Transferts énergétiques entre les ions Eu³⁺ ET UO₂²⁺ dans Ca_1-xEux/2Na_x/2UMo₄O₁₆

¹ Laboratoire des Eléments de Transition dans les Solides, CNRS 1 place A. Briand, 92195 Meudon Cedex France
² Laboratoire de Cristallochimie du Solide, Université P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05 France

Résumé

La fluorescence du molybdate Ca_1-x Eu_x/2Na_x/2 UMo₄0₁₆ (0 ≤ x ≤ 0,06) est étudiée à 77 K. Par spectroscopie en temps résolu et sous excitation sélective, on détermine pour Eu les niveaux associés aux transitions ⁵D_0,1 → ⁷F_0,1,2,3, ainsi que quatre niveaux émetteurs pour de UO₂²⁺. Les transitions présentent des raies vibroniques (v_as = 899cm^-1, V = 815 cm^-1,v_def = 250cm^-1). Plusieurs mécanismes de transfert sont proposés. Ils justifient l'intense fluorescence rouge émise quelle nue soit la fréquence d'excitation.

Abstract

The fluorescence of Ca ^1-x Eu_χ/2Nа_х/2UMo₄O₁₆ (0 ≤ x ≤ 0,06), with the Eu and active ions, has been studied at 77 K. By means of time-resolved spectroscopy and under selective excitation, all the levels involved in the transitions ⁵D_0,1 → ⁷F_0,1,2,3 of Eu³⁺are given, and four emetting levels of have been investigated. In agreement with the C₂ low symmetry, all the degeneracies are lifted. Nevertheless, the five fold pseudosymmetry of the Eu³⁺ site appears in the distribution of the sublevels. The intrinsic emissions of U022+ are always accompanied by vibronic lines of various orders including mainly the vibrations : v_as = 899cm^-1,v_s = 815cm^-1 and v_b = 250cm^-1. Cross resonance processes of energy transfer between the two ions have been proposed. The excitation of the level transfers to the uranyl ion, and conversely the ⁵D₂ level is populated by an excited level of U02 2+ • This explains the very strong red fluorescence (⁵D₀ → ⁷F₂) issued from the compound, independently of the frequency of the exciting radiation.