V. G. Semenov

Grazing incidence Mössbauer spectroscopy: new method for surface layers analysis: Part I. Instrumentation

Abstract The aims of this series of papers are to describe a spectrometer for simultaneous investigation of Mossbauer spectra from: (1) specularly reflected gamma-rays, (2) secondary electron, (3) characteristic X-ray, and (4) scattered gamma-rays (part I); to present a general theory of such spectra and to indicate some unusual characteristics and features of Mossbauer spectra at grazing angles (part II); and to give a quantitative analysis of experimental spectra for some 57 Fe films which shows that grazing incidence mossbauer spectroscopy (GIMS) is really a new method of surface investigation (part III).

Grazing incidence Mössbauer spectroscopy: new method for surface layers analysis: Part II. Theory of grazing incidence Mössbauer spectra

Abstract A general theory of grazing incidence Mossbauer spectroscopy (GIMS) spectra for an inhomogeneous layered medium is presented. The computer simulation based on this theory shows that the shape of the resonant spectrum of the reflected wave intensity, in the case of a resonant film, differs considerably from the case of a semiinfinite resonant mirror. This shape strongly depends on the thickness and the properties of the film and of the nonresonant substrate. The proposed theory of the lineshape of Mossbauer secondary radiation spectra from resonant films takes into account the influence of different types of photo- and conversion electrons on the resulting spectrum. It is shown that the fitting of grazing incidence Mossbauer spectra can be done only by means of numerical simulations.

Mechanism of formation of the complex oxide Gd2SrFe2O7

The mechanism of formation of the perovskite-like layered structure of the oxide Gd2SrFe2O7 was studied. The limiting stages are those of formation of phases with perovskite (GdFeO3, SrFeO3−x) and K2NiF4 (GdSrFeO4) structures. The Mössbauer study has shown that iron atoms exist in a heterovalent state (Fe3+ and Fe4+) only in the structure of SrFeO3−x).

Structural–chemical transformations of α-Fe2O3 upon transport reduction

Abstract Transient regions between iron oxide crystalline phases were found in various stages of α-Fe 2 O 3 structural–chemical transformation into FeO by Mossbauer spectroscopy. Particle sizes in those regions are less than 8 nm. The sequence of phase transformations was established for α-Fe 2 O 3 reduction to iron at temperatures higher and lower than the wustite stability point (∼570°C). Two phases Fe 1− x O and FeO were found to form upon the reduction within the wustite stability region (the stoichiometric phase is formed first). It was shown that reduction of hematite into magnetite occurs within temperature range 570–600°C by the multistage mechanism, while magnetite reduction to wustite occurs by a zonal mechanism, where three phases Fe 3 O 4 , FeO and Fe are present in a sample simultaneously.

Moessbauer reflectometry of ultrathin multilayer Zr(10 nm)/[57Fe(1.6 nm)/Cr(1.7 nm)×26]/Cr(50 nm) film using synchrotron radiation

Abstract Time-resolved nuclear resonant reflectivity from ultrathin multilayer Zr(10 nm)/[ 57 Fe(1.6 nm)/Cr(1.7 nm)×26]/Cr(50 nm) film at grazing incidence angles of synchrotron radiation has been investigated at the Nuclear Resonance Beamline of the European Synchrotron Radiation Facility. For interpretation of the results, complementary measurements of X-ray reflectivity and grazing incidence Moessbauer spectra of reflectivity and secondary electron yield are invoked. Peculiarities and depth-selectivity of the Moessbauer reflectometry method in energy and time domain are compared. It is shown that using this method the information about top layers of the multilayer structure can be obtained. Computer fitting of the energy and time spectra shows the decreasing of the hyperfine magnetic field in the upper layers of investigated multilayer structure.

Three-point multivariate calibration models by correlation constrained MCR-ALS: A feasibility study for quantitative analysis of complex mixtures

When it comes to address quantitative analysis in complex mixtures, Partial Least Squares (PLS) is often referred to as a standard first-order multivariate calibration method. The set of samples used to build the PLS regression model should ideally be large and representative to produce reliable predictions. In practice, however, the large number of calibration samples is not always affordable and the choice of these samples should be handled with care as it can significantly affect the accuracy of the predictive model. Correlation constrained multivariate curve resolution (CC-MCR) is an alternative regression method for first-order datasets where, unlike PLS, calibration and prediction stages are performed iteratively and optimized under constraints until the decomposition meets the convergence criterion. Both calibration and test samples are fitted into a unique bilinear model so that the number of calibration samples is no longer a critical issue. In this paper we demonstrate that under certain conditions CC-MCR models can provide for reasonable predictions in quantitative analysis of complex mixtures even when only three calibration samples are employed. The latter are defined as samples having the minimum, the maximum and the average concentration, providing for a simple and rapid strategy to build reliable calibration model. The feasibility of three-point multivariate calibration approach was assessed with several case studies featuring mixtures of different analytes in presence of interfering species. Satisfactory predictions with relative errors in the range 3-15% were achieved and good agreement with classical PLS models built from a larger set of calibration samples was observed.

SECONDARY RADIATION EMISSION AT MOSSBAUER TOTAL EXTERNAL REFLECTION

The effects of energy dependence of secondary radiation emission (SRE) followed by nuclear resonant and nonresonant photo-absorption at Mössbauer total external reflection (MTER) have been considered theoretically and checked experimentally. Numerical interpretation of a set of MTER and CEM spectra at different grazing angles has given the depth profiles of electronic density, photoabsorption coefficient and hyperfine interaction variations in 50 nm slightly oxidized57Fe film. Empirical functions of photo- and conversion electron yield are also determined. They appeared to be nonsimilar and nonmonotone. The energy dependencies of SRE are recalculated for time domain Mössbauer spectroscopy of synchrotron radiation (SR). The coherent nature of the TER effect reveals itself in the existence of delayed intensity of nonresonant SRE. The relation of nonresonant and resonant SRE, their energy, time and angular spectra strongly depend upon the depth of their creation, which opens a way for depth selective measurements as in the X-ray standing waves method.

Mössbauer and magneto-chemical study of solids formed by surface chemical reaction of OH-silica groups with iron diacetylacetonato chloride (C5H7O2)2FeCl

A reaction of iron(III) acetylacetonate with silica surface OH groups results in the formation of iron–organic groups on SiO2 surface. Large dimension organic ligands inhibit an interaction between iron(III) atoms. According to magnetic and Mossbauer spectroscopy data, samples with surface iron–oxygen groups differ from samples with iron–organic groups by the absence of detectable magnetic ordering.

Two-Level Micro-to-Nanoscale Hierarchical TiO2 Nanolayers on Titanium Surface

Joint replacement is being actively developed within modern orthopedics. One novel material providing fast implantation is bioactive coatings. The synthesis of targeted nanocoatings on metallic nanotitanium surface is reported in this paper. TiO2-based micro- and nanocoatings were produced by sol-gel synthesis using dip-coating technology with subsequent fast (shock) drying in hot plate mode at 400 °C. As a result of shock drying, the two-level hierarchical TiO2 nanolayer on the nanotitanium was obtained. This two-level hierarchy includes nanorelief of porous xerogel and microrelief of the micron-sized “defect” network (a crack network). The thickness of TiO2 nanolayers was controlled by repeating dip-coating process the necessary number of times after the first layer deposition. The state of the MS3T3-E1 osteoblast cell line (young cells that form bone tissue) on the two-level hierarchical surface has been studied. Particularly, adhesion character, adhesion time and morphology have been studied. The reported results may serve the starting point for the development of novel bioactive coatings for bone and teeth implants.

Structural features and stability of the Aurivillius phases Bi n + 1Fe n − 3Ti3O3n + 3

293 It has been shown that an increase in the thickness of the perovskite like block in the Aurivillius phases Bin + 1Fen – 3Ti3O3n + 3 to ~2 nm (n ≈ 5) brings about structural changes reflecting the fact that the distribu tion of titanium and iron ions between the internal and external layers of the perovskite block becomes closer to the random one. These structural changes correlate with the sharp decrease in the stability of the com pounds. With an increase in the perovskite block thickness to ~3.5 nm (n ≈ 9), the effective charges of the iron ions in the internal and external layers are equalized, and the Aurivillius phase thereby com pletely loses its stability.