The analysis of spin Hamiltonian and crystal field tensors for Fe3+ in crystals of LiCaAlF6 and LiSrAlF6

In single crystals of LiCaAlF6 and LiSrAlF6 doped with Fe3+ the trigonal EP R spectra with multiplicity K-M=1 were observed due to Fe3+ substituted for Al3+. The spin Hamiltonian parameters describing the fine structure and th e superhyperfine structure were determined. It is argued that the tensors B -2 and B-4 of the spin Hamiltonian for Fe3+ ions are essentially determined by the quadratic contributions of the crystal field at the substitution si te. The signs and the relative values of the elements in the spin Hamiltoni an tensor B-4 of rank L = 4 for Fe3+ are determined by the irreducible tens or product [V-4 x V-4](2) of the crystal field tensor V-4 of rank L = 4 at the substitution sites. The ratio between the invariant sum of the spin Ham iltonian tensor B-4 for Fe3+ in oxygen octahedra [FeO6] and that in fluorin e octahedra [FeF6] is directly proportional to the fourth power of the rati o between the effective charges of surrounding ions. The sign of the spin H amiltonian parameter B-20 corresponds to the sign of the element [V(4)xV(4) ](20) in the irreducible tensor product [V(4)xV(4)](2) of rank L = 2.