Energy transfer in europium and terbium compounds formed with (Cr(OH)$_6$ Mo$_6$O$_{18}$)$^{3-}$ and (Cr$_{0.1}$Al$_{0.9}$(OH)$_6$ Mo$_6$O$_{18}$)$^{3-}$ heteropolyanions

The new compounds Ln[M(OH)(6)Mo6O18].11H(2)O (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)(6)Mo6O18].11H(2)O and Tb[Cr(OH)(6)Mo6O18].11H(2)O. 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) = 1.33x10(6) s(-1) from Tb3+ to Cr3+ ions in Tb[Cr(OH)(6)Mo6O18].11H(2)O at 12 K is obtained