Abstract

A theoretical model to calculate intensities induced by the odd vibrational modes in centrosymmetric lanthanide complexes is developed and applied to octahedarl complex ions, LnX6(3-) such as occur in the hexachloroelpasolites Cs2NaLnCl6. Both the crystal field and the ligand polarisation contribution are evaluated using a standard set of symmetry coordinates.For the crystal field term a truncated expansion of the intermidiates states is employed rather than the more conventional closure approximation. Special care is necessary to ensure that the phases of the contribution are correctly determinated since the cross-term between the ligand polarisation and crystal fiel contributions is signed. General equations applicable to any f(n) complex ion are derived and an example of their application to the PrCl6(3-) ion is given. The agreement with experiment is satisfactory.