Mossbauer spectroscopy source materials

For a comparison of experimental Mossbauer isomer shifts, the values have to be referenced to a common standard. According to (4.23), the results of a measurement depend on the type of source material, for example, Co diffused into rhodium, palladium, platinum, or other metals. For Fe Mossbauer spectroscopy, the spectrometer is usually calibrated by using the known absorption spectrum of metallic iron (a-phase). Therefore, Fe isomer shifts are commonly reported relative to the centroid of the magnetically split spectrum of a-iron (Sect. 3.1.3). Conversion factors for sodium nitroprusside dihydrate, Na2[Fe(CN)5N0]-2H20, or sodium ferrocyanide, Na4[Fe(CN)]6, which have also been used as reference materials, are found in Table 3.1. Reference materials for other isotopes are given in Table 1.3 of [18] in Chap. 1. 

Let us now discuss some recent work by Sano and myself on completely characterizing barium stannate, a material first proposed by Plotnikova, Mitrofanov, and Shpinel (21), as a source for tin Mossbauer spectroscopy. It is easily prepared, is a stoichiometric compound and has all the properties one desires in a Mossbauer matrix. The recoil-free fraction at room temperature is about 0.55 with about a 10% error. The line width extrapolated to zero absorber thickness is about 6% larger than natural—i.e., the line width observed is ca. 0.318 mm./sec. at zero ab-... 

A substantial advantage of emission Mossbauer spectroscopy in comparison with the transmission technique is that if the material to be investigated contains heavy elements, then the required dopant concentration (e.g., Co) may be 1-2 orders of magnitude lower in the emission experiment than the Fe concentration in an analogous transmission experiment. This is in connection with the intensity loss of the Mossbauer radiation due to electronic absorption, which is always self absorption in the source and regular absorption in the absorber (Vertes and Homonnay 1997). Low dopant concentration is very important in impurity Mossbauer spectroscopy, where the investigated material does not contain a Mbssbauer element thus, a conveniently measurable Mbssbauer nuclide is introduced artificially as an impurity with a potential risk of perturbing the physicochemical properties of the host phase. 

Different from conventional Mossbauer spectroscopy, which is an ener gy-domain technique, NFS is a time-domain technique—it monitors the change of the nuclear decay signal from the nuclear excited states as a function of time. The use of SR in a time-domain approach eliminates the source contribution to the spectral linewidth, making the spectral resolution of NFS higher than the conventional Mossbauer. NFS has been applied to many different Mossbauer isotopes, and has been demonstrated as a promising new technique for studies in solid-state physics, materials science, geosciences, thin film, and bioinorganic chemistry [13-16]. 

Nuclear excitation and nuclear resonant scattering with synchrotron radiation have opened new fields in Mossbauer spectroscopy and have quite different aspects with the spectroscopy using a radioactive source. For example, as shown in Fig. 1.10, when the high brilliant radiation pulse passed through the resonant material and excite collectively the assemblies of the resonance nuclei in time shorter than the lifetime of the nuclear excited state, the nuclear excitons are formed and their coherent radiation decay occurs within much shorter period compared with an usual spontaneous emission with natural lifetime. This is called as speed-up of the nuclear de-excitation. The other de-excitations of the nuclei through the incoherent channels like electron emission by internal conversion process are suppressed. Synchrotron radiation is linearly polarized and the excitation and the de-excitation of the nuclear levels obey to the selection rule of magnetic dipole (Ml) transition for the Fe resonance. As shown in Fig. 1.10, the coherent de-excitation of nuclear levels creates a quantum beat Q given by... 

In conventional Mossbauer spectroscopy one uses a single-line source, e.g. Co embedded in a rhodium matrix in the case of Fe spectroscopy, and the iron containing material under study as absorber. This technique is termed Mossbauer Absorption Spectroscopy (MAS) in order to distinguish it from the so-called source experiment, also known as Mossbauer Emission Spectroscopy (MES). In a MES... 

The inherent excitation linewidth guarantees specificity of elemental detection. Most Mossbauer measurements have been made on materials in which the Mossbauer resonating element is a macroconstituent in the matrix. Some work has been done in which the matrix investigated constitutes the source of excitation for Mossbauer spectroscopy and in this sense constitutes an analysis of a trace impurity in the matrix. ° ... 

The nuclear decay of radioactive atoms embedded in a host is known to lead to various chemical and physical after effects such as redox processes, bond rupture, and the formation of metastable states [46], A very successful way of investigating such after effects in solid material exploits the Mossbauer effect and has been termed Mossbauer Emission Spectroscopy (MES) or Mossbauer source experiments [47, 48]. For instance, the electron capture (EC) decay of Co to Fe, denoted Co(EC) Fe, in cobalt- or iron-containing compormds has been widely explored. In such MES experiments, the compormd tmder study is usually labeled with Co and then used as the Mossbauer source versus a single-line absorber material such as K4[Fe(CN)6]. The recorded spectrum yields information on the chemical state of the nucleogenic Fe at ca. 10 s, which is approximately the lifetime of the 14.4 keV metastable nuclear state of Fe after nuclear decay. 

However, the source, which is supposed to be a material doped with a Mdssbauer nuclide in its excited state caimot be prepared because the half-life is so short that the source would completely decay before one would start the experiment (for Fe, T1/2 10 s). Thus, one has to find a parent nuclide with a reasonably long half-life, which produces the excited Mossbauer level by nuclear decay. This has the consequence that in an emission experiment the dopant element may be different from the Mossbauer active one. In Fe Mdssbauer spectroscopy, this parent nuclide is Co, which decays by electron capture, Co(EC) Fe, with a half-life of 9 months. 

We used Co(Rh) as the Mossbauer source for transmission mode spectroscopy. Pieces of the material were powdered and sandwiched between plastic disks. The Mossbauer spectrum for pristine BaFe2As2 shows a single line at room temperature and a magnetically split sextet at low temperatures, as expected (Fig. 26.2). On the other hand, Co-and Ni-doped materials exhibit single lines at 78 K and ambient temperature. The observed Mossbauer parameters for BaFe2As2 and Ni-doped material, obtained by the least-squares fitting procedure, are in concordance with those... 

An interesting application of Mossbauer emission spectroscopy is illustrated by the study of Co(pyridine)2Cl2 which was used as a source against a single-line absorber. The resulting emission spectra clearly reveal the presence of the phase transition and indicate that this polymeric material is able to withstand the damage associated with the electron capture decay of the Co. 

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