Tetragonal field, spin-orbit coupling

Figure 18 Splitting of a ground term by (a) cubic ligand.field, (b) tetragonal ligand field component, (c) spin—orbit coupling in second order and (d) magnetic fields parallel and perpendicular to the tetragonal component axis...

Fig. 4. The energy of the T2g multiplet as a function of the tetragonal field and the spin-orbit coupling, A. The negative and positive values of AJA correspond to a tetragonal compression and elongation respectively. The energy separations indicated on the left and right hand sides of the figure are the limiting values...

The actual S5nnmetry of the distorted MeFs units in most cases is lower than D4ft. But to describe the electronic spectra it will be sufficient to consider only the tetragonal field component, as minor structural differences disappear within the width of the absorption bands. The same holds for the splitting of terms through spin-orbit coupling which therefore can be neglected. 

Fig. 1. Lowest energy level splittings in paramagnetic

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