Mossbauer pairs

The most frequently used Mossbauer nuclide is Fe, originating from the Mossbauer source Co by electron capture (Fig. 10.1). Source and Mossbauer nuclide form a Mossbauer pair. The half-life of the first excited state of Fe at E = 14.4 keV is 98 ns (r = 1.4 x 10 s) and the natural linewidth is T = 4.6 10 eV. 

Ndi g5Ceo.i5Cu04 Localization-delocalization of copper pairs on Zn impurity centers in the copper sublattice of the HTc superconductor Ndj g5Ceo.i5Cu04 was observed by Zn Mossbauer emission spectroscopy... 

Fig. 8.14 Mossbauer spectra of (bt, S) recorded at 4.2 K in zero-field (a) and in a field of 50 kOe (b), of (bpym, S) recorded at 4.2 K in zero-field (c) and in a magnetic field of 50 kOe (d), of (bpym, Se) recorded at 4.2 K in zero-field (e) and in a magnetic field of 50 kOe (f). LS in [HS-LS] and [LS-LS] pairs (gray), HS in [HS-LS] pairs (light gray), HS in [HS-HS] pairs (dark gray) (from [27])...

These results obtained in applied field clearly prove that the ST in the dinuclear compounds under study proceeds via [HS-HS] O [HS-LS] O [LS-LS]. Simultaneous ST in both iron centers of the [HS-HS] pairs, leading directly to [LS-LS] pairs, apparently can be excluded, at least in the systems discussed above. This is surprising in view of the fact that these dinuclear complexes are centrosymmetric, that is, the two metal centers have identical surroundings and therefore, experience the same ligand field strength and consequently, thermal ST is expected to set in simultaneously in both centers. In other dinuclear iron(II) complexes, however, thermally induced direct ST from [HS-HS] to [LS-LS] pairs does occur and, indeed, has been observed by Mossbauer measurements [30, 31]. 

Figure 1. In situ Mossbauer spectra of the Na+ and NH4+ forms of the Si/Fe = 27 sample (the top two pairs of spectra were recorded at 300 K, the bottom pair at 490 K)...

The special feature of the spin crossover process in all bpym-bridged dinuclear compounds studied so far is the occurrence of a plateau in the spin transition curve. A reasonable assumption to account for this observation is that a thermal spin transition takes place successively in the two metal centres. However, it cannot be excluded that spin transition takes place simultaneously in the dinuclear units leading directly from [HS—HS] pairs to [LS-LS] pairs with decreasing temperature. Therefore, two possible conversion pathways for [HS—HS] pairs with decreasing temperature may be proposed [HS—HS]<->[HS—LS]<->[LS—LS] or [HS-HS] [LS-LS]. The differentiation of the existence of the [LS—LS], [HS—LS], and [HS—HS] spin pairs is not trivial and has recently been solved experimentally by utilisation of magnetisation versus magnetic field measurements as a macroscopic tool [9], and by Mossbauer spectroscopy in an applied magnetic field as a microscopic tool [11]. 

Previous attempts to distinguish between the different kinds of pairs by applying microscopic methods such as conventional Mossbauer spectroscopy were unsatisfactory, since the Mossbauer spectra corresponding to the HS state of the iron(II) atoms in the [HS-LS] and [HS-HS] spin pairs are indistinguishable. Zero-field Mossbauer spectroscopy applied to the bpym-bridged iron(II) dinuclear compounds only gives access to the total fraction... 

The main objective in carrying out LIESST experiments on (bt, S) was to elucidate the nature of excitations (metastable pairs) which appear at low temperatures after light irradiation of a ground [LS-LS] state. As is illustrated in Fig. 11a, at 4.2 K before irradiation the Mossbauer spectrum of a sample, which was enriched with 20% of 57Fe, reflects the presence of mainly LS species. The Mossbauer parameters obtained from the fitting of the spectrum are dLs=0.357(l) mms, AEq(ls)=0.452(2) mms. After irradiation of the sample for one hour (2=514 nm) at 4.2 K, the Mossbauer spectrum of (bt, S) shows a decrease in the intensity of the LS species (62.0%) in favour of an increase of the HS species (38.0%) (Fig. lib). Time-dependent measurements revealed the decay of the HS component (Fig. 11c, d), which... 

Important information relating to the intramolecular interaction in dinuclear units of [Fe(phdia)(NCS)2]2(phdia) can be drawn from the study of the LIESST effect at low temperature. The Mossbauer spectrum recorded at 4.2 K after slow cooling reveals 16.0% of HS and 84.0% of LS species (Fig. 17a). The spectrum recorded after the irradiation (1=488 nm) of the sample for one hour at 4.2 K shows an increase of the intensity of the HS doublet up to 25.0% (Fig. 17b). The spectrum recorded subsequently in a magnetic field reveals no [HS-HS] pairs it consists of 50.0% of [HS-LS] and 50% of [LS-LS] pairs (Fig. 17c). This is further evidence of the inherent stability of mixed pairs in [Fe(phdia)(NCS)2]2(phdia). ... 

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