Non-equivalent Pr³⁺ centres in Gd₃Ga₁₅O₁₂

The Pr³⁺ spectra of Gd₃Ga₅O₁₂ Czochralski-grown crystals, investigated by absorption, site-selective excitation and emission and fluorescence decays, shows a complex structure. Besides the spectra assigned to Pr³⁺ in dodecahedral c sites (D₂ local symmetry), Pr³⁺ in other sites is also significant. An attempt to assign the most intense lines to structural defects is made. Three of the most intense satellite lines are assigned to a Pr³⁺(c) ion perturbed by a nearby non-stoichiometric defect (Gd³⁺ in octahedral a sites) whose concentration is estimated to be about 7%. The spectra for perturbed Pr³⁺ centres show additional lines connected with a reduction in local symmetry. Two other lines have been assigned to Pr³⁺(c)-Pr³⁺(c) pairs. The ³P₀ emission decays are almost exponential with similar lifetimes, except one that presents a very strong quenching. This line has been assigned to strongly coupled Pr³⁺(c)-Pr³⁺(c) nearest-neighbour pairs. The influence of pair quenching on the relative quantum efficiency of ³P₀ emission is also estimated.