Electron Mossbauer spectroscopy contribution

Recently, molecular orbital calculations on some iron complexes have been made by Gray and his co-workers (2, 30). These values for ferrous complexes are plotted in Figure 2 as the dashed line [3d 4s (MO)]. Hence, Mossbauer spectroscopy provides estimates for the 4s electron contribution for molecular orbital calculations. This correlation does not hold for high spin complexes such as FeCb" and FeFe ". Bersuker (3, 4) has attempted to relate both S and AEq directly to molecular orbital parameters. Using the equations developed from this approach, Bersuker, Gordanskii, and Makarov (5) have concluded that in tin tetrahalides the role of dx-Px bonding is significant. 

As indicated in the previous discussion, Mossbauer spectroscopy provides information that when coupled with results using other structural techniques assists in determining the structure of the complex under analysis. The relationships between the various techniques are summarized in Table II. The Mossbauer chemical shift provides information about the 4 electron contribution to the bond between the metal and the ligands in a complex. Similar estimates can be obtained from the results of measurements on the fine structure in the x-ray absorption edge and nuclear magnetic resonance data. The number of unpaired electrons can be evaluated from magnetic susceptibility data, electron spin resonance, and the temperature coeflScient of the Mossbauer quadrupole splitting (Pr). 

The unique contribution of MOssbauer spectroscopy to chemistry is the direct determination of changes in the s electron density at the Mossbauer nucleus for various compounds by measuring their chemical shift. Interpreting the chemical shift for Sn compounds, in contrast to Fe, has resulted in a great controversy. 

Opacity of mixed-valence minerals. The opacities of many end-member Fe2+-Fe3+ oxide and silicate minerals result from electron hopping between neighbouring cations when they are located in infinite chains or bands of edge-shared octahedra in the crystal structures. Opaque minerals such as magnetite, ilvaite, deerite, cronstedtite, riebeckite and laihunite owe their relatively high electrical conductivities to thermally activated electron delocalization, contributing to intermediate valence states of iron cations which may be detected by Mossbauer spectroscopy. 

Because of the broad coverage of this article, we will limit ourselves to the study of stmctural and electronic properties of iron centers by Fe Mossbauer spectroscopy. If the reader is interested in the determination of dynamic properties both by conventional and synchrotron-based Mossbauer spectroscopy, we refer them to the articles by Parak, and Paulsen et as well as to the contribution of Scheidt and Sage in this volume see Nuclear Resonance Vibrational Spectroscopy (NR VS)). 

In this study no other sulfide-containing minerals, except the ones mentioned above, were detected in fresh coals by using Mossbauer spectroscopy. Pyrrhotite was detected in some heavy, weathered coal (Figure 3). The presence of Fe. S was determined by temperature-dependent measurements and the analysis of low-temperature ashes, both by X-ray diffraction and Mossbauer spectroscopy. Other minerals, like spharelite, chalcopyrite, and arsenopyrite, were not detectable in these experiments. Some of the latter minerals have been identified by using scanning electron microscopy, but their presence in the coal was too small to make their contribution to the Mossbauer spectrum significant. 

Equation 3.5 ignores distortion of y>s(0) hy the finite nuclear size which will cause an overestimation of the shift. It should also be mentioned that there is a second possible contribution to electron density at the nucleus if electronic states of the atom are occupied. However, this term will always be much less than the equivalent i-term. Furthermore, the electronic state is not usually encountered in applications of Mossbauer spectroscopy and will therefore not be considered f urther. 

Hyperfine interactions at U nuclei in uranium dipnictides are correlated with the magnetic sequence in their antiferromagnetic states. Because of high Neel temperature, the lattice contribution in the EFG tensor is difficult to estimate. Therefore, it has not been clarified experimentally whether the nuclear quadrupole interaction is caused by the cancellation between the 5f quadrupole moments and lattice contributions or not Considering the crystal structure of the series of the uranium dipnictides, the pnictogen dependence of the lattice contribution in the EFG tensor is small. The results ofthe U Mossbauer spectroscopy imply that the electronic structure thataffects the hyperfine interactions at nuclei... 

In this chapter, we will focus on the application of Mossbauer spectroscopy on perovskite-related systems where emission Mossbauer spectroscopy had a special contribution in exploring the structure and electronic or magnetic behavior of these materials. Some typical racamples are selected from the literature. 

In Fig. 2, the structural results of Fe Mossbauer spectroscopy and EXAFS are compared. It becomes apparent that EXAFS gives an average overview of all compounds that contribute to the Fe-N distance and Fe-Fe distance. On the other hand, the Fe Mossbauer spectra indicate that beside iron nitride (cOTitributing to Fe-N and Fe-Fe), three FeN4 centers can be distinguished. In aU cases, the irrm atoms have the same chemical envirrMiment (planar N4 coordination) but different electronic structures. 

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