Crystal effective point charge model

How to Determine the Crystal-Field Parameters 3. The Effective Point Charge Model... 

Various other experiments have been performed on HoP, including nuclear magnetic resonance (Jones 1969) and distortion measurements (Levy 1969). The distortion turned out to be very small. The crystal electric field fits in an effective point-charge model (Birgeneau et al. 1973). 

Brill et ah IS21 investigated the 3SC1(NQR) spectrum and used the point charge model to calculate the EFG.Semiempirical LCAO-MO calculations have also been used. An empirical Stemheimer factor of -10 for chlorine was used to explain the crystal field effects which amount to 10 % for y(35Cl) in [SnCl6]2. ... 

The crystal field splitting in a tetrahedral field is intrinsically smaller than diai in an octahedral held because there are only two-thirds as many ligands and they have a less direct effect on the d orbitals. The point-charge model predicts that for the same metal ion, ligands, and metal-ligand distances. As a result, orbhal splitting... 

The simplest description of the effect of pressure on crystal field strength assumes that the variation of Dq with R is given by the point charge model (Eq. 12) and that the variation of R with P can be directly obtained from the volume of the host lattice through its equation of state. According to this approach, the predicted effect of pressure on crystal field strength is given by... 

The point-charge model can be improved by considering the effects of higher multipole moments in eq. (46). Dipole and quadrupole contributions (terms with k - and 2 in eq. (46)) have been evaluated for PrCL (Hutchings and Ray, 1963). It is possible to include the dipole contributions to the crystal field in a self-consistent manner (Morrison, 1976) in that treatment, point charge and dipole contributions to the electric field at various lattice sites are evaluated, and the dipole moments at these sites... 

Pronounced crystal-field effects were observed in the paramagnetic phase NMR for intermetallic compounds of Ce, Pr, Sm or Tm. Crystal-field effects for Tm in TmAlj have been analysed by de Wijn et al. (1970) by a combination of the temperature dependences of the Al Knight shift and of the magnetic susceptibility. These authors also analysed the quadrupole interaction for the Al site in the cubic CujAu structure. Comparing the experimental value e qQ/h = 7.6 MHz with the point-charge model value (Abragam 1961)... 

This would not be expected simply on the basis of a crystal-field model, for the d orbitals will contract with increasing positive charge and hence interact less well with the ligand point charges . The modest decreases in bond length as one traverses the series (Eq. 6.9) are unlikely to compensate for, let alone override, the effects of such orbital contraction. Finally, to add to the confusion, we also note from Eq. (6.7) that zio -t values increase as we go down the periodic table (Eq. 6.10). 

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