Mossbauer effect, determination structure

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 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. 

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. 

NpAl3 is a ferromagnet below Tc = 62.5 K. The magnetic moment value p = 1.2/xB/Np was determined from Mossbauer effect measurements. However, the bulk magnetization does not saturate and a moment of only 0.43/xB/f.u. was observed in a field of 12 T at 4.2 K (Aldred et al. 1974). This discrepancy points to a non-collinear magnetic structure. The non-linearity of the 1 /x versus T curve was tentatively ascribed to crystal-field effects. A strong anisotropy should be considered as an alternative source for the explanation of x(T) measured on a polycrystal. 

In the second chapter (94) of the present volume, Dormann reviews the advances that have been made in NMR spectroscopy over the past 12 years, since Barnes overview on NMR, EPR and Mossbauer effect, in volume 2 of the HANDBOOK. Dormann s contribution is more specialized since it concentrates on NMR in inter-metallic, magnetically ordered compounds. As Dormann points out, considerable progress has been made in experimental techniques which enable experimentalists to study materials and perform measurements that could not have been done ten years ago, and this has led to a much better understanding and comprehension of these magnetic lanthanide materials. These experimental advances allow the determination of the temperature dependence of spontaneous magnetization, direction of easy magnetization, and the electronic structure of intermetallic compounds with non-magnetic components. 

Molecular geometry, determination of cis- and trans isomers, distortion from cubic symmetry (Jahn-Teller effect), effect of polymerization, finding the right structure of polynuclear complexes, identification of possible structural position occupied by the Mossbauer atom, determination of distribution of Mossbauer atoms among the sites, determination of site preference, determination of coordination number, effect of neighbors... 

Of the various techniques routinely available, IR and 13C NMR spectroscopy usually provide the most valuable information in terms of the determination of the most appropriate valence description (A-D, Chart 1) of the carbon fragment. Mossbauer spectroscopy has also been used with good effect with iron-containing poly-carbon complexes.89 This solution-based work is complemented by a significant number of solid-state structural studies, which are described in greater detail below. Electronic spectroscopic methods, including luminescence methods, have been used to probe the electronic structures of a small number of poly-yndiyl complexes and polymers.288 315 340 342 377 380 Selected IR, 13C NMR, and UV-vis data have been given in Tables I-VIII, above. 

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