Dopant-host relationships in rare-earth oxides and garnets doped with trivalent rare-earth ions -: art. no. 195108

The dopant-host interactions were considered in the Eu3+- and R3+-doped cubic C-type rare-earth oxides (R2O3) as well as in nondoped R3Ga5O12 and R3+-doped yttrium gallium garnets (Y3Ga5O12:R3+). The investigations were based mainly on the analyses of optical absorption and luminescence spectra using both new and previously published data. The concept of the crystal field (CF) strength parameter N-v was used in the considerations. When a R3+ dopant different from the host cation is embedded into the host lattice, a decrease in the R3+ site symmetry, distortions, and/or stresses are usually introduced. These local effects were found to affect significantly the strength of the crystal field in both R3+-doped Y2O3 and Y3Ga5O12 when compared to nondoped R2O3 and R3Ga5O12. With increasing positive size mismatch between the dopant and the host cation, the CF effect was found to be enhanced. The origin of this effect may be of electrostatic or covalent nature due to the decrease in dopant-oxygen distances or the delocalization of the 4f electrons, respectively. High-pressure investigations on Eu3+-doped R2O3 support the present conclusions. The formation of neighbor R3+-R3+ pairs in doped garnets induces additional lines in the optical spectra. For large ions such as Pr3+ and Nd3+ in Y3Al5O12 or Gd3Ga5O12, the pair lines compared to isolated ions result from modifications in the CF effect due to local distortions. The superexchange interaction for near-neighbor Pr3+ and Nd3+ pairs is manifested in dynamical behavior.