Mossbauer spectroscopy systematics

The ice produced by the slow freezing of solutions that contain Mossbauer atoms, and the glass produced by their rapid freezing, are also suitable for investigation by Mossbauer spectroscopy. Systematic Mossbauer examinations of ices were started with investigations of the polymorphic transformation... 

Until then, the purification of the Fepr protein had been a laborous job as a 240-L batch yielded only as little as 5 mg of protein. With the overexpression clones of the Fepr proteins, the range of proteinconsuming studies such as Mossbauer spectroscopy, EXAFS, and, last but not least, crystallization experiments was greatly extended. Thus, several groups set off to systematically investigate the spectroscopic properties of both Fepr proteins, poised at all four (proposed) redox states. 

There may, however, be a number of other reasons to pursue a predictive first principles theory of Mossbauer spectroscopy. For example, one may want to elucidate structure/spectroscopy correlations in the cleanest way. To this end one may construct in the computer a number of models with systematic variations in oxidation states, spin states, coordination numbers, and identity of hgands to name only a few chemical degrees of freedom. In such studies it is immaterial whether these molecules have been made or could be made what matters is that one can find out which structural details the Mossbauer parameters are most sensitive to. This can provide insight into the effects of geometry or covalency that are very difficult to obtain by any other means. 

In this chapter, we will first describe what the Mossbauer effect is, then explain why it can only be observed in the solid state and in a limited number of elements. Next we discuss the so-called hyperfine interactions between the nucleus and its environment, which make the technique so informative. After a few remarks on spectral interpretation we go systematically through a number of examples which show what type of information Mossbauer spectroscopy yields about catalysts. 

Dr. Erickson For those interested in coordination chemistry, certain other transition metal atoms are suitable for Mossbauer spectroscopy. One in particular is ruthenium which is just below iron in the Periodic Table. It is a difficult isotope to work with since it requires helium temperatures almost exclusively. I don t know whether it is possible to work at nitrogen temperatures or not, but Kistner at Brookhaven has examined various ruthenium compounds from the 2-j- to the 8+ oxidation states with interesting results. These are not published yet, but at least his work offers the possibility of going down one element below the other in the Periodic Table to study chemical effects. Osmium, which is below ruthenium, can also be Mossbauered. Some sort of systematic study like this involving elements in the various transition series would be extremely interesting. 

In Section I, C the different Mossbauer parameters were individually discussed with reference to possible catalytic applications. The purpose of that discussion was to provide a physical feeling for the parameters and an appreciation of their possible uses in catalysis. In general, however, for the study of a particular catalytic phenomenon the decision whether also to employ Mossbauer spectroscopy is not based only on the consideration of a single Mossbauer parameter. Thus, in the next sections we discuss, based on a number of examples, the manner in which various catalytic phenomena can be investigated through the systematic employment of the Mossbauer parameters. 

A systematic study of the metal dilution effect on the spin-crossover behaviour in the solid solutions [FexZnj x(2-pic)3 ] Cl2 EtOH (0.0009 < x < 1) has been performed employing S7Fe Mossbauer spectroscopy between 5 and 300 K 84.8S). The purpose was to find support for the cooperative domain model suggested earlier by Sorai and Seki34 87). Some representative Mossbauer spectra of the undiluted system (x = 1) as a function of temperature are displayed in Fig. 19. The spectra demonstrate that... 

Besides that, a systematic study of CoMo catalysts with emission Mossbauer spectroscopy (EMS) reported by Tops0e and co-workers, showed the existence of three different Co compounds in the catalyst cobalt contained in alumina support as aluminate (type 1), cobalt contained in CogSg (type 2), and a third cobalt species (type 3) associated in small amounts with the M0S2 phase. Topspe and co-workers (25) located the cobalt inside or on the edges of M0S2 crystallites and called the type 3 Co the CoMoS phase... 

Experimental determinations of for fine particles show some inconsistencies. This is not surprising, as detailed studies on ultrathin films have shown that changes may depend on several parameters. Generally, is lowered with respect to the saturation magnetization value for the corresponding bulk material. The decrease should be mainly due to spin canting at the surface, that is, disorientations of spins directions, as evidenced by Mossbauer spectroscopy experiments. " A decrease of the magnetic moment per spin at the surface is also possible for metallic particles. However, for the present it is difficult to predict such effects because of the lack of systematic studies on the different types of particles. 

Fig. 1. Upper part Lanthanide and actinide series. For shaded elements Mossbauer spectroscopy cannot be carried out. For underlined elements Mossbauer spectroscopy is well established and numerous systematic data exist. Lower part Widely used Mossbauer transitions (underlined elements) in the lanthanides and actinides. [After Stevens and Stevens (1976).]...

Reviews of Mossbauer spectroscopy applied to lanthanides (Ofer et al. 1968, Barnes 1979, Taneja and Kimball 1983) and actinides (Dunlap and Kalvius 1985) are availale, but progress has of course been made in recent years. The purpose of this article, however, is not to systematically update the information now available. We wish rather to discuss selected examples where the differences between lanthanides and actinides can best be contrasted. We shall deal exclusively with intermetallic compounds. 

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