Isomer Shift Studies

7 Mossbauer-Active Transition Metals Other than Iron 

Kaindl et al. [186] have plotted the isomer shift results for metallic hosts versus the number of outer electrons of the 3d, Ad, and 5d metals and found the transition energy to decrease when proceeding from a to a Ad and further to a 3d host metal in the same column of the periodic table. This systematic behavior is similar to that observed for isomer shifts of y-rays of Fe(14.4 keV) [193], Ru(90 keV), Pm (77 keV), and lr(73 keV) [194]. The changes of A(r ) = (r )e — (r )g for these Mossbauer isotopes are all reasonably well established. Kaindl et al. [186] have used these numbers to estimate, with certain assumptions, the A(r ) value for Ta (6.2 keV) and found a mean value of A(r ) = —5 10 fin with some 50% as an upper limit of error. The negative sign of A(r ) is in agreement with the observed variation of the isomer shift of LiTaOs, NaTaOs, and KTaOs, as well as with the isomer shift found for TaC [186]. 

The temperature dependence of the transition energy of the 6.2 keV y-rays in Ta requires particular attention, especially in studies over a wide temperature 

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. 

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From the applications of Ni Mossbauer spectroscopy in solid-state research, it is clear that (1) information from isomer shift studies is generally not very reliable because of the smallness of the observed isomer shifts and the necessity of SOD shift corrections which turn out to be difficult, and (2) useful information about magnetic properties and site symmetry is obtained from spectra reflecting magnetic and/or quadrupolar interactions. 

The resonance line from the 130 keV state is a factor of 5 narrower than that from the 99 keV transition and therefore more amenable to isomer shift studies. However, even at 20 °K, the 130 keV transition recoilfree fraction, f = 2.8%, is much smaller than that of the 99 keV transition, and the effect observed with a 0.011-inch absorber is only 0.26% (10). 

A unique situation is encountered if Fe-M6ssbauer spectroscopy is applied for the study of spin-state transitions in iron complexes. The half-life of the excited state of the Fe nucleus involved in the Mossbauer experiment is tj/2 = 0.977 X 10 s which is related to the decay constant k by tj/2 = ln2/fe. The lifetime t = l//c is therefore = 1.410 x 10 s which value is just at the centre of the range estimated for the spin-state lifetime Tl = I/Zclh- Thus both the situations discussed above are expected to appear under suitable conditions in the Mossbauer spectra. The quantity of importance is here the nuclear Larmor precession frequency co . If the spin-state lifetime Tl = 1/feLH is long relative to the nuclear precession time l/co , i.e. Tl > l/o) , individual and sharp resonance lines for the two spin states are observed. On the other hand, if the spin-state lifetime is short and thus < l/o) , averaged spectra with intermediate values of quadrupole splitting A q and isomer shift 5 are found. For the intermediate case where Tl 1/cl , broadened and asymmetric resonance lines are obtained. These may be the subject of a lineshape analysis that will eventually produce values of rate constants for the dynamic spin-state inter-conversion process. The rate constants extracted from the spectra will be necessarily of the order of 10 -10 s"F... 

The isomer shift is considered the key parameter for the assignment of oxidation states from Mossbauer data. The early studies, following the first observation of an isomer shift for Fe203 [7], revealed a general correlation with the (formal) oxidation state of iron. However, isomer shifts have also been found to depend on the spin state of the Mossbauer atom, the number of ligands, the cr-donor and the... 

If one pursues the calibration approach, one has to stick to a given combination of density functional and basis set, since the calibration will change for each such combination. Calibration curves have been reported for a number of widely used density functionals and basis sets. The results of a relatively comprehensive study are collected in Table 5.4. The standard deviation of the best fits is on the order of 0.08 mm s which appears to be the intrinsic reliability of DFT for predicting Mossbauer isomer shifts. 

Table 5.4 Linear fit data for Fe Mossbauer isomer shift predictions using the linear equation 5 = h (p - c) + a. A collection of 21 iron complexes with varying charge, oxidation- and spin-states have been studied (taken from [11])...

Mossbauer studies on gold compounds were first reported by Roberts et al. [337] and Shirley et al. [338, 339]. They observed rather large isomer shift changes in some simple gold (I) and gold (III) halides and in halogeno-complexes such as AuX (X = Cl, Br, I), AuXj (X = F, Cl), K[AuX4] (X = F, Cl, Br), and... 

The range of isomer shifts is larger for aurous, Au(l), than for auric, Au(lll), compounds, most probably due to the larger amount of -character in sp than in dsp hybrid orbitals, and also to the smaller variety of ligands in the auric compounds under study. 

Alloys of Pd-Au-Fe (2 at%) Mossbauer effect in Fe and Au, study of band filling, hyperfine fields, isomer shifts... 

We have learned from the preceding chapters that the chemical and physical state of a Mossbauer atom in any kind of solid material can be characterized by way of the hyperfine interactions which manifest themselves in the Mossbauer spectrum by the isomer shift and, where relevant, electric quadrupole and/or magnetic dipole splitting of the resonance lines. On the basis of all the parameters obtainable from a Mossbauer spectrum, it is, in most cases, possible to identify unambiguously one or more chemical species of a given Mossbauer atom occurring in the same material. This - usually called phase analysis by Mossbauer spectroscopy - is nondestructive and widely used in various kinds of physicochemical smdies, for example, the studies of... 

Mossbauer spectroscopy is particularly suitable to study ST since (1) the spectral parameters associated with the HS and LS states of iron(II) clearly differ and (2) the time-scale of the technique ( 10 s) allows the detection of the separate spin states in the course of the transition. Typically, Mossbauer spectra of HS iron(II) show relatively high quadrupole splitting (AEq 2-3 mm s ) and isomer shift (3 1 mm s ), while for LS iron(II), these parameters are generally smaller (AEq < 1 mm s 3 < 0.5 mm s ). Among the early applications of Mossbauer spectroscopy to study ST phenomena in iron(II) complexes is the work of Dezsi et al. [7] on [Fe (phen)2(NCS)2] (phen = 1,10-phenanthroline) as a function of temperature (Fig. 8.2). The transition from the HS ( 12) state (quadrupole doublet of outer two lines with AEq 3 mm s ) to the LS CAi) state (quadrupole... 

A remarkable number of Mossbauer studies have been published since the first spectra reported in 1966 [135], most of them performed on the p-form when not specified differently [131, 132, 136-139]. Also, high pressure has been applied [140] and thin Aims were prepared [141]. Because of the ambiguity concerning the crystalline phase, the values of the hyperfine parameters show some dispersion. The isomer shift, 5 = 0.4-0.6 mm s is found in between the t3q>ical values known for high-spin iron(II) and low-spin iron(II). The quadrupole splitting is large, A q = 2.4-3.0 mm s (Table 8.3), as one might expect because of the unusual non-cubic symmetry. Applied-field measurements revealed positive F . 

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