Mossbauer spectroscopy electric field gradient

Relativistic effects in core properties like electric field gradients can be extremely large [103]. As the isotope 197-Au is widely used is Mossbauer spectroscopy [104], an... 

Spiering, H. The Electric Field Gradient and Quadmpole Interaction. In Long, G. (ed.) Mossbauer Spectroscopy Applied to Inorganic Chemistry, p. 79. Plenum, New York (1984)... 

For nuclei possessing an electric quadrupole moment, the electric field gradient at the atomic nuclei can be measured accurately by techniques such as nuclear quadrupole resonance, Mossbauer spectroscopy, nuclear magnetic resonance, and, for gaseous species, by microwave spectroscopy. The diffraction data permit an... 

Maruthe,V.R. Trautwein, A. (1983) Calculation of charge density, electric field gradient and internal magnetic field using molecular orbital cluster theory. In Thosar, B.V. (ed.) Advances in Mossbauer spectroscopy. Elsevier, Amsterdam, 398-449 Matijevic, E. Cimas S. (1987) Formation of uniform colloidal iron(lll) oxides in ethylene... 

Mossbauer spectroscopy has proved to be a very valuable tool in tin chemistry. The isomer shift (IS) is a measure of the 5s electron density at the tin nucleus while the quadrupole splitting (QS) is a measure of the electric field gradient (due to the asymmetry of the electron cloud) at the tin nucleus. On going from the plus two to the plus four oxidation state tin loses 5s electron density. Since the tin nucleus expands on absorption of the 23.9 keV photon during the Mossbauer experiment, the greater the s electron density at the nucleus the more positive the isomer shift will be (56, 57). Therefore, tin(II) compounds have higher isomer shift values than do tin(IV) compounds. The isomer shift of /3 tin (2.56 mm/sec relative to BaSnOa) is commonly used as the dividing point between divalent and tetravalent tin (58-60). 

Figure 14 shows three Fe case studies of the time behavior of the photons reemitted in the forward direction and a comparison with the typical spectra obtained in Mossbauer spectroscopy. Figure 14a corresponds to the case for which there is no hyperfine interaction. The nuclear levels are not split, and only one transition between ground and excited state is possible. In that case, the Mossbauer spectrum shows a single-absorption line and contains only y-quanta of equal energy. In the presence of an electric field gradient (Fig. 14b), the splitting of the excited state is... 

The quadrupole splitting A Eg is the first Mossbauer parameter that could be predicted with reasonable accuracy by electronic structure calculations. The quadrupole sphtting can be related to the EFG at the Mossbauer nucleus according to equation (12). Since the nuclear quadrupole moment Q is very difficult to determine by experiment alone, the combination of Mossbauer spectroscopy and density functional calculations is currently the most accurate approach to estimate Q. A linear regression of measured quadrupole splittings and calculated electric field gradients has been performed by Blaha et al. for a large series of compounds... 

CEMS = conversion electron Mossbauer spectroscopy DFT = density functional theory EFG = electric field gradient EPR = electron paramagnetic resonance ESEEM = electron spin echo envelope modulation spectroscopy GTO = Gaussian-type orbitals hTH = human tyrosine hydroxylase MIMOS = miniaturized mossbauer spectrometer NFS = nuclear forward scattering NMR = nuclear magnetic resonance RFQ = rapid freeze quench SAM = S -adenosyl-L-methionine SCC = self-consistent charge STOs = slater-type orbitals TMP = tetramesitylporphyrin XAS = X-ray absorption spectroscopy. 

Mossbauer spectroscopy is also able to give local moment orientations, with respect to the crystalline lattice, or the correlations between moment orientations and local distortion axis orientations in a chemically disordered or amorphous material. This arises from the interplay between the structural (electric field gradient) hyperfine parameters and the magnetic hyperfine parameters. In this way, the spin flop Morin transition of hematite, for example, is easily detected and characterized (e.g., Dang et al. 1998). The noncollinear magnetic structures of nanoparticles can also be characterized. 

One less-well-known technique, which has many experimental aspects in common with Mossbauer spectroscopy, deserves special attention at this point, since it gives valuable information about the electric-field gradients and the magnetic hyperfine interactions of radioactive nuclei in solids at ambient conditions and under pressure. In this technique, two y-rays with different energies from two different transitions of an individual nucleus in a radioactive-decay cascade are recorded consecutively. The spatial and temporal perturbation of the emission probability by the hyperfine fields is registered in the corresponding perturbed angular correlation (PAC) spectra. 

Tennant WC, Finch J, Aldridge LP, Gairtsford GJ (1992) The electric field gradient and mean squared displacement terrsor in IM biotites investigated by Mossbauer spectroscopy. J Phys Cond Matter 4 5447-5459... 

The existence of an electric quadrupole interaction is one of the most useful features of Mossbauer spectroscopy. The energy levels in the presence of an electric field gradient (e.f.g.), q, are ... 

YbNiSn was intensively investigated by °Yb Mossbauer spectroscopy in the paramagnetic and in the ferromagnetically ordered state (Bonville et al. 1992, Bellot et al. 1992). The 1.48 K spectrum shows hyperfine field splitting with five well-resolved lines with an experimental linewidth of about 3 mm/s (fig. 16). Due to the non-cubic ytterbium site, a quadrupolar hyperfine term with an electric field gradient is observed. In the paramagnetic phase (6K measurement) the spectrum shows two resolved lines which result from quadrupole splitting. 

Besides the isomer shift, the experimental linewidth and the electric field gradient, information about the crystal-field anisotropy can also be obtained from Yb Mossbauer spectroscopy, as shown by Bonville et al. (1990) and Bonville and Hodges (1985) for YbCuAl. Furthermore, YbCuAl was investigated by Yb Mossbauer spectroscopy at pressures up to ISOkbar. The data give strong evidence for a valence transition towards the 4f (Yb ) configuration. At 4.2 K the transition is completed at about SOkbar (Schdppner et al. 1986). This behavior is paralleled by the pressure-dependent susceptibility measurements (Klaasse et al. 1977). 

A more general treatment of Mossbauer effect spectroscopy in amorphous solids is presented in the reviews by Coey (1974) and Litterst and Kalvius (1975). Details regarding the electric field gradient in amorphous solids are treated in the papers by Lines (1982) for Fe and for Gd in the paper by Czjzek et al. (1981). 

Spieting H. (1984) The Electric Field Gradient and the Quadrupole Interaction, In Long GJ (ed), Mossbauer spectroscopy applied to inorganic chemistry, Vol. 1. Plenum Press, New York, p 77. 

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