Isomer shift in Mössbauer spectra of iron compounds in different oxidation states in the octahedral or tetrahedral oxygen environment
Mössbauer study of oxo derivatives of iron in the Fe2O3-Na2O2 system. Pankratov D.A. //Inorganic Materials, 2014. V. 50. № 1. P. 82-89
As a rule, interpretation of poorly resolved Mössbauer spectra is an intricate, multiparametric problem. A measured spectrum is fitted using a particular model based on available information about the composition, structure, and properties of the sample. A criterion for whether the spectrum has been adequately decomposed into components is the agreement between the obtained parameters and expected ones for realistic physical and chemical models. A criterion for sufficiency is provided by statistical parameters of the deviation of results of the proposed model from experimental data. Clearly, a larger number of subspectra in a model and a smaller number of constraints for their components ensure better fitting quality. In some cases, when the iron atoms in a sample have an inhomogeneous environment and parameters of subspectra vary continuously in some range, or spectra contain overlapping components (resonance lines), a model-free description of spectra is used, instead of models. However, a model-free description of spectra also requires an adequate initial hypothesis as to the properties of the substance to be studied. In some cases, even with well-resolved spectra, an incorrect initial hypothesis may entail inadequate interpretation of experimental data.
To assign the corresponding subspectra to particular charge states of iron in the samples, consider the isomer shift as a function of formal oxidation state for various oxo iron compounds. To this end, in plots of the isomer shift against oxidation state we indicate the average of the experimentally observed isomer shifts for oxo compounds of iron in different oxidation states in the most widespread coordination polyhedra—tetrahedra and octahedra—at a particular temperature (Fig.). Data were selected over all values presented in the classic review by Menil  and were supplemented by more recent data [2, 3] for compounds of iron in higher oxidation states. On the whole, we used about 240 experimental data points in constructing the plot (Fig.). It is seen from the diagram thus obtained that, for most iron-containing oxo clusters with a given symmetry, the isomer shift is roughly a linear function of the oxidation state of the central atom. Despite the small amount of data for compounds of iron in oxidation states other than (+2) and (+3), such an empirical relation, is often used in predicting the isomer shift for compounds of iron in extreme oxidation states and builds on its connection with the electron density at the Mössbauer nucleus [4, 5, 6].
Nevertheless, analysis of the data in Fig. leads to some useful observations for interpretation of Mössbauer parameters of iron oxide compounds. In particular, in the case of iron oxo compounds a change in oxidation state by +1, with no symmetry changes, leads to a reduction in isomer shift by 0.34÷0.41 mm/s (except for the +2 → +3 transition). Further, raising the emperature from liquid_nitrogen temperature to room temperature leads to a reduction in isomer shift by 0.05÷0.26 mm/s. Finally, the isomer shift of iron atoms in tetrahedral coordination is typically smaller than that of iron atoms in octahedral coordination by 0.19–0.25 mm/s when there is no changes in the charge state of the central atom. (Although the difference is perhaps much smaller for compounds in the oxidation states (+5) and (+6) (see Fig.)).
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