Ion—Ion Interactions in Rare-Earth-Doped LaF3

A systematic study of quantum efficiencies and lifetimes has been carried out for the visible fluorescences of Pr3+ in single-crystal LaF3. An analysis of the results as a function of Pr3+ concentration is given in terms of a two-site model in which a Pr3+ ion can either be in an isolated site or in a site where it is coupled to other impurity ions. This model leads to ion—ion interaction ranges varying from ∼5 Å (nearest neighbor) to ∼12 Å depending on the fluorescent level studied. Throughout this range of interaction lengths, Dexter's model of electrostatic ion—ion interaction shows that the dominant contribution comes from the quadrupole—quadrupole term. Temperature effects in the interaction are consistent with the lack of exact resonance between the electronic levels. It appears that the lattice can readily absorb excess energy up to ∼1000 cm−1. A set of samples doped with a second rare earth (Ce3+, Nd3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+) has demonstrated the existence of selection rules for ion—ion interaction in addition to energy-matching requirements. The system 0.5% Pr3+: x% Ce3+ has been studied in more detail.