Mossbauer atoms

The recoil-free fraction depends on the oxidation state, the spin state, and the elastic bonds of the Mossbauer atom. Therefore, a temperature-dependent transition of the valence state, a spin transition, or a phase change of a particular compound or material may be easily detected as a change in the slope, a kink, or a step in the temperature dependence of In f T). However, in fits of experimental Mossbauer intensities, the values of 0 and Meff are often strongly covariant, as one may expect from a comparison of the traces shown in Fig. 2.5b. In this situation, valuable constraints can be obtained from corresponding fits of the temperature dependence of the second-order-Doppler shift of the Mossbauer spectra, which can be described by using a similar approach. The formalism is given in Sect. 4.2.3 on the temperature dependence of the isomer shift. 

Experimental isomer shifts, exp, should be corrected for the contribution of soD in order to avoid misinterpretations. The value of (5sod drops with temperature and becomes vanishingly small at liquid helium temperature, because is proportional to the mean kinetic energy of the Mossbauer atom. In practice, sod may already be negligible at liquid nitrogen temperature it rarely exceeds —0.02 mm s at 77 K. At room temperature, 3soo rnay be as large as —0.1 mm s or more (Fig. 4.2). 

The electron density i/ (0)p at the nucleus primarily originates from the ability of s-electrons to penetrate the nucleus. The core-shell Is and 2s electrons make by far the major contributions. Valence orbitals of p-, d-, or/-character, in contrast, have nodes at r = 0 and cannot contribute to iA(0)p except for minor relativistic contributions of p-electrons. Nevertheless, the isomer shift is found to depend on various chemical parameters, of which the oxidation state as given by the number of valence electrons in p-, or d-, or /-orbitals of the Mossbauer atom is most important. In general, the effect is explained by the contraction of inner 5-orbitals due to shielding of the nuclear potential by the electron charge in the valence shell. In addition to this indirect effect, a direct contribution to the isomer shift arises from valence 5-orbitals due to their participation in the formation of molecular orbitals (MOs). It will be shown in Chap. 5 that the latter issue plays a decisive role. In the following section, an overview of experimental observations will be presented. 

The isomer shift is considered the key parameter for the assignment of oxidation states from Mossbauer data. The early studies, following the first observation of an isomer shift for Fe203 [7], revealed a general correlation with the (formal) oxidation state of iron. However, isomer shifts have also been found to depend on the spin state of the Mossbauer atom, the number of ligands, the cr-donor and the... 

The electrons at the Mossbauer atom and the surrounding charges on the ligands cause an electric potential V(r) at the nucleus (located at r = (0,0,0). The negative value of the first derivative of the potential represents the electric field, E = — VV, which has three components in Cartesian coordinates, E (3V73x, dVIdy, dVIdz). 

Although the EFG of a given system can be easily determined from a Mossbauer spectrum, it may be rather difficult to relate it to the electronic structure of the Mossbauer atom. In order to visualize a few typical cases, the computation of the EFG is described in the following for some selected charge distributions. A comprehensive quantum chemical interpretation of the quadrupole sphtting will be given in Chap. 5. 

The EFG parameters Vzz and described by (4.42a) and (4.42b) do not represent the actual EFG felt by the Mossbauer nucleus. Instead, the electron shell of the Mossbauer atom will be distorted by electrostatic interaction with the noncubic distribution of the external charges, such that the EFG becomes amplified. This phenomenon has been treated by Stemheimer [54—58], who introduced an anti-shielding factor (1 —y 00) for computation of the so-called lattice contribution to the EFG, which arises from (point) charges located on the atoms surrounding the Mossbauer atom in a crystal lattice (or a molecule). In this approach,the actual lattice contribution is given by... 

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... 

Expression (5-6) indicates that the intensity of a Mossbauer spectrum is only proportional to the concentration of Mossbauer atoms if the term (1 -trnJAl l) 1. 

Polyakov 1997). Because the second-order Doppler shift is not the only factor controlling Mossbauer absorption frequencies, it is generally necessary to process data taken at a variety of temperatures, and to make a number of assumptions about the invariance of other factors with temperature and the form and properties of the vibrational density of states of the Mossbauer atom. Principles involved in analyzing temperature dependencies in Mossbauer spectra are extensively discussed in the primary literature (Hazony 1966 Housley and Hess 1966 Housley and Hess 1967) and reviews (e.g., Heberle 1971). 

Mossbauer spectroscopy should also be mentioned here as a very promising method for combining the structural and dynamic studies of biomolecular systems. The asymmetry of Mossbauer spectra caused by the anisotropy of vibrations of Mossbauer atoms allowed—for example, to find that the mean square amplitude of vibrations of Fe atoms normal to the plane of porphyrin ring (which are responsible for many important biological functions of hemoproteins) is about five times larger than in the... 

There has been to some degree the belief that Mossbauer spectroscopy, although in principle an ideal technique for catalyst studies, for practical purposes can only be applied to problems in catalysis if the catalyst contains either iron or tin. Therefore, one of the main purposes of this review is to show how Mossbauer spectroscopy can be directly extended to many additional Mossbauer atoms or isotopes (such as antimony, europium, nickel, ruthenium, gold, and tungsten) and, perhaps more importantly, how the technique can be extended to obtain information about systems that do not contain a Mossbauer atom. ... 

In the expression for the isomer shift, the term [c — nuclear constant, which has been determined either by direct nuclear measurement or by the measurement of the isomer shift for compounds with known electronic structures (57). Ideally then, a measure of 8 provides a determination of 0S(O) 2, the latter related to the electronic structure as expressed, for example, by the occupation numbers of the various orbitals, e.g., 3d74s1 for metallic iron (Fig. 7) (52). In this manner, it is often possible to identify the oxidation state of the Mossbauer atom and to deduce information concerning the bonding of this atom to its surroundings. In some cases, different electronic structures may have similar values of 0S(O) 2 (the low-spin Fe2+ and Fe3+ pair is an example), and electronic structure information from Mossbauer spectroscopy is most... 

Equation (5-6) indicates that the intensity of a Mossbauer spectrum is only proportional to the concentration of Mossbauer atoms if the term (1 — trnat/4ra) x 1. This is so when absorbers are thin, and when the lines are broader than the natural linewidth. For heavier samples, saturation effects come into play. In case of a sextet, the factor fo in Eq. (5-7) forms the reason that the outer peaks are more affected than the inner peaks, with the result that the line intensity ratios become lower than the expected value of 3 1. 

Spectra with a source of ThOa and absorbers of PajOj and PaOi at 4-2 K are shown in Fig. 18.2. Although there is some suggestion of hyperfine interactions, the lines are far broader than the natural width of 0-079 mm s , which makes analysis difficult. If the hyperfine interactions are large compared to the linewidth, as appears likely, the effects of radiation damage on the environment of the Mossbauer atoms could be a significant factor. 

Method sees all Mossbauer atoms in the sample and works very well with Fe. There is no Mossbauer-silent Fe. 

Mossbauer spectroscopy is based on the phenomenon of recoil-free resonant absorption of y rays by atomic nuclei, and the spectrum reflects the perturbation induced in the nuclear levels due to the interaction of the Mossbauer atom with its chemical environment. The Mossbauer elements, iron and tin, can be used conveniently as in situ probes in environmental and geochemical applications. Since Mossbauer spectroscopy has now become relatively familiar to chemists, I present here only a brief description of typical experimental techniques used in Mossbauer measurements. There are two types of Mossbauer measurements transmission method and scattering method. 

The relative abundances of the Mossbauer atoms (e.g., iron) distributed in various chemical states can be determined on the basis of the areas of the corresponding absorption or scattering peaks. 

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