Concentration dependence of energy transfer between Eu³⁺ ions occupying two symmetry sites in Lu₂O₃

A series of Lu₂O₃ samples doped with Eu was prepared in the form of sintered tablets. The activator concentration was 0.2, 1, 3, 5, 7 and 10% with respect to Lu. Emission spectra excited with light of various energies were taken at liquid nitrogen and room temperature. Excitation spectra of characteristic Eu emission lines for two different symmetry sites in Lu₂O₃ (C₂ and S₆) were also recorded. It was shown that Eu ions located in the two different symmetry sites possess characteristic excitation spectra in the regions both of the ⁷F₀→⁵D₁ absorption and of the charge transfer transitions. In both cases the levels of the Eu(S₆) ions were located above those of the Eu(C₂). In both emission and excitation spectra an energy transfer from Eu ions occupying the centrosymmetric S₆ site in Lu₂O₃ to Eu ions in the noncentrosymmetric C₂ site was observed. Some indications were found that an opposite back-transfer (from Eu in C₂ to Eu in the S₆ site) also occurs. Lower temperature significantly reduces the rate of the Eu(S₆)→ Eu(C₂) transfer. Decay kinetics indicated that even at room temperature the Eu(S₆) emission could be further reduced if the dopant content was higher than 10%. Both at room and liquid nitrogen temperatures the decay kinetics of the Eu(C₂) emissions are characterized by the time constants of about 1.0 ms for all concentrations, indicating that up to 10% of the dopant content no concentration or thermal quenching takes place. The liquid nitrogen Eu(S₆) luminescence is characterized by a time constant of about 7 ms for the 0.2% specimen. At room temperature this value decreases to 5.6 ms.