Mossbauer chemical isomer shift

An initial report that the crystal structure of [Co(NH3)e]-[FeCle] is isomorphous with that of ]Co(NH3)6][TlCl6] has been shown to be incorrect. However, the Mossbauer chemical isomer shifts for [Co(en)3][FeCl6] and [Co(NH3)e][FeCl6] of 0.60 + 0.10 and 0.59 0.10 mm./second, respectively, agree with the values found for other hexacoordinate iron(III) compounds and differ considerably from the value of 0.40 mm./... 

Fig. 1. Mossbauer spectrum showing the chemical isomer shift.

Hobson and Campbell reported that the Mossbauer spectrum of a sample of iron oxide on silica gel, which has been calcined at 500°C for 16 hr, showed (65) a small chemical isomer shift of 0.29 mm see -1 and an unusually large quadrupole splitting of 1.87 mm sec-1. This splitting was noted to be larger than 1.60 mm sec-1 calculated by Flinn et al. (17) for a ferric ion in an octahedral environment with 1 oxygen atom missing but less than 2.26 mm sec -1 calculated for the... 

Closer inspection of the Sb Mossbauer parameters (79, 19a) showed that the decreasing antimony(V) chemical isomer shifts with decreasing antimony content in materials calcined for long periods or at high temperatures were consistent with increasing electron density at the antimony nuclei. Such data are compatible with the accumulation of electron density over the cationic species as the antimony content approaches 10%. It has... 

Mossbauer studies of americium compounds have shown that changes in the chemical isomer shift, d, with the oxidation state of the americium may be attributed to increases in the electron density at the nucleus resulting from removal of 5/ electrons. ... 

The existence of a relativistic temperature-dependent contribution to the chemical isomer shift was pointed out independently by Pound and Rebka [6] and by Josephson [7]. The emitting or absorbing atom is vibrating on its lattice site in the crystal. The frequency of oscillation about the mean position is of the order of 10 per second, so that the average displacement during the Mossbauer event is zero. However, there is a term in the Doppler shift which depends on v, so that the mean value is non-zero. 

In a similar way to the chemical isomer shift and quadrupole splitting, the magnetic hyperfine effect is the product of a nuclear term which is a constant for a given Mossbauer transition, and a magnetic field which, as will now be shown, can be produced by the electronic structure. 

X-ray studies on FeCls at room temperature indicate a hexagonal unit cell. The chlorine atoms are hexagonally close-packed with the iron atoms occupying two-thirds of the possible octahedral sites in a layered array. The iron environment is not perfectly octahedral although no quadrupole splitting is resolved in the paramagnetic Mossbauer spectrum. The chemical isomer shift at room temperature [94] is 0-436 mm s which is to be compared with 0-442 mm s for FeCla-bHaO and 0-489 mm s for FeFa ... 

The complex [Fe(phen)2(CN)2] shows a smaller chemical isomer shift (0-18 mm s at 300 K) than does [Fe(phen)3](C104)2 (0-31 nun s" ) because of the greater 7i-bonding capabilities of the cyanide. The spectrum of so-called /ra 5 -[Fe(phen)2(CN)2] shows Mossbauer parameters identical to those of the cw-complex and from the arguments in Section 7.4 it can be concluded that the difference is not one of m-/ra j-isomerism [47]. 

The compound now formulated as [(OC)3FePMe2Ph]3 was found to be a trinuclear species rather than di- or tetra-nuclear from the similarity of its Mossbauer spectrum to Fe3(CO)i2 [5]. All three iron atoms have shown a change in chemical isomer shift, and this leads one to suggest that one terminal carbonyl group on each iron atom has been substituted. This has since been confirmed by an X-ray crystal structure analysis [28]. 

The first reported Mossbauer spectrum of a-Fe203 was by Kistner and Sunyar [1], who thereby recorded the first chemical isomer shift and electric quadrupole hyperfine interactions to be observed by this technique. With a single-line source the room-temperature spectrum comprises six lines from a hyperfine field of 515 kG the chemical isomer shift (Table 10.1) is... 

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