Optical characterization of the phosphate glasses containing pair transition ions

The paper deals with optical and electronic properties of the aluminophosphate glasses containing Fe–Mn and Fe–Cr ion pairs in different concentration. The influence of the mixed alkali ions over the electronic properties has been investigated. The optical behavior (optical transmission) of the glass samples has been studied by UV-VIS spectroscopy and the refractive index dependency on wavelength has been discussed. The transmission spectra show features specific for the doping transition ions (TM), revealing different oxidation states of iron (Fe²⁺/Fe³⁺), manganese (Mn²⁺/Mn³⁺) and chromium (Cr³⁺/Cr⁶⁺) in the vitreous network. Mössbauer spectroscopy offers information regarding the TM oxidation states, redox processes and the iron coordination symmetry in the vitreous network. In the case of Fe–Mn doped glasses, the percentage of Fe²⁺ is about 40% and a doubled iron content leads to an increasing of Fe²⁺ percentage up to 53%. The replacing of lithium ions by natrium ions (mixed alkali effect) provides an increasing of the Fe²⁺ percentage up to 56%. The occurrence of the tetrahedral or octahedral symmetry of Fe²⁺ ions bonded by O^2? ions depends on the transition ion nature and Li⁺/Na⁺ ratio. Infrared absorption spectra of the pair transition ions-doped aluminophosphate glasses reveal optical phonons specific for the phosphate glass matrix.     

Mössbauer study of Na0.82Co0.99Fe0.01O2

We studied by Mössbauer spectroscopy the Na_0.82CoO₂ compound using 1% ⁵⁷Fe as a local probe which substitutes for the Co ions. Mössbauer spectra at T=300 K revealed two sites which correspond to Fe³⁺ and Fe⁴⁺. The existence of two distinct values of the quadrupole splitting instead of a continuous distribution should be related with the charge ordering of Co⁺³, Co⁺⁴ ions and ion ordering of Na(1) and Na(2). Below T=10 K part of the spectrum area, corresponding to Fe⁴⁺ and all of Fe³⁺, displays broad magnetically split spectra arising either from short-range magnetic correlations or from slow electronic spin relaxation.     

Mossbauer, infrared and magnetic susceptibility studies of iron sodium borate glasses doped by sulphur

The affect of sulphur on the structural properties of iron sodium diborate glasses having the composition {(100−x)Na₂B₄O₇+xFe₂O₃}+yS, where x=0.05, 0.15 and 0.25 mol% and Y=0, 2.5 and 5 wt% was studied by infrared, Mossbauer spectroscopy and magnetic susceptibility measurements. It was found that, for samples having 5 mol% Fe₂O₃ and free from sulphur, the iron ions are present in both Fe²⁺ and Fe³⁺ states and also 92% of the total iron enters the glass network as a glass former. The ratio of Fe³⁺/Fe²⁺ increases with increasing the iron content for sulphur-free samples and others containing sulphur. This ratio also decreases with increasing the sulphur content. The magnetic susceptibility was found to decrease with increasing the sulphur content. Also, the increase of Fe₂O₃ content led to a less symmetrical environment of Fe³⁺ ions and vice versa for the Fe²⁺ environment.     

Deep reduction behavior of iron oxide and its effect on direct CO oxidation

Reduction of metal oxide oxygen carrier has been attractive for direct CO oxidation and CO₂ separation. To investigate the reduction behaviors of iron oxide prepared by supporting Fe₂O₃ on γ-Al₂O₃ and its effect on CO oxidation, fluidized-bed combustion experiments, thermogravimetric analyzer (TGA) experiments, and density functional theory (DFT) calculations were carried out. Gas yield (γCO₂) increases significantly with the increase of temperature from 693 K to 1203 K, while carbon deposition decreases with the increase of temperature from 743 K to 1203 K, where temperature is a very important factor for CO oxidation by iron oxide. Further, it were quantitatively detected that the interaction between CO and Fe₂O₃, breakage of O-Fe bonds and formation of new C-O bonds, and effect of reduction degree were quantitatively detected. Based on adsorptions under different temperatures and reducing processes from Fe³⁺ into Fe²⁺, Fe⁺ and then into Fe, it was found that Fe²⁺ → Fe⁺ was the reaction-controlling step and the high oxidation state of iron is active for CO oxidation, where efficient partial reduction of Fe₂O₃ into FeO rather than complete reduction into iron may be more energy-saving for CO oxidation.     

Ferromagnetic ordering of iron impurities in the valence-fluctuating semiconductor SmB6

The electron paramagnetic resonance (EPR) of the valence-fluctuating semiconductor SmB₆ doped by 1 at % Fe is studied. The EPR measurements are performed on a SmB₆ single crystal in a temperature range of 1.6–300.0 K. A number of resonance lines whose g factors indicate the presence of iron ions in the Fe⁰, Fe⁺, Fe²⁺, and Fe³⁺ states have been detected. The iron ions are ferromagnetically ordered below a Curie temperature T = 100 K, and this ordering can be caused by the exchange interaction of impurity ions due to matrix polarization (a similar mechanism is observed in PdFe alloys). This exchange interaction is estimated to be significantly higher than that in PdFe; this fact can result from a very high density of states in the narrow f band, which is characteristic of a valence-fluctuating material.     

Vibrations and chemical states of iron ions in soda‐lime glasses determined by element‐specific X‐ray analyses

We systematically study medium‐range structures including more than three neighboring atoms around iron ions (Fe²⁺ and Fe³⁺) in soda‐lime glass samples with low iron oxide concentrations (M_Fe2O3) and a wide number ratio of Fe²⁺ to all iron ions (Fe²⁺/Σ_nFe). The precise medium‐range structures around iron ions in glass have not yet been revealed because of a lack of the appropriate measurement methods. To avoid this problem, we used element‐specific nuclear resonant inelastic scattering (NRIS) with synchrotron X‐rays to observe the vibrations of iron ions (⁵⁷Fe). The vibrations are related to medium‐range structures with more than three neighboring atoms and to the potential asymmetry and the coordination environment, around iron ions. The NRIS method has high sensitivity and can measure over a wide concentration range. Linear combination fitting of the X‐ray absorption fine structure spectra, which measures only the first neighbors but is a faster than using the NRIS method, was also used additionally. A systematically produced set of glasses with 0.015–5 wt% M_Fe2O3 and 0–0.85 Fe²⁺/Σ_nFe was measured with these methods. It was found that the soda‐lime glass possessed two different medium‐range structures with different iron ion valences (~2+ or ~3+), which were determined by the Fe²⁺/Σ_nFe, and that these structures were generated during production of the glass. Moreover, these medium‐range structures were the same from 0.015 to 5 wt% M_Fe2O3.     

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.     

Effect of Sn on methane decomposition over Fe supported catalysts to produce carbon

In this work, alumina-supported Sn containing Fe catalysts were investigated in CVD reactions (Chemical Vapor Deposition) using methane for carbon production. The catalysts were prepared with 10 wt.% of Fe (as Fe₂O₃) and 3, 6 and 12 wt.% of Sn (as SnO₂) supported on Al₂O₃ named hereon Fe10Sn3A, Fe5Sn6A and Fe10Sn12A, respectively. These catalysts were characterized by SEM, TPCVD, TPR, TG, Raman, XRD and ⁵⁷Fe and ¹¹⁹Sn Mössbauer spectroscopy. Methane reacts with Fe10A catalyst (without Sn) in the temperature range 680?C900°C to produce mainly Fe⁰, Fe₃C and 20 wt.% of carbon deposition. TPR and TPCVD clearly showed that Sn strongly hinders the CH₄ reaction over Fe catalyst. ⁵⁷Fe Mössbauer suggested that in the presence of Sn the reduction of Fe^?+?3 by methane becomes very difficult. ¹¹⁹Sn Mössbauer showed Sn^?+?4 species strongly interact with metallic iron after CVD, producing iron-tin phases such as Fe₃SnC and FeSn₂. This interaction Sn?CFe increases the CVD temperatures and decreases the carbon yield leading to the production of more organized forms of carbon such as carbon nanotubes, nanofibers and graphite.     

A rapid combustion process for the preparation of MgSrAl10O17:Eu phosphor and related luminescence and defect investigations

Blue-emitting europium-ion-doped MgSrAl₁₀O₁₇ phosphor, prepared using the combustion method, is described. An efficient phosphor can be prepared by this method in a muffle furnace maintained at 500 °C in a very short time of few minutes. The phosphor is characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy and BET surface area measurements. Photoluminescence (PL) spectra revealed that europium ions were present in divalent oxidation state. The thermoluminescence (TL) glow curve shows two peaks at around 178 and at 354 °C. The defect centres formed in the phosphor are studied using electron spin resonance (ESR). The ESR spectrum indicates the presence of Fe³⁺ ions in the non-irradiated system. Irradiated MgSrAl₁₀O₁₇:Eu exhibits lines due to radiation-sensitive Fe³⁺ ion and a defect centre. The centre is characterized by an isotropic g-value of 2.0012 and is assigned to a F⁺ centre. The radiation-sensitive Fe³⁺ ion appears to correlate with the main TL peak at 178 °C. During irradiation an electron is released from Fe²⁺ and is trapped at an anion vacancy to form F⁺ centre. During heating, an electron is liberated from the defect centre and recombines with Fe³⁺ emitting light.     

Application of X-ray absorption spectroscopy to the investigation of charge states of iron ions in iron borate nanoceramics

The method of X-ray absorption spectroscopy has been used for the investigation of charge states of iron ions in iron borate nanoceramics prepared by shear deformation under pressure. The experimental Fe 2p X-ray absorption spectra have been presented in comparison with the calculation of atomic multiplets of iron ions taking into account the charge transfer from the 2p orbitals of oxygen to the 3d orbitals of iron and the crystal-field splitting of the 3d orbitals of iron. Our results indicate that, in addition to iron ions in the ground charge state Fe³⁺, nanostructured FeBO₃ contains a few percent of Fe²⁺ ions. It has been found that an increase in the degree of plastic deformation (the rotation angle of the anvils) leads to a decrease in the size of crystallites and to an increase in the concentration of Fe²⁺ ions without the formation of new phases. The results of this work agree with the magnetic and optical measurements and confirm high defectness of FeBO₃ nanoceramics.     

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.     

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.     

Theoretical study of local crystal structure in KZnF3:Fe system

An analysis of the relationship between the EPR trigonal-field parameters and the local crystal structure of KZnF₃:Fe³⁺system is presented by diagonalizing the complete energy matrices for a d⁵ configuration ion in a trigonal crystal field. We propose a two-layer-ligand model, in which the ligands consist of six nearest-neighbor F⁻ ions in the first layer and eight next nearest-neighbor K⁺ ions in the second layer. The calculation indicates that the local structure distortion of KZnF₃:Fe³⁺system is due to the displacement of a K⁺ ion along C₃ axis towards the Fe³⁺ ion, which leads to the shift of the F⁻ ions away from C₃ axis. By simulating the EPR low-symmetry parameters D and (a−F), the distorted angles between the Fe³⁺-F⁻ bonds and C₃ axis are determined, Δθ₁=2.58°, Δθ₂=−1.4° at room temperature (300 K) and Δθ₁=2.84°, Δθ₂=−1.4° at low temperature (77 K). Those results are in good agreement with the experimental findings Δθ₁=2.8±0.3°and Δθ₂=−1.1±0.3°.     

Formation of nickel and nickel,zinc-bearing ferrite in aqueous suspension by air oxidation

Stoichiometric Ni-bearing ferrite was formed by air oxidation of an iron(II) hydroxide suspension at an initial Ni : Fe_tot mol ratio (r_Ni) of 0.20 : 2.80 at pH 10.0 and 65°C. Most of products formed at r_Ni=0.40 : 2.60 and 0.60 : 2.40 were Ni-bearing ferrites, of which vacancies of Fe³⁺ ion on the lattice points may be considered. Only Ni, Zn-bearing ferrites were formed in the suspensions at initial (Ni + Zn)  : Fe_tot mol ratios (r_{Ni + Zn}) of 0.20 : 2.80–0.60 : 2.40 at pH 10.0 and 65°C. At higher r_Ni or r_{Ni + Zn} by-products containing Ni, Fe and O₄²⁻ were formed. The formation of the by-products was depressed in the suspensions containing chloride ions in the place of sulfate ions.     

Mössbauer study of ultrafine amorphous Fe−B alloys

The chemical composition of ultrafine amorphous Fe−B powders prepared by a chemical reduction depends on the mixed molar ratio of KBH₄ to Fe ions. We propose the following reaction processes for the formation of ultrafine Fe−B powders: (1) 4Fe²⁺+2BH₄₋+6OH⁻→2Fe₂B+6H₂O+H₂ and (2) 4Fe²⁺+2BH₄₋+7OH⁻→2Fe₃B+Fe+BO₂+5H₂O+5/2H₂.     

Effect of FeCl3 and acetone on the structure of Na–montmorillonite studied by Mössbauer and XRD measurements

⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction, X-ray fluorescence spectroscopy and infrared spectroscopy were used to study the effect of FeCl₃ and acetone on the structure of a Na–bentonite. XRD indicated the incorporation of Fe³⁺ ions into the interlayer space since the basal lattice spacing of montmorillonite increased to 1.6 from 1.24 nm after treatment with FeCl₃ dissolved in acetone. Interlayer Na⁺ ions could be exchanged to Fe³⁺. Magnetically split Mössbauer subspectra with internal magnetic fields 41 and 46 T at 74 K, were associated with two main Fe³⁺ microenvironments within the interlayer regions. The resultant Fe–montmorillonite was successfully applied as a catalyst in the preparation of 1,1-diacetates from aromatic aldehydes and acetic acid anhydride.     

HgSe:Fe—a mixed-valency system and the problem of the ground state

The dependence of the electron mobility on the iron impurity content N _Fe and temperature is studied for three variants of the ordering of Fe³⁺ ions in crystalline HgSe:Fe, a weakly correlated gas, states with near ordering like that in a strongly correlated Coulomb liquid, and long-range ordering. The electron mobilities owing to scattering on the correlated system of Fe³⁺ ions are determined. The temperature dependence of the mobility is analyzed for electron scattering on fluctuations in the charge density in a system of Fe²⁺-Fe³⁺ iron ions with mixed valency, and the correlation length is determined. It is shown that the ordering region for the Fe³⁺ ions encompasses only the first coordination sphere, i.e., near ordering in the position of the Fe³⁺ ions is established, as in a liquid. The coupling between the ordering of the Fe³⁺ ions and the formation of a correlation gap in the density of impurity d-states and its effect on the low-temperature behavior of the electron mobility in HgSe:Fe crystals are examined. Fiz. Tverd. Tela (St. Petersburg) 40, 425–432 (March 1998)     

Synthesis of nanocomposites based on nanotubes and silicates

In situ synthesis of nanocomposites based on carbon nanotubes and zeolite/montmorillonite was carried out in a hot filament CVD reactor where the precursors (methane and hydrogen) are activated by carbonized tungsten filaments heated up to 2200 °C. In nanocomposites formed both on zeolite and montmorillonite we observed cross-linking of the catalytic particles by nanotubes and creation of carbon nanotube bridges and three-dimensional networks. The length of nanotube bridges was in a range from several nm to nearly 10 μm. A high density of carbon nanotubes was observed in the whole volume of zeolite. The high catalytic efficiency of zeolite is most likely caused by its structure that allows anchoring of Fe³⁺ catalytic particles in the pores and prevents their migration from the sample. At the ends of the nanotubes grown on zeolite we observed particles of the catalyst. In montmorillonite, the particles catalyzing the growth of carbon nanotubes may be present not only on the external surface but also in the interlayer voids of the mineral. Its catalytic efficiency is enhanced as proved by the higher amount of CNTs and their bundles. In the course of CNTs synthesis probably also clumps of Fe³⁺ catalytic particles arise, which may be the reason for formation of bundles of nanotubes.     

Mixed valence of iron in natural vonsenite

The exchange of electrons between adjacent ions in different oxidation states in vonsenite was observed using Mössbauer spectroscopy. The Mössbauer spectra of a series of naturally occurring vonsenite were recorded over a temperature range of 120–773 K. Four quadrupole doublets were resolved by computer fitting and assigned to Fe²⁺(Fel), Fe²⁺(Fe3), Fe³⁺(Fe2, Fe4) and Fe²⁺-Fe³⁺(Fe2–Fe4). The percentage of iron sites participating in an electron exchange process increases from 17% between 120 and 298 K to 27% between 573 and 773 K.     

Surface analysis of heat-treated Mong Hsu rubies

Mong Hsu rubies have been heat treated in air at 1100, 1200, 1300, 1400, 1500 and 1600 °C. Their visual appearance and surface analysis (XPS) after each stage of heating have been monitored. The characteristic blue core regions of untreated ruby become slightly faded at 1100 °C and completely disappear at temperatures above 1500 °C. Trace amounts of Na, Ca, Si and Fe were found on the surface of untreated stones. Ti was first detected after heating to 1100 °C. Plots of detected surface concentrations of elements versus temperature show that the highest concentration of Fe occurred at 1300 °C while surface concentrations of Ti appeared to show two maxima near 1300 and 1500 °C. The results suggest that both the changing oxidation state of Fe²⁺ to Fe³⁺ and the diffusion of the Fe and Ti ions with temperature are responsible for the color changes through decreasing Fe²⁺ to Ti⁴⁺ charge transfer.