Energy crystal-field symmetry

The above equations have been obtained on the assumption that no orbital states have energies close to that of the ground state. This means that they should be applicable to d3, d5, and d8 for crystal fields which are close to octahedral in symmetry. They should be applicable to d4 and d9 also, when the distortion from octahedral symmetry is tetragonal, since in this case matrix elements of are zero between the ground state and the nearby excited state, d2, d6, and d1 in octahedral symmetry must be treated in a manner similar to that used for dl in Sec. III.D. For other crystal-field symmetries, the treatment used depends on whether the crystal field gives low-lying excited states that have nonzero matrix elements of with the ground state. 

In an octahedral crystal field, for example, these electron densities acquire different energies in exactly the same way as do those of the J-orbital densities. We find, therefore, that a free-ion D term splits into T2, and Eg terms in an octahedral environment. The symbols T2, and Eg have the same meanings as t2g and eg, discussed in Section 3.2, except that we use upper-case letters to indicate that, like their parent free-ion D term, they are generally many-electron wavefunctions. Of course we must remember that a term is properly described by both orbital- and spin-quantum numbers. So we more properly conclude that a free-ion term splits into -I- T 2gin octahedral symmetry. Notice that the crystal-field splitting has no effect upon the spin-degeneracy. This is because the crystal field is defined completely by its ordinary (x, y, z) spatial functionality the crystal field has no spin properties. 

An S term, like an s orbital, is non-degenerate. Therefore, while the effect of a crystal field (of any symmetry) will be to shift its energy, there can be no question of its splitting. The ground term for the configuration is S. In an octahedral crystal field, this is relabelled Aig, in tetrahedral symmetry, lacking a centre of inversion, it is labelled M]. 

Figure S6.2 Crystal field splitting of the energy of five d orbitals when the ion is placed in a site with octahedral symmetry. The magnitude of the splitting, A or 10Dg, depends upon the size of the octahedral site and the charges on the surrounding ions.

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