Energy transfer in europium and terbium compounds formed with [Cr(OH)₆Mo₆O₁₈]³⁻ and [Cr_0.1Al_0.9(OH)₆Mo₆O₁₈]³⁻ heteropolyanions

The new compounds Ln[M(OH)₆Mo₆O₁₈]·11H₂O (Ln = Tb, Eu and M = Al, Cr) were synthesized and their luminescence properties analysed and discussed. Results show that there is a very efficient energy transfer from Eu³⁺ (Tb³⁺) to Cr³⁺ ions in Eu[Cr(OH)₆Mo₆O₁₈]·11H₂O and Tb[Cr(OH)₆Mo₆O₁₈]·11H₂O. The transfer strongly quenches Eu³⁺ (Tb³⁺) luminescence and greatly shortens the lifetimes of the radiative levels. By using selective excitation at low temperature, the transfer mechanisms are investigated and models are presented. Based on the above model, a rate of ET k_C = 1.33 × 10⁶ s⁻¹ from Tb³⁺ to Cr³⁺ ions in Tb[Cr(OH)₆Mo₆O₁₈]·11H₂O at 12 K is obtained.