Paramagnetic Mossbauer spectra

The spin-Hamiltonian concept, as proposed by Van Vleck [79], was introduced to EPR spectroscopy by Pryce [50, 74] and others [75, 80, 81]. H. H. Wickmann was the first to simulate paramagnetic Mossbauer spectra [82, 83], and E. Miinck and P. Debmnner published the first computer routine for magnetically split Mossbauer spectra [84] which then became the basis of other simulation packages [85]. Concise introductions to the related modem EPR techniques can be found in the book by Schweiger and Jeschke [86]. Magnetic susceptibility is covered in textbooks on molecular magnetism [87-89]. An introduction to MCD spectroscopy is provided by [90-92]. Various aspects of the analysis of applied-field Mossbauer spectra of paramagnetic systems have been covered by a number of articles and reviews in the past [93-100]. 

X-ray studies on FeCls at room temperature indicate a hexagonal unit cell. The chlorine atoms are hexagonally close-packed with the iron atoms occupying two-thirds of the possible octahedral sites in a layered array. The iron environment is not perfectly octahedral although no quadrupole splitting is resolved in the paramagnetic Mossbauer spectrum. The chemical isomer shift at room temperature [94] is 0-436 mm s which is to be compared with 0-442 mm s for FeCla-bHaO and 0-489 mm s for FeFa ... 

The very low Neel temperature (77 K) of lepidocrocite ensures that this compound is paramagnetic at RT. The Mossbauer spectrum at RT consists of a doublet with a AEq of 0.53 mm s . Magnetically ordered lepidocrocite shows a sextet with a Bhf of 45-46 T at 4K that is lower than that of any other Fe oxide (Johnson, 1969 Murad Schwertmann, 1984). 

The Mossbauer spectrum of paramagnetic magnetite consists of one broad line which can be resolved into a component for the cubic A sites and a quadrupole split doublet for the B sites. Between the Gurie and Verwey temperatures (see Ghap. 6), magnetite is ferrimagnetic and the Mossbauer spectrum can be fitted with two sextets (Fig. 7.7),... 

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

Cleavage of the first species to the transient ferryl complex and a ferric hydroxo monomeric complex can explain the near 50% yield of the ferryl compound. The ferric monomer subsequently dimerizes to the paramagnetic dimer, 32, responsible for the 65% of the Mossbauer signal. The dimer formed after the oxidation is proposed to be similar to the starting material, based on comparisons of its Mossbauer spectrum with that of 31. 

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