Metallic iron Mossbauer spectra

The Debye temperature is usually high for metallic systems and low for metal-organic complexes. For metals with simple cubic lattices, for which the model was developed, is found in the range from 300 K to well above 10 K. The other extreme may be found for iron in proteins, which may yield d as low as 100-200 K. Figure 2.5a demonstrates how sharply/(T) drops with temperature for such systems. Since the intensity of a Mossbauer spectrum is proportional to the... 

Fig. 3.16 Schematic drawing of the MIMOS II Mossbauer spectrometer. The position of the loudspeaker type velocity transducer to which both the reference and main Co/Rh sources are attached is shown. The room temperature transmission spectrum for a prototype internal reference standard shows the peaks corresponding to hematite (a-Fe203), a-Fe, and magnetite (Fe304). The internal reference standards for MIMOS II flight units are hematite, magnetite, and metallic iron. The backscatter spectrum for magnetite (from the external CCT (Compositional Calibration Target) on the rover) is also shown...

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. 

In catalyst characterization, diffraction patterns are mainly used to identify the crystallographic phases that are present in the catalyst. Figure 6.2 gives an example where XRD readily reveals the phases in an Fe-MnO Fischer-Tropsch catalyst [7], The pattern at the top is that of an MnO reference sample. The diffraction pattern of the reduced Fe-MnO catalyst shows a peak at an angle 29 of 57°, corresponding to metallic iron, and two peaks which are slightly shifted and broadened in comparison with the ones obtained from the bulk MnO reference. The Mossbauer spectrum of the reduced catalyst contains evidence for the presence of Fe2+ ions in a mixed (Fe,Mn)0 oxide [7], and thus it appears justified to attribute the distortion of the XRD peaks to the incorporation of Fe into the MnO lattice. Small particle size is another possible reason why diffraction lines can be broad, as we discuss below. 

We also examined the effects of various heat treatments in both reducing and oxidizing atmospheres. An indochinite reduced for 12 hours in a hydrogen atmosphere at 950 °C. yielded the Mossbauer spectrum displayed in Figure 5. The curve shown is the result of the computer fit the data points are not given, but the closeness of fit is about the same as in the other spectra. The spectrum clearly shows that a certain amount of the original tektite remains the two broad lines at —0.11 and 1.83 mm./sec. match the original tektite lines closely. There is also clear evidence of the presence of metallic iron since the six narrow... 

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. 

The absolute velocity imparted to the drive shaft can be determined either directly or indirectly (30, 32, 87, 88). In the latter technique, the spectrum of a compound with well-established Mossbauer parameters is collected, and to the positions in the spectrum where resonances appear, specific absolute velocities can be assigned. The velocities at other positions in the spectrum are then inferred by interpolation between these known velocities. This indirect calibration is then used in the interpretation of other spectra obtained with the same drive unit. Unfortunately, compounds with well-established Mossbauer parameters may not be available for the Mossbauer isotope of interest. For 57Fe, however, this is not a problem, and metallic iron foils and sodium nitroprusside are often used for calibration purposes. Thus, the 57Fe resonance may be used to calibrate the drive unit, and this unit can then be used to study other Mossbauer isotopes if the drive unit is operated under identical conditions. 

The ferrocene-exchanged ZSM-5 zeolite shows a Mossbauer spectrum that when analyzed gives a 40% ferrocinium ion signal with an isomer shift of 0.53 mm/second. It is likely that water in the zeolite is being reduced as the ferrocene is oxidized. Reduction of this sample in hydrogen causes a complete loss of both the ferrocinium ion and the ferrocene Mossbauer features and a new appearance of alpha iron(0) metal and a new quadrupole split doublet of 0.4 mm/second and a quadrupole splitting of... 

The data for mixed metal zeolites as first prepared by Scherzer and Fort (18) shown in Tables I and XI are quite extensive. The reported isomer shifts and quadrupole splittings are for the iron atoms in the anionic state. Each of these unreduced samples show Mossbauer spectra that are in close agreement with literature values of the corresponding iron coordination complexes. Typical examples of unreduced and reduced samples are shown in Figures 3 and 4. We note here that preparations 16 through 22 are new and are developments of our laboratory and that 9 through 15 are preparations based on the work of Scherzer and Fort (18). Samples 16 and 17 show that this method can be extended to other zeolites like ZSM-5. If no transition metal cation is used in the synthesis, no Mossbauer spectrum for the corresponding anion is observed. Therefore, the nature of the cation is critical and complexation of the anion to a cation is necessary for anion inclusion. Certain transition metal cations (Ru + for instance) do not seem to bind the anion. 

Secondly, only 4 hours of reduction with hydrogen is necessary to completely reduce these samples. The most easily reduced anion is [FeCClOgNO2-] and for the corresponding Zn2+ and Cu2+ samples, complete reduction is noted in the Mossbauer spectrum. As indicated in Table II, the reported isomer shifts are all close to that of metallic iron and the hyperfine field strength, H, values are all very close to that of alpha iron. It should be noted here that whenever the cation was Fe2+ that reduction was never complete. We now believe that these ion-exchanged Fe2 ions are strongly bonded to the lattice and are not easily reduced. This is also the belief of several other investigators (2-5, 8, 10). 

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