The technique of Mossbauer spectroscopy

The Mossbauer effect is the emission and resonant absorption of nuclear y-ray s studied under conditions such that the nuclei have negligible recoil velocities when y-rays are emitted or absorbed. This is only achieved hywovkmgvi hsolidsampks in which the nuclei are held rigidly in a crystal lattice. The energy, and thus the frequency of the y-radiation involved, corresponds to the transition between the ground state and the short-lived excited state of the nuclide concerned. Table 3.4 lists properties of several nuclei which can be observed using Mdssbauer spectroscopy. 

We illustrate the study of the Mossbauer effect by reference to Fe spectroscopy. The basic apparatus includes 

If an image of a certain organ is required, it is important to find a contrast agent that targets that organ, e.g. gadolinium(III) complexes are used to target the liver. 

Albert, G.D. Cates, B. Driehuys, W. Mapper, B. Saam, C.S. Springer and A. Wishnia (1994) Nature, vol. 370, p. 199 - Biological magnetic resonance imaging using laser-polarized Xe . 

Caravan, J.J. Ellison, T.J. McMurry and R.B. Lauffer (1999) Chemical Reviews, vol. 99, p. 2293 - Gado-linium(III) chelates as MRI contrast agents structure, dynamics and applications . 

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The technique of Mossbauer spectroscopy is especially suited to chemical studies of implanted ions. The great amount of reference material allows in most cases rather detailed statements on the chemical bonding of an element implanted in compounds. The implantation dose necessary for Mossbauer spectra is not too high. That means that the lattice order is normally preserved. 

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.

The above discussion exemplifies how a study of the different Mbssbauer parameters and their temperature dependences can give detailed information about the location of a non-Mossbauer isotope, lead, in its surrounding structure. It should perhaps for comparison be mentioned that the conventional technique of structure analysis, X-ray diffraction, did not enable the above information to be obtained, again showing the advantage of Mossbauer spectroscopy in the study of catalyst systems, which often may show X-ray amorphous features. 

The status of Mossbauer spectroscopy is akin to that of NQR. It is restricted to solids, and it yields information about the electronic and chemical environment about certain nuclei. Where diffraction methods are inappropriate, Mossbauer may be the best technique for structure determination. 

The strength of Mossbauer spectroscopy is its ability to provide information about the environment of metal centres in large molecules, polymers and minerals, in both single- and multiphase specimens. The drawbacks of the technique are the limited number of elements to which it can be applied in practice, its insensitivity, its limitation to solid state studies, and the need for a suitable radioactive source. 

The nature and breadth of the physical techniques used to investigate solid catalysts continue to increase rapidly in complexity. This statement pertains specifically to Mossbauer spectroscopy, which was applied to the characterization of solid catalysts as early as 1971 (7). A retrospective analysis of the use of Mossbauer spectroscopy in catalysis showed that it has consistently accounted for 3-10% of the communications presented at the International Congresses on Catalysis (ICQ (7%t at the ICC in Paris in 2004). Such continuity over the years reflects the high value of this technique in catalyst characterization. 

This technique, besides allowing determination of the Lamb-Mossbauer factor, provides direct access to the density of phonon states for the probe isotope in a solid. It thus provides information about lattice dynamics that is excluded by the limitations of Mossbauer spectroscopy. This technique could be valuable in investigations of adsorption with the adsorbing element as the probe and showing the modifications brought about by the adsorbate on the dynamic properties of the probe. 

In this paper the application of Mossbauer spectroscopy ( Fe) to determine the iron-bearing minerals will be described, and a critical view of the advantages and disadvantages of the technique will be presented. In this study more than 200 coal samples were investigated and more than 2000 Mossbauer runs were carried out on those samples. Before going into the experimental results, a brief description of the Mossbauer parameters that give the necessary information to determine the compounds seems appropriate. 

The Mossbauer effect has been used as an analytical tool to characterize the diffrent iron-bearing minerals in coal. It has been pointed out that by the use of low-temperature measurements (in the presence of a large external magnetic field) and treatment of the coal samples, all the iron-bearing minerals can be identified correctly. The use of Mossbauer spectroscopy as a quantitative analytical tool presents several experimental difficulties. It is recommended that this spectroscopy be used as a complement to and not as a substitute for the standard techniques. 

We shall return to the theoretical discussion again in Chapter 3 to see how the spectrum can be influenced in detail by various properties of the resonant nucleus and by the extra-nuclear electrons. Before then, however, it will be convenient to outline the experimental techniques of Mossbauer spectroscopy and this forms the subject of the next chapter. 

Rudolph Mossbauer discovered the phenomenon of recoil-free nuclear resonance fluorescence in 1957-58 and the first indications of hyperfine interactions in a chemical compound were obtained by Kistner and Sunyar in 1960. From these beginnings the technique of Mbssbauer spectroscopy rapidly emerged and the astonishing versatility of this new technique soon led to its extensive application to a wide variety of chemical and solid-state problems. This book reviews the results obtained by MSssbauer spectroscopy during the past ten years in the belief that this will provide a firm basis for the continued development and application of the technique to new problems in the future. 

The last contribution (Chapter 7) deaHng with the role of Mossbauer spectroscopy in the science of molecular sieves was provided by Lovat V.C. Rees, one of the pioneers in this field. Although Mossbauer spectroscopy is appHcable in zeolite research only to a small extent because of the limited number of suitable Mossbauer nuclei, we are indebted to this technique for valuable knowledge of and a deeper insight into some special groups of zeoHtes and zeolite/guest systems. This is particularly true of molecular sieves, which contain the most important Mossbauer nucleus Fe in their framework and/or extra-framework guests (cations, adsorbates, encapsulated complexes, and so on). 

Chapter 12 provides a comprehensive review of the application of Mossbauer spectroscopy to metal-containing polymers and Chapter 13 reviews the application of a new mass spectrometty technique. The use of metal-containing polymers as catalysts is described in Chapters 1,9, and 10. Their use as precursors for advanced ceramics (Chapter 14), high temperature materials (Chapter 15), and flame retardants (Chapter 16) is also discussed. The unusual property of selected materials to spontaneously form fibers is described in Chapter 18. 

Ni SRPAC of Ni Ferrite The real difficulty for Mossbauer spectroscopy is to study Mossbauer isotopes with high transition energy because of the short lifetime of radioactive source and small Lamb-Mossbauer factor f[>. In the case of Ni, the radioactive source used ( Co) only has 99 min of lifetime, and fLM is 0.02 even in the liquid helium temperature. These make the application of Mossbauer spectroscopy to Ni extremely difficult. As discussed earlier, SRPAC signal is independent to fLM, and the use of SR eliminates the necessity of radioactive sources. It makes SRPAC an ideal technique to study Ni. The first Ni SRPAC experiment was done by Sergueev et al. on Ni-enriched Ni foil to reveal the magnetic hyperfine interactions [18]. Here we report the first application of Ni SRPAC toa Ni-enriched Ni ferrite to reveal the magnetic hyperfine interactions. 

In this chapter, the aim is to identify and quantify the iron mineral phases present in South African coal fractions by the use of Mossbauer spectroscopy, in conjunction with various other analytical techniques. Because the atomic weight of the carbon content in coal is low, Mossbauer spectroscopy is a convenient, and to a certain degree unique, analytical tool in the identification of iron-bearing minerals in coal with iron contents as low as 1%. With an understanding of the iron mineral phases present in the as-mined coal, the fete of these minerals during transportation, weathering, oxidation, and combustion or gasification can be better understood. 

The study of the structural properties of Fe3(CO)i2 provides a good example of the use of Mossbauer spectroscopy for elucidating the structure of a material for which detailed single-crystal X-ray structural results were not available. It also indicates how essential it is for the Mossbauer spectroscopist to consider information derived from other techniques. 

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