Site Assignment of Fe3+ in Low Symmetry Crystals. Application to NaAlSi3O8

A new method of site assignment of Fe³⁺ in low symmetry crystals is presented. This method is based on the fact that the fourth order spin‐Hamiltonian constants of Fe³⁺ are sensitive mainly to the immediate surroundings of the Fe³⁺. It is shown how it is possible to find the pseudosymmetry of the fourth order term of the spin Hamiltonian, while from the pseudosymmetry it is possible to define the orientation of the $\mathrm{Fe}–{\mathrm{O}}_n$ complex in the structure and to localize Fe³⁺ in the crystal. To use this method the spin‐Hamiltionian constants must be well known. The determination of the 20 constants of a general spin Hamiltonian is described. To apply our method in a low symmetry case, we studied Fe³⁺ in NaAlSi₃O₈, which has a triclinic lattice. Experiments were done on a single crystal at Q band frequency. The spin‐Hamiltonian constants are given. It is shown that Fe³⁺ is in a T₁(0) site and substitutes for Al³⁺.