Mossbauer Spectra Characterization

We have learned from the preceding chapters that the chemical and physical state of a Mossbauer atom in any kind of solid material can be characterized by way of the hyperfine interactions which manifest themselves in the Mossbauer spectrum by the isomer shift and, where relevant, electric quadrupole and/or magnetic dipole splitting of the resonance lines. On the basis of all the parameters obtainable from a Mossbauer spectrum, it is, in most cases, possible to identify unambiguously one or more chemical species of a given Mossbauer atom occurring in the same material. This - usually called phase analysis by Mossbauer spectroscopy - is nondestructive and widely used in various kinds of physicochemical smdies, for example, the studies of... 

These two examples illustrate how Mossbauer spectroscopy reveals the identity of iron phases in a catalyst after different treatments. The examples are typical for many applications of the technique in catalysis. A catalyst is reduced, carburized, sulfided, or passivated, and, after cooling down, its Mossbauer spectrum is taken at room temperature. However, a complete characterization of phases in a catalyst... 

In catalyst characterization, diffraction patterns are mainly used to identify the crystallographic phases that are present in the catalyst. Figure 6.2 gives an example where XRD readily reveals the phases in an Fe-MnO Fischer-Tropsch catalyst [7], The pattern at the top is that of an MnO reference sample. The diffraction pattern of the reduced Fe-MnO catalyst shows a peak at an angle 29 of 57°, corresponding to metallic iron, and two peaks which are slightly shifted and broadened in comparison with the ones obtained from the bulk MnO reference. The Mossbauer spectrum of the reduced catalyst contains evidence for the presence of Fe2+ ions in a mixed (Fe,Mn)0 oxide [7], and thus it appears justified to attribute the distortion of the XRD peaks to the incorporation of Fe into the MnO lattice. Small particle size is another possible reason why diffraction lines can be broad, as we discuss below. 

Mossbauer spectroscopy of the 57Fe nucleus has been extensively used to investigate aspects of spin equilibria in the solid state and in frozen solutions. A rigid medium is of course required in order to achieve the Mossbauer effect. The dynamics of spin equilibria can be investigated by the Mossbauer experiment because the lifetime of the excited state of the 57Fe nucleus which is involved in the emission and absorption of the y radiation is 1 x 10 7 second. This is just of the order of the lifetimes of the spin states of iron complexes involved in spin equilibria. Furthermore, the Mossbauer spectra of high-spin and low-spin complexes are characterized by different isomer shifts and quad-rupole coupling constants. Consequently, the Mossbauer spectrum can be used to classify the dynamic properties of a spin-equilibrium iron complex. 

In Figure 10 a Mossbauer spectrum for a mixture of szomolnokite (A) and rozenite (B) is shown. The sample was characterized by X-ray diffraction as well as Mossbauer spectroscopy. After the LTA was obtained, the Mossbauer spectrum shows the presence of szomolnokite (no rozenite) and ferric sulfate (Figure 11). This conversion was observed for all the coal samples studied that contained sulfates. In Table III a list of the iron sulfates and their respective Mossbauer parameters is given. 

The hydroxides of berklium(III), Bk(OH)3, and califomium(III), Cf(OH)s, behave in a similar fashion [3]. In their crystalline forms, Am(OH)s and Cm(OH)3 are anhydrous (as are hydroxides of light rare-earth elements), and are hexagonal, C 6h P s/m space group, a = 6.420 and 6.391 A, c = 3.745 and 3.712 A, for Am and Cm compounds, respectively. Due to self-irradiation, the unit-cell parameters increase with time, as does the sample amorphization. In the case of " Cm(OH)3, the stmcture decomposes within 1 day, but the same process for " Am(OH)3 takes up to 4-6 months [4]. The Mossbauer spectrum of Am(OH)3 [5] is characterized by 5 = 4.6 cm/c (relative to Am02). The nuclear magnetic resonance (NMR) studies indicate that, among the TUE(III) hydroxides, the Am compound has the most covalent chemical bonds. The TUE(III) hydroxides are readily soluble in different mineral acids under these conditions, the solutions of hydrated An ions are produced. 

Mossbauer spectroscopy is a rather exotic characterization method in polymer chemistry. We will therefore briefly outline the principles of this method with special emphasis on three principal parameters describing the Mossbauer spectrum. 

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