Abstract

The trivalent Pr ions substitute the divalent Cd ions. For reasons of charge compensation and the quasilinear structure of the crystal lattice only two types of ion pairs are formed predominantly, the weakly coupled symmetric pair Pr<SUP>3+</SUP>-Cd vacancy-Pr<SUP>3+</SUP> and the strongly coupled asymmetric pair Pr<SUP>3+</SUP>-Pr<SUP>3+</SUP>-Cd vacancy. Only the latter one showed up-conversion. The dynamics of the up-conversion cannot be explained in the standard picture of direct ion-ion cross relaxation. An alternative explanation is proposed in which electronic excitations of the host lattice are generated by a cooperative energy transfer from a doubly excited ion pair to the lattice. Up-conversion results by the back transfer of the energy to only one ion of the pair by the process of lowest order.

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