The zero-field splitting of the S state due to the virtual electron transfer processes from the ligands into an unfilled shell (l n ) of a paramagnetic ion has been calculated. A simple analytical formula has been deduced for the spin-Hamiltonian parameters.
Abstraet. In single crystal colquiriite LiCaA1F 6 doped with Gd 3+ ions two EPR speetra of the Gd 3+ ions with the Laue site-symmetry groups C i and C3i were observed. The spectrum angular dependente for trigonal Gd 3+ centre was investigated in detail and corresponding spin Hamiltonian parameters were fitted. From analysis of the spin Hamiltonian tensors B 4 and B 6 it was established that Gd 3 § with the Laue group C3~ substitutes at Ca 2 § site with the excess charge compensation by an ion located along the threefold axis from this site. The transformation formulas for a sixth-rank irreducible Hermitian tensor under coordinate rotation are tabulated in an explicit form. By using the EPR data for Gd 3~ substituted in a variety of host crystals, the fourth-rank and sixth-rank tensors of Gd 3+ spin Hamiltonians were tabulated and correlated with structures of the coordination polyhedra al substitution sites. The results suppose a predominante of quadratic crystal field contributions into the spin Hamiltonian tensor B 4 of Gd 3+.
In single crystals of LiCaA1F 6 and LiSrAIF6 doped with Fe" the trigonal EPR spectra with multiplicity KM = 1 were observed due to Fei substituted for Al'. The spin Hamiltonian parameters describing the fine structure and the superhyperfine structure were determined. It is argued that the tensors B2 and B4 of the spin Hamiltonian for Fei+ ions are essentially determined by the quadratic contributions of the crystal field at the substitution site. The signs and the relative values of the elements in the spin Hamiltonian tensor B4 of rank L = 4 for Fe' are determined by the irreducible tensor product [V4 ®V4]2 of the crystal field tensor V4 of rank L = 4 at the substitution sites. The ratio between the invariant sum of the spin Hamiltonian tensor B4 for Fe" in oxygen octahedra [Fe06] and that in fluorine octahedra [FeF6] is directly proportional to the fourth power of the ratio between the effective charges of surrounding ions. The sign of the spin Hamiltonian parameter B20 corresponds to the sign of the element [V4 ®V4]20 in the irreducible tensor product [V4 ®V4] 2 of rank L = 2.
Recently the first all-solid-state tunable UV laser with direct pumping has been reported [1) This laser is based on the allowed 5d-4f interconfigurational transitions of Ce3+ ions in LiCaAlFF6 (LiCAF:Ce) single crystal. Due to the geterovalent activation nature LiCAF:Ce and LiSrAlF6:Ce3+ (LiSAF) [2] laser media appeared to be quite complex subject for spectroscopic investigations with their laser characteristics strongly dependent on the growth conditions as well as on the presence of charge-compensating additions [2, 3]. Therefore investigations of a variety of the rare earth structure-nonequivalent doping centers in a LiCAF/LiSAF single crystals is of great practical importance.
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