Isomer shift systematics

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]. 

Wagner et al. [258] reported a systematic investigation of the isomer shift and quadrupole splitting in various osmium compounds. Of special interest is the comparison with similar or isoelectronic compounds of iridium and ruthenium. 

Fig. 8.26 (Left) Isomer-shift correlation diagram for a systematic series of [(Me3cyclam-acetate) FcOO]"" complexes, whereX is an azide (Na ) or a nitrido (=N) group. (Right) Orbital scheme of [(Me3cyclam-acetate)Fe =N] ...

Core electrons are highly relativistic and DFT methods may show systematic errors in calculating the charge density at the nucleus because of the inherent approximations. Fortunately, this does not hamper practical calculations of isomer shifts of unknown compounds, because only differences of li//(o)P are involved. In practice, the reliability of the results depends more on the number of compounds used for calibration and how wide the spread of their isomer shift values was. The isomer shift scale for several Mossbauer isotopes has been calibrated by this approach, among which are Au [1], Sn [4], and Fe [5-9]. For details on practical calculation of Mossbauer isomer shifts, see Chap. 5. 

The situation for tin-119 is more confused because of the uncertainty in AR/R. The first attempts to systematize isomer shifts for tin-119 were made by Cordey-Hayes and his co-workers (8). He assumed that ionic stannous compounds have a configuration 5s, Sn compounds or tetrahedral tin compounds have the configuration in which there are 4 p ... 

Reasonable results have also been obtained for Zn Mdssbauer isomer shifts in zinc chalcogenides from LMTO band calculations (Svane and Antoncik, 1986) and for Sb compounds (Ravenak et al., 1983) using the DYM-Aot method. The overall conclusion must be that the time is ripe for the systematic calculation of Fe isomer shift and electric-field-gradient values in Fe-0, Fe-S, and other polyhedra using ab initio Hartree-Fock-Roothaan or DVM-Aa methods. 

The potential of the SW approach to systematize inneratomic properties and processes can be easily illustrated by reconsidering chemically induced nuclear lifetime variations which, among others, are of relevance to the calibration problem of Moessbauer isomer shifts. Highly excited atom states carrying single or multiple vacancies in inner shells form another promising subject of SW simulations. In the latter case the results of a DV-Xa study of the K-shell x-ray satellite intensities of metal fluorides can be used for a comparative assessment of both methods. 

Compounds (6-8) contain the basic structural unit [Fe(py) —S2 2] they have three unpaired electrons and are probably penta-coordinate Fe(IIl) S = i compounds (structure V) similar to the bis-(N,N -dithiocarbamato)-iron(III) halides discussed in the preceding section. Both series show an approximately systematic variation in A with change in the ligand which is not matched by a corresponding variation in the chemical isomer shift, so that it seems unlikely that large changes in delocalisation are occurring. The very small temperature dependence of A in the S = i complexes makes it difficult to determine the electronic level separations. 

There are comparatively few data available for iron sulphides and other chalcogenides, though interesting comparisons can be made in those cases where there are equivalent oxide systems. The same broad trends in the Mossbauer systematics are observed. Thus for chemical isomer shifts... 

Correlation of the ionicity of the bonding with the chemical isomer shift in a number of similar compounds has revealed systematic relationships [30]. In series such as EuXa, EuOX, and EuXj (X = I, Br, Cl, F) the shift decreases as the ionicity increases. The reverse trend is found in EU2Y3 ( Y= Te, Se, S, O) and this was taken to indicate a participation of the 5p-electrons in the... 

Another useful systematic study has been made of a number of oxides, sulphides, and selenides by comparison of the Eu + chemical isomer shift with the mean Eu +-anion distance as estimated from the crystallographic lattice constants [31]. The correlation for the selenides is illustrated in Fig. 

A systematic study of the Eu/Yb and Eu/Ba alloys has been made [52, 53]. In the ytterbium system, the Curie temperature falls from 90 to 5 K and the saturation field also falls from 265 to 160 kG as the ytterbium content increases from 0 to 92 at. %. The relationships are linear apart from a discontinuity at 50 at. % where there is a phase change. Similarly for barium the Curie temperature falls from 90 to 40 K and the field from 265 to 206 kG as the barium content rises to 50 at. %. However, the chemical isomer shift is not significantly altered. The sign of the magnetic field is known to be negative from neutron diffraction data. Calculations suggest that a contribution of —340 kG to the field in europium metal arises from core polarisation, that +190 kG comes from conduction-electron polarisation by the atoms own 4/-electrons, and that —115 kG comes from conduction-electron polarisation, overlap, and covalency effects from neighbouring atoms. 

Some preliminary data have been given for other thulium compounds [165]. C-type TmzOa, Tm(benzoate)3, Tm(oxinate)3, and Tm(acac)3.3H20 all show two thuUum sites, only one of which is quadrupole split. Chemical isomer shifts of 12 and 37 mm s relative to the oxide were found only in TmF3 and TmC respectively, but insufficient information is available for systematic interpretation. 

The model compounds do not bind or reduce N2 or other nitrogenase However, some iron-vanadium-sulfur clusters will reduce hydrazine to ammonia. In a systematic evaluation of the metal M in double cubane clusters [M2Fe6Sg(SEt)9] (M = V, Nb, Mo, W, Re), it was found that the reduction potential is lower, and the Fe Mossbauer isomer shift higher, for Mo relative to However, the Fe isomer shifts are similar between MoFe and VFe proteins, suggesting that protein interactions (especially hydrogen bonding) are likely to play an important role in determining the electron density at the iron atoms in the FeMoco and... 

A relationship between the parameters was first proposed by May and Spijkerman (105), who examined the stannanes MenSnHi -n (n = 0-4). The correlation which was claimed hardly seems well founded, because the range of isomer shifts (1.43- 1.48 mm sec ) is within the experimental error. It was pointed out previously that the isomer shifts of RsSnY compounds are essentially independent of Y. Although the coupling constants appear to vary systematically with the electron-withdrawing character of Y, the fractional changes are small (ca. 5% about the mean), and changes of this order in the isomer shift would be comparable with the experimental errors. The best evidence so far available for such a correlation is the observation that... 

Our systematic XRD and Lns ( Eu and Gd) Mossbauer studies of DF oxides [18,19] also stand as one such. Their detailed summary up to 2006 [4] clarified several novel P-based local structure features of SZ(SH)s. These are basically (a) the presence of a broad Oq hump over the extended middle-y range, (b) Eu Mossbauer isomer-shift (IS) data of Zr-Eu and Hf-Eu exhibiting V-shaped minima at y 0.50, and (c) rich Gd Mossbauer data of Zr-Gd (IS, quadrupole-splitting (QS), and relative absorption area (RAA)) revealing its unique P/DF phase and structure relationships. 

In the An series the presence of IV is not definitely established. According to the rules for Ln the likely candidates would be Pa and Am. Unfortunately, little systematic data (not only Mossbauer data but information of solid state properties in general) exist for compounds of these elements. Like in Sm one may suspect IV to exist also in analogous Pu compounds and indeed there is some indication in that direction (Wachter et al. 1991). Unfortunately once more, a good Mossbauer isotope of Pu which would allow the measurement of isomer shifts does not exist. 

We turn now to the isomer shift. Its variation for NpX2 intermetallics (X = nonmagnetic) indicates a continuous change of electronic structure with lattice constant (or df p). The slope AS/Ad p on the right-hand side of fig. 36 is practically the same as the one found as a function of applied pressure in NpAl2- In essence, the systematics of isomer shift supports the continuous change of f electron structure outlined just above. As expected, the Hill limit is not noticeable in the isomer shift. It does not constitute a break in electron structure. 

Looking at the behavior of the isomer shift, the same general trends are manifested but no break in the linear correlation of shift with volume is observed between NpAs and NpSb. In contrast to the behavior of the hyperfine field, the isomer shift in none of the materials reach the theoretical free-ion Np value, stressing the lingering influence of ligand electrons within the whole series. The systematic difference in isomer shift between the pnictides and the chalcogenides corresponds to a higher covalency of the latter in accordance with the behavior of the hyperfine field. 

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