Localisation of Fe 3+ impurities in kyanite by EPR.
Résumé
An electron paramagnetic resonance (EPR) study of Fe3+ centers in kyanite AlSi2O5 has been performed on a natural blue single crystal at 34.40 GHz. Three Fe3+ centers have been observed. One of them, arising from 10% of the Fe3+ amount, presents weak signals and few resonance lines inside Kramers doublets, the weakness of the signals and the high value of the zero-field splitting between Kramers doublets allow only the determination of the second-order parameters of the spin Hamiltonian. For the two other centers all second- and fourth-order parameters are determined. The comparison of the pseudo-symmetries of the fourth-order parameters between EPR results and crystal field calculation does not allow one to define the location of all Fe3+ centers at Al octahedra. It is proved that Fe3+ is partly located at Al1 position (25%). For the other observed centers it was not possible to define their location in the structure. This study is a good example of the limitation of the pseudo-symmetry method.