Electron paramagnetic resonance measurements were performed, at room temperature, on crystals that were doped with Gd3+ or co-doped with Gd3+ and Li+. In the case of crystals doped only with Gd3+, a spectrum with tetragonal symmetry (A-center) was observed. In the case of co-doped crystals, new spectra with tetragonal (B-center) and monoclinic (C-center) symmetries were observed instead of the spectrum of the A-center. The latter was identified as being a substitutional Gd3+ ion on a Zn2+ site, in 6-fold coordination, without any local charge compensation in its near vicinity. According to a spin Hamiltonian separation analysis, the b2a(1) parameter for the C-center had a value that was close to the b20 parameter for the B-center. The B- and C-centers in co-doped crystals were attributed to a Gd3+ ion which replaced a Tl+ ion in 9-fold coordination, with the divalent excess positive charge on Gd3+ being compensated by a Li+ ion at a Zn2+ site along the c-axis. In the case of the C-center, another association with a Li+ ion was found at the nearest Zn2+ site along the [111] direction. The other parameter, b2a(2), for the C-center had the opposite sign and 6 times the magnitude of b20 for a Gd3+ center with trigonal symmetry along the [111] direction in RbCaF3. It was suggested that the large negative value was due to deviation of the ligands towards the Li+ ion at the nearest Zn2+ site.

EPR Study of Gd3+ Centers in Tl2ZnF4 Crystals. M.Arakawa, T.Nakano, H.Ebisu, H.Takeuchi: Journal of Physics - Condensed Matter, 2003, 15[22], 3779-92