Mossbauer nuclides

Up to the present time, the Mossbauer effect has been observed with nearly 100 nuclear transitions in about 80 nuclides distributed over 43 elements (cf. Fig. 1.1). Of course, as with many other spectroscopic methods, not all of these transitions are suitable for actual studies, for reasons which we shall discuss later. Nearly 20 elements have proved to be suitable for practical applications. It is the purpose of the present book to deal only with Mossbauer active transition elements (Fe, Ni, Zn, Tc, Ru, Hf, Ta, W, (Re), Os, Ir, Pt, Au, Hg). A great deal of space will be devoted to the spectroscopy of Fe, which is by far the most extensively used Mossbauer nuclide of all. We will not discuss the many thousands of reports on Fe... 

Values of the relativity factor S (Z) for Z = 1-96 have been compiled by Shirley [1] and others [2, 3]. For example, S (Z) = 1.32 for iron (Z = 26), 2.48 for tin (i = 50), and 19.4 for neptunium (Z = 93). The problem of relativistic corrections does not arise in Mossbauer effect studies, where one compares compounds of the same Mossbauer nuclide, because the relativity factor S (Z) is constant for all compounds of a given Mossbauer nuclide. 

Table I shows the various Mossbauer nuclides—i.e., the nuclides where the Mossbauer eflFect has actually been seen. Not all of these are as easy to exploit as the Fe and 9Sn cases referred to above. However, with improved techniques a number of these should prove accessible to the chemist. Representative elements of almost all parts of the periodic table are tractable by these techniques. It seems clear, however, that the methods of Mossbauer spectroscopy are no longer technique-oriented but that this field is becoming a problem-oriented discipline. In other words, the Mossbauer effect is now used successfully in many cases not only to demonstrate the effect or to corroborate physical evidence obtained by other means—NMR, or infrared, or kinetic studies— but also to solve new chemical problems.

Examination of the electronic states of catalysts and of rare earth dopants in phosphors are other applications which come to mind. When more is known about some of the yet unexplored properties of other possible Mossbauer nuclides, Mossbauer spectroscopy bids fair to being a powerful tool in rare earth chemistry. 

Fig. 3. Mossbauer spectrum resulting from a magnetic liyperfine interaction between electron paramagnetic moment and Mossbauer nuclide nuclear magnetic moments...

The most frequently used Mossbauer nuclide is Fe, originating from the Mossbauer source Co by electron capture (Fig. 10.1). Source and Mossbauer nuclide form a Mossbauer pair. The half-life of the first excited state of Fe at E = 14.4 keV is 98 ns (r = 1.4 x 10 s) and the natural linewidth is T = 4.6 10 eV. 

Figure 10.1. Transmutation of Co (Mossbauer source) into Fe (Mossbauer nuclide) Mossbauer level at 0.0144 MeV (excitation energy).

The original Mossbauer experiment has been carried out with Os as source and an iridium foil as absorber in an arrangement shown schematically in Fig. 10.3. The Mossbauer nuclide is Ir. Some of the 129 keV y rays emitted after decay of Os from the first excited state of Ir are absorbed by the atoms of ir in the foil, exeiting these atoms from the ground state to the first excited state (resonance absorption). The latter ehanges with a half-life of 0.13 ns to the ground state, reemitting 129 keV y-ray photons at random. As a result, a decrease of the intensity is measured by the deteetor. 

More information on this system came from non-destructive Mossbauer measure-ments In cis- and trans-(Co(en)2Cl2)N03 about 10 Co cm were implanted and the targets used as Mossbauer sources. The spectra were compared with emission spectra of the same compounds labelled with Co via chemical synthesis. One must keep in mind that the Mossbauer spectrum contains no direct information on the Co in the matrix but only on the actual Mossbauer nuclide pe arising from the Co -> Fe 3-decay. Because the corresponding compounds cis- and trans--(Fe(en)2Cl2)N03 were not accessible by synthesis, a comparison with the absorption spectra was not possible. The interpretation followed the line ... 

The sign and magnitude of bRJR are known for most of the Mossbauer nuclides. In general, IS values must always be given with respect to a reference material (for Fe, metallic iron and Na2 [Fe (CN)s NO] 2H2O are... 

To bridge the gap between ideal and practical catalysts, optical spectroscopies, electron spin resonance (ESR), nuclear magnetic resonance (NMR), and Mossbauer spectroscopy can be used. All have been reviewed recently (373, 396), and some examples have been cited earlier (107, 108). Electron spin resonance has been used in several studies of electroorganic reactions (357,371). It can detect short-lived radicals resulting from electron transfer. Recent application of Mossbauer spectroscopy in situ in electrochemical cells deserves mentioning, although it addressed only the anodic polarization and film stability of Co- and Sn-coated electrodes (397,398). Extension to electrocatalytic studies involving Mossbauer nuclides seems feasible. 

In conducting Mossbauer effect spectroscopy experiments on platinum-iridium catalysts, one might incorporate the Mossbauer nuclides 195Pt and/or 193lr in the catalysts. However, experiments with these nuclides are more difficult because of short-lived sources and the requirement for measurements... 

A full tabulation of relevant nuclear properties for all Mossbauer nuclides is given in Appendix 1. 

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