EPR of Gd3+ in single crystal colquiriite and analysis of the spin Hamiltonian tensors B4 and B6

In single crystal colquiriite LiCaAlF6 doped with Gd3+ ions two EPR spectra of the Gd3+ ions with the Laue site-symmetry groups C-i and C-3i were observed. The spectrum angular dependence for trigonal Gd3+ 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 Gd3+ with the Laue group C-3i substitutes at Ca2+ 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 Gd3+ substituted in a variety of host crystals, the fourth-rank and sixth-rank tensors of Gd3+ spin Hamiltonians were tabulated and correlated with structures of the coordination polyhedra at substitution sites. The results suppose a predominance of quadratic crystal field contributions into the spin Hamiltonian tensor B-4 of Gd3+.