Sternheimer antishielding

For nuclei that have perfect cubic site symmetry (e.g., those in an ideal rock salt, diamond, or ZB lattice) the EFG is zero by symmetry. However, defects, either charged or uncharged, can lead to non-zero EFG values in nominally cubic lattices. The gradient resulting from a defect having a point charge (e.g., a substitutional defect not isovalent with the host lattice) is not simply the quantity calculated from simple electrostatics, however. It is effectively amplified by factors up to 100 or more by the Sternheimer antishielding factor [25],... 

Consider first the effect of the atomic electrons. A filled or half-filled electron shell has a spherically symmetric electron distribution, and as such gives rise to no electric field gradient (except through external deformation, i.e., Sternheimer antishielding). Thus, of all the atomic electrons, only the... 

Here ay is the second-order Stevens factor, is the Sternheimer antishielding factor and a2 is a screening constant. 

In practice, one does not examine naked nuclei, but nuclei in atoms. Because of the intervening electrons, the nuclei are submitted usually to a magnified efg (an Ji 2 perturbation). This screening due to the atomic electrons leads to an antishielding effect which was first calculated by Sternheimer (2-5). Since the original work of Sternheimer (2-5) ca. 30 years of work, in numerous papers, have been devoted to this topic. Fortunately, a recent review is available (6). The Sternheimer antishielding factors Yoo have been calculated recently by a fully self-consistent Hartree-Fock treatment for the alkali metal cations, and we shall quote these results (7) (Table 5). 

Table 5. Sternheimer Antishielding Factors for Alkali Metal Ions with the Rare Gas Electronic Configurations (7)...

The example of Na illustrates the problem in locating in the literature accurate values of the quadrupole moment Q and of the Sternheimer antishielding factor Yoo After undertaking a critical survey, I feel that values of Q close to 0.15 barn (0.15 X 10-28 2) are erroneous. Taking the experimental values obtained for the atom in the 3p or 4p state (13,14) and correcting for the antishielding contribution yields values of Q 0.10 X 10-28 ra (15), in excellent accord with results from calculation (16). 

Perhaps the most serious limitation on both these approximate calculations is that the influence of both outer bound and coordinated charges must be scaled by a factor 7oo iu order to allow for the amplifying effect of inner electron motions on the externally applied field gradient. is called the Sternheimer antishielding factor, and lies typically between two and ten. Yet a further efg arises from the polarization of the inner orbitals by the electric quadrupole of I itself, but this term may readily be allowed for in any calculation. 

Sternheimer antishielding factor Chemical shift/ppm Difference (e.g., AS)... 

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