Electric field gradient lattice contribution

In Fe2+, the situation is more complicated. Here the six 3d-electrons dominate the magnitude of the electric field gradient, though in a way that is determined by the lattice symmetry. If a Fe2+ ion comes in a more asymmetric environment, the quadrupole splitting decreases in general, because the lattice contribution to the electric field gradient is smaller than the electronic contribution, and has the opposite sign. 

Other effects frequently encountered in inorganic systems that can severely affect line shape involve relaxation processes arising from interactions of nuclear quadrupole moments with electric field gradients. For quad-rupolar nuclei (I 1), the quadrupolar contribution to the spin-lattice relaxation time Ti is given approximately by... 

The quadrupolar splittings depend mainly on the electric field gradient at the nucleus, which is caused by the spatial distribution of the electrons around it, via electric Coulomb forces. Both electrons (valence contribution f/yai) and neighboring anions and cations (lattice contribution U x) in the vicinity of the nucleus contribute to this electric field gradient U, which can be expressed as follows ... 

As Perlow has mentioned, at least two corrections were neglected in his calculation of charges. One is the dependence of (r- >5p on charge state" (which may be as large as 15% increase per charge removed) the other is the lattice contribution to the electric field gradient. The field gradient q is written as... 

In 1996, Munn extended the microscopic theory of bulk second-harmonic generation from molecular crystals to encompass magnetic dipole and electric quadrupole effects [96] and included all contributions up to second order in the electric field or bilinear in the electric field and the electric field gradient or the magnetic field. This was accomplished by replacing the usual polarization of Refs. 72 and 84 by an effective polarization as well as by defining an effective quadrupole moment. Consequently, the self-consistently evaluated local electric field and electric field gradient were expressed in terms of various molecular response coefficients and lattice multipole tensor sums (up to octupole). In this... 

Generally the valence term is the major contribution to the electric field gradient unless the ion has the high intrinsic symmetry of an iS-state ion such as high-spin Fe ( i ). In the latter case the lattice term will be dominant. 

The direction of the spin axis with respect to the molecular axes has not been determined. The electric field gradient cannot be generated by the nonbonding electrons in the quartet term, and presumably originates from lattice contributions, but no analysis has been attempted. 

Evidence for magnetically induced nuclear quadrupole interaction Further evidence for non-s, especially orbital contributions to the hyperfine interaction in magnetically ordered intermetallic compounds is obtained from a closer inspection of nuclear quadrupole interactions. For an electric field gradient (EFG) of axial symmetry eq = caused, e.g., by the low-symmetry arrangement of ionic charges in the crystal lattice, the quadrupole interaction of the nuclear spin / with the quadrupole moment eQ (Barnes 1979, McCausland and Mackenzie... 

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