Phase composition and local environment of iron ions in gadolinium-doped iron oxide nanoparticles

The method of nuclear gamma resonance (NGR) spectroscopy was used in an investigation of iron-doped lithium metaniobate crystals of variable composition. The NGR spectra yielded the type of impurity centers and their relative concentrations. A mechanism of charge compensation of iron was suggested for these crystals.

In several studies the methods of x-ray diffraction analysis, neutron scattering, and gamma resonance spectroscopy have been used to study the crystalline structure and the local magnetic and electronic properties of iron atoms in ordered and unordered nitrides. Nitrided martensite, an unordered interstitial solid solution of nitrogen in an (~-Fe lattice, was investigated by nuclear gamma resonance in [1]. The local magnetic structure of the iron atoms in ~-FetN was treated in detail in [2, 3]. The magnetic and electronic properties of iron in ~-Fe3~ and the ~ nitride with the stoichiometric composition Fe2N were analyzed in [4, 5]. The question of the stages in the formation of the structure of the bulky nitride phases appearing when nitrogen is introduced interstitially into the lattice of metallic iron has been less thoroughly investigated. Nevertheless, these phases, which are deposited on iron catalysts in a number of syntheses, particularly of ammonia, may play an important role in the formation of the active surface of the catalyst [6]. In the present study, we used gamma resonance spectroscopy and x-ray diffraction analysis to investigate the stages in the formation of bulky nitride phases on defective portions of an activated iron catalyst when it is treated with ammonia in the 200-350~ range. EXPERIMENTAL An iron catalyst prepared by electrosmelting magnetite with promotors (V205, A1203) followed by reduction by hydrogen at 450~ (50 arm) [7] was used for nitriding. The nitriding was carried out at 200-350~ in a flow system with a fixed catalyst bed and an ammonia space velocity of 4000 h -i. The preparation conditions, the average concentration of the nitrogen in the samples, and the phase composition of the catalysts following the treatment are given in Table i. The nitrogen concentration in the catalysts was determined, as in [5], from the weight lost by the sample during heating in a vacuum at 950~ for 1.5 h. The amount of gas liberated from the sample was measured, and it was analyzed in a mass spectrometer at the same time. After a correction is made for the water liberated from the samples, the amount of nitrogen determined volumetrically coinced with the analogous value obtained from the weight loss of the sample to within ~+2~. The gamma-resonance spectra were obtained on an electrodynamic apparatus with a 5~Co source in chromium. The isomeric shifts were measured from the center of the hyperfine structure of ~-Fe. The high-temperature n'leasurements (above 20~ were carried out in a thermostated vacuum cuvette. The x-ray diffraction patterns of the catalysts were obtained on a URS-501 diffractometer with the use of Fe K s radiation.

The article reports the preparation and complex characterization of iron-containing phosphate glasses considered to be ecological materials, as they contain non-toxic compounds related to environment. The oxide system Li2O–MgO–(CaO)–Al2O3–P2O5–(FeO/Fe2O3) was investigated in respect to its structural changes caused by MgO replacement with CaO and by the iron addition. UV–vis–NIR (ultraviolet–visible–near infrared) spectroscopy as well as thermo-gravimetric (TG) measurements, differential thermo-analysis (DTA), X-ray diffraction (XRD) analysis, electronic paramagnetic resonance (EPR), and Mossbauer (nuclear gamma resonance) spectroscopy have been used to investigate redox states and coordination symmetry of iron, together with vitreous network changes during the heat treatment up to 1000 °C. UV–vis–NIR transmission spectroscopy revealed no structural modifications when MgO was substituted by CaO, but noteworthy absorption bands attributed to Fe2+/Fe3+ species. TG analysis made in the 20–1000 °C range shows low weight loss accompanied by several thermal effects, as evidenced by DTA. XRD patterns for the glass samples heat treated at about 700 °C revealed the presence of different phosphate crystalline phases containing Mg, Al, and Fe ions. EPR spectroscopy revealed the presence of paramagnetic Fe3+ ions and the change of the first coordination symmetry, when the samples are heated below the vitreous transition temperature. Mossbauer spectroscopy has evidenced two paramagnetic species, Fe2+ and Fe3+, both in octahedral coordination symmetry and a clustering process supported by only Fe3+ ions.

Vibronic interaction effects constitute a new field of investigation in the physics and chemistry of molecules and crystals that combines all the phenomena and laws originating from the mixing of different electronic states by nuclear displacements. This field is based on a new concept which goes beyond the separate descriptions of electronic and nuclear motions in the adiabatic approximation. Publications on this topic often appear under the title of the lahn-Thller effect, although the area of application of the new approach is much wider: the term vibronic interaction seems to be more appropriate to the field as a whole. The present understanding of the subject was reached only recently, during the last quarter of a century. As a result of intensive development of the theory and experiment, it was shown that the nonadiabatic mixing of close-in-energy elec tronic states under nuclear displacements and the back influence of the modified electronic structure on the nuclear dynamics result in a series of new effects in the properties of molecules and crystals. The applications of the theory of vibronic in of spectroscopy [including visible, ultraviolet, in teractions cover the full range frared, Raman, EPR, NMR, nuclear quadrupole resonance (NQR), nuclear gam ma resonance (NOR), photoelectron and x-ray spectroscopy], polarizability and magnetic susceptibility, scattering phenomena, ideal and impurity crystal physics and chemistry (including structural as well as ferroelectric phase transitions), stereochemistry and instability of molecular (including biological) systems, mechanisms of chemical reactions and catalysis.

Chapter 18 NMR, EPR and mössbauer effect: Metals, alloys and compounds

Using the nuclear gamma resonance (NGR) spectroscopy, a low-temperature (400-500 K) isothermal BCC⇔FCC transformation was found to occur in Fe - (31-32%) Ni alloys subjected beforehand to shear under a high pressure. The phase transformation was stimulated by irradiation with high-energy electrons (E ∼ 5.5 MeV, dose F - 5.10 18 e/cm -2 ). The BCC+FCC structure of the binary Fe-Ni alloy, which is produced under the deformation pressure, undergoes either the α→γ or γ→α radiation-induced martensitic transformation depending on the initial (before strain) state: y-phase or α-phase.

Short-range order in soft magnetic FeGa alloys containing from 3 to 25 at % Ga was studied using nuclear gamma resonance (Mössbauer) spectroscopy. The Mössbauer spectra were analyzed via fitting with subspectra corresponding to different configurations of the neighborhoods of the Fe atoms with Ga in the first and second coordination shells. It has been shown that in samples of alloys containing from 3 to 17 at % gallium, the short-range order is almost independent of the heat treatment conditions (quenching from a paramagnetic state or exposure in a ferromagnetic state) and is characterized by the presence of pairs of Ga atoms in the position of the second neighbors (B2-type clusters). At a Ga content of 17 to 21 at %, the portion of B2 clusters turns out to be significantly higher after quenching than after annealing, which correlates with the observed effect of heat treatment on the magnitude of magnetostriction. As the Ga concentration (21–25 at %) further increases, the observed features in the distribution of Ga atoms indicate the appearance and growth of D03 phase regions.

Paramagnetic resonance investigation of mono- and di-manganese-containing systems in biochemistry.

Capabilities of the emission nuclear gamma resonance (NGR) spectroscopy in studying grain boundaries in metals are considered. It is demonstrated that this method enables to evaluate grain-boundary segregation factors, to determine grain-boundary diffusion mechanisms, to estimate an effective diffusion coefficient in near-boundary areas, etc. Besides, this method permits to evaluate quantitatively differences in the properties of grain boundaries of polycrystalline and nanostructured materials.

In this paper, we have studied the crystal structure of strontium-substituted manganite and ytterbium ferromanganites Yb0.82Sr0.18Mn1 – xFe x O3 (x = 0–0.2) by using X-ray diffraction analysis. Magnetic microstructure has been studied by electron paramagnetic resonance and nuclear gamma-resonance (Mossbauer) spectroscopy. We have observed phase separation in the ceramics for antiferromagnetic, superparamagnetic, and ferromagnetic phases.

In this paper, we have studied the crystal structure of strontium-substituted manganite and ytterbium ferromanganites Yb_0.82Sr_0.18Mn_1 – x Fe_ x O_3 ( x = 0–0.2) by using X-ray diffraction analysis. Magnetic microstructure has been studied by electron paramagnetic resonance and nuclear gamma-resonance (Mössbauer) spectroscopy. We have observed phase separation in the ceramics for antiferromagnetic, superparamagnetic, and ferromagnetic phases.

The paper describes the influence of thermomagnetic treatment on the parameters of the amplitude dependence of internal friction, the crystal and magneto-crystalline structure of Fe alloys with ferrite structure: Fe – 8%Cr, Fe – 16%Cr, Fe – 19%Cr, Fe – 4%Cr – 2%V, Fe – 3%Cr – 3%Al, and Fe–1.2%C–2% Si–2%Al graphitic steel with graphite and ferrite structure. The thermomagnetic treatment was carried out at a temperature of 450ºС. The structure was studied by means of metallography, X-ray crystal structure analysis, ferromagnetic resonance, and nuclear gamma resonance spectroscopy. It is shown that, the larger the ferrite grain in alloys with ferrite structure is, the greater an increase in internal friction is, in the amplitude range up to approximately the amplitude of its maximum owing to thermomagnetic treatment. The change of the damping properties of graphitic steel caused by thermomagnetic treatment is relatively small, due to a large number of diamagnetic inclusions of graphite and quite fine-grained ferrite matrix. The results of the X-ray crystal structure analysis, the ferromagnetic resonance and nuclear gamma resonance spectroscopy have shown that, in the case of thermomagnetic treatment, diffusion redistribution of atoms of alloying elements, combined with the change of structural anisotropy, takes place in Fe-Cr alloys.

A new approach to analyze the nuclear gamma resonance (NGR) spectra is presented and justified in the paper. The algorithm successively spots the Lorentz lines in the experimental spectrum by a certain optimization procedures. In Mossbauer spectroscopy, the primary analysis is based on the representation of the transmission integral of an experimental spectrum by the sum of Lorentzians. In the general case, a number of lines and values of parameters in Lorentzians are unknown. The problem is to find them. In practice, before the experimental data processing, one elaborates a model of the Mossbauer spectrum. Such a model is usually based on some additional information. Taking into account physical restrictions, one forms the shape of the lines which are close to the normalized experimental Mossbauer spectrum. This is done by choosing the remaining free parameters of the model. However, this approach does not guarantee a proper model. A reasonable way to construct a structural NGR spectrum decomposition should be based on its model-free analysis. Some model-free methods of the NGR spectra analysis have been implemented in a number of known algorithms. Each of these methods is useful but has a limited range of application. In fact, the previously known algorithms did not react to hardly noticeable primary features of the experimental spectrum, but identify the dominant components only. In the proposed approach, the difference between the experimental spectrum and the known already determined part of the spectral structure defines the next Lorentzian. This method is effective for isolation of fine details of the spectrum, although it requires a well-elaborated algorithmic procedure presented in this paper.

The atomic structure of soft magnetic iron-aluminum alloys is studied by X-ray diffraction and nuclear gamma-resonance spectroscopy. The concentration dependence of the body-centered cubic lattice constant and the short-range order (SRO) parameters in the region of a disordered solid solution is monitored. It is shown that in the concentration range from 3 to 18 at.% Al, the lattice constant increases almost linearly. Discrete decomposition of nuclear gamma resonance spectra makes it possible to determine such SRO parameters as the relative fractions of contributions from coordinations without Al atoms and with one, two, and three Al atoms in the first and second coordination shells. The deviation of the values of these fractions from the average statistical probabilities indicates the presence of a chemical order in the arrangement of atoms. The largest deviations are observed at 12 and 15 at.% Al. Conditions of preliminary heat treatment, such as quenching from the paramagnetic state and holding in the ferromagnetic state, give very similar values of the SRO parameters. The method is characterized by a high resolution in the hyperfine field, while having a rather high sensitivity for determining the intensity of individual contributions.

The atomic structure of soft magnetic iron-aluminum alloys is studied by X-ray diffraction and nuclear gamma-resonance spectroscopy. The concentration dependence of the body-centered cubic lattice constant and the short-range order (SRO) parameters in the region of a disordered solid solution is monitored. It is shown that in the concentration range from 3 to 18 at.% Al, the lattice constant increases almost linearly. Discrete decomposition of nuclear gamma resonance spectra makes it possible to determine such SRO parameters as the relative fractions of contributions from coordinations without Al atoms and with one, two, and three Al atoms in the first and second coordination shells. The deviation of the values of these fractions from the average statistical probabilities indicates the presence of a chemical order in the arrangement of atoms. The largest deviations are observed at 12 and 15 at.% Al. Conditions of preliminary heat treatment, such as quenching from the paramagnetic state and holding in the ferromagnetic state, give very similar values of the SRO parameters. The method is characterized by a high resolution in the hyperfine field, while having a rather high sensitivity for determining the intensity of individual contributions Keywords: soft magnetic alloys, disordered Fe-Al solid solution, local ordering, X-ray diffraction, Mossbauer effect, distribution of atoms over coordination shells.