Effect of Fe3+-doped Ca12Al14O33 cement on optical and thermal properties

This study aims to investigate the effect Fe ions doped into Ca₁₂Al₁₄O₃₃ (C12A7, 12CaO-7Al₂O₃) cement compound on its thermal and optical properties. Polycrystalline samples of Ca₁₂Al_14?xFe_xO₃₃ (where x?=?0.0, 0.5, and 1.0) were prepared via a solid state reaction in an oxygen atmosphere. The lattice constant of Ca₁₂Al₁₄O₃₃ determined using an XRD technique was in excellent agreement with first-principles calculations. With increasing Fe concentrations, the lattice constants were found to have increased. Additionally, the optical gaps of Ca₁₂Al_14?xFe_xO₃₃, (x?=?0, 0.25, 0.5, and 1.0) were 3.9?eV, 3.77?eV, 3.75?eV and 3.63?eV, respectively. It was clearly seen that the optical gap decreased with increasing Fe concentrations. As revealing by first-principles calculations, the optical gap was directly related to the electronic transition from the occupied electronic state of extra-framework O^2? ions (as free O^2? ions inside nano-cage) to the conduction band. Moreover, we also found that the thermal conductivity Ca₁₂Al_14?xFe_xO₃₃ was reduced when the larger atomic mass and atomic radii Fe was substituted into Al sites. Hence, this indicated that Fe³⁺-substitution into Al³⁺ sites of Ca₁₂Al₁₄O₃₃ cement directly affected both its optical gap and thermal conductivity.     

Mössbauer spectra of Fe3+ and Fe2+ ions in hexagonal ferrite with w-structure

The hexagonal ferrite Fe₂W = BaFe₂²⁺Fe³⁺₁₆O₂₇ exhibits a sharp ⁵⁷Fe Mössbauer spectrum at 300 and 78 K. All seven sublattices in this complicated crystal structure are detected. Fast electron exchange between Fe²⁺ and Fe³⁺ ions gives rise to sharp lines and makes them indistinguishable. At 5 K the exchange is slow and the Fe²⁺ ions are detected from the presence of a weak subspectrum with broadened lines separated from the main spectrum of the Fe³⁺ ions. Analysis shows that the Fe²⁺ ions reside exclusively on one of the seven sublattices, which is occupied statistically by Fe²⁺ and Fe³⁺ ions in the ratio of 2 : 1. For SrFe²⁺₂Fe³⁺₁₆O₂₇ the situation is the same.     

Study on dielectric and magnetic properties of Ho3Fe5O12 ceramics

Ho₃Fe₅O₁₂ ceramics with garnet structure were prepared by the solid-state reaction method. The results revealed the existence of Fe²⁺ ions have intensive influence on dielectric and magnetic properties of Ho₃Fe₅O₁₂ ceramics, which could be further confirmed by oxygen treatment. With a magnetic field lower than 10 kOe, the ME coefficient reaches 33 ps m⁻¹ at room temperature. And the ME coupling was further verified by dielectric anomaly near Néel temperature.     

Dielectric and piezoelectric properties of Fe2O3-doped (Na0.5K0.5)0.96Li0.04Nb0.86Ta0.1Sb0.04O3 lead-free ceramics

The effect of a small amount Fe₂O₃ (0.1-2 mol%) doping on the electrical properties of (Na_0.5K_0.5)_0.96Li_0.04Nb_0.86Ta_0.1Sb_0.04O₃ (NKLNTS) ceramics was investigated. It was found that the B-site substitution of Fe³⁺ does not change the crystal structure within the studied doping level and all modified ceramics have a pure tetragonal perovskite structure at room temperature. The addition of Fe₂O₃ can promote the sintering of NKLNTS ceramics, and simultaneously cause the grain growth so that Fe³⁺-doped NKLNTS compositions show degraded densification at higher doping level. Furthermore, the dielectric properties of the NKLNTS ceramics do not show a significant change by Fe₂O₃ doping. However, the addition of Fe₂O₃ was found to have a significant influence on the electric fatigue resistance and the durability against water. The presence of oxygen vacancies caused by the replacement of Fe³⁺ for B-site ions makes the NKLNTS ceramics harder.     

EPR and magnetic susceptibility studies of iron ions in ZnO-Fe2O3-SiO2-CaO-P2O5-Na2O glasses

Room temperature electron paramagnetic resonance (EPR) spectra and temperature dependent magnetic susceptibility data have been obtained on bulk x(ZnO,Fe₂O₃)(65−x)SiO₂20(CaO, P₂O₅)15Na₂O (6≤x≤21 mole%) glasses prepared by melt quenching method. EPR spectra of the glasses revealed absorptions centered at g≈2.1 and 4.3. The variations of the intensity and line width of these absorption lines with composition have been interpreted in terms of the variation in the concentration of the Fe²⁺ and Fe³⁺ ions in the glass and the interaction between the iron ions. EPR and magnetic susceptibility data of the glasses reveal that both Fe²⁺ and Fe³⁺ ions are present in the glasses, with their relative concentration being dependent on the glass composition. The studies reveal superexchange type interactions in these glasses, which are strongly dependent on their iron content.     

Valence states of iron ions in nanostructured yttrium iron garnet Y3Fe5O12 studied by means of soft X-ray absorption spectroscopy

Nanostructured samples of yttrium iron garnet Y₃Fe₅O₁₂ obtained by plastic deformation method (high-pressure torsion) were studied with help of soft X-ray absorption spectroscopy using Fe 2p and O 1s spectra. Experimental spectra were compared with crystal field multiplet calculations for Fe ions. Some amount of Fe²⁺ ions in nanostructured Y₃Fe₅O₁₂ was found. The concentration of Fe²⁺ ions was found to be increased with the increase of the degree of plastic deformation.     

Arrangement of ferric and ferrous ions at low temperatures in FeCl3 intercalated graphite

Previous work has shown that electrons donated by the graphite to the FeCl₃ intercalant freeze onto some Fe³⁺ ions to change them into Fe²⁺ ions at low temperatures. We show that the created Fe²⁺ ions are not randomly situated. Indirect evidence suggesting the formation of a specific chemical superlattice of Fe³⁺ and Fe²⁺ ions is presented. A method for direct observation of this superlattice is suggested.     

Luminescence of Fe3+ in single crystals of LiAl5O8

We have measured the luminescent properties of single crystals of LiAl₅O₈:Fe³⁺. In addition to a zero-phonon line due to Fe³⁺ in A-sites, we have observed another sharp fluorescent line at 699.2 nm which we assign to Fe³⁺ occupying B-sites. The excitation spectrum of the B-site Fe³⁺ shows characteristics similar to those of the A-site Fe³⁺ but are also shifted towards longer wavelengths. The spectra of the single crystals are compared with those of ordered and disordered powder samples.     

Fluorination of perovskite-related SrFeO3−δ

We report here on the fluorination of the perovskite-related phase SrFeO_3−δ through a low temperature (400 °C) reaction with poly(vinylidene fluoride). X-ray powder diffraction shows that fluorination leads to an expansion in the unit cell which is consistent with partial replacement of oxygen by fluorine and consequent reduction in the oxidation state of iron giving SrFeO₂F. This reduction in oxidation state is confirmed by X-ray absorption- and Mössbauer-spectroscopy. The latter show complex magnetically split hyperfine patterns for the fluorinated sample, reflecting the interactions between Fe³⁺ ions which are not possible in oxides containing Fe⁴⁺.     

Electronic structure studies of the spinel CoFe2O4 by X-ray photoelectron spectroscopy

The spinel CoFe₂O₄ has been synthesized by combustion reaction technique. X-ray photoelectron spectroscopy shows that samples are near-stoichiometric, and that the specimen surface both in the powder and bulk sample is most typically represented by the formula (Co_0.4Fe_0.6)[Co_0.6Fe_1.4]O₄, where cations in parentheses occupy tetrahedral sites and those within square brackets in octahedral sites. The results demonstrate that most of the iron ions are trivalent, but some Fe²⁺ may be present in the powder sample. The Co 2p_3/2 peak in powder sample composed three peaks with relative intensity of 45%, 40% and 15%, attributes to Co²⁺ in octahedral sites, tetrahedral sites and Co³⁺ in octahedral sites. The O 1s spectrum of the bulk sample is composed of two peaks: the main lattice peak at 529.90 eV, and a component at 531.53 eV, which is believed to be intrinsic to the sample surface. However, the vanishing of the O 1s shoulder peak of the powder specimen shows significant signs of decomposition.     

Electron spin resonance investigation of the substitution of Fe3^＋ for Ti4^＋ ions in rutile TiO2 single crystal

A Fe doped rutile TiO 2 single crystal is grown in an O 2 atmosphere by the floating zone technique.Electron spin resonance (ESR) spectra clearly demonstrate that Fe 3+ ions are substituted for the Ti 4+ ions in the rutile TiO 2 matrix.Magnetization measurements reveal that the Fe:TiO 2 crystal shows paramagnetic behaviour in a temperature range from 5 K to 350 K.The Fe 3+ ions possess weak magnetic anisotropy with an easy axis along the c axis.The annealed Fe:TiO 2 crystal shows spin-glass-like behaviours due to the aggregation of the ferromagnetic clusters.     

Prediction of defect structure in lithiated tin- and titanium-doped α-Fe2O3 using atomistic simulation

The defect structure of lithiated tin- and titanium-doped α-Fe₂O₃ has been assessed using interatomic potential calculations. Of the models considered for lithiation, a model in which Li⁺ occupies an interstitial site balanced by the reduction of Fe³⁺ to Fe²⁺ on an Fe³⁺ site was found to be more favourable than the substitution of Li⁺ on an Fe³⁺ octahedral site balanced by an O²⁻ vacancy. Insertion of lithium into the interstitial site between two adjacent M⁴⁺ ions was particularly favourable. The calculated lattice parameters decrease on lithiation as has been observed experimentally.     

ESR study of Fe3+ and Mn2+ ions on trigonal sites in ilmenite structure MgTiO3

Electron spin resonance has been observed for Fe³⁺ and Mn²⁺ ions occupying sites with trigonal symmetry in undoped and doped Verneuil-grown crystals of the ilmenite type compound MgTiO₃. At 300 K, the fine structure parameters in the spin Hamiltonian are (in 10^?4cm^?1) D = +844 (± 1), (a? F) = +118 (± 1), a = 69 (± 7) for Fe³⁺ and D = +164 (± 1), (a ? F) = +10.2 (± l), a = 7.0 (± 1) for Mn²⁺. These values are compared with literature data for Fe³⁺ and Mn²⁺ in other oxides, especially Al₂o₃, with particular reference to the recent “superposition” theory of the effect of a trigonal distortion. From the orientation of the axes of cubic pseudosymmetry of the spin Hamiltonian, and with the assumption that a has the same sign for both ions, it is proposed that Fe³⁺ and Mn²⁺ occupy the same octahedral site, namely the Mg²⁺ site. Anomalous line splittings observed for one sample were attributed to twinning on (0001) or {1120} planes.     

Synthesis and laser patterning of Bi-doped Y3Fe5O12 crystals in germanosilicate glasses

New germanosilicate glasses giving the crystallization of yttrium iron garnet Y₃Fe₅O₁₂ (YIG) and Bi-doped YIG, 23Na₂O-xBi₂O₃-(12−x)Y₂O₃-25Fe₂O₃-20SiO₂-20GeO₂ (mol%), are developed, and the laser-induced crystallization technique is applied to the glasses to pattern YIG and Bi-doped YIG crystals on the glass surface. It is clarified from the Mössbauer effect measurements that iron ions in the glasses are present mainly as Fe³⁺. It is suggested from the X-ray diffraction analyses and magnetization measurements that Si⁴⁺ ions are incorporated into YIG crystals formed in the crystallization of glasses. The irradiations (laser power: 32-60 mW and laser scanning speed: 7 μm/s) of continuous wave Yb:YVO₄ fiber laser (wavelength: 1080 nm) are found to induce YIG and Bi-doped YIG crystals, indicating that Fe³⁺ ions in the glasses act as suitable transition metal ions for the laser-induced crystallization. It is suggested that YIG and Bi-doped YIG crystals in the laser irradiated part might orient. The present study will be a first step for the patterning of magnetic crystals containing iron ions in glasses.     

Semiconduction in xFe2O3.(1-x) [3B2O3.PbO] glasses

The electrical resistivity of iron lead borate glasses was measured over the temperature range 300–700 K. The resistivity increases with the iron content and is a function of the Fe²⁺/Fe_tot ratio, but the minimum does not appear for 0.28 ≤ c ≤ 0.50. The samples with x > 15 mol % Fe₂O₃ show the presence of two activation energies for conduction. A change in the activation energy can be explained by charge transfer between iron ions in similar positions at low temperatures, and between iron ions in different positions at higher temperatures. In order to analyse the conductivity data, we have considered in all the glasses a polaronic model for conduction.     

Cation distribution in the spinels CoxFe3−xO4

Mössbauer spectra of four spinels having the general formulae Co_xFe_3?xO₄ and x ~ 1.0, 1.5, 2.0 and 2.5 have been obtained at liquid helium temperature and in a large transverse magnetic field. Intensity data have been used to determine the distribution of iron ions between octahedral and tetrahedral sites. The results are examined on the basis of two models. For one there is a competition between the Fe³⁺ ions and the cobalt ions for the two sites; while for the other there is a competition between Fe³⁺ ions and Co²⁺ ions for the two sites. The relative merits of these two extreme models are discussed.     

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Transfert électronique d'intervalence entre sites inéquivalents: Étude de CaFe3O5 par spectrométrie Mössbauer

As in magnetite Fe₃O₄, calcium ferrite CaFe₃O₅ is an oxide in which electron transfer occurs between the iron ions ($\text{Fe}{}^{\mathrm{3+}}\text{Fe}{}^{\mathrm{2+}}\text{= 2}$). This intervalence exchange process has been studied by ⁵⁷Fe Mössbauer spectroscopy and by electrical conductivity measurements. In CaFe₃O₅, the Fe³⁺ and Fe²⁺ ions occupy different crystallographic sites and have a deformed octahedral coordination. Each Fe²⁺O₆ octahedron shares an edge with two Fe³⁺O₆ octahedra. In the antiferromagnetic region (T_N = 282 ± 2 K), the Fe³⁺ and Fe²⁺ ions are well differentiated. Thermally-activated electron transfer is observed above t_n, in the paramagnetic region and is well characterized by the Mössbauer spectra. These are analyzed using the hypothesis of an electron jump limited to a trimer Fe³⁺Fe²⁺Fe³⁺ which leads to a relaxation time of 180 ns at 298 K and 80 ns at 400 K. Within this temperature interval, the process follows the Arrhenius law with an activation energy of 0.10 eV. Electrical conductivity measurements lead to similar results with an activation energy of 0.09 ± 0.02 eV.     

Electronic structure studies of Mg0.95Mn0.05Fe2−2xTi2xO4 (0x0.8)

The electronic structure of Mg_0.95Mn_0.05Fe_2−2xTi_2xO₄ (0x0.8) compound is investigated using near edge X-ray absorption fine structure, (NEXAFS) spectroscopy measurements, carried out at O K, Fe and Ti L_3,2-edges at room temperature. The O K-edge spectra indicate that the Fe 3d orbitals have been considerably modified and a new spectral feature start dominating in the pre-edge region at higher Ti doping. The Fe 2p NEXAFS spectra exhibit a mixed valent Fe²⁺/Fe³⁺ states apart from the conversion of Fe³⁺ to Fe²⁺ with the substitution of Ti ions. The Ti L_3,2-edge spectra indicate that Ti ions remain unchanged at 4+ state. These variations in the host electronic structure due to Ti substitution are consistent with the dielectric and transport properties of the material.     

Magnetic,electrical transport and electron spin resonance studies of Fe-doped manganite LaMn0.7Fe0.3O3+δ

We have investigated the magnetic, electrical transport and electron spin resonance (ESR) properties of polycrystalline Fe-doped manganite LaMn_0.7Fe_0.3O_3+δ prepared by sol–gel method. A typical cluster-glass feature is presented by DC magnetization and AC susceptibility measurements and a sharp but shallow memory effect was observed. Symmetrical Lorentzian lines of the Mn/Fe spectra were detected above 120 K, where the sample is a paramagnetic (PM) insulator. When the temperature decreases from 120 K, magnetic clusters contributed from ferromagnetic (FM) interaction between Mn³⁺ and Mn³⁺/Fe³⁺ ions develop and coexist with PM phase. At lower temperature, these FM clusters compete with antiferromagnetic (AFM) ones between Fe³⁺ ions, which are associated with a distinct field-cooled (FC) effect in characteristic of cluster-glass state.