Mossbauer spectroscopy isomer shift hyperfine interactions

Mossbauer spectroscopy senses the hyperfine interactions, which are present at the nucleus of the Mossbauer isotope. The electrical monopole interaction causes the isomer shift and the electric quadrupole interaction leads to the quadrupole splitting, which in the case of Fe causes a two-line Mossbauer pattern. The magnetic dipole interaction leads to a magnetically split six-line pattern (Figure 4). In the following text, these interactions and their deduction from Mossbauer spectra will be discussed. 

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... 

Singwi KS, Sjolander A (1960) Phys Rev 120 1093 Smit PH, van Stapele RP (1982) Appl Phys A28 l 13 Speth J, Henning W, Kienle P, Meyer J (1978) In Shenoy GK, Wagner FE (eds) Mossbauer isomer shifts. Noth Holland, Amsterdam, New York, Oxford Spieling H, Alflen M, Giitlich P, Hauser A, Hennen C, Manthe U, Tuczek F (1990) Hyperfine Interact 53 113 Srivastava TS, Nath A (1976) J Phys Chem 80 529 Stemheimer RM (1950) Phys Rev 80 102 Stemheimer RM (1951) Phys Rev 84 244 Stemheimer RM (1963) Phys Rev 130 1423 Stevens JG, Shenoy GK (1981) Mossbauer spectroscopy and its chemical applications in advances in chemistry series, Vol 194. American Chemical Society, Washington DC... 

Another technique widely used for characterizing ferrisilicates is Fe Mossbauer spectroscopy, a very sensitive tool able to differentiate the nuclei in slightly different electronic environments. The electron density of the nuclei, determined by the oxidation state, the coordination number and the type of ligands produce a Mbssbauer isomer shift (IS), while nuclear and magnetic hyperfine interactions are responsible for the form of the spectrum. 

Figure 6.6 depicts a comprehensive compilation of isomer shifts for Fe compounds. From this we see a broad correlation between isomer shift and oxidation state, but values also depend on the spin state. Note that high-spin Fe(II) is the only commonly occurring form of iron that can be unequivocally identified by Mossbauer spectroscopy, as all low-spin iron compounds effectively cover the same range of isomer shifts. However, in such cases other Mossbauer hyperfine interactions, such as quadrupole splitting (Section 6.4.2), can help to identify the nature of the iron atom in such compounds. 

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