Mossbauer effect, determination

Johnson, C.E. and Glasby, G.P., 1969. Mossbauer Effect determination of particle size in microcrystalline iron-manganese nodules. Nature, 222 376-377. 

The recollless fraction, that Is, the relative number of events In which no exchange of momentum occurs between the nucleus and Its environment. Is determined primarily by the quantum mechanical and physical structure of the surrounding media. It Is thus not possible to observe a Mossbauer effect of an active nucleus In a liquid, such as an Ion or a molecule In solution. This represents a serious limitation to the study of certain phenomena It allows, however, the Investigation of films or adsorbed molecules on solid surfaces without Interference from other species In solution. This factor In conjunction with the low attenuation of Y-rays by thin layers of liquids, metals or other materials makes Mossbauer spectroscopy particularly attractive for situ studies of a variety of electrochemical systems. These advantages, however, have not apparently been fully realized, as evidenced by the relatively small number of reports In the literature (17). 

The recoilless nuclear resonance absorption of y-radiation (Mossbauer effect) has been verified for more than 40 elements, but only some 15 of them are suitable for practical applications [33, 34]. The limiting factors are the lifetime and the energy of the nuclear excited state involved in the Mossbauer transition. The lifetime determines the spectral line width, which should not exceed the hyperfine interaction energies to be observed. The transition energy of the y-quanta determines the recoil energy and thus the resonance effect [34]. 57Fe is by far the most suited and thus the most widely studied Mossbauer-active nuclide, and 57Fe Mossbauer spectroscopy has become a standard technique for the characterisation of SCO compounds of iron. 

The intensity of the Mossbauer effect is determined by the recoil-free fraction, or /factor, which can be considered as a kind of efficiency. It is determined by the lattice vibrations of the solid to which the nucleus belongs, the mass of the nucleus and the photon energy, Ea and is given by ... 

In conclusion, Mossbauer spectroscopy has matured into one of the classical techniques for catalyst characterization, although its application is limited to a relatively small number of elements which exhibit the Mossbauer effect. The technique is used to identify phases, determine oxidation states, and to follow the... 

The Mossbauer effect involves the resonance fluorescence of nuclear gamma radiation and can be observed during recoilless emission and absorption of radiation in solids. It can be exploited as a spectroscopic method by observing chemically dependent hyperfine interactions. The recent determination of the nuclear radius term in the isomer shift equation for shows that the isomer shift becomes more positive with increasing s electron density at the nucleus. Detailed studies of the temperature dependence of the recoil-free fraction in and labeled Sn/ show that the characteristic Mossbauer temperatures Om, are different for the two atoms. These results are typical of the kind of chemical information which can be obtained from Mossbauer spectra. 

The Mossbauer effect, although not a substitute for other analytical methods such as x-ray diffraction, can be used to obtain several kinds of structural information about solids. In favorable cases, it is possible to obtain rather detailed information about the electronic configuration of atoms and the local symmetry of their sites by measuring the isomer shift and quadrupole splitting. If more than one valence state of a given atom is present, a semiquantitative determination of the amount of each kind is possible. In solid solutions, the amount of local or long range order can be estimated, and in certain defect structures the relation between the active atoms and the defects can be studied. 

One also sees that at room temperature the two peaks of the quadru-pole split pattern are different in intensity. This difference disappears at 77°K. It is an example of what is known as the Gordanskii effect 10) and is caused by the fact that the amplitude of vibration is different, parallel to the surface and normal to the surface, so that the effective resonant fraction is different for the two halves of the quadrupole splitting. This effect has been extensively studied by the Russians. In a recent paper, Suzdalev and others report a study of tin in the surface of silica gel 18), They put calcium ions in the surface of silica gel, then replaced them with divalent tin by ion exchange. The result was a mixture of stannous and stannic ions, and of course, the use of the Mossbauer effect made it possible to determine accurately the relative amounts of the two. They studied the amplitude of vibration of the two kinds of atoms and found, for example, for the stannous ions, the r.m.s. amplitude of vibration parallel to the surface was 0.07 A., and the r.m.s. amplitude of vibration perpendicular to the surface was about twice as great—about 0.13 A. Karasev and others have also worked on the chemistry of adsorbed... 

The Mossbauer effect in Te can provide information on both the nuclear properties of the 35.6-k,e,v, first excited state of Te and the chemical properties of pure Te and Te compounds. Nuclear properties which have already been determined include the quadrupole moment, Qj = 0.20 0.03 barn, and the magnetic moment, = - -0.60 0.02 nm. Information on chemical bonding of Te can be obtained from the Te Mossbauer spectra of various Te compounds. The isomer shift which is related to the valence s electrons gives a measure of the ionic character while the quadrupole splitting can provide information on ionic character and hybridized covalent bonds. 

In systems containing a number of physically inequivalent sites, Mossbauer effect spectroscopy (MES) can often allow the determination of the properties of the individual sites. This also proved to be the case here [24],... 

Fig. 333 Ground-state splitting of Fe in Au determined by Mossbauer effect (Violet and Borg 1966) (a) observed (b) Brillouin function for s= 1.

In principle the deviation <5 can be determined by the use of usual analytical chemistry or a highly sensitive thermo-balance. These methods, however, are not suitable for very small deviations. In these cases the following methods are often applied to detect the deviation physico-chemical methods (ionic conductivity, diffusion constant, etc.), electro-chemical methods (coulometric titration, etc.), and physical methods (electric conductivity, nuclear magnetic resonance, electron spin resonance, Mossbauer effect, etc.), some of which will be described in detail. 

The energy of the emitted y ray is determined by the energy difference between the nuclear excited and ground states. This energy difference is not altered by the first term, eZU, since this term equally affects the nuclear excited and ground states in both the source and absorber. In addition, atomic nuclei do not possess electric dipole moments, and therefore the second term in Eq. (9) must be equal to zero. Thus, when considering the Mossbauer effect, Ee can be written in the form... 

The advantage of the technique is that the particle size may be determined with the sample in a controlled atmosphere and at a temperature different from 300 K, i.e., in situ particle size measurement, and measurement of changes in particle size may be possible. The problem, however, is that the quantitative relation between the Mossbauer parameters and particle size is rather complex and in some cases not theoretically available. Therefore, the application of the Mossbauer effect to particle size measurement is often facilitated through an experimental calibration of the Mossbauer parameters to particle size for the particular catalyst system of interest, i.e., the measurement of the parameters for a set of samples of known particle size as determined by other experimental methods. This point will become clearer below, as the effects of particle size on the Mossbauer parameters are discussed. 

The utility, and also the limitations, of Mossbauer spectroscopy in surface structure measurement can now be seen. While this determination using the Mossbauer effect alone may be possible, it is often difficult. The ultimate determination of surface structure and changes thereof, however, can be deduced through combined studies using the Mossbauer effect and other physical methods. 

Mossbauer spectroscopy has matured into one of the classical techniques for catalyst characterization, although its application is limited to a relatively small number of elements which exhibit the Mossbauer effect. The technique is used to identify phases, determine oxidation states, and to follow the kinetics of bulk reactions. Mossbauer spectra of super-paramagnetic iron particles in applied magnetic fields can be used to determine particle sizes. In favorable cases, the technique also provides information on the structure of catalysts. The great advantage of Mossbauer spectroscopy is that its high-energy photons can visualize the insides of reactors in order to reveal information on catalysts under in-situ conditions. 

Application of the Mossbauer effect, which is essentially a bulk phenomenon, to the study of surfaces has received significant attention in recent years. The usefulness of this technique lies in its ability to determine the electronic environment and symmetry of the surface nucleus, and it offers a method of investigation that is clearly complementary to other physical methods for the characterization of solid surfaces. Mossbauer spectroscopy has the attractive advantage that it may be used at a variety of pressures and can be applied to the study of heterogeneous catalysis and adsorption processes to probe the nature of the solid surface and its electronic modification when holding adsorbed species. 

Crystal field levels in rare earth metals have been determined by measurements of properties such as (i) specific heat, (ii) Van Vleck susceptibility, (iii) magnetization in high magnetic fields, (iv) paramagnetic resonance, (v) Mossbauer effect, (vi) inelastic neutron scattering and (vii) miscellaneous methods. 

The magnitude of the observed Mossbauer effect is determined by the effective thickness of the absorber, which is a dimensionless quantity given by... 

The most common use of the Mossbauer effect in mineralogy and geology has been the determination of the oxidation states of iron in various minerals (2). The study of the Mossbauer spectral area also gives valuable information on the concentration of the different minerals in rocks (2). Recently the Mossbauer effect was applied to the study of iron-bearing minerals in coal to determine the amount of pyritic sulfur (3, 4, 5). 

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