Electron paramagnetic resonance measurements were performed on as-grown monocrystals which were doped with Gd3+. A new spectrum, with a large fine-structure splitting of orthorhombic symmetry, was observed in each crystal. The new center was attributed to a Gd3+ ion which replaced a Cd2+-F- divalent cation, with an O2- ion at its ligand site, in the c-plane. In the case of Rb2ZnF4, no signals from the uncompensated Gd3+ center were observed. This contrasted with the cases of Cs2CdF4 and Rb2CdF4. The formation of a Gd3+-O2- center in Rb2ZnF4 was explained in terms of a spin-Hamiltonian separation analysis. The ligand octahedron was deduced to be elongated, along the c-axis, by the presence of the nearest-neighbor O2- ion in the c-plane. The similar formation of a Gd3+-O2- center occurred in Rb2ZnF4, without the formation of an uncompensated Gd3+ center. This center was suggested to be the first reported example of Gd3+ substituting for Zn2+, with 6-fold coordination, in perovskite fluorides. The substitution of Gd3+ for Zn2+ was permitted only by the presence of O2- at the ligand site on a- or b-axes. The elongated ligand octahedron made it possible for a Gd3+ ion to substitute for Zn2+, as the space available for Gd3+ was thereby increased.

M.Arakawa, H.Ebisu, H.Takeuchi: Journal of Physics - Condensed Matter, 1996, 8[50], 11299-308