Ligand field parameters for distorted environments

The ligand field parameter for cations in a regular octahedral environment, 10 Dq, expressed by eq. (2.17), 

The Dt parameter is a measure of the difference between the equatorial and axial Dq parameters. In an elongated octahedron, the energy separations between pairs of 3d orbital energy levels are expressed by 

3 Energy level diagrams and crystal field spectra 

Analogous expressions apply to crystal field states for electronic configurations involving these orbitals (cf. fig. 3.13). These energy separations are shown in fig. 3.21. All three parameters, Dq(eq)y Dt and Ds, may be determined experimentally provided that energies of three absorption bands originating from transition metal ions in tetragonally distorted octahedral sites occur in the crystal field spectra. Equation (3.15) yields the Dq( a) parameter, so that the mean Dq parameter, Dq(m) may be calculated from 

This method of evaluating ligand field parameters for transition metal ions in tetragonally distorted octahedra is particularly applicable to minerals containing Mn3+ and Cr2+, as well as Fe2+ ions in some very distorted octahedra, because their crystal field spectra enable 8j, 82 and 83 (eq. 3.16)) to be determined experimentally. Examples are discussed in chapter 5 particularly when crystal field spectra of Mn3+-bearing minerals are described ( 5.10.5). 

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