Evaluation of Mossbauer Spectra

The evaluation of a Mossbauer spectrum implies the determination of the physical parameters, the isomer shift, Debye-Waller factor, and electric and magnetic hyperfine fields. When flue- 

Combined magnetic dipole and electric quadrupole interaction 

Fitting of Mossbauer spectra requires the existence of a model for the hyperfme interaction at the Mossbauer nuclei in the material under study. Computer programs are available for a variety of models. The interested reader is referred to the homepage of the Mossbauer community http //www.kfki.hu/mixhp/. 

The weighted mean square deviation between the experimental data points and the corresponding theoretical values ATi (as explained below) has to be minimized, i.e., 

Here = square of the statistical variation of count rate in the ith 

A versatile Mossbauer Data Analysis (MDA) user oriented computer simulation program was developed by Jernberg and Sundqvist (1983) in which the calculations consider a number of experimental situations and the comparisons is made by least squares sums or by plotting the simulated and the measured spectrum. The fitting routine minimizes the least square sums to find the parameters characterizing the measured spectrum. 

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Contents Basic Physical Concepts. - Hyperfine Interactions. - Experimental. - Mathematical Evaluation of Mossbauer Spectra. - Interpretation of Mossbauer Parameters of Iron Compounds. - Mossbauer-Active Transition Metals Other than Iron. - Some Special Applications. 

Table 3 Results of the computer evaluation of Mossbauer spectra...

The low-temperature Mossbauer spectra of the spinel type oxides, NiCr204 [14,18] (Fig. 7.6b) and NiFe204 [3, 18], have been found to exhibit combined magnetic dipole and electric quadrupole interaction (Fig. 7.7). For the evaluation of these spectra, the authors have assumed a small quadrupolar perturbation and a large magnetic interaction, as depicted in Fig. 7.3 and represented by the Hamiltonian [3]... 

Typical Mossbauer spectra of (H-3) type Sn-Pt/Si02 catalysts with different Sn/Pts ratios are shown in Figure 3, the corresponding computer evaluation of the spectra is given in Table 3. ... 

O. Hupe, M.A. Chuev, H. Bremers, J. Hesse, A.M. Afanas ev, Magnetic properties of nanostructured ferromagnetic FeCuNbB alloys revealed by a novel method for evaluating complex Mossbauer spectra. J. Phys. Condens. Matter 11, 10545-10556 (1999)... 

For the evaluation of magnetically split Mossbauer spectra within the spin-Hamiltonian formalism, the purely -dependent Hamiltonian must be extended by an appropriate... 

A pronounced hysteresis of ATC = 9.5 K has been observed (Tc = 181.86 K for rising and Tc = 172.33 K for falling temperature). The authors attempted to describe the results with the thermodynamic model for spin transitions of Slichter and Drickamer 91 Different diffraction patterns were recorded above and below Tc, where only one spin isomer was present (see Fig. 39). This provides evidence that a crystallographic phase change accompanies the abrupt spin phase transition in this compound. The temperature dependence of the peak profiles was found to follow that of the HS fraction as derived from the Mossbauer spectra, which shows that the crystallographic phase change is directly associated with the interconversion of the two spin phases. The Debye-Waller factors were evaluated for the two spin phases they showed at Tc a discontinuity of Af 35% on going from the HS to the LS phase. The shape of the transition curve near Tc and Tc itself, were somewhat different in three independently prepared samples. 

Mossbauer spectra were recorded in constant acceleration mode. Isomer shift values are related to the centre of 300 K spectrum of metallic a-iron. Lorentzian line shapes were assumed, and no positional parameters were constrained at the evaluation. The estimated accuracy of isomer shift (IS) and quadrupole splitting (QS) data is 0.03 mm/s. 

Relative densities of extended defects were evaluated by X-Ray Small Angle Scattering method (SAXS) using Cu K radiation with a nickel filter and an amplitude analyzer [4,8] Mossbauer spectra of Fe (including experiments with oxides doped with this ion tracer) were acquired using an NF-640 spectrometer in the temperature range 298-4.2 K [7, 10]. 

Gatte and Phillips (16) evaluated the modeling routine used to obtain the iron phases from Mossbauer spectra and concluded that this is fraught with difficulties. In spite of this, they conclude that there is ample evidence to support the conclusion that at least one phase of octahedral carbide, normally called e carbide, forms during Fischer-Tropsch synthesis however, they cautioned that the spectra of e -carbide probably consists of at least two sextuples. 

The Mossbauer parameters are derived from the peak parameters (base line parameters, peak position, peak width, and peak area/height) via the fitting process by computer evaluation of spectra in the case of the so-called model-dependent evaluation. In this case, an exact a priori knowledge about the spectrum decomposition (peak-shape function, number, and type of subspectra corresponding to the interactions assumed for each microenvironment in the model) is inevitably necessary. (Incorrectly chosen number of peaks renders the analysis itself incorrect.)... 

Mossbauer spectra of normal human adult (a), rabbit (b), and pig (c) oxyhemoglobins measured at 90 K in 4096 channels and presented in 1024 channels (components are the results of the best fit 1—a-subunits, 2—(3-subunits, 3—carboxyhemoglobin) and differences of Mossbauer hyperfine parameters for normai human adult (A), rabbit (O), and pig ( ) oxyhemoglobins at T = 90 K (approximation with one quadrupoie doubiet (d) and with two quadrupole doubiets for a-subunits (e) and for (3-subunits (f) indicated errors are instrumental errors for hyperfine parameters evaluated for the spectra presented in 1024 channels or calculated errors obtained during the spectra fitting if these errors exceeded instrumental errors) [95],... 

G. Le Caer, J.M. Dubois, Evaluation of hyperfine distributions liom overlapped Mossbauer spectra of amorphous alloys. J. Phys. E. Sci. Instrum. 12, 1083-1090 (1979)... 

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