Interpretation and Computation of Electric Field Gradients

The experimentally observed quadrupole splitting AEq for Fe in inorganic compounds, metals, and solids reaches from 0 to more than 6 mm s [30, 32]. The range of AEq for other Mossbauer isotopes may be completely different because of the different nuclear quadrupole moment Q of the respective Mossbauer nucleus, and also because the EFG values may be intrinsically different due to markedly different radial distributions of the atomic orbitals (vide infra). As Q is constant for a given isotope, variations in the quadrupole coupling constants eQV can only arise from 

Although the EFG of a given system can be easily determined from a Mossbauer spectrum, it may be rather difficult to relate it to the electronic structure of the Mossbauer atom. In order to visualize a few typical cases, the computation of the EFG is described in the following for some selected charge distributions. A comprehensive quantum chemical interpretation of the quadrupole sphtting will be given in Chap. 5. 

The influence of a noncubic electronic charge distribution interacting with a Mossbauer nucleus may be exemplified by using point charges, for which the EFG is easy to calculate. A point charge 7 at a distance r = +y from a 

a charge q located on the z-axis at point (0, 0, r) produces an EFG at the nucleus (Fig. 4.7b) which has the from 

The off-diagonal elements are zero for this arrangement and Vxx = Vyy = — l/214z- As expected, the adopted coordinate system is a principal axes system (PAS) of the EFG, and the asymmetry parameter rj is zero. 

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