Electron paramagnetic resonance measurements were made of Gd3+ centers, in monocrystals at room temperature, by using an X-band spectrometer. In crystals which were doped with Gd3+, spectra with both cubic and tetragonal symmetry were observed. These centers were attributed to Gd3+ ions which substituted for Rb+ ions in cubic crystals, with no local charge compensation and with a vacancy at the nearest Rb+ site, respectively. In some as-grown crystals which were co-doped with Gd3+ and Li+, signals from a new trigonal center were observed instead of those from cubic and tetragonal centers in the cubic phase. The trigonal center was identified as being a Gd3+-Li+ pair which substituted for the Zn2+-Zn2+ bond in a Zn2F9 unit in the hexagonal phase. It was recalled that the cubic perovskite structure was normally stabilized at room temperature, and the results suggested that the Gd3+-Li+ pairs in Zn2F9 units might be related to growth of the hexagonal structure in the matrix.

M.Arakawa, H.Ebisu, H.Takeuchi: Journal of Physics - Condensed Matter, 1997, 9[24], 5193-204