Mossbauer temperature dependence

Mossbauer spectroscopy can only be used to obtain rates of interconversion if the lifetimes are close to 10 7 second. As described in Section III,E a few examples satisfying this condition have been found. Some questions remain over the quantitative interpretation of the data. Nevertheless, spin-equilibrium relaxation lifetimes have been estimated from Mossbauer temperature-dependent linewidths for two salts of an iron(III) complex, [Fe(acpa)2]+. The lifetimes are of the order 10 5—10 7 second over temperature ranges from 100 to 300 K (109, 111). 

For example, octahedral quadrupole splitting observed for the cis-octahedral analogs 7,8). More recently, temperature-dependent Mossbauer measurements have been used in conjunction with Raman spectroscopy to determine molecular weights 453) and lattice rigidity 460) of various organotin compounds. 

A major discrepancy that remains unresolved in the excited-state properties of the [Fe384]° cluster in D. gigas Fdll concerns the existence of a low-lying, fully valence-delocalized state that becomes populated at temperatures above 25 K. 8uch a state is clearly apparent in the temperature-dependent Mossbauer studies of reduced D. gigas Fdll (29) and P. furiosus 3Fe Fd (198) and is represented by one quad-rupole doublet with AEq 0.9 mm/s and S = 0.45 mm/s. 8uch a... 

The Mossbauer spectra of the complex [Fe(acpa)2]PF6 shown in Fig. 26 have also been interpreted on the basis of a relaxation mechanism [168]. For the calculations, the formalism using the modified Bloch equations again was employed. The resulting correlation times x = XlXh/(tl + Xh) are temperature dependent and span the range between 1.9 x 10 s at 110 K and 0.34 x 10 s at 285 K. Again the correlation times are reasonable only at low temperatures, whereas around 200 K increase of the population of the state contributes to... 

The recoil-free fraction depends on the oxidation state, the spin state, and the elastic bonds of the Mossbauer atom. Therefore, a temperature-dependent transition of the valence state, a spin transition, or a phase change of a particular compound or material may be easily detected as a change in the slope, a kink, or a step in the temperature dependence of In f T). However, in fits of experimental Mossbauer intensities, the values of 0 and Meff are often strongly covariant, as one may expect from a comparison of the traces shown in Fig. 2.5b. In this situation, valuable constraints can be obtained from corresponding fits of the temperature dependence of the second-order-Doppler shift of the Mossbauer spectra, which can be described by using a similar approach. The formalism is given in Sect. 4.2.3 on the temperature dependence of the isomer shift. 

An important accessory in many applications of Mossbauer spectroscopy is a cryostat for low temperature and temperature-dependent measurements. This may be necessary to keep samples frozen or to overcome small Debye-Waller factors of the absorbers at room temperature in the case of an isotope with high y-energy. Paramagnetic samples are measured at liquid-helium temperatures to slow down... 

In addition to the four detectors used to detect backscattered radiation from the sample, there is a fifth detector to measure the transmission spectrum of the reference absorber (a- Fe, a- Fe203, Fc304 see Fig. 3.16). Sample and reference spectra are recorded simultaneously, and the known temperature dependence of the Mossbauer parameters of the reference absorber can be used to give a measurement of the average temperature inside the SH, providing a redundancy to measurements made with the internal temperature sensor (see Sect. 3.3.4). 

Fig. 4.2 Temperature dependence of the isomer shift due to the second-order Doppler shift, sod- The curves are calculated for different Mossbauer temperatures 0m by using the Debye model whereby the isomer shift was set to (5 = 0.4 mm s and the effective mass to Meff =100 Da, except for the dashed curve with Meff = 57 Da...

Figure 6.13 shows the Mossbauer spectra of ferritin [51], which is an iron-storage protein consisting of an iron-rich core with a diameter around 8 nm with a structure similar to that of ferrihydrite and which is surrounded by a shell of organic material. At 4.2 K essentially all particles contribute to a magnetically split component, but at higher temperatures the spectra show the typical superposition of a doublet and a sextet with a temperature dependent area ratio. At 70 K the sextet has disappeared since all particles have fast superparamagnetic relaxation at this temperature. 

Fig. 6.19 Temperature dependence of the total magnetic field, 5tot, for the three sextets in the Mossbauer spectra of Mno.25Zno.75Fe204, shown in Fig. 6.18. (Adapted with permission from [84] copyright 2003 by Elsevier)...

With the observed temperature shift data for (dSldT)p and calculated (within the framework of the Debye model) numbers for the temperature shift of SOD and with the known thermal expansion coefficient as well as results from Ta Mossbauer experiments under pressure, the authors [191] were able to evaluate the true temperature dependence of the isomer shift, (dSisIdT) as —33 10 " and —26 10 " mm s degree for Ta and W host metal, respectively. 

Another study of the temperature dependence of the 6.2 keV Mossbauer resonance of Ta has been carried out by Salomon et al. [197] for sources of WAV metal and W/Ta metal in the temperature range from 15 to 457 K. In more recent investigations, Salomon et al. [198] have extended such studies of the temperature behavior of the 6.2 keV Mossbauer transition of Ta in tantalum metal to temperatures up to 2,300 K which has been the highest temperature range for any Mossbauer study so far. 

Mossbauer spectroscopy with started only in 1965, when Harris et al. [322] measured the Mossbauer absorption spectra of the 99 keV transition of Pt in platinum metal as a function of temperature (between 20 and 100 K) and of absorber thickness and derived the temperature dependence of the Debye-Waller factor. 

The chemistry of iron(IV) in solid-state materials and minerals is restricted to that of oxides, since other systems such as iron(IV)-halides are not stable [186]. Iron(lV) oxides are easy to handle because they are usually stable in air, but they often have a substoichiometric composition, with oxygen vacancies contributing to varying degrees. Moreover, the samples may contain different amounts of iron(lll) in addition to the intended iron(IV) oxide, a complication which may obscure the Mossbauer data [185]. Even iron(V) was found in iron(IV) oxides due to temperature-dependent charge disproportionation [188, 189]. 

The structural picture that was envisaged to represent the temperature-dependent fluctuations of the EFG tensor [15] is based on the X-ray structure of MbOa that exhibits a geometric disorder of Fe02 with two different positions of the terminal O-atom [28]. Within this stmcture, the projection of the 0-0 bond on the heme plane is rotated by about 40° in position 2 compared to 1 (Fig. 9.10). Conventional Mossbauer studies of single crystals of Mb02 have shown that the principal component of the FFG tensor lies in the heme plane and is oriented along the projection of the 0-0 bond onto this plane [29]. If the terminal O-atom is located in position 2, the EFG should be of the same magnitude as in position 1, but its orientation is different. The EFG fluctuates between positions 1 and 2 with a rate that depends on temperature. 

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