Fe-doped TiO2 prepared by mechanical alloying

The effect of different milling conditions on the formation of Fe-doped TiO₂ powders by mechanical alloying was investigated by Mössbauer spectrometry. The milling conditions investigated were ball to powder weight ratio, milling time, rotation velocity of supporting disc, and the type of starting reactive iron and its concentration. X-ray diffraction shows that high energy mechanical milling of undoped anatase TiO₂ induce the anatase to rutile phase transformation via high pressure srilankite. Mössbauer spectra for the majority of the doped samples were decomposed into one sextet and one or two doublets. The sextets was attributed to the presence of α-Fe or hematite impurities. The doublets were assigned to Fe^3?+? incorporated in the TiO₂ structure, and to the Fe^2?+? located either at the surface or the interstitial sites of TiO₂. A greater incorporation of Fe in the TiO₂ structure was observed when samples were prepared from hematite instead of α-Fe.     

Relationship between structural and magnetic properties in (Ti,Fe)O2 powders obtained by mechanical milling

Fe-doped TiO₂ samples with different Fe content were prepared by mechanical alloying starting from TiO₂ rutile and FeO. The samples were structurally and magnetically characterized by XRD, Mössbauer spectroscopy, X-ray absorption spectroscopy (XAS), AC-susceptibility and magnetization measurements. XAS results showed that Fe ions were incorporated into the rutile phase with oxygen coordination that was lower than that expected in this phase. The oxygen coordination number decreased with the increase of Fe²⁺ ions such as it was previously found in the milled samples of TiO₂ doped with hematite. The RT Mössbauer spectra were reproduced using two paramagnetic interactions, one corresponding to Fe²⁺ (δ∼0.87 mm/s) and the other to Fe³⁺ (δ∼0.31 mm/s). Magnetometry measurements showed the presence of paramagnetic and ferromagnetic-like interactions at room temperature. Although saturation and coercivity of the ferromagnetic phase increased with iron, the effective magnetic moment per iron atom decreased, probably due to the precipitation of Fe rich antiferromagnetic structures.     

The relationship between magnetic behaviour and local structure around Fe ions in Fe-doped TiO2 rutile

The magnetic and structural characterization of Ti_1−xFe_xO₂ (x=0.025, 0.05, 0.07, 0.125, and 0.15) samples prepared by mechano-synthesis using TiO₂ and Fe₂O₃ as starting materials are reported. XANES measurements performed at the Fe K-edge show that Fe ions are in 3+ oxidation state in the 7 at% Fe-doped sample and in a mixture of 2+ and 3+ oxidation states in the other samples. EXAFS results show the incorporation of Fe ions substituting Ti ones in the rutile TiO₂ structure. They also reveal a strong correlation between the number of oxygen nearest neighbours and the Fe²⁺ fraction, i.e the number of oxygen near neighbours decreases when the Fe²⁺ fraction increases. All samples present ferromagnetic-like behaviour at room temperature. We found a clear dependence between saturation magnetization and coercivity with the fraction of Fe²⁺ and/or the number of Fe near neighbour oxygen vacancies.     

Effect of thermal treatment in vacuum on Fe-doped SnO2 powders

A sample of 10 at% Fe-doped SnO₂ powder was prepared by mechanical alloying and then thermally treated at 773 K in vacuum. The fit of the diffraction patterns and X-ray absorption spectroscopy measurements revealed that the as milled sample was pure doped rutile. Fe dissolved into SnO₂ was found in Fe²⁺/Fe³⁺ ionic valence with mainly paramagnetic behavior. After the thermal treatment all techniques indicate the formation of the ternary Sn_0.36Fe_2.64O₄ spinel phase, which is responsible for the observed ferromagnetism.     

Mixing of iron with various metals by high-energy ball milling of elemental powder mixtures

The alloying of Fe with T=V, Cr and Mn by high-energy ball milling of elemental powder mixtures has been studied from the scale of a powder particle down to the atomic scale using X-ray and neutron diffraction, Mössbauer spectrometry and magnetic measurements for Fe_1?x T _x alloys with x=0.50, 0.65 for T=V, x=0.50, 0.70 for T=Cr and x=0.72 for T=Mn. Different alloying behaviours are observed according to T once powder particles have the final composition. The rather fast mechanical alloying of Fe with Mn reflects the statistical nature of the milling process in contrast to the slow mixing of Fe with V and of Fe with Cr. Hyperfine magnetic field distributions remain stationary in shape in the last milling stage at room temperature both for T=V and T=Cr. Magnetic measurements evidence the persistence with milling time of a large population of nanometer-sized Fe-Cr zones that are superparamagnetic at room temperature and at 400 K. By contrast, room-temperature Mössbauer spectra show only a single line for long milling times. The unmixed stationary state of milled p-Fe_0.7Cr_0.3 is discussed in the light of a recent model of systems driven by competing dynamics.     

Effect of Sr concentration on microstructure and magnetic properties of (Ba1−xSrx)(Ti0.3Fe0.7)O3 ceramics

Fe-doped (Ba_1−xSr_x)TiO₃ ceramics were prepared by solid-state reaction, and ferromagnetism was realized at room temperature. The microstructure and magnetism were modified by the Sr concentration control (0≤x≤75 at%) at a fixed Fe concentration, and the relevant magnetic exchange mechanism was discussed. All the samples are shown to have a single perovskite structure. When increasing the Sr concentration, the phase structure is transformed from a hexagonal perovskite into a cubic perovskite, with a monotonic decrease in lattice parameters induced by ionic size effect. The room-temperature ferromagnetism is expected to originate from the super-exchange interactions between Fe³⁺ on pentahedral and octahedral Ti sites mediated by the O²⁻ ions. The increase in Sr addition modifies two main influencing factors in magnetic properties: the ratio of pentahedral to octahedral Fe³⁺ and the concentration of oxygen vacancies, leading to a gradually enhanced saturation magnetization. The highest value, obtained for Fe-doped (Ba_0.25Sr_0.75)TiO₃, is an order of magnitude higher than that of the Fe-doped BaTiO₃ system with similar Fe concentration and preparation conditions, which may indicate (Ba_1−xSr_x)TiO₃ as a more suitable matrix material for multiferroic research.     

Formation and characterization of boride coatings thermochemically grown on the Fe64Ni36 alloy

Zirconium oxide (zirconia) exists in three crystalline forms of monoclinic, tetragonal and cubic structures at atmospheric pressures. The cubic form of zirconia is well known for its mechanical, electrochemical and optical applications. Fe-doped cubic zirconia (high temperature phase) compositions are synthesized by microwave combustion method. Here, we present a Mössbauer investigation of Zr_1???x Fe _x O₂ composition within a range of Fe (0.03 < x < 0.09). ⁵⁷Fe Mössbauer spectra were recorded at room temperature and at low temperature (77 K) for all samples. 3% Fe-doped ZrO₂ shows doublet and the corresponding 6% and 9% Fe-doped ZrO₂ samples show superimposed sextet and doublets. The isomer shift and quadrupole moment indicate, Iron to be in III oxidation state and to occupy different octahedral sites, associated with some amount of disorder. X-ray powder diffraction pattern of Fe-doped ZrO₂ samples appear as very well crystalline. The Miller indices refer to the cubic fluorite-type ZrO₂ structure. The magnetic behavior shows increase in moment and decrease in coercivity, with increase in Fe concentration. The M vs. H plots of the as-prepared Zr_1-x Fe _x O₂ essentially show typical hysteresis loops, indicating room temperature ferromagnetism. Thus, the introduced microwave combustion route is an effective process to achieve multifunctional Fe-doped Zirconia with coexistent magnetic properties.     

Low temperature magnetic ordering in Fe-doped TiO 2 samples

Fe-doped TiO₂ samples with different Fe content prepared by mechanical alloying have been investigated by means of Mössbauer spectroscopy at 300 and 4.2 K. The results indicate the coexistence of Fe^2?+? and Fe^3?+? ions in paramagnetic states at room temperatures in the rutile structure. All samples present magnetic order at 4.3 K. When the Fe concentration increases the Fe ions in the rutile matrix became closer giving the possibility of strong magnetic interactions between them. The temperature evolution of the magnetic order was followed for the 15 at.% of Fe sample. The Fe-doped oxide formed for this composition orders below 20 K reaching an almost totally magnetic ordered state at 4.3 K.     

Preparation and characterization of hollow TiO2 nanospheres: The effect of Fe3+ doping on their microstructure and electronic structure

Hollow Fe-doped TiO₂ spheres with various Fe dopant concentrations, 0.25, 0.50 and 1.0 wt%, were synthesized via a hydrothermal method using carbon spheres as templates. The prepared samples were calcined in air at temperatures of 500 and 550 °C for 3 h and characterized using XRD, SEM, TEM, SAED, EDX, XPS and UV–vis spectroscopy. The analytical results showed that the presence of a very low concentration of Fe³⁺ incorporated into hollow nanoporous TiO₂ spheres inhibited the growth of nanocrystals as well as the anatase to rutile phase transformation inside the anatase TiO₂ lattice. Doping with 1.0 wt% Fe³⁺ resulted in a reduction of the TiO₂ sphere diameter from 205.71 ± 25.29 nm to 68.70 ± 7.07 nm. The optical energy gap of the samples was determined from UV–Vis absorption spectra. The results showed that the absorption edge of TiO₂@Fe was shifted toward the visible light region with increased Fe content.     

Photooxidation of different organic dyes (RB, MO, TB, and BG) using Fe(III)-doped TiO2 nanophotocatalyst prepared by novel chemical method

The nano-structured Fe(III)-doped TiO₂ photocatalysts with anatase phase have been developed for the oxidation of non-biodegradable different organic dyes like methyl orange (MO), rhodamine B (RB), thymol blue (TB) and bromocresol green (BG) using UV-Hg-lamp. The different compositions of Fe_xTi_1−xO₂ (x = 0.005, 0.01, 0.05, and 0.1) nanocatalysts synthesized by chemical method (CM), have been characterized by X-ray diffraction (XRD), UV-vis diffuse reflectance spectra, specific surface area (BET), transmission electronic microscopy (TEM) analysis, XPS, ESR and zeta potential. From XRD analysis, the results indicate that all the compositions of Fe(III) doped in TiO₂ catalysts gives only anatase phase not rutile phase. For complete degradation of all the solutions of the dyes (MO, RB, TB, and BG), the composition with x = 0.005 is more photoactive compared all other compositions of Fe_xTi_1−xO₂, and degussa P25. The decolorization rate of different dyes decreases as Fe(III) concentration in TiO₂ increases. The energy band gap of Fe(III)-doped TiO₂ is found to be 2.38 eV. The oxidation state of iron has been found to be 3+ from XPS and ESR show that Fe³⁺ is in low spin state.     

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.     

Mössbauer study on Zn1 − x Fe x O semiconductors prepared by high energy ball milling

We present the preparation of massive Zn_1???x Fe _x O ternary oxides using the mechanical mill. The Fe atom is a particular dopant since it presents two different oxidation states which allow us to vary the starting materials: Fe₂O₃, $\upalpha $ -Fe or FeO. Parameters such as initial concentrations, atmosphere and milling times were varied. X-ray diffraction and ⁵⁷Fe Mössbauer spectrometry (MS) were applied in order to analyze the structure evolution and iron incorporation in the wurtzite crystalline structure with milling time. At final stages, Fe atoms seem to be incorporated in the ZnO structure for those samples milled under Ar atmosphere. In all cases, two paramagnetic components, attributed to Fe atoms in both valence states, were observed by MS.     

Studies on Structural, Magnetic and Thermal Properties of xFe2TiO4-(1−x)Fe3O4 (0≤x≤1) Pseudo-binary System

The xFe₂TiO₄-(1−x)Fe₃O₄ pseudo-binary systems (0≤x≤1) of ulvöspinel component were synthesized by solid-state reaction between ulvöspinel Fe₂TiO₄ precursors and commercial Fe₃O₄ powders in stochiometric proportions. Crystalline structures were determined by X-ray powder diffraction (XRD) and it was found that the as-obtained titanomagnetites maintain an inverse spinel structure. The lattice parameter a of synthesized titanomagnetite increases linearly with the increase in the ulvöspinel component. ⁵⁷Fe room temperature Mössbauer spectra were employed to evaluate the magnetic properties and cation distribution. The hyperfine magnetic field is observed to decrease with increasing Fe₂TiO₄ component. The fraction of Fe²⁺ in both tetrahedral and octahedral sites increases with the increase in Ti⁴⁺ content, due to the substitution and reduction of Fe³⁺ by Ti⁴⁺ that maintains the charge balance in the spinel structure. For x in the range of 0 ≤x≤0.4, the solid solution is ferrimagnetic at room temperature. However, it shows weak ferrimagnetic and paramagnetic behavior for x in the range of 0.4<x≤0.7. When x>0.70, it only shows paramagnetic behavior, with the appearance of quadrupole doublets in the Mössbauer spectra. Simultaneous differential scanning calorimetry and thermogravimetric analysis (DSC-TGA) studies showed that magnetite is not stable, and thermal decomposition of magnetite occurs with weight losses accompanying with exothermic processes under heat treatment in inert atmosphere.     

Mössbauer characterization of Fe-doped ZnO prepared by mechanical milling

The preparation of massive oxides Zn₁??? _x Fe _x O by means of mechanical milling in diverse gaseous atmospheres and starting with different powder mixtures was investigated. ZnO powder with 10 at.% admixture of Fe, FeO or Fe₂O₃, were milled during 1, 4 or 16 h and characterized by X-ray diffraction and Mössbauer spectroscopy. Different phases were obtained according to the initial conditions (precursor used, atmosphere, etc.) and phase’s formation enthalpies.     

Comparative Study by MS and XRD of Fe50Al50 Alloys Produced by Mechanical Alloying, Using Different Ball Mills

In this work we report a comparative study of the magnetic and structural properties of Fe₅₀Al₅₀ alloys produced by mechanical alloying using two different planetary ball mills with the same ball mass to powder mass relation. The Fe₅₀Al₅₀ sample milled during 48 h using the Fritsch planetary ball mill pulverisette 5 and balls of 20 mm, presents only a bcc alloy phase with a majority of paramagnetic sites, whereas that sample milled during the same time using the Fritsch planetary ball mill pulverisette 7 with balls of 15 mm, presents a bcc alloy phase with paramagnetic site (doublet) and a majority of ferromagnetic sites which include pure Fe. However for 72 h of milling this sample presents a bcc paramagnetic phase, very similar to that prepared with the first system during 48 h. These results show that the conditions used in the first ball mill equipment make more efficient the milling process.     

Properties of Fe–Ga based powders prepared by mechanical alloying

Alloys of Fe–Ga with starting compositions of 17, 19, 21, 23, and 25 at% Ga and Fe₈₁Ga₁₇Z₂ (Z=Si, Sn) have been prepared by mechanical alloying. Samples were milled in a SPEX Model 8000 mill with a ball to sample weight ratio of about 4:1. Phase formation as a function of milling time has been investigated for the 19 at% Ga sample and suggests that milling times of 12 h produce fully alloyed samples. Alloys have been studied by electron microprobe, X-ray diffraction, vibrating sample magnetometery and ⁵⁷Fe Mössbauer effect spectroscopy. Fully milled powders have measured compositions of Fe_100−xGa_x with x=15.7, 17.0, 19.0, 22.4, and 24.0 and Fe_83.1Ga_15.2Z_1.7 (for both Z=Si and Sn). X-ray diffraction showed the presence of a disordered bcc phase with no indication of an ordered D0₃ phase. However, the latter is difficult to observe with X-ray diffraction because of the low intensity of the fcc superlattice peaks. A bimodal Fe hyperfine field distribution as obtained from Mössbauer effect spectra indicated the presence of two discrete Fe environments. The results suggested a lower degree of Ga clustering than has been previously observed in Fe–Ga alloys, of similar composition, prepared by melt spinning. The microstructure is similar to that of Fe–Ga thin films prepared by combinatorial sputtering. Some samples have also been studied after annealing at 800 °C for 8 h. No changes were observed in X-ray diffraction patterns after annealing. However, Mössbauer effect studies show the formation of D0₃ and L1₂ order in annealed samples analogous to the phases observed in melt spun ribbons of similar composition.     

Synthesis and characterization of indium oxide-hematite magnetic ceramic solid solution

Indium oxide-doped hematite xIn₂O_3*(1-x)??-Fe₂O₃ (molar concentration x = 0.1?C0.7) solid solutions were synthesized using mechanochemical activation by ball milling. XRD patterns yield the dependence of lattice parameters and grain size as function of milling time. After 12 h of milling, the completion of In^3?+? substitution of Fe^3?+? in hematite lattice occurs for x = 0.1. For x = 0.3, 0.5 and 0.7, the substitutions between In^3?+? and Fe^3?+? into hematite and respectively, In₂O₃ lattices occur simultaneously. The lattice parameters of ??-Fe₂O₃ (a and c) and In₂O₃ (a) vary with milling time. For x = 0.1, Mössbauer spectra were fitted with one, two, or three sextets versus milling time, corresponding to gradual substitution of In^3?+? for Fe^3?+? in hematite lattice. For x = 0.3, Mössbauer spectra after milling were fitted with three sextets and two quadrupole-split doublets, representing In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in two different sites of In₂O₃ lattice. For x = 0.5 and 0.7, Mössbauer spectra fitting required two sextets and one quadrupole-split doublet, representing coexistence of In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in indium oxide lattice. The recoilless fraction studied versus milling time for each molar concentration exhibited low values, consistent with the occurrence of nanoparticles in the system. SEM/EDS measurements revealed that the mechanochemical activation by ball milling produced xIn₂O_3*(1-x)??-Fe₂O₃ solid solution system with a wide range of particle size distribution, from nanometer to micrometer, but with a uniform distribution of Fe, In, and O elements.     

57Fe mössbauer study of high-T c Y−Ba−Cu oxide superconductors

⁵⁷Fe Mössbauer measurements have been performed for Fe-doped YBa₂ (Cu_1-x Fe_x)₃O_7-v (x=0.005–0.10) superconducting oxides. Fe atoms mainly substitute at Cul chain sites and the fraction of Fe at Cu2 plane sites is discussed thermodynamically. The spin direction of Fe at Cu2 was determined to be normal to thec-axis using an oxygen deficient specimen having a strong texture along thec-axis. A magnetically broadened spectrum at 4.2 K for Fe at Cul is compared with a simulated one which takes the distribution of hyperfine fields into account.     

Observation of photoexcitation of Fe-oxide grown on TiO<Subscript>2</Subscript>(100) by visible light irradiation

Electronic excitation of materials is of fundamental and technological importance and interest in terms of photoinduced phase transition, photovoltaics, and photocatalysis. In the present study, photoexcitation of Fe₂ O ₃ epitaxially grown on rutile TiO₂(100) was investigated with conversion electron Mössbauer spectroscopy (CEMS) under dominantly visible-light irradiation. ⁵⁷Fe was deposited on the substrate at a substrate temperature of 973 K, and the resulting film was characterized by RHEED and XPS. After deposition of Fe on TiO₂(100), it was found that Fe was oxidized to Fe ³⁺, and the structure was analyzed to be the rhombohedral phase of Fe₂ O ₃. While the CEMS spectrum without light irradiation showed a quadrupole splitting of 0.80 mm/s with an isomer shift of +0.25 mm/s, an additional component with a quadrupole splitting of 0.85 and an isomer shift of +0.67 mm/s was observed under light irradiation. The latter component corresponds to a reduced state of Fe at the octahedral site surrounded by oxygen atoms. The lifetime of this photoexcited state is discussed.     

Structure and magnetic properties of nanocrystalline Fe75Si25 powders prepared by mechanical alloying

Nanocrystalline Fe₇₅Si₂₅ powders were prepared by mechanical alloying in a planetary ball mill. The evolution of the microstructure and magnetic properties during the milling process were studied by X-ray diffraction, scanning electron microscope and vibrating sample magnetometer measurements. The evolution of non-equilibrium solid solution Fe (Si) during milling was accompanied by refinement of crystallite size down to 10 nm and the introduction of high density of dislocations of the order of 10¹⁷ m⁻². During the milling process, Fe sites get substituted by Si. This structural change and the resulting disorder are reflected in the lattice parameters and average magnetic moment of the powders milled for various time periods. A progressive increase of coercivity was also observed with increasing milling time. The increase of coercivity could be attributed to the introduction of dislocations and reduction of powder particle size as a function of milling time.