doped strontium ferrite system. While maintaining the dielectric loss of the main phase, the produced heterogeneous phase acts as a companion for dielectric and magnetic losses. The synthesized SrFe12-2xPrxDyxO19 ferrite demonstrates its potential as a microwave absorbing material, exhibiting excellent absorption effects over a wide range of Pr–Dy doping (x = 0.05–1.5). The XRD results show that the synthesized materials include both pure M-phase and coexistence of M-phase and heterophases, and the generated heterophases are mainly perovskite type ferrite and garnet type ferrite. SEM and EDS results indicate that Pr–Dy co-doping can significantly refine the grain size, reduce the grain size of the main and heterophases, and lead to a decrease in oxygen content. The XPS analysis shows that some praseodymium ions are in the Pr4+ oxidation state, leading to the existence of Fe2+ ions and oxygen defects (Od). The VSM results indicate that at different calcination temperatures, the saturation magnetization decreases and the coercivity shows an increasing trend as the Pr–Dy doping amount increases. Pr–Dy doping not only increases the dielectric and magnetic losses of the M phase, but also generates heterophases that can enhance the dielectric and magnetic losses of the material, jointly improving the absorption effect. This results in excellent absorption performance of the material within a large range of praseodymium dysprosium doping contents (x = 0.05–1.5, RLmin = −52.51 dB, EABmax = 4.81 GHz).

IIIA-IIB multicomponent perovskite rare earth ferrites with promising electromagnetic wave absorption properties

Perovskite-type rare-earth ferrites (REFeO3) are promising materials for absorbing electromagnetic (EM) wave pollution. However, insufficient dielectric loss and poor impedance matching are key factors that limit the broader implementation of REFeO3. Herein, a series of multicomponent perovskite-type ferrites with strong EM wave absorption capabilities was prepared. Through the synergistic effect of chemical constitution regulation and entropy regulation, optimization of the dielectric loss and impedance matching is achieved by strengthening the structural defect mechanism, thus further adjusting the EM wave absorption performance. Compared with (LaGdSmNdBa)FeO3 (HE-1) and (LaGdPrSmNdBa)FeO3 (HE-2), (LaGdBa)FeO3 (ME-1) and (LaGdSmBa)FeO3 (ME-2) exhibit favorable performance, with optimal minimum reflection loss (RLmin) of −56.35 dB (at 11.12 GHz) and −63.25 dB (at 7.22 GHz) and effective absorption bandwidth (EAB) of 4.46 and 4.72 GHz, respectively. This multicomponent design provides a new strategy for the development of EM wave absorption materials.

Electrochemical determination of SrFe12O19 impurity in perovskite ferrites

A strong negative effect of SrFe12O19 impurity presence in perovskite-type ferrites on the results of the coulometric titration has been revealed. A multistage decomposition of this impurity during reductive titration causes distortion of oxygen content isotherms and makes the obtained data inapplicable for the defect equilibrium analysis. Even a small amount of SrFe12O19 undetected by X-ray diffraction has been shown to affect data considerably. In this study, a method has been developed to determine the amount of impurity and recover the oxygen content data in perovskite-type ferrites from measurements of these ferrites contaminated with SrFe12O19. The method has been verified on the perovskite-type NdxSr1−xFe0.8Mo0.2O3−δ (x = 0, 0.1, 0.2), the latter two compositions of which have been found to be contaminated with ~0.9 and 1.8 wt% of SrFe12O19, respectively, whereas 5.8 wt% of hexaferrite has been intentionally admixed to the x = 0 composition. The recovered data have been used for thermodynamic modeling of the defect equilibrium in NdxSr1−xFe0.8Mo0.2O3−δ. Good agreement of all obtained thermodynamic parameters with the published results for similar compositions and their monotonous change with neodymium content confirms the correct functionality of the method.

High-temperature transport properties of LaSr3Fe3−xMoxO10−δ

The effect of substituting molybdenum for iron in triple-layer Ruddlesden–Popper ferrite Sr3LaFe3O10−δ has been examined. The solubility of molybdenum in LaSr3Fe3−xMoxO10−δ synthesized under an air atmosphere has been found to be at about x = 0.2. The oxygen non-stoichiometry, thermal expansion and electrical conductivity of the single-phase compositions (x = 0.1 and x = 0.2) have been studied from room temperature to 950 °C. In contrast to perovskite-type ferrites, in which substituting molybdenum for iron suppresses ion transport, it results in an increase in the ion conductivity in LaSr3Fe3−xMoxO10−δ, in which the x = 0.2 composition approaches 0.1 S·cm−1 at 950 °C. An increase in the molybdenum content from x = 0.1 to x = 0.2 has been shown to result in a 2.5-fold increase in the electron conductivity of LaSr3Fe3−xMoxO10−δ under reducing conditions.

SrFe12O19 as an impurity in perovskite-type ferrites

A decrease of oxygen content in SrFe12O19 and its subsequent decomposition were studied at 750–950 °C by coulometric titration. The lowest oxygen content that strontium hexaferrite endures is about 18.5. Further oxygen removal from oxide lead to its reductive decomposition with the formation of Fe3O4 and Sr4Fe6O13+δ. The stability limits of SrFe12O19 under reducing conditions was determined. The continuation of the reductive titration was shown to result in the transformation Fe3O4 → Fe1−xO followed by Fe1−xO → Fe. The significant release of oxygen during stepwise decomposition can be considered as a specific characteristic of SrFe12O19. It is argued that the double stepwise fall, which is often observed on oxygen content isotherms of perovskite-type ferrites, indicates the presence of SrFe12O19 impurity.

Study of effect of preparation method and composition on the catalytic properties of complex oxides (Gd,Sr) n + 1Fe n O3n + 1 for dry reforming of methane

The catalytic activity of nanostructured perovskite-type ferrites of gadolinium and strontium, has been studied. The intercorrelation between the catalytic activity and the method used to obtain ferrites has been shown: the products formation rates are higher in the samples obtained by a ceramic technology, while the sol-gel method makes it possible to obtain ferrites with a greater hydrogen selectivity, which is accounted for by their nanocrystalline state and porous structure. It is shown that the specific catalytic activity of the catalysts, which are prepared by the same method and are of approximately the same chemical composition, is constant. It is shown that a nonisovalent replacement of gadolinium by strontium causes symmetry lowering in the perovskite structure and the appearance of a heterovalent state of iron atoms, which affects ferrite catalytic activity.

Structural characterization of a new vacancy ordered perovskite modification found for Ba3Fe3O7F (BaFeO2.333F0.333): Towards understanding of vacancy ordering for different perovskite-type ferrites

The new vacancy ordered perovskite-type compound Ba3Fe3O7F (BaFeO2.33F0.33) was prepared by topochemical low-temperature fluorination of Ba2Fe2O5 (BaFeO2.5) using stoichiometric amounts of polyvinylidene difluoride (PVDF). The vacancy order was found to be unique so far for perovskite compounds, and the connectivity pattern can be explained by the formula Ba3(FeX6/2)(FeX5/2)(FeX3/2X1/1), with X=O/F. Mössbauer measurements were used to confirm the structural analysis and agree with the presence of Fe3+ in the above mentioned coordination environments. Group–subgroup relationships were used to build a starting model for the structure solution and to understand the relationship to the cubic perovskite structure. Furthermore, a comparison of a variety of vacancy-ordered iron-containing perovskite-type structures is given, highlighting the factors which favour one structure type over the other depending on the composition.

The Microwave Electromagnetic Properties of Sr<sup>2+</sup> Doped DyFeO<sub>3</sub>

Perovskite type ferrite doped with Sr2+, Dy1-xMxFeO3(x = 0.0, 0.05, 0.15, 0.25) were prepared by the conventional solid-state reaction. The structure and electromagnetic properties of the calcined samples were studied using powder X-ray diffraction (XRD, Rigaku D/Max-2500) and network analyzer (Agilent 8722ET). All the XRD patterns showed the single phase of the perovskite type ferrite without other intermediate phase when x≤0.25. The electromagnetic properties of the samples have been studied at the frequency range from 2 GHz to 18 GHz. The ε′ and ε′′ values were improved significantly. Both the values of ε′ and ε′′ increase with the increasing content of the doped cations Sr2+, and the peak value of ε′′ was about 2.2 at 12.1 GHz position when x = 0.25.

Oxygen nonstoichiometry, Mössbauer spectra and mixed conductivity of Pr 0.5Sr 0.5FeO 3−δ

The oxygen deficiency of perovskite-type Pr 0.5Sr 0.5FeO 3− δ , studied by coulometric titration, thermogravimetry and Mössbauer spectroscopy, is significantly higher than that in La 0.5Sr 0.5FeO 3− δ at 973–1223 K. The variations of hole mobility and Seebeck coefficient in oxidizing atmospheres, where the total conductivity of praseodymium-strontium ferrite is predominantly p-type electronic, suggest progressive delocalization of the p-type charge carriers on increasing oxygen chemical potential. As for other perovskite-type ferrites, reduction leads to the co-existence of vacancy-ordered and disordered domains. The n-type electronic conductivity of Pr 0.5Sr 0.5FeO 3− δ at reduced p(O 2) and the hole transport under oxidizing conditions are both lower compared to the La-containing analogue. Analogous conclusion was drawn for the ionic conductivity, calculated from the steady-state oxygen permeation data under oxidizing conditions and from the p(O 2)-dependencies of total conductivity in the vicinity of electron-hole equilibrium points where the average iron oxidation state is 3+. The similar activation energies for partial ionic and electronic conductivities in Ln 0.5Sr 0.5FeO 3− δ (Ln=La, Pr) indicate that the presence of praseodymium does not alter any of the conduction mechanisms but decreases the charge-carrier mobility due to the smaller radius of Pr 3+ cations stabilized in the perovskite lattice.

Mixed Conductivity of Ce- and Nb-Substituted (La, Sr)FeO<sub>3-δ</sub>

Due to relatively high oxygen permeability, lanthanum-strontium ferrite phases are of interest as ceramic membrane materials for the partial oxidation of natural gas. This work was focused on the study of perovskite-type ferrites co-doped with Sr2+ and Ce4+ or Nb5+, with particular emphasis on the ionic transport and thermodynamic stability limits at low oxygen chemical potentials. Dense membranes of La0.5-2xCexSr0.5+xFeO3-δ (x = 0 - 0.2) and La0.5-2ySr0.5+2yFe1-yNbyO3-δ (y = 0 - 0.1) were characterized employing X-ray diffraction (XRD), scanning electron microscopy (SEM), dilatometry, oxygen permeation and faradaic efficiency studies, and the measurements of total conductivity and Seebeck coefficient in the oxygen partial pressure range from 10-20 to 0.5 atm. The incorporation of Ce4+ or Nb5+ was found to decrease thermal expansion and electronic transport parameters, whereas the ionic conductivity behavior is complex, indicating the relevance of redox interactions of the variable-valence cations and the concentration of mobile oxygen vacancies.

The Electrochemical Oxidation : a New Way for Preparing Highly Oxidized Ferrites

The electrochemical oxidation is depicted as a novel and powerful route for preparing highly oxidized ferrites. The reaction is achieved at room temperature, in air, in alkaline solution (1M KOH or NaOH). This process has been used for preparing various new compounds and especially perovskite-type ferrites such as SrMO 3 ( M = Fe or/and Co ) or Sr 2 LaFe 3 0 9.8 . The characterization of the samples shows that they remain well crystallized after the electrochemical treatment and bulky oxidized. The reaction can be described as an intercalation process of oxygen atoms into the host lattice of the starting material. For instance, the chemical analysis as well as Mossbauer spectroscopy data of SrFeO 3 or of Sr 2 FeCoO 6 reveal iron cations to be exclusively tetravalent. On the other hand, Sr 2 LaFe 3 0 9-e exhibits at decreasing temperature a disproportionation of the mixed valence of iron. Potentiostatic and galvanostatic experiments have shown that the amount of intercalated oxygen can be controlled and that the process is reversible. A mechanism involving the formation of O - species that can diffuse inside the bulk of the material, is proposed.

Oxidation of Reduced Perovskite-type Ferrites

The oxidation of the reduced solid solution Sr1-xMxFeO2.5+x/2(MY3+ and La3+) was studied by means of the high temperature X-ray diffractometry and the thermogravimetry. The end member SrFeO2.5, which is isostructural with the brownmillerite having orthorhombic symmetry, changes to the cubic perovskite-type structure when Sr is substituted for Y in the composition range of 0.15<x<0.25 and for La in the range of 0.30<x<0.60. When the samples were heated at a constant rate in air, the oxidation reaction of SrFeO2.5 started at 356°C and ceased at 458°C and the structure of SrFeO2.5 then transformed into the cubic perovskite-type. The increase of the M content in this solid solution lowered both the starting and the ceasing temperature of the oxidation. Oxygen self-diffusion coefficient was successfully evaluated by a diffusion equation derived from the observation of the weight gains due to the incorporation of oxygen. The activation energy for the mobility of oxygen in the oxygen-deficient perovskite- type structure was obtained to be 6∼8 kcal/mole. The energy required for the structural change from the brownmillerite-type phase to the cubic perovskite-type one was estimated to be 43 kcal/mol which decreased with the increasing M content.