Room Temperature 57Fe Research Articles

Experimental Testing of the Theoretically Predicted Magnetic Properties for Kagomé Compounds in the Li-Fe-Ge System.

Investigating material properties is essential to assessing their application potential. While computational methods allow for a fast prediction of the material structure and properties, experimental validation is essential to determining the ultimate material potential. Herein, we report the synthesis and experimental magnetic properties of three previously reported Kagome compounds in the Li-Fe-Ge system. LiFe6Ge4, LiFe6Ge5, and LiFe6Ge6 were predicted to have ferromagnetic or antiferromagnetic ground states. The hydride route that replaces the ductile Li metal with salt-like LiH proved to be an excellent alternative for the facile synthesis of the Li-Fe-Ge powders with appreciable purity, permitting the investigation of their bulk magnetic properties. Magnetometry below room temperature and room-temperature 57Fe Mössbauer spectroscopy collectively indicate an antiferromagnetic ground state for the three compounds with ordering temperatures above 300 K, contrary to the prediction of ferromagnetic ground states. Moreover, Mössbauer spectroscopy reveals a magnetization of 1.1-1.3 μB/Fe atom for the Li-Fe-Ge compounds, while higher moments of 1.63-2.90 μB/Fe atom were theoretically predicted. Experimental (in)validation addresses the issue of inaccuracy in determining material properties in silico only and helps to improve the prediction power of the computational models. This work underlines that the contribution of experimentalists continues to be valuable for the accurate determination of structure-property relationships in solid-state materials.

Structural and magnetic characterization of Co2+ substituted Ni-Zn ferrite nanoparticles synthesized by refluxing co-precipitation and their potential application as gas sensors

This work investigates the effect of Co2+ content on the structural, magnetic, and gas sensing properties of Ni0.3Zn0.7-xCoxFe2O4 (x=0.00, 0.05, 0.15, 0.25, 0.35) nanoferrites synthesized by refluxing co-precipitates of metal chlorides. X-ray diffraction and electron microscopy were used to study the structure and morphological aspects of the materials. X-ray diffraction (XRD) patterns revealed single-phase nanomaterial with shrinking lattice constants from 8.41 to 8.38 Å with increasing Co2+ content. The crystallite sizes varied slightly between 9.95 and 10.06 nm. The vibrating sample magnetometer (VSM) was used to extract information of the magnetization of the materials at room temperature. Saturation magnetization increased from 39.5 to 48.5 emu/g from x = 0 to 0.25 and dropped at x = 0.35 to 46.2 emu/g. Furthermore, the coercivity, effective anisotropy constant and remnant magnetization increased from 170 to 400 Oe, 696 to 1925 erg/Oe, and 1.15 to 2.72 emu/g respectively. A distinct correlation between the inverse initial magnetic susceptibility and unit cell volume of the materials was found. Room temperature 57Fe Mössbauer spectra revealed a magnetic transition from paramagnetic to ordered magnetic phase with increased Co2+ content. Except for sample with x = 0 and 0.15, where suspected co-existence of Fe3+ and Fe2+ was observed Fe3+ ions were observed. Zn2+ ions were found to largely occupy the B sites. Specific surface area and the average pore size distribution increased from 61 – 85 m2 g−1 and 22 – 25 nm respectively, with the addition of Co2+ content. The composition x= 0 showed exceptional response to propanol suggesting good suitability for sensor fabrication. All samples revealed the selectivity preferences in the order of propanol > ethanol > methanol.

Study of structural, electrical and hyperfine properties of Dy doped YFeO3

Investigations on effect of Dysprosium (Dy) on structural and electrical properties of polycrystalline Yttrium Iron Oxide (YFeO3) are presented in this paper. Y1xDyxFeO3(x = 0,0.2,0.4,0.6,0.8) samples are prepared through sol-gel process. Rietveld refined XRD plots confirms formation of mono phase orthorhombic structures. From the Rietveld data, lattice parameter values are found to increase with increase in Dy concentration. Raman spectroscopic study enables us to understand the subtle changes occurring in crystal structure due to Dy doping and also confirms the pure phase of samples. When Dy+3 is doped in place of Y+3, Raman bands are seen to be shifted to lower frequencies, which indicates an increase in lattice constants in the samples. SEM morphology investigations demonstrate that grains are not homogeneous in size and are irregular in shape. Up to x = 0.4, J-E graphs indicate a rise in leakage current density but however on further substitution of Dy, leakage current is found to decrease. From ln J vs ln E plots, it is observed that Ohmic conduction dominates the whole electric field range. Room temperature 57Fe Mössbauer studies dismiss any existence of magnetic impurities and confirm that samples have magnetic ordering.

Synthesis, crystal structure, 57Fe Mössbauer spectroscopy and magnetic properties of high-spin iron(III) anionic complexes [Fe(azp)2]- (H2azp = 2,2′- dihydroxyazobenzene) with organic cations

Two new iron(III) complexes, (TrBA)[Fe(azp)2] (1) and (TrPA)[Fe(azp)2] (2) (H2azp = 2,2′- dihydroxyazobenzene, TrBA = tributylammonium and TrPA = tripropylammonium) were prepared by reaction of the azo dye H2azp, tributylamine (or tripropylamine) and FeCl3 in methanolic solution. The crystal structure of 1 was determined by single-crystal X-ray analysis. The asymmetric unit contains one tributylamonnium cation and one [Fe(azp)2]- anion with an octahedrally coordinated Fe(III) ion with an FeN2O4 chromophore – the tridentate ligand azp2- is coordinated by two deprotonated phenolic oxygen atoms and one nitrogen atom from the azo group. The single-crystal X-ray analysis of 1 suggests that an NH⋅⋅⋅O hydrogen bonding interaction between the TrBA cation and the [Fe(azp)2]- anion may induce distortion of the coordination polyhedron resulting in a high-spin complex. Magnetic investigation (DC magnetic data) revealed a high-spin state in the studied temperature range (2–300 K) for both 1 and 2. AC susceptibility measurements for 1 showed a field-induced slow relaxation of magnetization with an atypical temperature dependence of the relaxation time. Moreover, the cryogenic and room temperature 57Fe Mössbauer spectroscopy parameters confirmed the high-spin state of 1. The study was also supported by theoretical calculations at the DFT level of theory.

Low temperature hydrothermal synthesis of Na3Fe2(PO4)2F3 and its cathode electrochemistry in Na- and Li-ion batteries

Na3Fe2(PO4)2F3 has been synthesized under mild hydrothermal conditions. The structure of Na3Fe2(PO4)2F3 has been solved using single-crystal X-ray diffraction and the phase purity has been evaluated by Rietveld refinement from high resolution synchrotron powder X-ray diffraction data. Both methods establish the accuracy of P42/mnm space group and synthesis of so called beta2-polymorph. Room temperature 57Fe Mössbauer spectroscopic studies confirm 3+ oxidation state of the compound. The electrochemical properties have been investigated with respect to Li- and Na-ion batteries. Galvanostatic charge-discharge studies indicate that up to ~0.6 lithium ions and only ~0.4 sodium ions per Fe can be inserted at an average voltage of 2.82 and 2.42 ​V with respect to Li+/Li and Na+/Na in lithium and sodium ion batteries, respectively. The cycle-life studies indicate that Li-ion batteries show very stable capacity retention over long cycles while Na-ion batteries tend to lose capacity after ~35 cycles at C/5 rate.

Synthesis and characterization of the structural and magnetic properties of the Sm3-xGdxFe5O12(x = 0.0–1.0) garnets using solid-state reaction and citrate methods

The present work focused on synthesis and characterization of the new garnet-type materials with formula Sm3-xGdxFe5O12 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0), using both the citrate and solid-state reaction methods. The influence of the substituting cation and the synthesis routes on the structural, morphological, and magnetic properties were evaluated by X-ray diffraction (XRD), Raman spectroscopy, vibrating sample magnetometry (VSM), scanning electron microscopy (SEM) and 57Fe Mössbauer spectroscopy. The results showed that Gd3+ substitution allows obtaining single-phase materials, with cubic structure and space-group symmetry Ia3¯d (230). The Gd3+ substitution modified the magnetic response of the Sm3Fe5O12 garnet, generating a decrease of the saturation magnetization (Ms), coercive field (Hc), remaining magnetization (Mr), besides, the sample with x = 1.0 exhibited a Tcomp of ≈60 K. The citrate method allowed obtaining smaller particle sizes, leading to an increase of the Hc and Mr, and decreasing the Ms, this compared with the samples obtained by the solid-state reaction method. The ferrimagnetic behavior of the materials was confirmed by room temperature 57Fe Mössbauer spectroscopy, which showed the existence of magnetic hyperfine fields and Fe3+ in the different crystallographic sites. This work shows that the use of the citrate method allows obtaining garnets of high purity and with outstanding magnetic and crystalline properties, making them applicable for transformers and microwave switches production. On the other hand, the garnets were synthesized at lower temperatures (300 °C) and less time (18 h) than those used in the solid-state reaction method, decreasing the production costs.

57Fe Mössbauer Spectroscopy for the Study of Nanostructured Mixed Mn-Co Spinel Ferrites.

An oleate-based solvothermal approach has been employed to produce pure CoFe₂O₄ and MnFe₂O₄ and mixed Co-Mn ferrites having manganese content in the range 0.13-0.65 and crystallite size of about 8-9 nm. The structural and magnetic properties, studied by powder X-ray diffraction and room temperature 57Fe Mössbauer spectroscopy, have allowed to ascertain the formation of a unique spinel phase in which cobalt and manganese are present and to get insights on the cation distribution and the correlated magnetic properties.

Mechanical Amorphization and Recrystallization of Mn-Co(Fe)-Ge(Si) Compositions

Mechanical alloying using a planetary ball mill allowed us to obtain two homogeneous systems formed by units with nanometer size and MnCo0.8Fe0.2Ge1−xSix stoichiometry (x = 0 and 0.5). The phase evolution of the systems with the milling time was analyzed using X-ray diffraction. Thermal stability of the final products was studied using differential scanning calorimetry. Room temperature 57Fe Mössbauer spectroscopy was used to follow the changes in the Fe environments. A paramagnetic Co-based amorphous phase developed in both alloys as milling progressed. However, while the presence of Si stabilized the Mn-type phase, mechanical recrystallization was observed in a Si-free composition leading to the formation of a MnCo(Fe)Ge intermetallic (Pnma space group) with a crystal size of 7 ± 1 nm. Mössbauer results indicate that Fe atoms migrate from the initial bcc phase to the amorphous and intermetallic phases.

Soft chemical routes to electrochemically active iron phosphates.

New iron phosphates with related structures have been synthesized using hydrothermal and ion-exchange routes, and their electrochemical properties were investigated. First, NaFe(HPO4)2 was synthesized employing a hydrothermal route and its structure was determined from single-crystal X-ray diffraction data. Subsequent Na+ and partial proton ion exchange with Li+ ion produced a known phase, Li2Fe(H0.5PO4)2, and complete deprotonation of Li2Fe(H0.5PO4)2 with Li+ by employing a solid-state ion-exchange route produced the new phase Li3Fe(PO4)2. The structure of the latter was solved from synchrotron powder X-ray data by employing ab initio methods. All of these phases are highly crystalline, built up of similar connectivities between FeO6 octahedra and PO4 tetrahedral units. Magnetic susceptibility measurements and room-temperature 57Fe Mössbauer spectroscopic studies confirm the 3+ oxidation state of the compounds and their antiferromagnetic ordering with Li2Fe(H0.5PO4)2 showing some interesting metamagnetic behavior. The compounds are stable up to 400 °C and undergo facile electrochemical lithium/sodium insertion through the reduction of Fe3+ to Fe2+. Galvanostatic charge-discharge studies indicate that up to 0.6 lithium ion and 0.5 sodium ion per formula unit can be inserted at average voltages of 3.0 and 2.75 V for lithium and sodium ion batteries, respectively, for NaFe(HPO4)2. The partially Li ion exchanged compound Li2Fe(H0.5PO4)2 showed better cycle life and experimentally achievable capacities up to 0.9 Li insertion with strong dependence on particle size. The electrochemical Li insertion in Li3Fe(PO4)2 was also investigated. The electrochemistry of these three related phases were compared with each other, and their mechanism of Li insertion was investigated by ex situ PXRD.

Electrochemistry of Illusive Barbosalite, Fe2+Fe3+2(PO4)2(OH)2: An Iron Phosphate Related to Lipscombite Structure

Barbosalite, an iron hydroxy-phosphate belonging to the family of Lazulite has been synthesized using hydrothermal route and its electrochemical property is investigated for the first time with respect to Li-ion batteries. The structure, as determined from single-crystal X-ray diffraction data, built up of undulating layers of FeO6 octahedra, consisting of trimers and PO4 tetrahedral units. Magnetic susceptibility measurements show predominant overall anti-ferromagnetic interactions and room temperature 57Fe Mössbauer spectroscopic studies confirm the mixed 3+ and 2+ oxidation states of Fe in the compound. The compound is stable up to 400°C and undergo facile electrochemical lithium insertion. Galvanostatic charge-discharge studies indicate that up to 0.7 lithium ions per formula unit can be inserted at an average voltage of 2.6 V. The barbosalite phase seems to undergo irreversible phase transition upon lithiation as evident from the powder X-ray diffraction pattern of the reduced and oxidized phases.

An Investigation of the Fe-Mn-Si System for Li-Ion Battery Negative Electrodes

Fe-Mn-Si alloys prepared by ball milling were investigated as negative electrodes for Li-ion batteries. X-ray diffraction and room temperature 57Fe Mössbauer measurements were used to characterize alloy structure, which revealed the formation of ternary (Fe,Mn,)11Si19 solid solutions. During cycling in Li cells, the formation of crystalline Li15Si4 was effectively suppressed by inactive transition metal silicide phases, resulting in good cycling performance. In addition, Fe was found to be superior to Mn in avoiding excessive side reactions with electrolyte during cycling.

Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys exposed to air at 870 K studied by TMS, CEMS and XPS

The room temperature 57Fe Mössbauer and XPS spectra were measured for polycrystalline Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys which were exposed to air at 870 K. The spectra were collected using three techniques: the transmission Mössbauer spectroscopy (TMS), the conversion electron Mössbauer spectroscopy (CEMS) and the X-ray photoelectron spectroscopy (XPS). The combination of these experimental methods allows to determine changes in atomic composition in the bulk, in the subsurface layer and on the surface of the studied alloys. The results obtained by XPS confirm the strong chromium segregation process on the surface of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys. At the same time, the CEMS and TMS measurements reveal that in both samples, during the exposure to air at 870 K, iron oxides were formed. These findings lead to conclusion that the formation of a stable Cr oxide film on the surface of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys, does not prevent atmospheric corrosion at 870 K. Moreover, the observed by CEMS and TMS, oxidation process of iron atoms is slower for Fe0.88Cr0.12 than for Fe0.85Cr0.15. This fact is in agreement with XPS results which show that Cr surface segregation process is more effective in case of Fe0.88Cr0.12.

Studies of Si-Fe-C Electrode Materials Prepared by Combinatorial Sputter Deposition

Four libraries of the Si-Fe-C system were produced using combinatorial sputtering methods. X-ray diffraction was used to study the structure of the thin films and showed that all portions of the films were amorphous or nanostructured. Room temperature 57Fe Mössbauer spectroscopy was utilized to study the atomic environment around the Fe atoms. Cyclic voltammetry measurements were performed to study the behavior of these materials as negative electrodes for Li-ion batteries. Through a combined consideration of Mössbauer effect measurements and electrochemical data, it is suggested that the alloys are a combination of Si-Fe and Si-C phases and active Si and C nanometer-scale grains. Some of these electrodes (for example, Si66Fe16C18) show initial specific reversible capacities of near 1500 mAh/g and specific capacities of near 1200 mAh/g after 25 cycles.

Mössbauer spectroscopic studies of Al3+ ions substitution effects in superparamagnetic Mg0.2Mn0.5Ni0.3AlyFe2−yO4 compositions

Nanoparticles of Al3+ ions substituted MgMnNi materials with compositions Mg0.2Mn0.5Ni0.3AlyFe2−yO4 (y = 0.15–0.25) were synthesized by citrate precursor technique. Samples were characterized by X-ray diffraction, transmission electron microscopy, vibrating sample magnetometer and room temperature 57Fe Mössbauer spectroscopy. Saturation magnetization decreases with increasing Al3+ ions concentration because replacement of Fe3+ ions by Al3+ ions at octahedral B-site weaken sublattice interaction and lowers magnetic moments. Mössbauer spectral studies show that as-prepared nano-sized samples are superparamagnetic at room temperature. Superparamagnetic relaxation was observed for small crystallite in samples with higher Al content, which is attributed to weakening of A–B exchange interaction. Mössbauer spectra at 300 K show a gradual collapse of magnetic hyperfine splitting typical for superparamagnetic relaxation. An increase in inversion parameter is observed with increasing Al3+ ions substitution, which is attributed to decrease in crystallite size.

Mössbauer and Electrochemical Investigations of Carbon-Rich Fe1-xCx Films

A thin film binary library of carbon-rich Fe1-xCx (0.47≤x≤0.97) alloys was prepared by combinatorial sputtering of carbon and iron. The sputtered library was characterized by X-ray diffraction and room temperature 57Fe Mössbauer effect spectroscopy to determine its microstructure. X-ray diffraction results show that the Fe1-xCx film is amorphous in the whole composition range of the library. For 0.52≤x≤0.59, a hyperfine field distribution and a quadrupole splitting distribution as obtained from Mössbauer spectra indicate the presence of a ferromagnetic phase and a paramagnetic phase in this regime. With increasing of carbon content, for 0.61≤x≤ 0.97, the sextet disappears and two paramagnetic doublets splitting appear suggesting two different Fe sites. The electrochemical performance of the Fe1-xCx film was investigated in lithium cells and the presence of Fe was found to increase the reversible capacity per mass of carbon over that of a pure carbon electrode.

Mechanically Milled Fe-Si-Zn Alloys as Negative Electrodes for Li-Ion Batteries

Fe-Si-Zn alloys prepared by ball milling were investigated as negative electrodes for Li-ion batteries. X-ray diffraction and room temperature 57Fe Mössbauer measurements were used to characterize alloy structure. Electrochemistry of the alloys was determined at 30°C. Formation of crystalline Li15Si4 was suppressed during cycling for compositions with greater than 10 at. % Fe. Unlike sputtered films of the same composition, the incorporation of Zn in these ball milled alloys did not suppress Li15Si4 formation. Instead, Zn behaved as if it were in a physical mixture with the other alloy components. This study shows that sputtered and ball milled alloys of the same composition can have very different electrochemical properties.

Oxidation and surface segregation of chromium in Fe–Cr alloys studied by Mössbauer and X-ray photoelectron spectroscopy

The room temperature 57Fe Mössbauer and XPS spectra were measured for polycrystalline iron-based Fe–Cr alloys. The spectra were collected using three techniques: the transmission Mössbauer spectroscopy (TMS), the conversion electron Mössbauer spectroscopy (CEMS) and the X-ray photoelectron spectroscopy (XPS). The combination of these experimental techniques allows to determine changes in Cr concentration and the presence of oxygen in bulk, in the 300nm pre-surface layer and on the surface of the studied alloys.

Mössbauer effect studies of Fe–C combinatorially sputtered thin films

Alloys of Fe1− x C x were produced using combinatorial sputtering methods. The composition of the films as a function of position was determined using electron microprobe techniques and the results have shown that a composition range of about 0.35 < x < 0.75 was obtained. X-ray diffraction methods were employed to study the structure of the thin films and showed that all portions of the films were amorphous or nanostructured. Room temperature 57Fe Mössbauer spectroscopy was utilized to study the atomic environment around the Fe atoms. Hyperfine field distributions of ferromagnetic alloys, as extracted from the Mössbauer analysis, suggested the existence of two classes of Fe sites: (1) classes of Fe sites that have primarily Fe neighbours corresponding to a high-field component in the distribution and (2) classes of Fe sites that have a greater number of C neighbours, corresponding to a low-field component. The magnetic splitting decreased as a function of increasing carbon concentration and alloys with x greater than about 0.68 were primarily paramagnetic in nature. These spectra exhibited distributions of quadrupole splitting with mean splitting in excess of 1.0 mm/s. This indicates a higher degree of local asymmetry around the Fe sites than typically seen in other Fe-metalloid systems.

Structure and phase analysis of one-pot hydrothermally synthesized FePO4-SBA-15 as an extremely stable catalyst for harsh oxy-bromination of methane

FePO4-SBA-15 (OP) was directly synthesized via a one-pot hydrothermal technique, using Fe(NO3)3 and H3PO4 as the precursors. FePO4/SBA-15 (IMP) was also prepared as a reference, using an impregnation method and commercially available SBA-15 as the support. The yielding samples were employed to catalyze the harsh oxy-bromination of methane (OBM) reaction, showing similar initial catalytic performances. The fresh and spent samples after catalytic reaction were thoroughly characterized by N2-physisoption, inductively coupled plasma, wide- and small-angle X-ray diffraction, transmission electron microscopy, diffuse reflectance UV–vis spectroscopy, temperature-programmed oxidation, and room temperature 57Fe Mössbauer spectroscopy. It was found that the FePO4 was in good crystalline in the OP sample while it was in amorphous for the IMP catalyst. Despite this difference, both FePO4 phases in the fresh samples were transformed into Fe7(PO4)6 and Fe2P2O7 in the spent ones. Furthermore, the OP catalyst showed excellent stability in a period of 1000h time-on-stream performance without apparent deposition of cokes. The losses of P and Fe after the catalytic evaluation were only 9.5% and 15.5%, respectively, while the ratio of P/Fe remained close to 1.0. N2-adsorption and TEM observations confirmed that the mesoporous pores were extremely stable under the harsh reaction ambience, which might play a crucial role in the stability test.

Cation distribution and micro level magnetic alignments in the nanosized nickel zinc ferrite

Ni0.5Zn0.5Fe2O4 nanoparticles were synthesized through sol–gel synthesis method. Sintering at elevated temperatures was employed to obtain samples with higher particle sizes. The structural characterization using X-ray diffraction suggests that the three samples possess uniformal cation distribution. The variation of magnetization as a function of applied magnetic field at room temperature is studied using a vibrating sample magnetometer and all the samples found to exhibit nearly zero remanence and zero coercivity suggesting a superparamagnetic behaviour. The variation of magnetizations of the three different samples with temperature was studied by ZFC–FC technique at two different applied fields of 10Oe and 1000Oe. In order to study the possibility of structural changes, chemical and coordination differences of iron in the nanocrystalline Ni–Zn ferrite particles, room temperature 57Fe Mössbauer spectroscopy study was used. The application of 5T magnetic field at 5K temperature resolves the two sub spectra and canting angle of 27o and 12o were observed which clearly indicates the noncollinear magnetic alignment in nano crystallites of Ni0.5Zn0.5Fe2O4. The neutron diffraction study was performed at seven different temperatures 20, 50, 100, 150, 200, 250 and 300K. Rietveld refinement of the neutron diffraction data was performed to deduce the basic structural parameters, cation distribution and micro level magnetic alignments in the nanosized nickel zinc ferrite.