Mossbauer effect quadrupole splitting

Figure 4.54 The effect of an electric field gradient (EFG) creating asymmetry in the electron distribution round a gold nucleus, leading to a quadrupole splitting in the Mossbauer spectrum. (Reproduced with permission from Gold Bull., 1982,15, 53, published by World Gold Council.)...

The results of the XRD measurement showed that the Fe jAl, jPO catalyst was almost in amorphous state. Only a very broad peak at 29 of ca. 23 degree was observed. The Mossbauer spectroscopic study on this catalyst showed one doublet of iron with the isomeric shift of 0.31 mm s (a-Fe was used as the reference) and the quadrupole splitting of 0.62 mm s. These parameters are very close to those observed for FePO [13, 14], suggesting that the iron cation in the catalyst is tetrahedrally coordinated with oxygen and isolated by four PO tetrahedral units. Such coordination circumstance was suggested to be a key factor for the iron site effective for the oxidation of CH to CHjOH by H -Oj gas mixture [15]. 

In a conventional Fe Mossbauer experiment with a powder sample, one would observe a so-called quadrupole doublet with two resonance lines of equal intensities. The separation of the lines, as given by (4.36), represents the quadrupole splitting The parameter Afg is of immense importance for chemical applications of the Mossbauer effect. It provides information about bond properties and local symmetry of the iron site. Since the quadrupole interaction does not alter the mean energy of the nuclear ground and excited states, the isomer shift S can also be derived from the spectrum it is given by the shift of the center of the quadrupole spectrum from zero velocity. 

Two other publications on Ir (73 keV) Mossbauer spectroscopy of complex compounds of iridium have been reported by Williams et al. [291,292]. In their first article [291], they have shown that the additive model suggested by Bancroft [293] does not account satisfactorily for the partial isomer shift and partial quadrupole splitting in Ir(lll) complexes. Their second article [292] deals with four-coordinate formally lr(l) complexes. They observed, like other authors on similar low-valent iridium compounds [284], only small differences in the isomer shifts, which they attributed to the interaction between the metal-ligand bonds leading to compensation effects. Their interpretation is supported by changes in the NMR data of the phosphine ligands and in the frequency of the carbonyl stretching vibration. 

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... 

Since in appropriate cases of the rare earths it is possible to observe quadrupole splittings of considerable magnitude (i) as well as significant isomer shifts, there are many possibilities for applying the Mossbauer effect to rare earth chemistry. Our first attempt to do this (and our first venture into the Mossbauer field) was actually an attempt to examine the chemistry of a rare earth system by an iron-57 probe. 

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. 

Mossbauer spectroscopy The Mossbauer effect is resonance absorption of 7 radiation of a precisely defined energy, by specific nuclei. It is the basis of a form of spectroscopy used for studying coordinated metal ions. The principal application in bioinorganic chemistry is Fe. The source for the 7 rays is Co, and the frequency is shifted by the Doppler effect, moving it at defined velocities (in mm/s) relative to the sample. The parameters derived from the Mossbauer spectrum (isomer shift, quadrupole splitting, and the hyperfine coupling) provide information about the oxidation, spin and coordination state of the iron. 

This internal pressure effect may actually be quite general in Mbssbauer effect studies of small particles, as discussed by Schroeer et al. for the recoil-free fraction (156) and the isomer shift (157). In addition, Schroeer (152) has summarized a number of origins for Mossbauer parameters being particle size dependent. Thus, from the above discussion, it seems apparent that a priori particle size determination using the recoil-free fraction, quadrupole splitting, or isomer shift is not possible for an arbitrary catalytic system. However, the "experimental calibration of these parameters, which not only facilitates particle size measurement, may also provide valuable information about the chemical state (e.g., electronic, defect, stress) of the small particles. This point will be illustrated later. 

---