Assessment of the optimum degree of Sr3Fe2MoO9 electron-doping through oxygen removal: An X-ray powder diffraction and Fe Mössbauer spectroscopy study

We describe the preparation and structural characterization by X-ray powder diffraction (XRPD) and Mössbauer spectroscopy of three electron-doped perovskites Sr₃Fe₂MoO_9−δ with Fe/Mo = 2 obtained from Sr₃Fe₂MoO₉. The compounds were synthesized by topotactic reduction with H₂/N₂ (5/95) at 600, 700 and 800 °C. Above 800 °C the Fe/Mo ratio changes from Fe/Mo = 2-1 < Fe/Mo < 2. The structural refinements of the XRPD data for the reduced perovskites were carried out by the Rietveld profile analysis method. The crystal structure of these phases is cubic, space group , with cationic disorder at the two different B sites that can be populated in variable proportions by the Fe atoms. The Mössbauer spectra allowed determining the evolution of the different species formed after the treatments at different temperatures and confirm that Fe ions in the samples reduced at 600, 700 and 800 °C are only in the high-spin Fe³⁺ electronic state.     

Mössbauer studies on Zn-substituted iron molybdate

Mössbauer spectra of Fe_2−yZn_yMoO₄ spinel ferrites were recorded in their paramagnetic state. Paramagnetic Mössbauer spectra of all the samples show broad absorption peaks due to the presence of Fe²⁺ and Fe³⁺ ions on both sites (A and B) of the spinel lattice. All the spectra have been fitted with four doublets, using a least squares fitting program. The isomer shift and quadrupole splitting values show that Fe_A²⁺, Fe_A³⁺, Fe_B³⁺ and Fe_B^2.5+ ions are present. Fe_B^2.5+ represents the presence of the Fe²⁺ and Fe³⁺ ions on the B-sites, which take part in charge hopping. The results of electrical resistivity and magnetic measurements support charge hopping between Fe²⁺ and Fe³⁺ ions on B-sites.     

XRD and Sn Mössbauer spectroscopy characterization of SnSe obtained from a simple chemical route

Crystalline tin selenide semiconductor was synthesized by a chemical route. Selenium powder reacted with potassium boronhydride, giving a soluble selenium species potassium seleniumhydride. The reaction of potassium seleniumhydride with tin chloride produced crystalline tin selenide, which was characterized by X-ray diffraction, ¹¹⁹Sn Mössbauer spectroscopy and scanning electronic microscopy. The material was thermally treated, in nitrogen flow, at 300 and 600 °C for 2 h and the particle size evolution was studied by X-ray diffraction. The X-ray diffraction and ¹¹⁹Sn Mössbauer results showed that a mixture of tin oxides and orthorhombic tin selenide was obtained.     

Impact of transition metal doping on Mössbauer,electrical and dielectric parameters of structurally modified lithium ferrite nanomaterials

Zr–Mn doped spinel lithium ferrites Li_0.5Fe_2.5−2xZr_xMn_xO₄ (0.0 ≤ x ≤ 0.5) are synthesized using the citrate precursor method. The spinel ferrite is formed at a relatively lower annealing temperature (873 K) compared to those synthesized by other conformist methods. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis are carried out to determine the cell parameters, crystallite size and grain growth. Cation distribution and site preferences for the doped ions are determined by Mössbauer spectroscopy at room temperature. The impact of doping of Li_0.5Fe_2.5O₄ with the binary mixtures of transition metals (Mn, Zr) on hyperfine interaction parameters (δ, Δ and H_int), electrical resistivity (ρ), dielectric constant (?) and dielectric loss tangent (tan δ) over the frequency range of 100 Hz to 3 MHz is discussed in details. Zr–Mn doping enhanced the DC electrical resistivity and decreased the loss tangent value which is considered useful for technological application in microwave and telecommunication devices.     

Fine Fe16N2 powder prepared by low-temperature nitridation

α-Fe was prepared by reduction of a fine γ-Fe₂O₃ powder under hydrogen at 500 °C for 8 h. The α-Fe fine powder, about 100 nm in crystallite size, was then nitrided under an ammonia flow at 130 °C for 100 h. X-ray single-phase Fe₁₆N₂ was obtained with a magnetization value of 225 emu/g at room temperature under a magnetic field of 15 kOe. The Mössbauer spectrum at room temperature could be resolved into three sets of hyperfine fields with an average magnetic moment of 2.52 μ_B. An additional paramagnetic component was present in the spectrum with an area ratio of 19%.     

Structural and magnetic properties of Mn nanoparticles prepared by arc-discharge

Mn nanoparticles are prepared by arc discharge technique. MnO, α-Mn, β-Mn, and γ-Mn are detected by X-ray diffraction, while the presence of Mn₃O₄ and MnO₂ is revealed by X-ray photoelectron spectroscopy. Transmission electron microscopy observations show that most of the Mn nanoparticles have irregular shapes, rough surfaces and a shell/core structure, with sizes ranging from several nanometers to 80 nm. The magnetic properties of the Mn nanoparticles are investigated between 2 and 350 K at magnetic fields up to 5 T. A magnetic transition occurring near 43 K is attributed to the formation of the ferrimagnetic Mn₃O₄. The coercivity of the Mn nanoparticles, arising mainly from Mn₃O₄, decreases linearly with increasing temperature below 40 K. Below the blocking temperature T_B ≈ 34 K, the hysteresis loops exhibit large coercivity (up to 500 kA/m), owing to finite size effects, and irreversibility in the loops is found up to 4 T, and magnetization is not saturated up to 5 T. The relationship between structure and the magnetic properties are discussed.     

Mössbauer spectrometry study of early stage spinodal decomposition in Fe-Cr-Co alloy under high magnetic field

Local structure in a low-cobalt-type Fe-25Cr-12Co-1Si ferromagnetic alloy spinodal decomposed under an external magnetic field up to 120 kOe was investigated by Mössbauer spectrometry. The high magnetic field was found to significantly affect the local structure in the alloy formed at the early stage of phase decomposition. It was found that high magnetic field favors the acceleration of phase decomposition of ferromagnetic alloy at the early stage, resulting in the enhancement of average hyperfine field. The effect of high magnetic field on spinodal decomposition in ferromagnetic alloy was initially interpreted based on the free energy analyses.     

Synthesis and characterization of Ni1−xZnxFe2O4 spinel ferrites from tailored layered double hydroxide precursors

In this paper, a series of pure Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrites have been synthesized successfully using a novel route through calcination of tailored hydrotalcite-like layered double hydroxide molecular precursors of the type [(Ni + Zn)_{1 − x − y}Fe_y²⁺Fe_x³⁺(OH)₂]^x+(SO₄²⁻)_x/2·mH₂O at 900 °C for 2 h, in which the molar ratio of (Ni²⁺ + Zn²⁺)/(Fe²⁺ + Fe³⁺) was adjusted to the same value as that in single spinel ferrite itself. The physico-chemical characteristics of the LDHs and their resulting calcined products were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy. The results indicate that calcination of the as-synthesized LDH precursor affords a pure single Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrite phase. Moreover, formation of pure ferrites starting from LDHs precursors requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring, because all metal cations on the brucite-like layers of LDHs can be uniformly distributed at an atomic level.     

Iron-stabilized nanocrystalline ZrO2 solid solutions: Synthesis by combustion and thermal stability

The synthesis of Fe³⁺-stabilized zirconia by the nitrate/urea combustion route was investigated. Using several characterization techniques, including X-ray diffraction, field-emission-gun scanning electron microscopy and notably Mössbauer spectroscopy, it was possible to determine the appropriate amount of urea that allows to obtain a totally stabilized Zr_0.9Fe_0.1O_1.95 solid solution. The nanocrystalline zirconia solid solution is mostly tetragonal, but the presence of the cubic phase could not be ruled out. An in-depth study of the thermal stability in air showed that the Fe³⁺ solubility in the stabilized solid solution starts to decrease at about 875 °C which results in the formation of hematite (possibly containing some Zr⁴⁺) at the surface of the zirconia grains and further provokes the progressive transformation into the monoclinic zirconia phase.     

Hybrid inorganic/organic composites of Mg-Al layered double hydroxides intercalated with citrate, malate, and tartrate prepared by co-precipitation

Inorganic/organic composite materials composed of Mg-Al layered double hydroxides (Mg-Al LDHs) intercalated with citrate (C₆H₅O₇³⁻), malate (C₄H₄O₅²⁻), or tartrate (C₄H₄O₆²⁻) anions were prepared by a co-precipitation technique and characterized by X-ray diffraction and compositional analyses. The molecular orientation of the organic component in the interlayer space was determined. At low intercalation levels, citrate ion was inclined at an angle of 26° relative to the brucite-like layers of Mg-Al LDH. At higher solution concentrations, both the Mg(C₆H₅O₇)⁻ complex and free C₆H₅O₇³⁻ were isotropically oriented in the interlayer space. For both malate·Mg-Al LDH and tartrate·Mg-Al LDH, the organic anions were inclined at 26° relative to the Mg-Al LDH layers regardless of anion concentration.     

Characterization and magnetic properties of Sm- and Gd-substituted CoFe2O4 nanoparticles prepared by forced hydrolysis in polyol

Pure nanoparticles of the CoFe_2−xRE_xO₄ (RE = Gd, Sm; x = 0.0, 0.1) system have been prepared by forced hydrolysis in polyol. The insertion of Sm³⁺ and Gd³⁺ cations into the cobalt ferrite structure has been investigated. X-ray micro-analysis (EDX) shows that the RE contents are close to the nominal ones. X-ray diffraction (XRD) evidences a cell size increase with slight distortions in the spinel-like lattice indicating the entrance of RE³⁺ ions. Micro-Raman spectroscopy confirms the cubic inverse-spinel structure and rules out the existence of impurities like hematite. Magnetic measurements (SQUID) show important differences in the magnetic properties of the unsubstituted and substituted particles. All the particles are superparamagnetic at room temperature and ferrimagnetic at low temperature. However, their main magnetic characteristics appear to be directly dependent on the RE content.     

Catalytic activity and magnetic properties of barium hexaferrite prepared from barite ore

Barium hexaferrite (BaFe₁₂O₁₉) has traditionally been used in permanent magnets and more recently used for high density magnetic recording. The classical ceramic method for the preparation of barium hexaferrite consists of firing mixture of chemical grade iron oxide and barium carbonate at high temperature. In this paper a mixture of chemical grade hematite, barium oxide and predetermined mixtures of iron oxide ore and barite ore containing variable amounts of coke were used to prepare barium hexaferrite (BaFe₁₂O₁₉) as a permanent magnetic material. The mixtures were mixed in a ball mill and fired for 20 h in a tube furnace at different temperatures (1100, 1150, 1200 and 1250 °C). XRD, magnetic properties, porosity measurements and catalytic activity were used for characterization of the produced ferrite. The results of experiments showed that the optimum conditions for the preparation of barium hexaferrite are found at 1200 °C for the mixture of chemical grade hematite and barium oxide. It was also found that the barium hexaferrite can be prepared from the iron and barite ores at 1200 °C. The addition of coke enhanced the yield of barium hexaferrite and improved its physicochemical properties. Samples prepared from ores with coke% = 0 show the most acidic active sites, they show a higher catalytic activity towards H₂O₂ decomposition. With addition of coke the catalytic activity decreases due to the poisoning effect of carbon on the available active site.     

Hydrothermal synthesis of nanocrystalline MxZn1−xFe2O4 (M=Ni, Mn, Co; x=0.40-0.60) powders

A series of nanocrystalline M_xZn_1−xFe₂O₄ (M=Ni, Mn and Co; x=0.40-0.60) powders have been successfully prepared via hydrothermal process and characterized by XRD, TEM and IR techniques. The effects of reaction temperature and the initial pH value of the starting suspension solution on the particulate properties such as the particle size and morphology are discussed. IR spectra indicate that there are no hydroxyl in as-prepared Ni_xZn_1−xFe₂O₄ and Co_xZn_1−xFe₂O₄ powders, while there are obvious hydroxyl adsorption on the IR spectrum of Mn_xZn_1−xFe₂O₄ powder.     

Synthesis of fine magnetite powder using reverse coprecipitation method and its heating properties by applying AC magnetic field

Nano-sized FeFe₂O₄ ferrite powder for local thermal coagulation therapy was synthesized by a reverse coprecipitation method. The crystal diameter for the samples from the reverse coprecipitation method was smaller than that from a normal coprecipitation method. The crystal diameter increased with an increase in the synthesized temperature for both methods. The maximum increasing temperature under the AC magnetic field was at around a 12 nm crystal diameter. The FeFe₂O₄ powder was oxidized to Fe₂O₃ by calcination at 600 °C in ambient air. The heating ability almost depended on the hysteresis loss value. Although the increased temperature under the AC magnetic field for the fresh FeFe₂O₄ sample was very close to that for the commercialized MgFe₂O₄ powder, it was gradually decreased for FeFe₂O₄ with time in ambient air.     

Eu-Mössbauer spectroscopic and X-ray diffraction study on EuyM1−yO2−x (0 ≤ y ≤ 1.0) (M = Th, U)

¹⁵¹Eu-Mössbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the Eu_yM_1−yO_2−x (M = Th and U) systems over the entire composition range of 0 ≤ y ≤ 1.0. The XRD results of the Eu-Th system showed that a very wide defect-fluorite (DF) type phase in which oxygen vacancies (V_O) are disordered (x = y/2) is formed for 0 ≤ y < 0.5 and that two-phase regions sandwitching a narrow C-type (C) single phase around y ≈ 0.8 appear for 0.5 < y < 0.8 (DF + C) and 0.82 < y < 1.0 (C + B-type (monoclinic) Eu₂O₃). The Mössbauer results show that the isomer shifts (ISs) of Eu³⁺ in this system smoothly increase with Eu composition, y. The decrease of average coordination number (CN) of O²⁻ around Eu³⁺ with increasing y (CN = 8 − 2y) (x = y/2) results in the decrease of the average EuO bond length, which is due to the decrease of repulsion force between O²⁻ anions. This result confirms that the IS of Eu³⁺ correlates well with the average EuO bond length in oxide systems. For the Eu-U system, the lattice parameter, a₀, of the system decreases almost linearly with y, in accordance with the calculated a₀ versus y curve for the oxygen-stoichiometric (i.e. x = 0) fluorite-type dioxide (CN = 8). The ISs of Eu³⁺ in this composition range remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu₂Zr₂O₇ and Eu₂Hf₂O₇ (y = 0.5)) with O²⁻-eight-fold coordinated Eu³⁺(CN = 8).     

The preparation of magnetite Fe3O4 and its morphology control by a novel arc-electrodeposition method

Magnetite Fe₃O₄ ultrafine powder, a magnetic material, was synthesized by a novel arc-electrodeposition method. The products were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experimental results showed that the morphologies of the Fe₃O₄ produced were greatly influenced by the diameters of the metallic iron filament electrodes and the electrolyte composition.     

A study of the influence of crystallite size on the electrical and magnetic properties of CuFe2O4

An attempt has been made to clarify the fundamental assumption that the properties of materials change as the crystallite size of the material is reduced below 100 nm. CuFe₂O₄ samples of different crystallite sizes were prepared by the sol–gel and combustion methods and then analyzed by X-ray diffraction (XRD), thermal analyses (TGA/DTG) and scanning electron microscopy (SEM) techniques. The magnetic properties were studied by measuring the AC magnetic susceptibility (χ) and the Mössbauer spectroscopy. The DC electrical resistivity, dielectric constant, dielectric loss tangent, Curie temperature and hyperfine splitting of the samples change with the crystallite size. The change in the electrical properties is attributed to the formation of discrete energy levels instead of the bands. However, the magnetic parameters change due to the existence of non magnetic surface layers. The isomer shift and the hyperfine splitting show gradual increase with the increase in crystallite sizes.     

Structural and magnetic properties of a mechanochemically activated Ti-Fe2O3 solid mixture

The mechanochemical effects on the reactivity and properties of a titanium/hematite powder mixture with molar ratio of 1/2 are investigated. Crystalline-phase structure, composition, hyperfine and magnetic behaviors were analyzed as a function of activation time by means of X-ray diffraction, scanning electron microscopy, Mössbauer spectroscopy and vibrating sample magnetometry. The results showed that at relatively short activation times metallic Ti reduces part of the ferric ions, yielding a complex product formed mainly by a mix of two solid solutions Fe_3−xTi_xO₄ (titanomagnetites), both with very different x values (0 < x < 1). Also metallic iron and superparamagnetic hematite particles were detected by Mössbauer spectroscopy. As the mechanical treatment extends the composition of the reactive mixture changes, prevailing in the end the solid solution with higher x value. In contrast, when these activated samples are thermally treated the fraction of the solid solution which is richer in Ti diminishes. This fact produces a significant variation of the saturation magnetization of the obtained material.     

Dielectric and magnetic properties of ZnCo-substituted X hexaferrites prepared by citrate sol-gel process

Ba₂Zn_2−xCo_xFe₂₈O₄₆ hexaferrites with x=2.0, 1.6, 1.2, 0.8, 0.4 and 0.0 were prepared by citrate sol-gel process. They were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetry-differential scanning calorimetry (TG-DSC). The frequency-response complex dielectric constant and complex permeability of Ba₂Zn_2-xCo_xFe₂₈O₄₆ sintered at 1000-1200 °C had been investigated in the range from 100 MHz to 6 GHz. The pronounced natural resonance phenomena were observed in μ″ spectrum for the samples annealed at 1100 and 1200 °C. The natural resonance frequency of Ba₂Zn_2−xCo_xFe₂₈O₄₆ ferrites was intensively affected by the substitution of Zinc ion and annealing temperature.     

Structural properties and Mössbauer spectra of metastable NdFe12B6

Metastable ternary NdFe₁₂B₆ compound was prepared by the annealing of the amorphous Nd-Fe-B ribbons and has a main phase of NdFe₁₂B₆ with some impurities of α-Fe and Nd₅Fe₁₈B₁₈. The NdFe₁₂B₆ was found to have a SrNi₁₂B₆ structure with space group R3¯m. Mössbauer spectra of the sample were measured between 300 K and 20 K. The result indicated that an abrupt drop with increasing temperature took place at about 205 K for the hyperfine field at both Fe(18 g) and Fe(18 h) sites. The corresponding hyperfine fields were found nearly constant at ferromagnetic state about 16.9(6) T and 19.3(5) T for Fe(18 g) and Fe(18 h) sites, respectively. Accordingly, the relative size of the Fe moments was deduced.